The Emergence of Staphylococcus aureus ST398

Abstract

The epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) has changed dramatically over the past 15 years. Initially a nosocomial pathogen, newly emergent strains of MRSA have become increasingly common in the community among individuals lacking contact with healthcare. More recently, a third group of MRSA strains have been identified in association with livestock, particularly swine. These strains, termed livestock-associated MRSA, have now been identified in Europe, North America, and Asia in humans and animals. One molecular type, ST398, has been the dominant strain of livestock-associated MRSA identified to date. The emergence of this strain in animals and humans will be described in this review, including colonization and clinical infections caused by this strain. We also discuss lingering research questions and implications for controlling spread of this bacterium in an agricultural environment and beyond.

Introduction

S taphylococcus aureus is a common cause of skin and soft tissue infection (SSTI) in humans. The bacterium is widespread, colonizing approximately a third of the population (Graham et al. 2006, Gorwitz et al. 2008), with the most common carriage site being the anterior nares (Wertheim et al. 2005). Recent studies have shown that methicillin-resistant S. aureus (MRSA) represents a major infectious disease burden in the United States, causing an estimated 94,000 infections and 18,000 deaths in 2005 (Klevens et al. 2007). MRSA has also been documented in a number of animal species, including horses (Weese et al. 2005a, 2006), dogs and cats (Baptiste et al. 2005), elephants (CDC 2009), and even marine mammals (Faires et al. 2009, Schaefer et al. 2009). In many cases, colonization or infection by MRSA in animal species appears to be due to zoonotic transfer from human owners or caretakers. However, some species have the capacity to act as reservoirs of the organism, and potentially transmit the bacterium to humans who work in close contact with colonized animals. MRSA in this group have been referred to as livestock-associated MRSA, to distinguish them from hospital-associated and community-associated MRSA (CA-MRSA) types (Wulf and Voss 2008). Livestock-associated MRSA were first recognized in cattle (Devriese and Hommez 1975), and substantial literature exists on S. aureus in this species, as it is a main cause of bovine mastitis and can lead to economic losses (Nickerson 2009). Additionally, zoonotic transmission between cows and humans has been documented (Juhasz-Kaszanyitzky et al. 2007).

Swine have recently emerged as another reservoir of S. aureus, including MRSA. The majority of swine-associated S. aureus belong to the same multi locus sequence type (MLST): ST398. This sequence type has also been referred to as non-typeable MRSA due to the inability of the SmaI restriction enzyme to cut DNA from these strains due to the presence of a unique methylase (Bens et al. 2006).

S. aureus ST398 in Swine and Swine Workers

ST398 was originally reported in swine farmers in France, in a study examining carriage of S. aureus in farmers and controls (Armand-Lefevre et al. 2005). Farmers had a higher S. aureus carriage rate than controls (44.6% vs. 24.1%); most isolates were methicillin-susceptible S. aureus (MSSA). Nineteen different MLST types were identified, including MRSA ST398. Strains collected from farmers included ST398 and ST9, another type that has subsequently been found in association with pigs (Armand-Lefevre et al. 2005, Battisti et al. 2009, Cui et al. 2009, Neela et al. 2009, Riesen and Perreten 2009, Wagenaar et al. 2009).

Many of the early studies on swine MRSA were carried out in The Netherlands. In the initial report from this country, the presence of MRSA in screening cultures from a 6-month-old girl admitted to a hospital was described (Voss et al. 2005). Subsequently, the girl's parents, who lived on a swine farm, were found to be colonized with MRSA. MRSA is uncommon in The Netherlands: 0.03% of patients were found to be colonized upon hospital admission in one study (Wertheim et al. 2004), while a population-based approach found 0.09% colonized (Ham ten et al. 2006). Therefore, an investigation began to determine the source of colonization, surveying MRSA in regional pigs and pig farmers. The study revealed that 6/23 farmers (23%) and 1 pig were colonized with MRSA, and all isolates were ST398. In the months subsequent to sampling, two additional human MRSA cases were found: one pig farmer in a different region, and a second in the son of a swine veterinarian. A similar situation was reported by Huijsdens and colleagues (2006).

A follow-up study examined pigs at nine Dutch slaughterhouses, representing a variety of farm types (farrowing farms, finishing farms, farrow-to-finish, rearing farms, and an insemination center); MRSA was isolated from 11% of the pigs and 23% of the farms (de Neeling et al. 2007).

Similar studies in other European countries have been carried out in Denmark (Bagcigil et al. 2007, Guardabassi et al. 2007, Lewis et al. 2008), Belgium (Denis et al. 2009, Van Hoecke et al. 2009), Germany (Kehrenberg et al. 2009, Kock et al. 2009b), Portugal (Pomba et al. 2009), Italy (Battisti et al. 2009), Switzerland (Riesen and Perreten 2009), and Sweden (Eliasson-Selling et al. 2008), with MRSA ST398 found in every study except two (Eliasson-Selling et al. 2008, Riesen and Perreten 2009). However, MSSA ST398 was identified in the Swiss study (Riesen and Perreten 2009).

A prevalence study examining swine breeding farms in most European Union countries found that MRSA was present in pig facilities in Austria, Belgium, Cyprus, the Czech Republic, Denmark, Finland, France, Germany, Hungary, Italy, Luxembourg, The Netherlands, Poland, Portugal, Slovakia, Slovenia, and Spain, whereas farms tested in Estonia, Ireland, Latvia, Lithuania, Sweden, and the United Kingdom were negative (EFSA 2009).

More recently, investigators have begun farm sampling of animals in the Americas. The first such study was carried out in Canada, sampling 285 pigs and 25 workers on 20 farms in Ontario. Twenty-five percent of pigs and 20% of humans were found to carry MRSA, with the majority of these coming from spa types associated with ST398 (Khanna et al. 2008).

In the United States, Smith and colleagues examined 299 animals and 20 workers in two swine production pyramids in Iowa and Illinois. Forty-nine percent of the animals and 45% of the workers were found to carry MRSA; all isolates typed from swine and humans were found to be ST398 (Smith et al. 2009). An additional study carried out in Brazil sampled 100 pigs, but did not find any MRSA (Baker et al. 2008).

In contrast to studies in Europe and the Americas, ST398 does not appear to be the dominant strain in Asian pigs. Nine farms in the Sichuan Province of China were studied, using dust samples rather than swabbing individual animals. Five of nine farms (55.6%) were positive for MRSA; these were found to be spa type t899, which had been previously associated with ST398 (van Duijkeren et al. 2008, Wulf et al. 2008b, de Boer et al. 2009, Kock et al. 2009a, Pan et al. 2009, Van Den Broek et al. 2009, van Wamel et al. 2009) (see also Table 1). However, in this research, when they carried out MLST typing for confirmation, all isolates were found to be either ST9 or a single-locus variant of this type (ST1376) (Wagenaar et al. 2009). Therefore, t899 has been found in both ST398 and ST9 swine-associated molecular types. Similar results were found in a second Chinese study (Cui et al. 2009). A third study sampling 100 swine carcasses at wet markets in Hong Kong also found t899 and ST9, but no ST398 (Guardabassi et al. 2009).

Table 1.

ST398-Associated Staphylococcus aureus

ST398-associated Spa types	SCCmec types	Country and sample types	Reference
t937	ND	Cape Verde islands, humans	Aires-de-Sousa et al. (2006)
t034, t1250 (reported in [Huijsdens et al. 2006])	ND	France, humans	Armand-Lefevre et al. (2005)
t011, t108	V	Spain, humans	Aspiroz et al. (2010)
t034	N/A	Denmark, pigs	Bagcigil et al. (2007)
t011, t034, t108, t899, t2510, t2922, t4838	IVb, V	Italy, pigs	Battisti et al. (2009)
t571, t3625	N/A	United States, Dominican Republic, humans	Bhat et al. (2009)
t011, t108, t567, t899, t943, t1457, t2503	ND	The Netherlands, pigs	Broens et al. (2008)
t034, t571, t1250	N/A	China, humans	Chen et al. (2010)
t899 but ST9, no ST398	III^a	China, pigs, cattle, humans	Cui et al. (2009)
t011	IVa	Austria, horses, humans	Cuny et al. (2008)
t011, t034, t108, t571, t1197, t1451, t2974	ND	Germany, pigs, humans	Cuny et al. (2009)
t011, t034, t108, t567, t779, t899, t1255, t1451, t1456, t1457, t2970, t3015, t3119, t4208	ND	The Netherlands, meat	de Boer et al. (2009)
t011, t108	ND	The Netherlands, human	Declercq et al. (2008)
t011, t034, t108, t943, t1254, t1255	IVa, V, III^a	The Netherlands, pigs	de Neeling et al. (2007)
t011, t034, t567	IVa, V, NT	Belgium, humans, pigs	Denis et al. (2009)
t011, t034, t108	IV, V	The Netherlands, Belgium, Germany, humans	Deurenberg et al. (2009)
t011, t034, t571	ND	Denmark, humans, pigs, cattle, lamb, turkey	de Vries et al. (2009)
t034, t571, t1456, t2383	N/A	The Netherlands, humans	Donker et al. (2009)
t011, t034, t108, t567, t571, t899, t1197, t1250, t1255, t1344, t1451, t1456, t1457, t1793, t1939, t2320, t2329, t2346, t2922, t2370, t2510, t3479, t4659, t4838, t4854, t4872	ND	European Union countries, pig farms	Authority EFS (2009)
t011, t2383	IV	The Netherlands, humans	Fanoy et al. (2009)
t011, t034, t2576	IV, V, NT	Germany, cattle, humans	Fessler et al. (2010)
Not reported (t034, personal com)	V	Norway, humans	Fossum Moen et al. (2009)
t034, t1250	V	Canada, humans	Golding et al. (2010)
t011, t034, t1451	V	Austria, humans	Grisold et al. (2009a, 2009b)
t034, t1793	ND	Denmark, pigs	Guardabassi et al. (2007)
t899 (ST9)	IV, V	Hong Kong, pig carcasses	Guardabassi et al. (2009)
t034	ND	Denmark, humans, pigs	Hartmeyer et al. (2010)
t011, t034, t108, t571, t1255, t1793	ND	Denmark, cattle, pigs, poultry	Hasman et al. (2009)
t011, t034	N/A	Hong Kong, humans	Ho et al. (2008)
t108	V	The Netherlands, pigs, humans	Huijsdens et al. (2006)
t011, t034, t108, t567, t571, t588, t779, t899, t943, t1184, t1255, t1451, t1456, t1457, t2123, t2287, t2389, t2330, t2383, t2582, t2748, t2971, t3013, t3014, t3053, t3146, t3208	IV, V, NT	The Netherlands, humans	Huijsdens et al. (2009)
ND	II, IVa	Hong Kong, humans	Ip et al. (2005)
t011, t034, t571, t1197, t1250, t1451, t1456, t2510	ND	Germany, pigs	Kadlec et al. (2009)
t034	V	Germany, pigs	Kehrenberg et al. (2009)
t034, t571, t1255	ND	Canada, pigs, humans	Khanna et al. (2008)
t011, t034, t108, t567, t899, t1451, t2011	ND	Germany, The Netherlands, humans	Kock et al. (2009a)
t011, t034, t108, t1451, t2510	IV, V	Germany, pigs, humans	Kock et al. (2009b)
t011, t034, t2346	IV, V	Austria, humans	Krziwanek et al. (2009)
t034	V, NT	Denmark, humans	Larsen et al. (2009)
t034, t108, t1793	IV, V	Denmark, pigs, humans	Lewis et al. (2008)
t011	IVa	UK, horses	Loeffler et al. (2009)
t011, t1197	V	Spain, food	Lozano et al. (2009)
ND	IVa	Italy, humans	Mammina et al. (2010)
t011, t034, t108, t1451	III,^a V	Germany, pigs	Meemken et al. (2009)
t034	ND	Germany, Switzerland, cattle	Monecke et al. (2007)
t011, t034	NT	Denmark, humans	Moodley et al. (2008)
t011, t034, t108, t1456	ND	Netherlands, chickens, humans	Mulders et al. (2010)
t011, t567	NT, III,^a IVa, V	Belgium, chickens	Nemati et al. (2008)
t937	N/A	Netherlands, humans	Nulens et al. (2008)
t108, t899	IVa, V	Italy, human, pigs	Lozano et al. (2009)
t108, t899	IVa, V	Italy, human, pig farm	Pan et al. (2009)
t1456	ND	Belgium, chickens	Persoons et al. (2009)
t011	V	Portugal, pigs, humans	Pomba et al. (2009)
t034, t899, t1939	N/A	Switzerland, pigs	Riesen and Perreten (2009)
t034	N/A	Netherlands, humans	Rijnders et al. (2009)
t011, t034	ND	Germany, cattle, pigs, horses, dogs, cats	Schwarz et al. (2008)
ND	V	Hong Kong, pigs, humans	Sergio et al. (2007)
t034	V	United States, humans, pigs	Smith et al. (2009)
t899	V	Italy, humans	Soavi et al. (2010)
t034	V	Denmark, humans	Stegger et al. (2009)
t011, t034, t108, t567, t571, t898, t1254, t1255	III,^a IV, IVa, V	The Netherlands, Humans, pigs, gorilla	van Belkum et al. (2008)
t011, t108, t567, t588, t899, t2330, t2741	ND	The Netherlands, pigs, humans	Van Den Broek et al. (2009)
t011, t1451	IVa, V	Belgium, horses	Van den Eede et al. (2009)
t011, t567,	IVa, V, NT	Belgium, cattle	Vanderhaeghen et al. (2010)
t011, t108, t567, t899, t1939	IV, V	The Netherlands, pigs, humans	van Duijkeren et al. (2008)
t011	IV	The Netherlands, pigs, humans	van Duijkeren et al. (2007)
t011, t2123	IVa	The Netherlands, horses, humans	van Duijkeren et al. (2009)
t011	ND	The Netherlands, Belgium, rats	van de Giessen et al. (2009)
t011	ND	Belgium, humans, pigs	Van Hoecke et al. (2009)
t011, t034, t108, t571, t567, t898	III,^a IV, V	The Netherlands, humans	van Loo et al. (2007a)
t108	ND	The Netherlands, meat samples	van Loo et al. (2007b)
ND	ND	The Netherlands, humans	van Rijen et al. (2008)
t011, t108, t567, t571	ND	The Netherlands, humans	van Rijen et al. (2009)
t011, t034, t108, t567, t571, t898, t899, t1254, t1255, t1939, t2123	IV, IVa, V	The Netherlands, horse, pigs, cattle, human	van Wamel et al. (2009)
t108, t1197	ND	Spain, humans	Vindel et al. (2009)
t108, t567, t943	ND	The Netherlands, pigs, humans	Voss et al. (2005)
t034	N/A	China, pig farms	Wagenaar et al. (2009)
t011	V	Germany, horses	Walther et al. (2009)
t034	ND	Sweden, humans	Welinder-Olsson et al. (2008)
t011, t034, t1197	IVa, V	Austria and Germany. Human, dog, horses, pig	Witte et al. (2007)
t011, t034, t571, t1255, t1451	N/A	China, humans	Wu et al. (2010)
t011, t034, t108	ND	The Netherlands, veterinarians and veterinary students	Wulf et al. (2006)
t011, t034, t108, t571, t567, t899	IVa, V, III,^a NT	Multiple, veterinarians	Wulf et al. (2008b)
t567	ND	The Netherlands, humans	Wulf et al. (2008c)
t108	ND	The Netherlands, humans	Wulf et al. (2008a)
ND	III, IV	China, humans	Yu et al. (2008)

Several type III isolates typed using a previous method (Zhang et al. 2005) were subsequently found to be type V (Jansen et al. 2009). Most MRSA ST398 have been SCCmec types IV or V.

ND, not determined; NT, nontypeable; MRSA, methicillin-resistant Staphylococcus aureus.

In Malaysia, 100 weaned pigs (age 4–5 weeks) were tested on each of 5 pig farms; S. aureus was found in 24.6%, but only 4/123 isolates (0.8%) were MRSA. These were not tested to determine strain type (Khalid et al. 2009). A second study in this country found a similar prevalence of MRSA in pigs (1%) and 5.5% in human handlers; like the Chinese studies, all were ST9 and no ST398 isolates were identified (Neela et al. 2009).

However, these studies do not prove an absence of ST398 from Asian swine. MRSA ST398 was previously found in Singapore, in a laboratory pig used for diabetes research (Sergio et al. 2007). Samples taken from the animal herd, research staff, animal holding rooms, and a slaughterhouse in Singapore found additional MRSA isolates, including ST398 and ST22. The authors hypothesized that the organism could have originated in pigs imported from Indonesia, but this was not confirmed.

Why have MRSA in swine gone unrecognized until just recently? Typically, S. aureus has not been thought to cause much illness in carrier pigs. MRSA may not have been detected until recently because many veterinary diagnostic labs will only report presence or absence of coagulase-positive Staphylococcci, and may not type them out to species or follow-up with a full panel of resistance testing. Further, skin infections on pig farms are more commonly caused by Staphylococcus hyicus than S. aureus. However, skin infections have occasionally been documented in pigs (van Duijkeren et al. 2007). An outbreak of exudative epidermititis among piglets was attributed to MRSA ST398 in The Netherlands (van Duijkeren et al. 2007). Further, isolates from pig infections with S. aureus were included as a comparison group to colonized pig farmers and bank or insurance workers in 2006 (Armand-Lefevre et al. 2005); 4/14 of these clinical swine isolates were ST398 (3 urine isolates and 1 from cutaneous infection). Other studies (Armand-Lefevre et al. 2005, de Vries et al. 2009, Hasman et al. 2009, Kadlec et al. 2009, Meemken et al. 2009) also retrospectively examined the epidemiology of S. aureus from various swine clinical isolates.

ST398 in Other Animals

Although ST398 has been mainly associated with pigs, it has also been found in other animal species. Several studies have identified contact with veal calves as a risk for carriage of ST398 (Wulf et al. 2006, 2008b, van Loo et al. 2007a, Lewis et al. 2008). Although sampling of live cattle has not commonly identified this strain (Lewis et al. 2008, Cui et al. 2009, Hasman et al. 2009), ST398 isolates of cattle origin have been analyzed in several publications (Monecke et al. 2007, de Vries et al. 2009, van Wamel et al. 2009, Fessler et al. 2010, Vanderhaeghen et al. 2010).

In addition to livestock, ST398 has been reported in one turkey sample (de Vries et al. 2009) and in chickens. Contemporary S. aureus isolates were examined from broiler chickens and historic isolates from breeder chickens in Belgium (Nemati et al. 2008). Ten MRSA isolates were found in the contemporary isolates; all were ST398. MRSA isolates were also found on 5/39 (12.8%) of the farms tested. However, zoonotic transmission was not examined as human handlers were not tested. Persoons and colleagues similarly found ST398 in 8 broiler chickens on 2/14 (14.3%) farms in Belgium (Persoons et al. 2009). While the Persoons and Nemati studies found ST398-associated strains exclusively, Mulders et al. found both ST398-associated as well as ST9-associated spa types in broiler chickens and slaughterhouse personnel in The Netherlands (Mulders et al. 2010). Conversely, investigators found no ST398 in 85 clinical isolates from poultry in Denmark (Hasman et al. 2009), and several studies have shown that ST5, rather than ST398, appears to be a dominant strain in poultry (Hasman et al. 2009, Lowder et al. 2009).

ST398 has been isolated from horses in Germany (Witte et al. 2007, Walther et al. 2009), Austria (Witte et al. 2007, Cuny et al. 2008), Belgium (Van den Eede et al. 2009), the United Kingdom (Loeffler et al. 2009), and The Netherlands (van Duijkeren et al. 2009). No MRSA was detected in healthy horses in Slovenia (Vengust et al. 2006), and in research carried out in Canadian and United States horses, CMRSA-5 (USA500) was found, but no ST398 (Weese et al. 2005a, 2005b, Weese et al. 2006). Several studies documented or suspected nosocomial transmission of MRSA in equine hospitals (Cuny et al. 2008, Loeffler et al. 2009, van Duijkeren et al. 2009), and human staff were found to be colonized in two (Cuny et al. 2008, van Duijkeren et al. 2009), suggesting roles for both environmental contamination and spread by human carriers in these facilities.

Finally, ST398 has occasionally been recovered from clinical isolates in other animal species, including a lamb (de Vries et al. 2009), a dog with a skin infection (Witte et al. 2007), rats on a swine farm (van de Giessen et al. 2009), and a gorilla ear isolate (van Belkum et al. 2008), documenting a wide range of species that may be colonized or infected with ST398.

ST398 in Veterinarians

A number of studies have examined carriage of MRSA in veterinarians. A 2006 study examined 179 attendees at a veterinary conference (Wulf et al. 2006). Seven of 179 (3.9%) were found to carry MRSA. All had regular or recent contact with pigs and cows, whereas no MRSA carriage was found among individuals who reported no livestock contact; all were spa types corresponding to ST398. Similarly, veterinarians were examined at an international conference for pig health in Denmark, where 34/272 individuals (12.5%) were found to carry MRSA (Wulf et al. 2008b). Positive participants came from Belgium, Denmark, Canada, Germany, Italy, The Netherlands, Spain, and Thailand; 31/34 (91.2%) of the isolates were ST398.

Danish veterinary practitioners were sampled at several veterinary conferences, including veterinarians who worked with livestock as well as those who worked with companion animals (Moodley et al. 2008). Cattle farmers as well as persons unexposed to animals were also enrolled. Of the 702 participants, 9 (3.9%) of the veterinary practitioners and 2 (0.7%) of the unexposed group were positive for MRSA. Interestingly, pig exposure was not found to be a risk factor. Four of 11 isolates recovered in this study were ST398, from 3 veterinarians and from 1 unexposed individual (Moodley et al. 2008). Further, a German study of 86 occupationally exposed individuals (veterinarians, meat inspectors, and field station workers involved with pig disease diagnostics and herd health management) found that 20/86 (23%) were colonized with MRSA ST398 (Blaha et al. 2008).

However, even when individuals working with large animals are sampled, ST398 is not always found. Investigators tested 417 individuals from 19 countries at an international veterinary conference in Baltimore, Maryland (Hanselman et al. 2006). Although MRSA was isolated from 15 individuals in large animal practice (15.6%), none were ST398. Similarly, research carried out at an international equine veterinarian conference found that 10% of study participants were colonized with MRSA, but none of the isolates were ST398 (Anderson et al. 2008). In the Czech Republic, an examination of attendees at a veterinary meeting found a low rate of MRSA carriage (0.7%) and no ST398, despite having a considerable number of attendees report daily contact with cows and pigs (Zemlickova et al. 2009).

ST398 in Meat Products

As ST398 has been found in food-producing animals, meat products have the potential to act as vectors for transmission of MRSA from the farm ino the general human population. Although a relatively small number of studies have been carried out examining retail meats specifically for MRSA, several have documented the presence of MRSA ST398 on raw retail meat products. The initial study from The Netherlands examined 79 samples of pork and beef, finding 2 MRSA isolates (2.5%), both from pork samples (van Loo et al. 2007b). One isolate was ST398; the other was USA300, a CA-MRSA isolate. A second study in The Netherlands examined a much larger sample of retail meats (n = 2217), including beef, pork, veal, lamb/mutton, chicken, turkey, fowl, and game (de Boer et al. 2009). The overall MRSA prevalence was 264/2217 (11.9%). Interestingly, the highest prevalence was found in turkey samples (41/116 samples, 35.3%), followed by chicken (16.0%; 83/520 samples), veal (15.2%; 39/257 samples), pork (10.7%; 33/309 samples), and beef (10.6%; 42/395 samples). A lower percentage of lamb/mutton, fowl, and game were positive for MRSA. Eighty-five percent of all strains found were spa types that had been associated with ST398.

In Spain, 318 raw meat samples were tested, including chicken, pork, veal, lamb, turkey, rabbit, and wild game (Lozano et al. 2009). MRSA was found in 5/318 samples (1.6%): one each from pork, chicken, rabbit, veal, and wild boar. Two strains of ST398 were detected, in pork and veal. Studies carried out in Japan (Kitai et al. 2005), Korea (Kwon et al. 2006), and the United States (Pu et al. 2008) all found a low prevalence of MRSA in meat samples, but no ST398 was identified.

ST398 in Humans: Transmission and Duration of Carriage

Although many studies have been carried out examining colonization with ST398 on farms and in individuals with live animal contact, an increasing number of reports have shown ST398 isolated from patients in hospitals, and found in individuals with no known contact with live animals. It remains unclear how transmissible ST398 is between humans, especially in relationship to other common human types, such as USA100 or USA300 (ST5 or ST8, respectively). Cuny and colleagues examined potential transmission events by determining the prevalence of MRSA ST398 colonization in individuals in contact with MRSA-positive pigs; individuals living on farms but not in direct pig contact; and schoolchildren in towns in the area of the identified MRSA-positive farms (Cuny et al. 2009). Veterinarians attending these farms and their family members were also sampled. Within the farm families, five families were found to have transfer events (4.3% of those examined). Four events (9% of those examined) were seen among the veterinarian population. Only 3/462 schoolchildren carried MRSA ST98, but all colonized children lived on pig farms. The authors concluded that dissemination of ST398 MRSA from colonized farm workers into the general community is relatively infrequent (Cuny et al. 2009). This study suffers from the limitation that it was only a point prevalence study and not longitudinal in design; therefore, additional transmission events may have been uncovered with additional sampling over time.

Several other studies have similarly suggested the presence of secondary transmission within families, including research from The Netherlands (Voss et al. 2005, Huijsdens et al. 2006) in which neither of two individuals found to have ST398 infections reported direct animal exposure, but both lived on pig farms and had family members in direct contact with livestock. Other studies (Huijsdens et al. 2006, Lewis et al. 2008, Bhat et al. 2009, Krziwanek et al. 2009, Van Den Broek et al. 2009, Aspiroz et al. 2010, Hartmeyer et al. 2010) have also suggested the occurrence of secondary transmission of ST398. In several other studies, no direct contact with farm animals was reported in individuals colonized or infected with ST398 (Wulf et al. 2008a, Bhat et al. 2009, Fanoy et al. 2009, Golding et al. 2010, Mammina et al. 2010).

While studies have suggested secondary spread among humans, it remains unclear how long carriage persists in colonized individuals, or if persistent exposure to an animal reservoir is necessary to maintain colonization. Van den Broek and colleagues examined this on a small scale, swabbing veterinary assistants who carried out sampling on swine farms (Van Den Broek et al. 2009). Forty-one percent of these assistants were colonized on at least one occasion, but colonization was of short duration: most tested negative the day after sampling. Pomba and coworkers similarly documented a veterinarian who tested positive, but was negative 1 week later (Pomba et al. 2009). As S. aureus has been found in farm air samples (Gibbs et al. 2004, 2006) and dust sampling is now being frequently used rather than testing of individual animals (EFSA 2009, Krause and Cavaco 2009, Wagenaar et al. 2009), this suggests that some of these positive results in humans may be due to airborne contamination of nasal passages, rather than true sustained colonization with ST398.

Examination of transmission within the healthcare setting has been carried out in several studies. The first documented hospital-associated outbreak of ST398 occurred in The Netherlands (Wulf et al. 2008a). The authors found 5 patients with non-typeable MRSA colonization and/or infection, while 5 of 238 health care workers were found to be colonized; all were found to be spa type t567. While none of the patients reported contact with pigs or veal calves, one health care worker lived on a pig farm. She reported that neither she nor her partner came into direct contact with pigs, providing a suggestive but unconfirmed origin of the outbreak. Another outbreak occurred at a Dutch residential care facility (Fanoy et al. 2009). In addition to the index case, two residents and three staff members were colonized with ST398. The source of this outbreak is likewise unknown, though live goats and rabbits present on the premises were screened and found to be negative; however, live chickens at the facility were not screened. Finally, at Amphia Hospital in The Netherlands, transmission of MRSA ST398 was examined relative to typeable MRSA strains (van Rijen et al. 2008). Ninety-five cases of MRSA carriage were documented, and the authors found that individuals colonized with ST398 were significantly less likely to transmit the bacterium to other individuals in the hospital setting compared to typeable MRSA strains. These results suggest that more typical human strains of MRSA may be more likely to spread in communities than ST398.

ST398 in Humans: Human Symptomatic Infections

While not always in an outbreak setting, spa types associated with ST398 have been found in a number of hospital and other medical care surveillance reports in many different countries, including the Cape Verde islands (Aires-de-Sousa et al. 2006); Hong Kong (Ip et al. 2005, Ho et al. 2008); China (Yu et al. 2008, Chen et al. 2010, Wu et al. 2010); Sweden (Welinder-Olsson et al. 2008); the Dominican Republic (Bhat et al. 2009); The Netherlands (van Loo et al. 2007a, van Rijen et al. 2008, Wulf et al. 2008a, Wulf et al. 2008c, Deurenberg et al. 2009, Donker et al. 2009, Huijsdens et al. 2009, Kock et al. 2009a, Nulens et al. 2009, Rijnders et al. 2009, van Rijen et al. 2009); Scotland (Edwards et al. 2008); Denmark (Lewis et al. 2008, de Vries et al. 2009, Larsen et al. 2009, Hartmeyer et al. 2010); Austria (Grisold et al. 2009a, Krziwanek et al. 2009); Spain (Vindel et al. 2009, Aspiroz et al. 2010); Norway (Fossum Moen et al. 2009); Belgium (Van Hoecke et al. 2009); Germany (Witte et al. 2007, Kock et al. 2009a, 2009b); Italy (Pan et al. 2009, Mammina et al. 2010, Soavi et al. 2010); and Canada (Golding et al. 2010). To date, no reports of human symptomatic infections with ST398 have been published in the United States, despite confirmation of this strain in this country (Smith et al. 2009); this may be due in part to lack of systematic nationwide surveillance in this country, particularly in rural areas.

A spectrum of infections have been documented in these publications, ranging from relatively minor or localized infections including abscesses (van Belkum et al. 2008, Welinder-Olsson et al. 2008, Fanoy et al. 2009, Fossum Moen et al. 2009, Grisold et al. 2009b, Pan et al. 2009) and various SSTI (Declercq et al. 2008, Ho et al. 2008, van Rijen et al. 2008, Krziwanek et al. 2009, Stegger et al. 2009, Aspiroz et al. 2010, Golding et al. 2010, Hartmeyer et al. 2010, Soavi et al. 2010, Wu et al. 2010), urinary tract infections (van Belkum et al. 2008), wound infections (Witte et al. 2007, van Belkum et al. 2008, Welinder-Olsson et al. 2008, Wulf et al. 2008a, Yu et al. 2008, Denis et al. 2009, Grisold et al. 2009b), mastitis (Huijsdens et al. 2006), and conjunctivitis (Grisold et al. 2009a), as well as more serious or invasive infections, including bacteremia (Ip et al. 2005, Nulens et al. 2008, van Belkum et al. 2008, de Vries et al. 2009, Soavi et al. 2010), pneumonia (Witte et al. 2007, van Rijen et al. 2008, Hartmeyer et al. 2010, Mammina et al. 2010) (including necrotizing pneumonia) (Huijsdens et al. 2006), osteomyelitis (van Rijen et al. 2008, Grisold et al. 2009a), pyomyositis (Pan et al. 2009), otomastoiditis (Van Hoecke et al. 2009), and postoperative infections (Krziwanek et al. 2009). Although most of these have been documented in the past 5 years, one MSSA ST398 bacteremia isolate from Denmark dates back to 1992 (de Vries et al. 2009).

Despite the diverse array of infection types, it has been suggested that ST398 may not cause as much disease (relative to colonization) as other human strains, such as USA300 (Cuny et al. 2009). However, by focusing mainly on skin and wound infections, we may be missing a section of ST398 diseases, as one study has shown that ST398 was more likely to be associated with respiratory disease rather than skin infections (van Loo et al. 2007a). Additional research needs to be done in this area to determine whether this finding is replicated in larger studies.

In The Netherlands, a dramatic increase has been observed in the prevalence of ST398 in MRSA isolates recovered in the country, which accounted for 0% in 2002, and 30% in 2007 following increased surveillance instituted in 2006 (van Loo et al. 2007a, Huijsdens et al. 2009). Similarly, in Germany ST398-associated spa types increased from 13% in 2005 to 22.4% in 2008 (Kock et al. 2009b). In both countries, contact with swine and cattle were risk factors for ST398 carriage. Interestingly, one publication found MSSA ST398 in The Netherlands in a retrospective survey dating back to 1997 (Rijnders et al. 2009). These isolates were taken from intensive care unit patients, in a collection of S. aureus isolates from 14 hospitals in the country. Both ST398 isolates were from the southern region of the country, which is an agricultural region. Whether these dramatic increases will occur in other countries where ST398 has been found remains to be seen.

ST398 Evolution and Genetic Diversity

Significant amounts of genetic diversity among spa and SCCmec cassette types have been documented in ST398. For instance, SCCmec types II, III, IV, IVa, and V have all been reported, as well as nontypeable SCCmec cassettes (see Table 1). However, it should be noted that type III may have been misidentified in some of these studies due to the typing method used (Jansen et al. 2009). ST398 appears to have evolved by multiple acquisitions of the SCCmec element. For example, in The Netherlands, two farms were found to have MRSA ST398 with identical spa types, but different SCCmec types, suggesting that divergent SCCmec elements were inserted into the (clonal) MSSA (van Duijkeren et al. 2008). Similarly, MSSA ST398 (spa type t899), MRSA ST398-IVa (spa type t899), and MRSA ST398-V (spa type t108) were found in dust samples, nasal swabs, and a blood isolate from workers on the same pig farm (Pan et al. 2009), suggesting multiple acquisitions of SCCmec cassettes by MSSA precursors. This is supported by other publications (Bagcigil et al. 2007, Nemati et al. 2008, van Belkum et al. 2008). It has been suggested that coagulase-negative Staphylococci in the farming environment could serve as a source of SCCmec (Zhang et al. 2009), and that the progeny of such emerging MRSA strains are spreading locally rather than globally (Fitzgerald et al. 2001, Oliveira et al. 2002, Nubel et al. 2008).

While SCCmec acquisition seems to be fairly common in MRSA ST398, the transfer of staphylococcal toxin genes appears to be rarer. Several studies have found that ST398 typically lacks many previously identified toxin genes (Monecke et al. 2007, Sergio et al. 2007, Lewis et al. 2008, van Duijkeren et al. 2008, Fossum Moen et al. 2009, Kadlec et al. 2009, Rijnders et al. 2009, Stegger et al. 2009, Varshney et al. 2009, Walther et al. 2009), including the Panton-Valentine leukocidin gene (pvl). Only a handful of studies have found pvl-positive ST398 (Lewis et al. 2008, van Belkum et al. 2008, Welinder-Olsson et al. 2008, Yu et al. 2008, Huijsdens et al. 2009, Stegger et al. 2009, Chen et al. 2010). However, as toxin genes may be present on mobile elements such as phage or plasmids, it is possible that isolates of ST398 may acquire such genes, or that they may already possess their own set of novel, as-yet unidentified toxin genes. Additionally, horizontal transfer of the protein A gene has been suggested, due to the finding of spa type t899 in both ST398 strains and ST9 strains (Cui et al. 2009, Guardabassi et al. 2009, Neela et al. 2009, Wagenaar et al. 2009).

Diversity within ST398 remains an area of debate. Using multi-locus variable number tandem repeat analysis, it has been suggested that variation within the ST398 complex is very low (Schouls et al. 2009). However, using whole genomic sequencing, considerable differences between strains of ST398 have been identified (Schijffelen et al. 2008), while others have identified novel mobile genetic elements in pvl-positive ST398 isolates (Stegger et al. 2009). An examination of pulsed field gel electrophoresis (PFGE) types has also shown that strains with identical PFGE patterns may have differing spa types (van Wamel et al. 2009), documenting the need for multiple molecular assays to investigate the epidemiology of ST398.

Presence of Novel Antibiotic-Resistance Genes

Several studies have documented the presence of novel antibiotic resistance genes in ST398. Kadlec and Schwarz found the novel plasmid-borne trimethoprim resistance gene dfrK in ST398 (Kadlec and Schwarz 2009a). This gene was located close to tetL, which would allow for selection of these genes either by use of tetracycline or trimethoprim, both of which are used in veterinary medicine. The same team also found a novel ABC transporter gene, vga(C), on this plasmid (Kadlec and Schwarz 2009a).

The multidrug resistance gene, cfr, was found in two porcine isolates of S. aureus from Germany, one MRSA ST398 and one MSSA ST9 (Kehrenberg et al. 2009). This gene was previously found in staphylococci from animals in Europe (Kehrenberg and Schwarz 2006, Kehrenberg et al. 2007) and in humans in Columbia and the United States (Toh et al. 2007, Mendes et al. 2008). cfr confers resistance to a number of antibiotics, including oxazolidinones and pleuromutilins, phenicols, lincosamides, and streptogramin A antibiotics.

Unanswered Questions

While ST398 has been the subject of epidemiologic research implemented on farms and hospitals, numerous important questions remain unanswered. First, it is unknown what is the primary driving factor for the maintenance of colonization on farms, or the spread of the organism between farms—or, more likely, what the multiple factors are that play a role in these phenomena. As this remains uncertain, it complicates efforts to control spread, both between farms and between animals (including zoonotic transmission between animals and humans). Interestingly, several studies examining risk factors for human colonization have provided unexpected results, including a Belgian study, which found that wearing gloves and an apron and reporting regular or occasional hand disinfection with an antimicrobial product actually increased the risk of MRSA colonization (Denis et al. 2009), and a United States study, which found that workers who do not obtain blood or other specimens from swine were actually at a higher risk of carrying MRSA than staff who did do these chores (and presumably had close, sustained contact with colonized animals) (Smith et al. 2009). Another publication found that disinfection measures did not influence human colonization with MRSA (Van Den Broek et al. 2009). Whether these are spurious findings or will be repeatable using larger population sizes remains to be seen.

Little is known about the ecology of ST398 on farms. While data from several studies suggest multiple introductions of the SCCmec cassette into MSSA ST398 (Bagcigil et al. 2007, Nemati et al. 2008, van Belkum et al. 2008, van Duijkeren et al. 2008, Pan et al. 2009), it is not known if other S. aureus strains already on the farm served as the reservoir for this cassette, or if coagulase negative staphylococci serve as a main reservoir for these antibiotic resistance genes. Additionally unknown is the relative importance of de novo generation of MRSA ST398 on farms from MSSA ST398 precursors, versus transmission of MRSA ST398 between farms or animals. An examination of different farm chains in The Netherlands found that of farms purchasing animals from MRSA-positive farms, 66.7% of farms were positive for MRSA, but only 20% of farms purchasing animals from MRSA-negative farms were positive themselves (Broens et al. 2008). Similar results were found by others (van Duijkeren et al. 2008). Further, the role of wildlife in the maintenance and spread of ST398 has not been systematically evaluated, but one recent study has found that farm rats can carry MRSA and MSSA (van de Giessen et al. 2009). This study found rats colonized with ST398-associated strains (t011, t937) as well as other types previously implicated in zoonotic transmission (t002 and t337, associated with ST5 and ST9) (Armand-Lefevre et al. 2005, Khanna et al. 2008, Aarestrup et al. 2009, Cui et al. 2009, EFSA 2009, Lowder et al. 2009, Wagenaar et al. 2009).

Another unknown is the transmissibility of ST398 among humans. Although a handful of studies have examined this, and from these it appears to transmit less frequently than common human strains of S. aureus (van Rijen et al. 2008, Cuny et al. 2009), more robust longitudinal studies examining carriage and transmission of both human strains and ST398 are needed. Similarly, surveillance for ST398 disease in countries where this has been identified should be increased, focusing not only on the most serious diseases from hospitalized patients (as a recent study from Iowa has done) (Van De Griend et al. 2009), but involving rural physicians in areas of high livestock density to capture more mild infections, such as abscesses or minor respiratory infections. These types of studies will allow for a better comparison of the potential for ST398 to cause symptomatic infection, and will allow for an indication of whether ST398 infections remain limited to individuals with animal contact, or if other routes of transmission (including food) may be important.

Further, while much of the focus of ST398 research to date has been in Europe and associated with livestock, the Chinese situation (both in humans and animals) bears further examination. Several ST398 isolates found in China, or in individuals who spent time in China, are pvl positive (Yu et al. 2008, Stegger et al. 2009, Chen et al. 2010, Wu et al. 2010). A comparison of ST398 isolates from Danish and Chinese origin found differences in mobile genetic elements between the strains (Stegger et al. 2009), suggesting diversification from a common ancestor. While the strains compared were both MRSA ST398 carrying SCCmec type V, there have been several reports from China of MSSA ST398 (Chen et al. 2010, Wu et al. 2010). MSSA ST398 accounted for 18.9% of all MSSA causing infections at sterile body sites at Peking Union Medical College Hospital (Chen et al. 2010), while at Beijing Childrens' Hospital, ST398-associated spa types accounted for 13.3% of CA-MRSA SSTI isolates (Wu et al. 2010). Because these were retrospective hospital-based studies, livestock contact was not investigated. Additional research is needed to determine if the high levels of MSSA ST398 are due to livestock contact or foodborne transmission, or if they represent sustained human-to-human transmission of ST398 in China.

It has been assumed that the use of antibiotics in animals and on farms has been the key force driving the emergence and spread of MRSA ST398, but additional empirical data need to be collected to quantify this aspect. Several findings support the hypothesis that antibiotic use on farms drives antibiotic resistance, including the observation that almost all isolates of ST398 are resistant to tetracycline, an antibiotic commonly used in swine farming. In one study examining antibiotic use on farms, it was found that 6/10 swine farms that used antimicrobials were positive for MRSA, while only 1/21 farms using no standard medication were positive (van Duijkeren et al. 2008). In a retail meat investigation, lower prevalence of MRSA in biological meats was found compared to conventional meats, potentially reflecting prevalence on farms (de Boer et al. 2009). It should be noted that there are a number of possible differences among farms that employ antibiotics and those that do not in addition to antibiotic use, including differences in pig movement between farms, variations in feed ingredients (including the presence of antimicrobial metals) (Ito et al. 2001, Aarestrup et al. 2009), and often a disparity in farm size as well; these factors need to be more carefully evaluated in future studies, as this information is critical to best implement maximally protective prevention procedures. Further, antibiotic use needs to be examined not only on the farm, but also in humans. Currently, two studies have found no association between antibiotic use in humans and carriage of MRSA in individuals exposed to MRSA-positive pigs, suggesting that the latter is the more significant risk factor (Cuny et al. 2009, Smith et al. 2009).

Finally, while ST398 has been the most commonly reported MRSA strain found in association with livestock in recent years, surveillance needs to be instituted to examine any strains that may arise as a consequence of animal farming. MRSA ST9 appears to be more common in pigs in China and Malaysia than ST398 (Cui et al. 2009, Neela et al. 2009, Wagenaar et al. 2009), can also colonize humans (Armand-Lefevre et al. 2005), and has been found to cause infection in humans in Miami (Chung et al. 2004), the United Kingdom (Guardabassi et al. 2009), and The Netherlands (van Loo et al. 2007a). The t002 spa type has also been found in swine (Khanna et al. 2008) and on swine farms (Aarestrup et al. 2009, van de Giessen et al. 2009). While this has been assumed to represent an instance of human-to-swine transmission of MRSA, a t002 strain has become adapted to poultry after transmission from humans several decades ago (Lowder et al. 2009). ST8, a common human strain, has also been found in animals (EFSA 2009, Sunde 2009, van Duijkeren et al. 2009). Whether swine may represent reservoirs of t002/ST5 and ST8 remains to be seen. We are only beginning to scratch the surface in our examination of the diversity of S. aureus in animals, and understanding the role of these strains may play in the epidemiology of human S. aureus colonization and disease.

Footnotes

Acknowledgments

T.C.S. has received research funding from the University of Iowa, the National Pork Board, and NIOSH via the Heartland Center for Occupational Health and Safety and the Great Plains Center for Agricultural Health. None provided specific funding for this publication.

Disclosure Statement

No competing financial interests exist.

References

Aarestrup

, Cavaco

, Hasman

. Decreased susceptibility to zinc chloride is associated with methicillin resistant Staphylococcus aureus CC398 in Danish swine. Vet Microbiol, 2010; 142:455–457.

Aires-de-Sousa

, Conceicao

, de Lencastre

. Unusually high prevalence of nosocomial Panton-Valentine leukocidin-positive Staphylococcus aureus isolates in Cape Verde Islands. J Clin Microbiol, 2006; 44:3790–3793.

Anderson

, Lefebvre

, Weese

. Evaluation of prevalence and risk factors for methicillin-resistant Staphylococcus aureus colonization in veterinary personnel attending an international equine veterinary conference. Vet Microbiol, 2008; 129:410–417.

Armand-Lefevre

, Ruimy

, Andremont

. Clonal comparison of Staphylococcus aureus isolates from healthy pig farmers, human controls, and pigs. Emerg Infect Dis, 2005; 11:711–714.

Aspiroz

, Lozano

, Vindel

, Lasarte

et al. Skin lesion caused by ST398 and ST1 MRSA, Spain. Emerg Infect Dis, 2010; 16:157–159.

Authority

EFS

. Analysis of the Baseline Survey on the Prevalence of Methicillin-Resistant Staphylococcus aureus (MRSA) in Holdings with Breeding Pigs, in the EU, 2008 Part A: MRSA Prevalence Estimates. Parma: Italy, 2009.

Bagcigil

, Moodley

, Baptiste

, Jensen

, Guardabassi

. Occurrence, species distribution, antimicrobial resistance and clonality of methicillin- and erythromycin-resistant staphylococci in the nasal cavity of domestic animals. Vet Microbiol, 2007; 121:307–315.

Baker

, Masson

GCIH

, Guilherme de Oliveira

, Fernando de Oliveira

, Davies

. A Pilot Study of the Prevalence of Methicillin-Resistant Staphylococcus aureus in Swine Populations of Brazil and the United States. San Diego, CA: American Association of Swine Veterinarians, 2008.

Baptiste

, Williams

, Willams

, Wattret

et al. Methicillin-resistant staphylococci in companion animals. Emerg Infect Dis, 2005; 11:1942–1944.

10.

Battisti

, Franco

, Merialdi

, Hasman

et al. Heterogeneity among methicillin-resistant Staphylococcus aureus from Italian pig finishing holdings. Vet Microbiol, 2010; 142:361–366.

11.

Bens

, Voss

, Klaassen

. Presence of a novel DNA methylation enzyme in methicillin-resistant Staphylococcus aureus isolates associated with pig farming leads to uninterpretable results in standard pulsed-field gel electrophoresis analysis. J Clin Microbiol, 2006; 44:1875–1876.

12.

Bhat

, Dumortier

, Taylor

, Miller

et al. Staphylococcus aureus ST398, New York City and Dominican Republic. Emerg Infect Dis, 2009; 15:285–287.

13.

Blaha

, Cuny

, Witte

, Meemken

. Occurrence of MRSA in humans occupationally exposed to pigs in the northwest of Germany. International Pig Veterinary Society Annual Conference. Durban: South Africa, 2008.

14.

Broens

, Graat

EAM

, van der Wolf

, van Duijkeren

et al. Transmission of NT-MRSA in the pig production chain in the Netherlands. International Pig Veterinary Society Annual Conference. Durban, South Africa, 2008.

15.

CDC. Methicillin-resistant Staphylococcus aureus skin infections from an elephant calf—San Diego, California, 2008. MMWR Morb Mortal Wkly Rep, 2009; 58:194–198.

16.

Chen

, Liu

, Jiang

, Chen

, Wang

. Rapid change of methicillin-resistant Staphylococcus aureus clones in a tertiary care hospital of China over a fifteen-year period. Antimicrob Agents Chemother, 2010; 54:1842–1847.

17.

Chung

, Dickinson

, De Lencastre

, Tomasz

. International clones of methicillin-resistant Staphylococcus aureus in two hospitals in Miami, Florida. J Clin Microbiol, 2004; 42:542–547.

18.

Cui

, Li

, Hu

, Jin

et al. Isolation and characterization of methicillin-resistant Staphylococcus aureus from swine and workers in China. J Antimicrob Chemother, 2009; 64:680–683.

19.

Cuny

, Nathaus

, Layer

, Strommenger

et al. Nasal colonization of humans with methicillin-resistant Staphylococcus aureus (MRSA) CC398 with and without exposure to pigs. PLoS ONE, 2009; 4:e6800.

20.

Cuny

, Strommenger

, Witte

, Stanek

. Clusters of infections in horses with MRSA ST1, ST254, and ST398 in a veterinary hospital. Microb Drug Resist, 2008; 14:307–310.

21.

de Boer

, Zwartkruis-Nahuis

, Wit

, Huijsdens

et al. Prevalence of methicillin-resistant Staphylococcus aureus in meat. Int J Food Microbiol, 2009; 134:52–56.

22.

Declercq

, Petre

, Gordts

, Voss

. Complicated community-acquired soft tissue infection by MRSA from porcine origin. Infection, 2008; 36:590–592.

23.

de Neeling

, van den Broek

, Spalburg

, van Santen-Verheuvel

et al. High prevalence of methicillin resistant Staphylococcus aureus in pigs. Vet Microbiol, 2007; 122:366–372.

24.

Denis

, Suetens

, Hallin

, Catry

et al. Methicillin-resistant Staphylococcus aureus ST398 in swine farm personnel, Belgium. Emerg Infect Dis, 2009; 15:1098–1101.

25.

Deurenberg

, Nulens

, Valvatne

, Sebastian

et al. Cross-border dissemination of methicillin-resistant Staphylococcus aureus, Euregio Meuse-Rhin region. Emerg Infect Dis, 2009; 15:727–734.

26.

Devriese

, Hommez

. Epidemiology of methicillin-resistant Staphylococcus aureus in dairy herds. Res Vet Sci, 1975; 19:23–27.

27.

de Vries

, Christensen

, Skov

, Aarestrup

, Agerso

. Diversity of the tetracycline resistance gene tet(M) and identification of Tn916- and Tn5801-like (Tn6014) transposons in Staphylococcus aureus from humans and animals. J Antimicrob Chemother, 2009; 64:490–500.

28.

Donker

, Deurenberg

, Driessen

, Sebastian

et al. The population structure of Staphylococcus aureus among general practice patients from The Netherlands. Clin Microbiol Infect, 2009; 15:137–143.

29.

Edwards

, Cosgrove

, Girvan

. ST398 MRSA infections in Scotland–no pig association apparent. International Symposium on Staphylococci and Staphylococcal Infections: Cairns, Australia, 2008.

30.

EFSA. Analysis of the Baseline Survey on the Prevalence of Methicillin-Resistant Staphylococcus aureus (MRSA) in Holdings with Breeding Pigs, in the EU, 2008 Part A: MRSA Prevalence Estimates. Parma: Italy, 2009.

31.

Eliasson-Selling

, Gronlund Andersson

, Greko

, Franklin

. Screening for Methicillin-Resistant Staphylococcus aureus (MRSA) in Swedish Pigs. Durban, South Africa: International Pig Veterinary Society, 2008.

32.

Faires

, Gehring

, Mergl

, Weese

. Methicillin-Resistant Staphylococcus aureus in Marine Mammals. Emerg Infect Dis, 2009; 15:2071–2072.

33.

Fanoy

, Helmhout

, van der Vaart

, Weijdema

et al. An outbreak of non-typeable MRSA within a residential care facility. Euro Surveill, 2009; 14:pii: 19080.

34.

Fessler

, Scott

, Kadlec

, Ehricht

et al. Characterization of methicillin-resistant Staphylococcus aureus ST398 from cases of bovine mastitis. J Antimicrob Chemother, 2010; 65:619–625.

35.

Fitzgerald

, Sturdevant

, Mackie

, Gill

, Musser

. Evolutionary genomics of Staphylococcus aureus: insights into the origin of methicillin-resistant strains and the toxic shock syndrome epidemic. Proc Natl Acad Sci USA, 2001; 98:8821–8826.

36.

Fossum Moen

, Saltyte Benth

, Alm-Kristiansen

, Bukholm

. Exotoxin-encoding gene content in community-acquired and hospital-acquired methicillin-resistant Staphylococcus aureus. Clin Microbiol Infect, 2009; 15:1139–1145.

37.

Gibbs

, Green

, Tarwater

, Mota

et al. Isolation of antibiotic-resistant bacteria from the air plume downwind of a swine confined or concentrated animal feeding operation. Environ Health Perspect, 2006; 114:1032–1037.

38.

Gibbs

, Green

, Tarwater

, Scarpino

. Airborne antibiotic resistant and nonresistant bacteria and fungi recovered from two swine herd confined animal feeding operations. J Occup Environ Hyg, 2004; 1:699–706.

39.

Golding

, Bryden

, Levett

, McDonald

et al. Livestock-associated methicillin-resistant Staphylococcus aureus sequence type 398 in humans, Canada. Emerg Infect Dis, 2010; 16:587–594.

40.

Gorwitz

, Kruszon-Moran

, McAllister

, McQuillan

et al. Changes in the prevalence of nasal colonization with Staphylococcus aureus in the United States, 2001–2004. J Infect Dis, 2008; 197:1226–1234.

41.

Graham

3rd , Lin

, Larson

. A U.S. population-based survey of Staphylococcus aureus colonization. Ann Intern Med, 2006; 144:318–325.

42.

Grisold

, Zarfel

, Hoenigl

, Krziwanek

et al. Occurrence and genotyping using automated repetitive-sequence-based PCR of methicillin-resistant Staphylococcus aureus ST398 in Southeast Austria. Diagn Microbiol Infect Dis, 2009a. 66:217–221.

43.

Grisold

, Zarfel

, Stoeger

, Feierl

et al. Emergence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in Southeast Austria. J Infect, 2009b. 58:168–170.

44.

Guardabassi

, O'Donoghue

, Moodley

, Ho

, Boost

. Novel Lineage of Methicillin-Resistant Staphylococcus aureus, Hong Kong. Emerg Infect Dis, 2009; 15:1998–2000.

45.

Guardabassi

, Stegger

, Skov

. Retrospective detection of methicillin resistant and susceptible Staphylococcus aureus ST398 in Danish slaughter pigs. Vet Microbiol, 2007; 122:384–386.

46.

Ham ten

PBG

, Hoebe

, Nys

, Driessen

et al.

Prevalence of antibiotic resistance in the community. International Symposium on Staph and Staph infections (ISSSI)

Maastricht: The Netherlands, 2006.

47.

Hanselman

, Kruth

, Rousseau

, Low

et al. Methicillin-resistant Staphylococcus aureus colonization in veterinary personnel. Emerg Infect Dis, 2006; 12:1933–1938.

48.

Hartmeyer

, Gahrn-Hansen

, Skov

, Kolmos

. Pig-associated methicillin-resistant Staphylococcus aureus: family transmission and severe pneumonia in a newborn. Scand J Infect Dis, 2010; 42:318–320.

49.

Hasman

, Moodley

, Guardabassi

, Stegger

et al. spa type distribution in Staphylococcus aureus originating from pigs, cattle and poultry. Vet Microbiol, 2010; 141:326–331.

50.

, Chuang

, Choi

, Lee

et al. Community-associated methicillin-resistant and methicillin-sensitive Staphylococcus aureus: skin and soft tissue infections in Hong Kong. Diagn Microbiol Infect Dis, 2008; 61:245–250.

51.

Huijsdens

, Bosch

, van Santen-Verheuvel

, Spalburg

et al. Molecular characterisation of PFGE non-typable methicillin-resistant Staphylococcus aureus in The Netherlands, 2007. Euro Surveill, 2009; 14:pii: 19335.

52.

Huijsdens

, van Dijke

, Spalburg

, van Santen-Verheuvel

et al. Community-acquired MRSA and pig-farming. Ann Clin Microbiol Antimicrob, 2006; 5:26.

53.

, Yung

, Ng

, Luk

et al. Contemporary methicillin-resistant Staphylococcus aureus clones in Hong Kong. J Clin Microbiol, 2005; 43:5069–5073.

54.

Ito

, Katayama

, Asada

, Mori

et al. Structural comparison of three types of staphylococcal cassette chromosome mec integrated in the chromosome in methicillin-resistant Staphylococcus aureus. Antimicrob Agents Chemother, 2001; 45:1323–1336.

55.

Jansen

, Box

, Fluit

. SCCmec typing in methicillin-resistant Staphylococcus aureus strains of animal origin. Emerg Infect Dis, 2009; 15:136–137; author reply 136–137.

56.

Juhasz-Kaszanyitzky

, Janosi

, Somogyi

, Dan

et al. MRSA transmission between cows and humans. Emerg Infect Dis, 2007; 13:630–632.

57.

Kadlec

, Ehricht

, Monecke

, Steinacker

et al. Diversity of antimicrobial resistance pheno- and genotypes of methicillin-resistant Staphylococcus aureus ST398 from diseased swine. J Antimicrob Chemother, 2009; 64:1156–1164.

58.

Kadlec

, Schwarz

. Identification of a novel trimethoprim resistance gene, dfrK, in a methicillin-resistant Staphylococcus aureus ST398 strain and its physical linkage to the tetracycline resistance gene tet(L) Antimicrob Agents Chemother, 2009a. 53:776–778.

59.

Kadlec

, Schwarz

. Novel ABC transporter gene, vga(C), located on a multiresistance plasmid from a porcine methicillin-resistant Staphylococcus aureus ST398 strain. Antimicrob Agents Chemother, 2009b. 53:3589–3591.

60.

Kehrenberg

, Aarestrup

, Schwarz

. IS21-558 insertion sequences are involved in the mobility of the multiresistance gene cfr. Antimicrob Agents Chemother, 2007; 51:483–487.

61.

Kehrenberg

, Cuny

, Strommenger

, Schwarz

, Witte

. Methicillin-resistant and -susceptible Staphylococcus aureus strains of clonal lineages ST398 and ST9 from swine carry the multidrug resistance gene cfr. Antimicrob Agents Chemother, 2009; 53:779–781.

62.

Kehrenberg

, Schwarz

. Distribution of florfenicol resistance genes fexA and cfr among chloramphenicol-resistant Staphylococcus isolates. Antimicrob Agents Chemother, 2006; 50:1156–1163.

63.

Khalid

, Zakaria

, Toung

, McOrist

. Low levels of methicillin-resistant Staphylococcus aureus in pigs in Malaysia. Vet Rec, 2009; 164:626–627.

64.

Khanna

, Friendship

, Dewey

, Weese

. Methicillin resistant Staphylococcus aureus colonization in pigs and pig farmers. Vet Microbiol, 2008; 128:298–303.

65.

Kitai

, Shimizu

, Kawano

, Sato

et al. Characterization of methicillin-resistant Staphylococcus aureus isolated from retail raw chicken meat in Japan. J Vet Med Sci, 2005; 67:107–110.

66.

Klevens

, Morrison

, Nadle

, Petit

et al. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA, 2007; 298:1763–1771.

67.

Kock

, Brakensiek

, Mellmann

, Kipp

et al. Cross-border comparison of the admission prevalence and clonal structure of methicillin-resistant Staphylococcus aureus. J Hosp Infect, 2009a. 71:320–326.

68.

Kock

, Harlizius

, Bressan

, Laerberg

et al. Prevalence and molecular characteristics of methicillin-resistant Staphylococcus aureus (MRSA) among pigs on German farms and import of livestock-related MRSA into hospitals. Eur J Clin Microbiol Infect Dis, 2009b. 28:1375–1382.

69.

Krause

, Cavaco

. Laboratory Protocols MRSA Training Course. Isolation of MRSA from Dust Samples. DTU Food National Food Institute, 2009.

70.

Krziwanek

, Metz-Gercek

, Mittermayer

. Methicillin-resistant Staphylococcus aureus ST398 from human patients, upper Austria. Emerg Infect Dis, 2009; 15:766–769.

71.

Kwon

, Park

, Jung

, Youn

et al. Characteristics of methicillin resistant Staphylococcus aureus isolated from chicken meat and hospitalized dogs in Korea and their epidemiological relatedness. Vet Microbiol, 2006; 117:304–312.

72.

Larsen

, Stegger

, Bocher

, Sorum

et al. Emergence and characterization of community-associated methicillin-resistant Staphyloccocus aureus infections in Denmark, 1999 to 2006. J Clin Microbiol, 2009; 47:73–78.

73.

Lewis

, Molbak

, Reese

, Aarestrup

et al. Pigs as source of methicillin-resistant Staphylococcus aureus CC398 infections in humans, Denmark. Emerg Infect Dis, 2008; 14:1383–1389.

74.

Loeffler

, Kearns

, Ellington

, Smith

et al.

First isolation of MRSA ST398 from UK animals: a new challenge for infection control teams?

J Hosp Infect, 2009; 72:269–271.

75.

Lowder

, Guinane

, Ben Zakour

, Weinert

et al. Recent human-to-poultry host jump, adaptation, and pandemic spread of Staphylococcus aureus. Proc Natl Acad Sci USA, 2009; 106:19545–19550.

76.

Lozano

, Lopez

, Gomez-Sanz

, Ruiz-Larrea

et al. Detection of methicillin-resistant Staphylococcus aureus ST398 in food samples of animal origin in Spain. J Antimicrob Chemother, 2009; 64:1325–1326.

77.

Mammina

, Cala

, Plano

, Bonura

et al. Ventilator-associated pneumonia and MRSA ST398, Italy. Emerg Infect Dis, 2010; 16:730–731.

78.

Meemken

, Blaha

, Tegeler

, Tenhagen

et al. Livestock associated methicillin-resistant Staphylococcus aureus (LaMRSA) isolated from lesions of pigs at necropsy in northwest Germany between 2004 and 2007. Zoonoses Public Health, 2009Ahead of print.

79.

Mendes

, Deshpande

, Castanheira

, DiPersio

et al. First report of cfr-mediated resistance to linezolid in human staphylococcal clinical isolates recovered in the United States. Antimicrob Agents Chemother, 2008; 52:2244–2246.

80.

Monecke

, Kuhnert

, Hotzel

, Slickers

, Ehricht

. Microarray based study on virulence-associated genes and resistance determinants of Staphylococcus aureus isolates from cattle. Vet Microbiol, 2007; 125:128–140.

81.

Moodley

, Nightingale

, Stegger

, Nielsen

et al. High risk for nasal carriage of methicillin-resistant Staphylococcus aureus among Danish veterinary practitioners. Scand J Work Environ Health, 2008; 34:151–157.

82.

Mulders

, Haenen

, Geenen

, Vesseur

et al. Prevalence of livestock-associated MRSA in broiler flocks and risk factors for slaughterhouse personnel in The Netherlands. Epidemiol Infect, 2010; 138:743–755.

83.

Neela

, Arif

, Nor Shamsudin

, van Belkum

et al. Prevalence of ST9 MRSA among pigs and pig handlers in Malaysia. J Clin Microbiol, 2009; 47:4138–4140.

84.

Nemati

, Hermans

, Lipinska

, Denis

et al. Antimicrobial resistance of old and recent Staphylococcus aureus isolates from poultry: first detection of livestock-associated methicillin-resistant strain ST398. Antimicrob Agents Chemother, 2008; 52:3817–3819.

85.

Nickerson

. Control of heifer mastitis: antimicrobial treatment-an overview. Vet Microbiol, 2009; 134:128–135.

86.

Nubel

, Roumagnac

, Feldkamp

, Song

et al. Frequent emergence and limited geographic dispersal of methicillin-resistant Staphylococcus aureus. Proc Natl Acad Sci USA, 2008; 105:14130–14135.

87.

Nulens

, Stobberingh

, Smeets

, van Dessel

et al. Genetic diversity of methicillin-resistant Staphylococcus aureus in a tertiary hospital in The Netherlands between 2002 and 2006. Eur J Clin Microbiol Infect Dis, 2009; 28:631–639.

88.

Nulens

, Stobberingh

, van Dessel

, Sebastian

et al. Molecular characterization of Staphylococcus aureus bloodstream isolates collected in a Dutch University Hospital between 1999 and 2006. J Clin Microbiol, 2008; 46:2438–2441.

89.

Oliveira

, Tomasz

, de Lencastre

. Secrets of success of a human pathogen: molecular evolution of pandemic clones of methicillin-resistant Staphylococcus aureus. Lancet Infect Dis, 2002; 2:180–189.

90.

Pan

, Battisti

, Zoncada

, Bernieri

et al. Community-acquired methicillin-resistant Staphylococcus aureus ST398 infection, Italy. Emerg Infect Dis, 2009; 15:845–847.

91.

Persoons

, Van Hoorebeke

, Hermans

, Butaye

et al. Methicillin-resistant Staphylococcus aureus in poultry. Emerg Infect Dis, 2009; 15:452–453.

92.

Pomba

, Hasman

, Cavaco

, da Fonseca

, Aarestrup

. First description of methicillin-resistant Staphylococcus aureus (MRSA) CC30 and CC398 from swine in Portugal. Int J Antimicrob Agents, 2009; 34:193–194.

93.

, Han

, Ge

. Isolation and characterization of methicillin-resistant Staphylococcus aureus from Louisiana retail meats. Appl Environ Microbiol, 2008; 75:265–267.

94.

Riesen

, Perreten

. Antibiotic resistance and genetic diversity in Staphylococcus aureus from slaughter pigs in Switzerland. Schweiz Arch Tierheilkd, 2009; 151:425–431.

95.

Rijnders

, Deurenberg

, Boumans

, Hoogkamp-Korstanje

et al.

The population structure of Staphylococcus aureus isolated from intensive care unit patients in The Netherlands over an eleven-year period (1996–2006)

J Clin Microbiol, 2009; 47:4090–4095.

96.

Schaefer

, Goldstein

, Reif

, Fair

, Bossart

. Antibiotic-resistant organisms cultured from Atlantic bottlenose dolphins (Tursiops truncatus) inhabiting estuarine waters of Charleston, SC and Indian River Lagoon, FL. EcoHealth, 2009; 6:33–41.

97.

Schijffelen

, Boel

, van Strijp

, Fluit

. Whole genome analysis reveals substantial diversification between pig-associated methicillin-resistant Staphylococcus aureus ST398 isolates. International Symposium on Staphylococci and Staphylococcal Infections, Cairns: Australia, 2008.

98.

Schouls

, Spalburg

, van Luit

, Huijsdens

et al. Multiple-locus variable number tandem repeat analysis of Staphylococcus aureus: comparison with pulsed-field gel electrophoresis and spa-typing. PLoS ONE, 2009; 4:e5082.

99.

Schwarz

, Kadlec

, Strommenger

. Methicillin-resistant Staphylococcus aureus and Staphylococcus pseudintermedius detected in the BfT-GermVet monitoring programme 2004–2006 in Germany. J Antimicrob Chemother, 2008; 61:282–285.

100.

Sergio

, Koh

, Hsu

, Ogden

et al. Investigation of methicillin-resistant Staphylococcus aureus in pigs used for research. J Med Microbiol, 2007; 56:1107–1109.

101.

Smith

, Male

, Harper

, Kroeger

et al. Methicillin-resistant Staphylococcus aureus (MRSA) strain ST398 is present in midwestern U.S. swine and swine workers. PLoS ONE, 2009; 4:e4258.

102.

Soavi

, Stellini

, Signorini

, Antonini

et al. Methicillin-resistant Staphylococcus aureus ST398, Italy. Emerg Infect Dis, 2010; 16:346–348.

103.

Stegger

, Lindsay

, Sorum

, Gould

, Skov

. Genetic diversity in CC398 methicillin-resistant Staphylococcus aureus isolates of different geographical origin. Clin Microbiol Infect, 2009[Epub ahead of print].

104.

Sunde

. Colonization and Persistence of MRSA Sequence Type 8 (ST8) on a Pig Farm. Methicillin-Resistant Staphylococci in Animals: Veterinary and Public Health Implications. London, UK, 2009.

105.

Toh

, Xiong

, Arias

, Villegas

et al. Acquisition of a natural resistance gene renders a clinical strain of methicillin-resistant Staphylococcus aureus resistant to the synthetic antibiotic linezolid. Mol Microbiol, 2007; 64:1506–1514.

106.

van Belkum

, Melles

, Peeters

, van Leeuwen

et al. Methicillin-resistant and -susceptible Staphylococcus aureus sequence type 398 in pigs and humans. Emerg Infect Dis, 2008; 14:479–483.

107.

van de Giessen

, van Santen-Verheuvel

, Hengeveld

, Bosch

et al. Occurrence of methicillin-resistant Staphylococcus aureus in rats living on pig farms. Prev Vet Med, 2009; 91:270–273.

108.

Van De Griend

, Herwaldt

, Alvis

, DeMartino

et al. Community-associated methicillin-resistant Staphylococcus aureus, Iowa, USA. Emerg Infect Dis, 2009; 15:1582–1589.

109.

Van Den Broek

, Van Cleef

, Haenen

, Broens

et al. Methicillin-resistant Staphylococcus aureus in people living and working in pig farms. Epidemiol Infect, 2009; 137:700–708.

110.

Van den Eede

, Martens

, Lipinska

, Struelens

et al. High occurrence of methicillin-resistant Staphylococcus aureus ST398 in equine nasal samples. Vet Microbiol, 2009; 133:138–144.

111.

Vanderhaeghen

, Cerpentier

, Adriaensen

, Vicca

et al. Methicillin-resistant Staphylococcus aureus (MRSA) ST398 associated with clinical and subclinical mastitis in Belgian cows. Vet Microbiol, 2010[Epub ahead of print].

112.

van Duijkeren

, Ikawaty

, Broekhuizen-Stins

, Jansen

et al. Transmission of methicillin-resistant Staphylococcus aureus strains between different kinds of pig farms. Vet Microbiol, 2008; 126:383–389.

113.

van Duijkeren

, Jansen

, Flemming

, de Neeling

et al. Methicillin-resistant Staphylococcus aureus in pigs with exudative epidermitis. Emerg Infect Dis, 2007; 13:1408–1410.

114.

van Duijkeren

, Moleman

, Sloet van Oldruitenborgh-Oosterbaan

, Multem

et al. Methicillin-resistant Staphylococcus aureus in horses and horse personnel: an investigation of several outbreaks. Vet Microbiol, 2010; 141:96–102.

115.

Van Hoecke

, Piette

, De Leenheer

, Lagasse

et al. Destructive otomastoiditis by MRSA from porcine origin. Laryngoscope, 2009; 119:137–140.

116.

van Loo

, Huijsdens

, Tiemersma

, de Neeling

et al. Emergence of methicillin-resistant Staphylococcus aureus of animal origin in humans. Emerg Infect Dis, 2007a. 13:1834–1839.

117.

van Loo

, Diederen

, Savelkoul

, Woudenberg

et al. Methicillin-resistant Staphylococcus aureus in meat products, The Netherlands. Emerg Infect Dis, 2007b. 13:1753–1755.

118.

van Rijen

, Bosch

, Heck

, Kluytmans

. Methicillin-resistant Staphylococcus aureus epidemiology and transmission in a Dutch hospital. J Hosp Infect, 2009; 72:299–306.

119.

van Rijen

, Van Keulen

, Kluytmans

. Increase in a Dutch hospital of methicillin-resistant Staphylococcus aureus related to animal farming. Clin Infect Dis, 2008; 46:261–263.

120.

van Wamel

, Hansenova Manaskova

, Fluit

, Verbrugh

et al. Short term micro-evolution and PCR-detection of methicillin-resistant and -susceptible Staphylococcus aureus sequence type 398. Eur J Clin Microbiol Infect Dis, 2010; 29:119–122.

121.

Varshney

, Mediavilla

, Robiou

, Guh

et al. Diverse enterotoxin gene profiles among clonal complexes of Staphylococcus aureus isolates from the Bronx, New York. Appl Environ Microbiol, 2009; 75:6839–6849.

122.

Vengust

, Anderson

, Rousseau

, Weese

. Methicillin-resistant staphylococcal colonization in clinically normal dogs and horses in the community. Lett Appl Microbiol, 2006; 43:602–606.

123.

Vindel

, Cuevas

, Cercenado

, Marcos

et al. Methicillin-resistant Staphylococcus aureus in Spain: molecular epidemiology and utility of different typing methods. J Clin Microbiol, 2009; 47:1620–1627.

124.

Voss

, Loeffen

, Bakker

, Klaassen

, Wulf

. Methicillin-resistant Staphylococcus aureus in pig farming. Emerg Infect Dis, 2005; 11:1965–1966.

125.

Wagenaar

, Yue

, Pritchard

, Broekhuizen-Stins

et al. Unexpected sequence types in livestock associated methicillin-resistant Staphylococcus aureus (MRSA): MRSA ST9 and a single locus variant of ST9 in pig farming in China. Vet Microbiol, 2009; 139:405–409.

126.

Walther

, Monecke

, Ruscher

, Friedrich

et al. Comparative molecular analysis substantiates zoonotic potential of equine methicillin-resistant Staphylococcus aureus. J Clin Microbiol, 2009; 47:704–710.

127.

Weese

, Archambault

, Willey

, Hearn

et al. Methicillin-resistant Staphylococcus aureus in horses and horse personnel, 2000–2002. Emerg Infect Dis, 2005a. 11:430–435.

128.

Weese

, Caldwell

, Willey

, Kreiswirth

et al. An outbreak of methicillin-resistant Staphylococcus aureus skin infections resulting from horse to human transmission in a veterinary hospital. Vet Microbiol, 2006; 114:160–164.

129.

Weese

, Rousseau

, Traub-Dargatz

, Willey

et al. Community-associated methicillin-resistant Staphylococcus aureus in horses and humans who work with horses. J Am Vet Med Assoc, 2005b. 226:580–583.

130.

Welinder-Olsson

, Floren-Johansson

, Larsson

, Oberg

et al. Infection with Panton-Valentine leukocidin-positive methicillin-resistant Staphylococcus aureus t034. Emerg Infect Dis, 2008; 14:1271–1272.

131.

Wertheim

, Melles

, Vos

, van Leeuwen

et al. The role of nasal carriage in Staphylococcus aureus infections. Lancet Infect Dis, 2005; 5:751–762.

132.

Wertheim

, Vos

, Boelens

, Voss

et al. Low prevalence of methicillin-resistant Staphylococcus aureus (MRSA) at hospital admission in The Netherlands: the value of search and destroy and restrictive antibiotic use. J Hosp Infect, 2004; 56:321–325.

133.

Witte

, Strommenger

, Stanek

, Cuny

. Methicillin-resistant Staphylococcus aureus ST398 in humans and animals, Central Europe. Emerg Infect Dis, 2007; 13:255–258.

134.

, Wang

, Yang

, Geng

et al. Epidemiology and molecular characteristics of community-associated methicillin-resistant and methicillin-susceptible Staphylococcus aureus from skin/soft tissue infections in a children's hospital in Beijing, China. Diagn Microbiol Infect Dis, 2010; 67:1–8.

135.

Wulf

, van Nes

, Eikelenboom-Boskamp

, de Vries

et al. Methicillin-resistant Staphylococcus aureus in veterinary doctors and students, The Netherlands. Emerg Infect Dis, 2006; 12:1939–1941.

136.

Wulf

, Voss

. MRSA in livestock animals-an epidemic waiting to happen? Clin Microbiol Infect, 2008; 14:519–521.

137.

Wulf

, Markestein

, van der Linden

, Voss

et al. First outbreak of methicillin-resistant Staphylococcus aureus ST398 in a Dutch hospital. June 2007. Euro Surveill, 2008a. 13pii8051.

138.

Wulf

, Sorum

, van Nes

, Skov

et al. Prevalence of methicillin-resistant Staphylococcus aureus among veterinarians: an international study. Clin Microbiol Infect, 2008b. 14:29–34.

139.

Wulf

, Tiemersma

, Kluytmans

, Bogaers

et al. MRSA carriage in healthcare personnel in contact with farm animals. J Hosp Infect, 2008c. 70:186–190.

140.

, Chen

, Liu

, Zhang

et al. Prevalence of Staphylococcus aureus carrying Panton-Valentine leukocidin genes among isolates from hospitalised patients in China. Clin Microbiol Infect, 2008; 14:381–384.

141.

Zemlickova

, Fridrichova

, Tyllova

, Jakubu

, Machova

. Carriage of methicillin-resistant Staphylococcus aureus in veterinary personnel. Epidemiol Infect, 2009; 137:1233–1236.

142.

Zhang

, McClure

, Elsayed

, Louie

, Conly

. Novel multiplex PCR assay for characterization and concomitant subtyping of staphylococcal cassette chromosome mec types I to V in methicillin-resistant Staphylococcus aureus. J Clin Microbiol, 2005; 43:5026–5033.

143.

Zhang

, Agidi

, LeJeune

. Diversity of staphylococcal cassette chromosome in coagulase-negative staphylococci from animal sources. J Appl Microbiol, 2009; 107:1375–1383.