Isolation of Enterotoxigenic Staphylococcus aureus from Pet Dogs and Cats: A Public Health Implication

Introduction

S taphylococcal food poisoning is considered one of the most common foodborne illnesses worldwide. The disease is caused by ingestion of contaminated food containing already performed staphylococcal enterotoxins that were produced by enterotoxigenic strains of Staphylococcus aureus (Hennekinne et al. 2009). Symptoms are characterized by sudden onset, including nausea, vomiting, stomach cramps, and diarrhea. The disease is usually mild, although in some cases may be severe enough to require hospitalization (Murray 2005). However, many kinds of foods were involved in staphylococcal food poisoning outbreaks, including meat and milk products; humans remain the principal source of food contamination with enterotoxins producing strains of S. aureus during handling and preparation of food either through direct contact or via coughing and sneezing, leading to contamination of food by these strains, which subsequently produce their enterotoxins in food, resulting in staphylococcal food poisoning (Le Loir et al. 2003). Although many different types of S. aureus enterotoxins (SEs) have been already discovered, only few types were usually implicated in food poisoning outbreaks, including SE type A, SE type B, SE type C, and SE type D (Balban and Rasooly 2000, Smyth et al. 2005). S. aureus is a ubiquitous organism and widely distributed among human and animals; pets, especially dogs and cats, are common households with intimate contact to family members; this permits the zoonotic transmission of many infections among them. The present study investigates the prevalence of enterotoxigenic S. aureus among pet dogs and cats as well as their human contacts, indicating its public health implications.

Materials and Methods

Nasal, oral, and wound swabs were collected from randomly selected 70 dogs and 47 cats admitted to different veterinary clinics in Cairo, Egypt, in addition to nasal swabs from 26 persons in intimate contact with those pets (either pet owners or veterinarians). All swabs were collected in Cary-Blair transport medium (BD) and brought back to the laboratory for investigation. Swabs were directly streaked on mannitol salt agar medium (Oxoid, Ltd.) and incubated at 37°C for 24 h. Yellow colonies indicating mannitol fermentation were picked up to prepare bacterial films and then Gram's stain was applied to confirm Gram-positive cocci arranged in irregular clusters. Then, the cultures were tested for catalase enzyme, and catalase-positive colonies were subcultured on 5% sheep blood agar medium (Oxoid, Ltd.) to observe the hemolytic activity. A tube coagulase test was performed on pure β hemolytic colonies using lyophilized rabbit plasma (BD) (Quinn et al. 2002). Identified colonies were confirmed as S. aureus when reacted positively with commercial latex agglutination test, Staphaurex (Remel). S. aureus isolates were then characterized for their enterotoxins production by using RIDASCREEN^® SET A, B, C, D, E (r–biopharm), which is highly sensitive (the minimal detectable amount of toxins was 0.1 ng/mL), specific (specificity 100%), and rapid ELISA for determination of staphylococci enterotoxins A, B, C, D, and E (Pakr et al. 1994, Bennett 2005). The test could detect and characterize the type of produced toxins in the bacterial cultures following the manufacturer's protocol.

Results

The prevalence of enterotoxigenic S. aureus was 10% and 2.1% for pet dogs and cats, respectively, whereas that for human contacts was 7.7% (Table 1).

Moreover, all types of SEs (A, B, C, D, and E) could be detected in different ratios in the isolates recovered from dogs, cats, and human contacts as shown in (Table 1). The majority of animal isolates were obtained from oral mucosa rather than other sites, whereas there is no nasal/oral co-colonization in dogs was detected (Table 2).

Discussion

The results of the present study revealed higher prevalence of enterotoxigenic strains of S. aureus among pet dogs (10%) rather than those of cats (2.1%), whereas the nasal carriage rate in human contacts was 7.7% (Table 1). Therefore, pet dogs may be considered as an important reservoir for enterotoxin-producing strains of S. aureus. Most of the animal isolates were obtained from the mouth of the apparently healthy dogs denoting high oral carriage rate of such pathogen in dogs, and the same was found in cats (Table 2). Consequently, saliva of such animals may be regarded as a source that sheds the pathogen and contaminates the surrounding environment. Moreover, all types of SEs were produced by the dog isolates, whereas three of them were found to produce more than one type of SEs considering SE type C, which was the most prevalent one, whereas the cat isolate produced only SE type A. On the other hand, the two human isolates were recovered from nasal mucosa of two veterinarians in contact with the examined animals and were found to produce all types of SEs, and SE type C was the most prevalent one. Unfortunately, no animal/human co-colonization was detected between pets and their owners, as the owners of positive animals were not available to be sampled; however, their veterinarians were positives as previously mentioned. Pet dogs and cats are common households with intimate contact with their owners. So, isolation of enterotoxogenic strains of S. aureus from these animals highlights the possibility of zoonotic transmission of such strains to human contacts. The transmission could be either through direct contact with the infected animals or indirectly via contact with contaminated household stuffs, accordingly leading to human nasal carriage and/or hand carriage of the pathogen, which is considered an important potential source for food contamination and regarded as the cornerstone in the initiation of household staphylococcal food poisoning outbreaks (Colombari et al. 2007). The detected enterotoxins were types A, B, C, D, and E in different ratios (Table 1). A special attention should be paid to enterotoxin type A that has been produced by isolates recovered from both dogs and cats, as it is considered the common cause of staphylococcal food poisoning worldwide (Balban and Rasooly 2000). Likewise, enterotoxin type E, which has been isolated from a dog as well as a human case, could be a serious cause of food poisoning like what was recorded in France recently (Ostyn et al. 2010) when six outbreaks of household staphylococcal food poisoning had been occurred.

Moreover, the public health impact of our findings may extend beyond the possible role of enterotoxigenic S. aureus isolated from pets in staphylococcal food poisoning outbreaks, since SEs were incriminated in several cases of sudden infant death syndrome (SIDS), specially enterotoxins types A, B, and C (Zorgani et al. 1999, Highet and Goldwater 2009). So, whenever we detect these enterotoxins in the animal isolates, especially those recovered from dogs, we should consider that such strains may get their way from pets to infants by different routes, resulting in nasal and/or intestinal colonization that may bear a risk of developing SIDS (Murrell et al. 1993).

In conclusion, the high prevalence of enterotoxigenic S. aureus among pet animals, especially dogs with high nasal carriage rate in human contacts, highlights the potential role played by these animals in the zoonotic transmission of such strains to human contacts and thus may be implicated in the epidemiology of certain conditions of public health burden such as household staphylococcal food poisoning outbreaks and SIDS.