Outbreaks Caused by Sprouts,United States,1998–2010: Lessons Learned and Solutions Needed

Abstract

After a series of outbreaks associated with sprouts in the mid-1990s, the U.S. Food and Drug Administration (FDA) published guidelines in 1999 for sprouts producers to reduce the risk of contamination. The recommendations included treating seeds with an antimicrobial agent such as calcium hypochlorite solution and testing spent irrigation water for pathogens. From 1998 through 2010, 33 outbreaks from seed and bean sprouts were documented in the United States, affecting 1330 reported persons. Twenty-eight outbreaks were caused by Salmonella, four by Shiga toxin–producing Escherichia coli, and one by Listeria. In 15 of the 18 outbreaks with information available, growers had not followed key FDA guidelines. In three outbreaks, however, the implicated sprouts were produced by firms that appeared to have implemented key FDA guidelines. Although seed chlorination, if consistently applied, reduces pathogen burden on sprouts, it does not eliminate the risk of human infection. Further seed and sprouts disinfection technologies, some recently developed, will be needed to enhance sprouts safety and reduce human disease. Improved seed production practices could also decrease pathogen burden but, because seeds are a globally distributed commodity, will require international cooperation.

Introduction

Outbreaks of foodborne infections associated with sprouts—including alfalfa, mung bean, radish, and fenugreek sprouts, among others—have been caused by many pathogens and have been a recurrent problem in the United States and other countries (Taormina et al., 1999). In 1995, a large international outbreak of Salmonella serotype Stanley infections associated with alfalfa sprouts focused public health, industry, and regulatory concern on the food safety challenge of alfalfa and other seed sprouts (Mahon et al., 1997). In 1996, an even larger outbreak caused by radish sprouts occurred in Japan, with more than 7000 cases of Shiga toxin–producing Escherichia coli (STEC) O157:H7 infection (Itoh et al., 1998; Michino et al., 1999; Watanabe et al., 1999). In 2011, an outbreak of Escherichia coli O104:H4 infections in Europe associated with fenugreek sprouts caused 845 cases of hemolytic-uremic syndrome, 2971 cases of acute gastroenteritis, and 54 deaths (Buchholz et al., 2011; Frank et al., 2011). Sprouts are produced rapidly from seed, a raw agricultural product, often by small-scale sprouting firms; they are sold intact, alive, and perishable and are typically consumed without further processing or treatment. Bacterial pathogens on the seed can grow to high densities in the warm, moist, and nutrient-rich conditions of sprouting (Jacquette et al., 1996).

Because of concern over sprouts-associated illnesses, steps have been taken in the United States to make sprouts safer. In 1999, the US Food and Drug Administration (FDA) recommended soaking seeds with an antimicrobial agent such as 20,000 mg/L (ppm) calcium hypochlorite solution before sprouting and testing the spent sprout irrigation water for Salmonella spp. and STEC O157:H7 before distribution (FDA, 1999). This 20,000 mg/L (ppm) calcium hypochlorite treatment was the most effective chemical seed treatment known (Beuchat, 1997), although various other chemical, physical, and biological control methods have been identified subsequently. Although lot-by-lot testing is generally not advocated as a safety strategy for other foods, it was advised for sprouts because seed antimicrobial treatments did not guarantee complete elimination of pathogens and because the rapid bacterial amplification that can occur during sprouting makes detection more likely (FDA, 1999; Thomas et al., 2003; Fu et al., 2001). A previous review summarized sprouts-associated outbreaks from 1973 through 1997 (Taormina et al., 1999). We evaluate sprouts-associated outbreaks from 1998 to 2010 to assess the effect of the recommended control measures, and we also review new seed and sprouts treatment technologies that offer the possibility of enhanced control of sprouts contamination.

Methods

Sprouts-associated outbreaks

Data on foodborne disease outbreaks in the United States were obtained from the Centers for Disease Control and Prevention's (CDC) Foodborne Disease Outbreak Surveillance System (FDOSS). FDOSS collects information reported voluntarily by local, state, tribal, and territorial health departments, defining an outbreak as two or more illnesses associated with consumption of a common food. This analysis includes outbreaks occurring between 1998 and 2010 reported to FDOSS by July 5, 2012. Information routinely collected includes the magnitude (number of illnesses, hospitalization, and deaths), location, pathogenic etiology, and food vehicle of the outbreak. All outbreaks reported as sprouts-associated that identified a pathogenic etiology were included. Information on the location of the sprouts grower, the origin of seed, and the grower's implementation of FDA guidelines were obtained from FDA when available. Finally, the literature was searched for additional sprouts-associated outbreaks during this period that had not been reported to CDC or FDA and for further information on reported outbreaks.

Whether guidelines for sprouts antimicrobial treatment had been implemented was determined from FDA reports, or, when these reports were not available, from summaries by the investigating state health departments. Although FDA guidelines for sprouts include multiple additional recommendations, sprouts growers were categorized as having complied with key guidelines if they both (1) conducted an antimicrobial treatment such as soaking seeds in 20,000 mg/L calcium hypochlorite solution and (2) tested the spent sprout irrigation water for Salmonella and STEC O157 before distribution.

Seed and sprouts treatment strategies

The scientific literature was reviewed for chemical, physical, biological, or combination treatments to inactivate bacterial human pathogens on sprouts, alfalfa seeds, and mung beans. Public and private literature databases were searched: PubMed (U.S. National Library of Medicine, National Institutes of Health), Scopus (Elsevier B.V., Amsterdam, The Netherlands), and Google Scholar (Mountain View, CA). Search terms included “alfalfa seeds,” “mung beans,” “sprouts,” or “seed sprouts,” in combinations with “pathogens,” “Salmonella,” or “Escherichia coli,” and “disinfection,” “sanitization,” or “decontamination.” Treatments that were reported to cause ≥2-log colony-forming units (CFU)/g reductions of the bacterial pathogens Salmonella, STEC O157:H7, or the native bacterial flora on seeds without significant (p<0.05) reduction in germination percentage are summarized.

Results

Sprouts-associated outbreaks

From 1998 through 2010, 14,956 U.S. foodborne disease outbreaks were reported to FDOSS. Among the 8366 for which a food vehicle was reported, 33 were sprouts associated. The sprouts-associated outbreaks were associated with 1330 illnesses (median of 22 illnesses per outbreak), 123 hospitalizations, and 2 deaths (Table 1). Alfalfa sprouts were implicated in 20 outbreaks, bean sprouts in 8, clover sprouts in 3, alfalfa and clover together in 1, and unspecified sprouts in 1. Fourteen were single-state outbreaks, and 19 were outbreaks in which the exposure occurred in more than 1 state.

Table 1.

Reported Sprouts-Associated Outbreaks in the United States, 1998–2010

Year	Pathogen	Sprout type	Illnesses	Hospitalizations	Deaths	Reporting states	Sprouts grower compliance with FDA guidelines ^a	Seed source, if known ^b
1998	Salmonella serovar Cubana/ Salmonella serovar Havana (suspected)	Alfalfa	40	4	1	Multistate	Unknown	California
	STEC O157:NM	Alfalfa and clover	8	2	0	CA	No	Alfalfa: AustraliaClover: Oregon
1999	Salmonella serovar Mbandaka	Alfalfa	89	9	0	Multistate	No	California
	Salmonella serovar Typhimurium	Clover	112	3	0	CO	No	Oregon
	Salmonella serovar Saintpaul	Clover	36	2	0	CA	Yes	Unknown
	Salmonella serovar Muenchen	Alfalfa	157	16	0	Multistate	No	Oklahoma, Texas
2000	Salmonella serovar Enteritidis	Mung bean	75	3	0	Multistate	No	China or Australia
	Salmonella spp.	Alfalfa	3	0	0	FL	Unknown	Unknown
2001	Salmonella serovar Enteritidis	Mung bean	22	0	0	HI	No	China or Australia^c
	Salmonella serovar Kottbus	Alfalfa	32	3	0	Multistate	Unknown	Australia
	Salmonella serovar Enteritidis	Mung bean	35	2	0	Multistate	Unknown	China
2002	STEC O157:H7	Alfalfa	5	3	0	CA	No	Australia
	Salmonella serovar Enteritidis	Mung bean	15	1	0	ME	No	China
2003	Salmonella serovar Saintpaul	Alfalfa	16	0	0	Multistate	Unknown	Unknown
	Salmonella serovar Chester	Alfalfa	26	3	1	Multistate	Unknown	Unknown
	STEC O157:H7	Alfalfa	20	3	0	Multistate	Unknown	Australia
2004	STEC O157:NM	Alfalfa	2	0	0	GA	No	Australia
	Salmonella serovar Bovismorbificans	Alfalfa	35	5	0	Multistate	Yes	Australia
2005	Salmonella serovar Braenderup	Mung bean	2	0	0	MA	No	China, other^c
2006	Salmonella serovar Braenderup	Bean	4	0	0	OR	Yes	China
2007	Salmonella serovar Montevideo	Bean	24	3	0	CA	Unknown	Unknown
	Salmonella serovar Mbandaka	Alfalfa	15	0	0	Multistate	Unknown	Unknown
	Salmonella serovar Mbandaka	Bean	20	0	0	CA	Unknown	Unknown
2008	Salmonella serovar Typhimurium	Alfalfa	24	4	0	Multistate	Unknown	Unknown
	L. monocytogenes	Alfalfa	20	16	0	Multistate	Unknown	Not explored
2009	Salmonella serovar Saintpaul	Alfalfa	256	8	0	Multistate	No	Italy
	Salmonella serovar Oranienburg	Alfalfa	25	0	0	Multistate	No	Australia
	Salmonella serovar Cubana	Sprouts, unspecified	2	0	0	MN	Unknown	Arizona
	Salmonella serovar Typhimurium	Alfalfa	14	2	0	MI	Unknown	Not explored
2010	Salmonella serovar Newport	Alfalfa	44		0	Multistate	No	California
	Salmonella serovar 1,4,[5],12:i:-	Alfalfa	140	31	0	Multistate	No	Not explored
	Salmonella serovar Cubana	Alfalfa	3	0	0	Multistate	Unknown	Not explored
	Salmonella serovar Newport	Clover	9	0	0	Multistate	No	Not explored

The key FDA guidelines include (1) disinfecting seeds with a treatment such as soaking seeds in 20,000 ppm calcium hypochlorite solution and (2) testing spent sprout irrigation water for pathogens. Compliance is recorded as “no” unless both key guidelines were followed by all sprouts growers involved.

When all seed came from the same supplier, traceback to seed origin was not always explored because outbreak source was deemed controlled.

Exact seed source could not be determined.

FDA, U.S. Food and Drug Administration; STEC, Shiga toxin–producing Escherichia coli.

Compared with outbreaks associated with other foods, sprouts-associated outbreaks had more illnesses (median of 22 versus 7) and hospitalizations (median of 2 versus 0) and were more likely to involve multiple states (58% versus 2%). The annual number of sprouts-associated outbreaks ranged from one to four, with no apparent trend over time (Fig. 1).

FIG. 1.

Sprout outbreaks and implementation of U.S. Food and Drug Administration (FDA) guidelines by year, 1998–2010.

Twenty-eight of the sprouts-associated outbreaks were caused by Salmonella, four by STEC O157, and one by Listeria. Fifteen serotypes of Salmonella were reported; the most common were Enteritidis (4), Mbandaka (3), Typhimurium (3), Cubana (3), Saintpaul (3), Newport (2), and Braenderup (2). Of the four STEC O157 outbreaks, two were caused by O157:NM and two by O157:H7.

Information on the origin of the seeds was available for 20 outbreaks. Six were caused by sprouts produced from domestically grown seeds, 13 by sprouts produced from imported seeds, and 1 by sprouts produced from seeds of both domestic and foreign origin. Three countries—Australia, China, and Italy—were identified as the source of imported seed. Two of the 13 outbreaks from imported seed had 2 or more reported seed sources; therefore, a source could not be definitively identified. Seeds from Australia were implicated in all outbreaks caused by STEC in which the seed source was known.

Eighteen outbreak reports included information about whether sprouts growers had implemented FDA's guidelines at the time of the outbreak. In 3 outbreaks, sprouts growers reportedly followed FDA guidelines; in 15 outbreaks, at least 1 sprouts grower that produced sprouts involved in the outbreak did not adhere to the guidelines. Data regarding the time interval between seed harvest and sprouting were not available for any outbreaks.

Seed and sprouts treatment strategies

Numerous studies of seed treatments have reported reductions of Salmonella and STEC O157:H7 by >2.0-log CFU/g from mung bean or alfalfa seed while preserving germination. Fewer studies reported ≥5-log reductions (Table 2), as evidenced by microbiologic enrichment of seeds or analysis of sprouts produced from treated seeds. A combination of peroxyacid, caprylic and capric acids, lactic acid, and glycerol monolaurate eliminated >6.90-log CFU/g of Salmonella and STEC O157:H7 from alfalfa seeds (Pierre and Ryser, 2006). Dry-heating alfalfa seeds at 55°C for 8 days eliminated 2-log CFU/g of Salmonella and >8-log CFU/g of STEC O157:H7 (Feng et al., 2007), and similar results were reported for mung beans (Weiss et al., 2007). Other treatments that achieved elimination of >5 logs of both pathogens on alfalfa seeds included high hydrostatic pressure of 500 MPa for 2 min at 45°C (Neetoo and Chen, 2010) and the combination of dry heating followed by 600 MPa for 2 min at 35°C (Neetoo and Chen, 2011).

Table 2.

Summary of Most Promising ^a Treatments to Reduce Bacterial Human Pathogens on Alfalfa and Mung Bean Seeds

			Log reductions
Treatment	Treatment parameters	Seed type	Salmonella	E. coli O157:H7	Native bacterial flora on seed	Reference
Chemical	20,000 ppm Ca(OCl)₂ 3–30 min	Alfalfa & mung bean combined means	3.21	2.81		Montville and Schaffner, 2004
	2.5–3% (wt/vol) Ca(OCl)₂ unbuffered, 10 min	Alfalfa	4.5^b	3.9^b		Fett, 2002
	3.0% (wt/vol) Ca(OCl)₂, in buffer, 15 min	Mung bean	4.65–5.02^b	3.93–4.03^b		Fett, 2002
	8% H₂O₂	Alfalfa	3.2			Weissinger and Beuchat, 2000
	1% Ca(OH)₂	Alfalfa	2.8			Weissinger and Beuchat, 2000
	1% calcinated calcium	Alfalfa	2.9			Weissinger and Beuchat, 2000
	5% organic acids	Alfalfa	2.8–3.2			Weissinger and Beuchat, 2000
	1% Ca(OH)₂+1% Tween 80	Alfalfa	4.1			Weissinger and Beuchat, 2000
	1% Ca(OH)₂ at 55°C for 5 min	Alfalfa	3.14–4.40			Beuchat and Scouten, 2002
	200 ppm stabilized oxychloro-based sanitizer for 24 h at 28°C	Mung bean	3–4^c	3–4^c		Hora et al., 2007
	Combinations of peroxyacid, caprylic and capric acids, lactic acid and glycerol monolaurate	Alfalfa	>6.90^c	>6.96^c		Pierre and Ryser, 2006
	Combination of ClO₂, O₃, and thyme essential oil	Alfalfa		2.80^b		Singh et al., 2003
	20,000 ppm commercial citrus-related products	Alfalfa	3.56–3.74^b	3.42–3.46^b		Fett and Cooke, 2003
	300 mg ammonia/L of air at 20°C	Alfalfa	2–3^b	2–3^b		Himathongkham, 2001
		Mung bean	5–6^b	5–6^b
	800 ppm acidified sodium chlorite 45 min	Alfalfa	3.9^b		1.84	Liao, 2009
Biological	10,000 AU semicrude colicin (bacteriocin) per 10 g seed	Alfalfa		5^b		Nandiwada, et al., 2004
Heat	Dry heat at 55°C for 8 days	Alfalfa	5^b	>5^c		Feng et al., 2007
			2^c	>8^c
	55–80°C for 2–20 min	Mung bean	5	5		Weiss and Hammes, 2005
	53–64°C for 0.5–8 min	Alfalfa	5	5
	55°C for 5–7 days	Mung bean	3.0^c	5.0^c		Hu et al., 2004
	80°C for 30 s followed by chilled water for 30 s	Mung bean	3.17^b	5.08^b		Bari et al., 2008
High Hydrostatic Pressure	475 MPa at 40°C for 8 min	Alfalfa		2		Ariefdjohan et al., 2004
	600 MPa for 15 min at 20°C with presoaking	Alfalfa		>5^c		Neetoo et al., 2009
	650 MPa for 15 min at 20°C	Alfalfa		>5^c		Neetoo et al., 2008
	550 MPa for 2 min at 40°C	Alfalfa		>5^c		Neetoo et al., 2009
	500 MPa for 2 min at 45°C	Alfalfa	5^c	5^c		Neetoo and Chen, 2010
γ irradiation	2.0 kGy	Alfalfa			2–4	Rajkowski and Thayer, 2001
	1.0 kGy	Mung bean	4.85^b			Saroj et al., 2007
	2.0 kGy	Mung bean	5.85^c
	2.0 kGy	Alfalfa	2	3		Thayer et al., 2003
Combination	85°C, 40 s+soaking 2 h in 2000 ppm chlorine	Mung bean	4.81^c	4.69^c		Bari et al., 2010
	Acidic electrolyzed water at 55°C 10 min	Alfalfa		3.4^b		Kim et al., 2006
	Heat at 75°C with phytic or oxalic acid (20+20 s), 3 repeats	Mung bean		4.38		Bari et al., 2009
	Dry heat at 60/65°C for 24/12 h followed by 600 MPa for 2 min at 35°C^d	Alfalfa	5^c	5^c		Neetoo and Chen, 2011
	Sonication and electrolyzed oxidized water (84 ppm active chlorine) at 24°C for 10 min	Alfalfa	3.8^b			Kim et al., 2003
	250 MPa for 10 min of seeds soaked in 18,000 ppm Ca(OCl)₂ and 1500 ppm carvacrol	Mung bean			>5	Peñas et al., 2010
	200 MPa and 18,000 ppm Ca(OCl)₂	Alfalfa			4.5–5	Peñas et al., 2009
	γ irradiation at 1.5 kGy followed by 16,000 ppm chlorine 10 min	Alfalfa	5.66^b			Thayer et al., 2006
	Low-dose γ irradiation (0.75–1.5 kGy) either before or after washing with acidified sodium chlorite (0.5–1.2 g/L)^d	Mung bean		>5.0^b		Nei et al., 2010
	50°C for 1 h followed by γ irradiation at 2.0 kGy	Alfalfa		5.69^c		Bari et al., 2003
	50°C for 1 h followed by sonication then hot acidic electrolyzed water	Mung bean		4.56^b
	50°C for 1 h followed by sonication then hot acidic electrolyzed water	Alfalfa		4.29^b

Treatments causing ≥2-log reductions of both pathogens (colony-forming units per gram) on seeds without significant reduction in germination percentage.

Incomplete reduction (i.e., recovery by microbiologic enrichment of treated seeds and/or detection of pathogen on sprouts subsequently grown from treated seeds).

Complete reduction (i.e., elimination from seed as determined by microbiologic enrichment of treated seed samples and/or absence of pathogen on sprouts subsequently grown from treated seeds).

Sprout yield and/or lengths significantly affected by treatment.

Direct treatments of sprouts have also been evaluated. Blanching alfalfa sprouts eliminated STEC O157:H7 from 72% of contaminated samples (Neetoo and Chen, 2011), and hot water or 5% acetic acid treatments eliminated Salmonella from mung beans and alfalfa sprouts (Pao et al., 2008). Waje et al. (2009) reported that γ-irradiation at 1 kGy eliminates Salmonella but not STEC O157:H7 in mature broccoli and red radish sprouts. A solution of 200 ppm stabilized oxychloro-based sanitizer eliminated both Salmonella and STEC O157:H7 from mung bean sprouts (Hora et al., 2007). Continuously sparging, submersing samples in process water injected with small bubbles of ozone gas, at an ozone concentration of 21 ppm O₃ for 2–64 min followed by high hydrostatic pressure treatment of alfalfa sprouts reduced E. coli O157:H7 by 2-log CFU/g (Sharma et al., 2003).

Discussion

This study shows that outbreaks of bacterial enteric infections associated with contaminated seed sprouts are an ongoing challenge in the United States. Compared with other foodborne-disease outbreaks, sprouts-associated outbreaks tend to be larger and are more likely to involve multiple states. This is especially striking given that, in a 2006–2007 survey, only 8% of the general population reported consuming any kind of sprouts during the previous week (CDC, 2006–2007). Although some of the outbreaks we reviewed might have been prevented if the sprouts grower implemented the FDA guidance to pre-soak sprout seeds with a treatment such as 20,000 mg/L (ppm) calcium hypochlorite solution and to test spent irrigation water, other outbreaks were linked to sprouts produced by growers who said they had implemented the guidelines (Thomas et al., 2003). The FDA recognizes the inability of this process to completely eliminate pathogens (FDA, 1999).

Our review of research on processes and control measures to ensure sprouts safety shows that several new approaches are promising. The sprouting industry itself has been instrumental in collaborating with researchers on many of these investigations. The National Advisory Committee on the Microbiological Criteria for Foods in the United States recommended seed disinfection using methods demonstrated to achieve ≥5-log reductions in the numbers of STEC O157 and Salmonella (National Advisory Committee, 1999). Therefore, numerous studies have aimed for ≥5-log reduction of both pathogens on seeds intended for sprouting—without reducing germination or sprouts quality. For several effective seed disinfection treatments, especially irradiation, cost and consumer concerns may preclude adoption. Most treatments disinfect incompletely, allowing low-level survival of pathogens on seeds, even though organisms are susceptible to chlorine. Combination treatments, such as heat and high pressure or heat and chemical treatment, tended to be most effective in eliminating pathogens on seeds (Peñas et al., 2008, 2010); using multiple control points may be most successful in producing safe sprouts. During sprouting, seed treatment could be followed by a sprouts-disinfection step. Treatment of maturing sprouts, such as application of Salmonella-specific lytic bacteriophage (Kocharunchitt et al., 2009; Ye et al., 2010) or commensal bacteria (Wilderdyke et al., 2004; Weiss et al., 2007), holds promise for limiting growth of any pathogens that survive seed treatment.

Seed contamination is an inherent risk. Salmonella and STEC, the pathogens most often identified in sprouts-associated outbreaks, are typically harbored in animals' intestinal tracts (Swartz, 2002). Between seed production and sprouts consumption, frequent opportunities, many of them likely unapparent, exist for contamination. On the farm, birds, rodents, and other animals, such as grazing livestock or deer, may defecate pathogens onto the plant producing the seeds or may defecate intact seeds along with pathogens in the manure. It has been suggested that Salmonella may cycle between herbivores and plants in this way (Fletcher, 2013). Alfalfa (i.e., lucerne) is typically grown primarily for animal feed. Only a small proportion of a given seed lot may be used to produce sprouts for human consumption (NACMCF, 1999). Consequently, the seed used for sprouting often is not grown, harvested, and cleaned using good agricultural practices to prevent contamination of the seed with human pathogens. While most developed countries have traceability guidelines, they usually suggest only a “one-step back” and “one-step forward” approach. Since the international seed trade includes multiple steps through brokers (Gill et al., 2003; Emberland et al., 2007), sprouts growers could unknowingly obtain seed that was produced for animal feed.

Several international outbreaks have been traced to single seed supplies (Mahon et al., 1997; Taormina et al., 1999). In our review, two outbreaks that occurred in Minnesota and Colorado at different times in 2003 were caused by STEC O157:NM with indistinguishable pulsed-field gel electrophoresis patterns. The implicated alfalfa seed, although sprouted in different facilities, originated from the same seed supplier in Australia (Ferguson et al., 2005), strongly suggesting contamination before sprouting. STEC O157:NM was isolated in half of STEC outbreaks. While STEC O157:NM only accounts for 2% of all E. coli O157 isolates from humans in the United States (Unpublished data, U.S. Department of Health and Human Services, CDC, 2012), it is the dominant type in Australia (78%) (Fegan and Desmarchelier, 2002).

After seeds are harvested, they are often scarified by rubbing between two hard surfaces to create microscopic cracks to facilitate water uptake and more rapid and consistent germination of the seed during sprouting. This process may lead to infiltration of bacteria into areas that allow evasion of chemical treatment (Holliday et al., 2001). Pathogens can be internalized into the seed and remain dormant until sprouting (Ferguson et al., 2005). The sprouting process, which occurs in a warm, moist environment, can amplify the number of bacteria on the sprout (Fu et al., 2008). Five- to 6-log increases of Salmonella (Jacquette et al., 1996) and STEC O157:H7 (Taormina et al., 1999) from inoculated seed have been observed during sprouting. Seeds remain viable for years, so they may be transported long distances and held for a long time. In addition, human pathogens such as Salmonella can remain viable on alfalfa seeds in dark storage for 2 years (Inami et al., 2001). If pathogenic bacteria are already present in or on the seed, one contaminated lot distributed widely can lead to contaminated sprouts produced over time and in disparate locations, adding to the challenge of detecting and investigating associated outbreaks.

Because most seed disinfectant treatments reduce risk of contamination but do not always eliminate pathogens, the FDA has maintained that testing spent irrigation water for pathogens is a key to verifying that pathogens are not present in sprouts and preventing contaminated sprouts from entering commerce (FDA, 1999). Sprouts-growing firms could conduct environmental testing for Listeria monocytogenes as an additional precaution.

In January 2013, the FDA published a proposed regulation on Standards for Produce Safety, which would establish science-based minimum standards for the safe growing, harvesting, packing, and holding of produce on farms. This proposed regulation would establish specific requirements for sprouts production that reflect the fact that sprouts production provides a hospitable environment for growth of many human pathogens. For example, sprouts growers would be required to treat seeds or beans before sprouting using a validated method to reduce pathogen load. In addition, sprouts growers would be required to test either a representative sample of spent irrigation water from each batch of sprouts for pathogens (Salmonella spp. and E. coli O157:H7) or an appropriate sample of the sprouts themselves from each batch. Also, sprouts growers would be required to test the sprouts production environment regularly for either Listeria species or L. monocytogenes (FDA, 2013).

This review of sprouts-associated outbreaks has certain limitations, and several important unanswered questions remain. An assessment of trends in the risk of an outbreak or illness per unit of sprouts consumed would be of great interest but for several reasons is not possible with our data. First, the advent of PulseNet, the nationwide molecular subtyping network for bacterial foodborne pathogens that was launched in 1996, greatly increased the detection of dispersed foodborne outbreaks, such as many of the sprouts-associated outbreaks we report (Olsen et al., 2000). Second, FDOSS was enhanced in 1998, leading to a marked increase in outbreak reports (Gould et al., 2013). Both advances complicate comparisons of our data with previous reports. Third, although information about trends in sprouts consumption in the United States would provide a helpful context for our findings, such information is not available. It is also important to note that the true burden of sprouts-associated illnesses is undoubtedly substantially greater than we report. Foodborne outbreak detection systems rely heavily on detection of laboratory-confirmed infections and may not detect outbreaks caused by pathogens for which testing is not routine. Geographically dispersed cases may not be recognized as part of an outbreak, and even since 1998, many outbreaks that are likely foodborne are not investigated or reported. Sprouts are sometimes referred to as a “stealth” vehicle, meaning persons may not be aware of having eaten them in, for example, a sandwich or a salad. It is not uncommon for less than 40% of ill persons in sprouts-associated outbreaks to recall sprouts consumption (Mahon et al., 1997; Van Beneden et al., 1999). In the German outbreak of E. coli O104:H4 infections in 2011, an analysis based on the ingredients of menu items was required to implicate the sprouts, as few diners remembered eating them (Buchholz et al., 2011). Data on seed sources and implementation of FDA guidelines are not available for some outbreak investigations, in most cases because of inadequate records for definitive traceback, but it seems unlikely that these outbreaks would differ dramatically from the outbreaks for which information was available. In seed disinfection studies, the varying methods of seed and sprouts inoculation, treatment, and pathogen enumeration may make it difficult to compare efficacy directly across studies (Wu et al., 2001).

Although progress has been made in the sprouts industry to reduce contamination, the continued occurrence of outbreaks shows that room still exists for improvement. Sprouts safety is an international concern; while pathogen control measures for sprouts production vary, guidelines or regulatory requirements for disinfection of seed exist in many other industrialized countries (Freshfel Europe, 2014). International cooperation, including among countries that export seed and those that import it, will be essential. To this end, determining the source of seed implicated in sprouts-associated outbreaks will continue to be important. In addition, ongoing educational efforts for the sprouts industry, which is largely composed of small firms, some of which lack personnel with formal food safety training, will be key to improving the safety of sprouts in the food supply. Continued progress is vital both to reduce illnesses and to retain consumer confidence in the sprouts industry itself.

Footnotes

Acknowledgments

We thank Heena Joshi and Leeza Kondos for their contributions in data gathering.

Disclosure Statement

No competing financial interests exist.

References

Ariefdjohan

, Nelson

, Singh

, Bhunia

, Balasubramaniam

, Singh

. Efficacy of high hydrostatic pressure treatment in reducing Escherichia coli O157 and Listeria monocytogenes in alfalfa seeds. J Food Sci, 2004; 69:M117–M120.

Bari

, Enomoto

, Nei

, Kawamoto

. Scale-up seed decontamination process to inactivate Escherichia coli O157:H7 and Salmonella Enteritidis on mung bean seeds. Foodborne Pathog Dis, 2010; 7:51–56.

Bari

, Inatsu

, Isobe

, Kawamoto

. Hot water treatments to inactivate Escherichia coli O157:H7 and Salmonella in mung bean seeds. J Food Prot, 2008; 71:830–834.

Bari

, Nazuka

, Sabina

, Todoriki

, Isshiki

. Chemical and irradiation treatments for killing Escherichia coli O157:H7 on alfalfa, radish, and mung bean seeds. J Food Prot, 2003; 66:767–774.

Bari

, Sugiyama

, Kawamoto

. Repeated quick hot-and-chilling treatments for the inactivation of Escherichia coli O157:H7 in mung bean and radish seeds. Foodborne Pathog Dis, 2009; 6:137–143.

Beuchat

, Scouten

. Combined effects of water activity, temperature and chemical treatments on the survival of Salmonella and Escherichia coli O157:H7 on alfalfa seeds. J Appl Microbiol, 2002; 92:382–395.

Beuchat

. Comparison of chemical treatments to kill Salmonella on alfalfa seeds destined for sprout production. Int J Food Microbiol, 1997; 34:329–333.

Buchholz

, Bernard

, Werber

, Böhmer

, Remschmidt

, Wilking

, Deleré

, an der Heiden

, Adlhoch

, Dreesman

, Ehlers

, Ethelberg

, Faber

, Frank

, Fricke

, Greiner

, Höhle

, Ivarsson

, Jark

, Kirchner

, Koch

, Krause

, Luber

, Rosner

, Stark

, Kühne

. German outbreak of Escherichia coli O104:H4 associated with sprouts. NEJM, 2011; 365:1763–1770.

[CDC] Centers for Disease Control and Prevention. Foodborne Active Surveillance Network (FoodNet) Population Survey Atlas of Exposures. Washington D.C.: U.S. Department of Health and Human Services, 2006–2007.

10.

Emberland

, Ethelberg

, Kuusi

, Vold

, Jensvoll

, Lindstedt

, Nygard

, Kjelso

, Torpdahl

, Sorensen

, Jensen

, Lukinmaa

, Niskanen

, Kapperud

. Outbreak of Salmonella Weltevreden infections in Norway. Denmark and Finland associated with alfalfa sprouts, July–October 2007. Euro Surveill, 2007; 12:E071129.4.

11.

[FDA] Food and Drug Administration. Guidance for Industry: Reducing Microbial Food Safety Hazards for Sprouted Seeds and Guidance for Industry: Sampling and Microbial Testing of Spent Irrigation Water During Sprout Production. Washington D.C.: Federal Registrar, U.S. Government Printing Office. Federal Register 1999; 64:57893–57902.

12.

FDA. Food Safety Modernization Act Proposed Rule for Produce Safety: Standards for the Growing, Harvesting, Packing, and Holding of Produce for Human Consumption. Washington D.C.: Federal Registrar, U.S. Government Printing Office, 2013.

13.

Fegan

, Desmarchelier

. Comparison between human and animal isolates of Shiga toxin–producing Escherichia coli O157 from Australia. Epidemiol Infect, 2002; 128:357–362.

14.

Feng

, Churey

, Worobo

. Thermal inactivation of Salmonella and Escherichia coli O157:H7 on alfalfa seeds. J Food Prot, 2007; 70:1698–1703.

15.

Ferguson

, Scheftel

, Cronquist

, Smith

, Woo-Ming

, Anderson

, Knutsen

, De

, Gershman

. Temporally distinct Escherichia coli O157 outbreaks associated with alfalfa sprouts linked to a common seed source—Colorado and Minnesota, 2003. Epidemiol Infect, 2005; 133:439–447.

16.

Fett

. Factors affecting the efficacy of chlorine against Escherichia coli O157:H7 and Salmonella on alfalfa seed. Food Microbiol, 2002; 19:135–149.

17.

Fett

, Cooke

. Reduction of Escherichia coli O157:H7 and Salmonella on laboratory-inoculated alfalfa seed with commercial citrus-related products. J Food Protect, 2003; 66:1158–1165.

18.

Fett

. Reduction of Escherichia coli O157:H7 and Salmonella spp. on laboratory-inoculated mung bean seed by chlorine treatment. J Food Prot, 2002; 65:848–852.

19.

Fletcher

, Leach

, Eversole

, Tauxe

. Human pathogens on plants: Designing a multidisciplinary strategy for research. Phytopathology, 2013; 103:306–315.

20.

Frank

, Werber

DJP

, Askar

, Faber

, an der Heiden

, Bernard

, Fruth

, Prager

, Spode

, Wadl

, Zoufaly

, Jordan

, Kemper

, Follin

, Müller

, King

, Rosner

, Buchholz

, Stark

, Krause G

, S Investigation

Team

. Epidemic profile of Shiga-toxin–producing Escherichia coli O104:H4 outbreak in Germany. NEJM, 2011; 365:1771–1780.

21.

Freshfel Europe, The European Fresh Produce Association. Comparison of different guidelines and practices for sprout production. 2011. Available at: www.efsa.europa.eu/en/search/doc/203eax1.pdf, accessed January 13, 2014 .

22.

, Stewart

, Reineke

, Ulaszek

, Schlesser

, Tortorello

. Use of spent irrigation water for microbiological analysis of alfalfa sprouts. J Food Prot, 2001; 64:802–806.

23.

, Reineke

, Chirtel

, VanPelt

. Factors influencing the growth of Salmonella during sprouting of naturally contaminated alfalfa seeds. J Food Prot, 2008; 71:888–896.

24.

Gill

, Keene

, Mohle-Boetani

, Farrar

, Waller

, Hahn

, Cieslak

. Alfalfa seed decontamination in Salmonella outbreak. Emerg Infect Dis, 2003; 9:474–479.

25.

Gould

, Walsh

, Vieira

, Herman

, Williams

, Hall

, Cole

. Surveillance for foodborne disease outbreaks—United States, 1998–2009. MMWR Surveill Summ, 2013; 62:1–34.

26.

Himathongkham

, Nuanualsuwan

, Riemann

, Cliver

. Reduction of Escherichia coli O157:H7 and Salmonella Typhimurium in artificially contaminated alfalfa seeds and mung beans by fumigation with ammonia. J Food Protect, 2001; 64:1817–1819.

27.

Holliday

, Scouten

, Beuchat

. Efficacy of chemical treatments in eliminating Salmonella and Escherichia coli O157:H7 on scarified and polished alfalfa seeds. J Food Prot, 2001; 64:1489–1495.

28.

Hora

, Kumar

, Kostrzynska

, Dixon

, Warriner

. Inactivation of Escherichia coli O157:H7 and Salmonella on artificially or naturally contaminated mung beans (Vigna radiata L) using a stabilized oxychloro-based sanitizer. Lett Appl Microbiol, 2007; 44:188–193.

29.

, Churey

, Worobo

. Heat treatments to enhance the safety of mung bean seeds. J Food Protect, 2004; 67:1257–1260.

30.

Inami

, Lee

SMC

, Hogue

, Brenden

. Two processing methods for the isolation of Salmonella from naturally contaminated alfalfa seeds. J Food Protect, 2001; 64:1240–1243.

31.

Itoh

, Sugita-Konishi

, Kasuga

, Iwaki

, Hara-Kudo

, Saito

, Noguchi

, Konuma

, Kumagai

. Enterohemorrhagic Escherichia coli O157:H7 present in radish sprouts. Appl Environ Microbiol, 1998; 64:1532–1535.

32.

Jacquette

, Beuchat

, Mahon

. Efficacy of chlorine and heat treatment in killing Salmonella Stanley inoculated onto alfalfa seeds and growth and survival of the pathogen during sprouting and storage. Appl Environ Microbiol, 1996; 62:2212–2215.

33.

Kim

, Feng

, Kushad

, Fan

. Effects of ultrasound, irradiation, and acidic electrolyzed water on germination of alfalfa and broccoli seeds and Escherichia coli O157:H7. J Food Sci, 2006; 71:M168–M173.

34.

Kim

, Hung

, Brackett

, Lin

. Efficacy of electrolyzed oxidizing water in inactivating Salmonella on alfalfa seeds and sprouts. J Food Prot, 2003; 66:208–214.

35.

Kim

, Hung

, Brackett

, Lin

. Efficacy of electrolyzed oxidizing water in inactivating Salmonella on alfalfa seeds and sprouts. J Food Prot, 2003; 66:208–214.

36.

Kocharunchitt

, Ross

, McNeil

. Use of bacteriophages as biocontrol agents to control Salmonella associated with seed sprouts. Int J Food Microbiol, 2009; 128:453–459.

37.

Liao

. Acidified sodium chlorite as an alternative to chlorine for elimination of Salmonella on alfalfa seeds. J Food Sci, 2009; 74:M159–M164.

38.

Mahon

, Ponka

, Hall

, Komatsu

, Dietrich

, Siitonen

, Cage

, Hayes

, Lambert-Fair

, Bean

, Griffin

, Slutsker

. An international outbreak of Salmonella infections caused by alfalfa sprouts grown from contaminated seeds. J Infect Dis, 1997; 175:876–882.

39.

Michino

, Araki

, Minami

, Takaya

, Sakai

, Miyazaki

, Ono

, Yanagawa

. Massive outbreak of Escherichia coli O157:H7 infection in schoolchildren in Sakai City, Japan, associated with consumption of white radish sprouts. Am J Epidemiol, 1999; 150:787–796.

40.

Montville

, Schaffner

. Analysis of published sprout seed sanitization studies shows treatments are highly variable. J Food Prot, 2004; 67:758–765.

41.

Nandiwada

, Schamberger

, Schafer

, Diez-Gonzalez

. Characterization of an E2-type colicin and its application to treat alfalfa seeds to reduced Escherichia coli O157:H7. Int J Food Microbiol, 2004; 93:267–279.

42.

[NACMCF] National Advisory Committee on Microbiological Criteria for Foods. Microbiological safety evaluations and recommendations on sprouted seed. Int J Food Microbiol, 1999; 52:123–153.

43.

Neetoo

, Chen

. Inactivation of Salmonella and Escherichia coli O157:H7 on artificially contaminated alfalfa seeds using high hydrostatic pressure. Food Microbiol, 2010; 27:332–338.

44.

Neetoo

, Chen

. Individual and combined application of dry heat with high hydrostatic pressure to inactivate Salmonella and Escherichia coli O157:H7 on alfalfa seeds. Food Microbiol, 2011; 28:119–127.

45.

Neetoo

, Pizzolato

, Chen

. Elimination of Escherichia coli O157:H7 from alfalfa seeds through a combination of high hydrostatic pressure and mild heat. Appl Environ Microbiol, 2009; 75:1901–1907.

46.

Neetoo

, Ye

, Chen

. Potential application of high hydrostatic pressure to eliminate Escherichia coli O157:H7 on alfalfa sprouted seeds. Int J Food Microbiol, 2008; 128:348–353.

47.

Neetoo

, Ye

, Chen

. Factors affecting the efficacy of pressure inactivation of Escherichia coli O157:H7 on alfalfa seeds and seed viability. Int J Food Microbiol, 2009; 131:218–223.

48.

Nei

, Bari

, Inatsu

, Kawasaki

, Todoriki

, Kawamoto

. Combined effect of low-dose irradiation and acidified sodium chlorite washing on Escherichia coli O157:H7 inoculated on mung bean seeds. Foodborne Pathog Dis, 2010; 7:1217–1223.

49.

Olsen

, MacKinnon

, Goulding

, Bean

, Slutsker

. Surveillance for foodborne-disease outbreaks—United States, 1993–1997. MMWR CDC Surveill Summ, 2000; 49:1–62.

50.

Pao

, Kalantari

, Khalid

. Eliminating Salmonella enterica in alfalfa and mung bean sprouts by organic acid and hot water immersions. J Food Process Preserv, 2008; 32:335–342.

51.

Peñas

, Gómez

, Frías

, Vidal-Valverde

. Application of high-pressure treatment on alfalfa (Medicago sativa) and mung bean (Vigna radiata) seeds to enhance the microbiological safety of their sprouts. Food Control, 2008; 19:698–705.

52.

Peñas

, Gómez

, Frías

, Vidal-Valverde

. Efficacy of combinations of high pressure treatment, temperature and antimicrobial compounds to improve the microbiological quality of alfalfa seeds for sprout production. Food Control, 2009; 20:31–39.

53.

Peñas

, Gómez

, Frías

, Vidal-Valverde

. Effects of combined treatments of high pressure, temperature and antimicrobial products on germination of mung bean seeds and microbial quality of sprouts. Food Control, 2010; 21:82–88.

54.

Pierre

, Ryser

. Inactivation of Escherichia coli O157:H7, Salmonella Typhimurium DT104, and Listeria monocytogenes on inoculated alfalfa seeds with a fatty acid-based sanitizer. J Food Prot, 2006; 69:582–590.

55.

Rajkowski

, Thayer

. Alfalfa seed germination and yield ratio and alfalfa sprout microbial keeping quality following irradiation of seeds and sprouts. J Food Prot, 2001; 64:1988–1995.

56.

Saroj

, Hajare

, Shashidhar

, Dhokane

, Sharma

, Bandekar

. Radiation processing for elimination of Salmonella Typhimurium from inoculated seeds used for sprout making in India and effect of irradiation on germination of seeds. J Food Prot, 2007; 70:1961–1965.

57.

Sharma

, Demirci

, Beuchat

, Fett

. Application of ozone for inactivation of Escherichia coli O157:H7 on inoculated alfalfa sprouts. J Food Process Preserv, 2003; 27:51–64.

58.

Singh

, Singh

, Bhunia

. Sequential disinfection of Escherichia coli O157:H7 inoculated alfalfa seeds before and during sprouting using aqueous chlorine dioxide, ozonated water, and thyme essential oil. Food Sci Technol, 2003; 36:235–243.

59.

Swartz

. Human diseases caused by foodborne pathogens of animal origin. Clin Infect Dis, 2002; 34(Suppl 3):S111–S122.

60.

Taormina

, Beuchat

, Slutsker

. Infections associated with eating seed sprouts: An international concern. Emerg Infect Dis, 1999; 5:626–634.

61.

Thayer

, Boyd

, Fett

. Synergy between irradiation and chlorination in killing of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes . J Food Sci, 2006; 71:R83–R87.

62.

Thayer

, Rajkowski

, Boyd

, Cooke

, Soroka

. Inactivation of Escherichia coli O157:H7 and Salmonella by gamma irradiation of alfalfa seed intended for production of food sprouts. J Food Prot, 2003; 66:175–181.

63.

Thomas

, Palumbo

, Farrar

, Farver

, Cliver

. Industry practices and compliance with U.S. Food and Drug Administration guidelines among California sprout firms. J Food Prot, 2003; 66:1253–1259.

64.

Van Beneden

, Keene

, Strang

, Werker

, King

, Mahon

, Hedberg

, Bell

, Kelly

, Balan

, MacKenzie

, Fleming

. Multinational outbreak of Salmonella enterica serotype Newport infections due to contaminated alfalfa sprouts. JAMA, 1999; 281:158–162.

65.

Waje

, Jun

, Lee

, Kim

, Han

, Jo

, Kwon

. Microbial quality assessment and pathogen inactivation by electron beam and gamma irradiation of commercial seed sprouts. Food Control, 2009; 20:200–204.

66.

Watanabe

, Ozasa

, Mermin

, Griffin

, Masuda

, Imashuku

, Sawada

. Factory outbreak of Escherichia coli O157:H7 infection in Japan. Emerg Infect Dis, 1999; 5:424–428.

67.

Weiss

, Hammes

. Efficacy of heat treatment in the reduction of salmonellae and Escherichia coli O157:H7 on alfalfa, mung bean and radish seeds used for sprout production. Eur Food Res Technol, 2005; 221:187–191.

68.

Weiss

, Hertel

, Grothe

, Ha

, Hammes

. Characterization of the cultivable microbiota of sprouts and their potential for application as protective cultures. Syst Appl Microbiol, 2007; 30:483–493.

69.

Weissinger

, Beuchat

. Comparison of aqueous chemical treatments to eliminate Salmonella on alfalfa seeds. J Food Prot, 2000; 63:1475–1482.

70.

Wilderdyke

, Smith

, Brashears

. Isolation, identification, and selection of lactic acid bacteria from alfalfa sprouts for competitive inhibition of foodborne pathogens. J Food Prot, 2004; 67:947–951.

71.

, Beuchat

, Wells

, Slutsker

, Doyle

, Swaminathan

. Factors influencing the detection and enumeration of Escherichia coli O157:H7 on alfalfa seeds. Int J Food Microbiol, 2001; 71:93–99.

72.

, Kostrzynska

, Dunfield

, Warriner

. Control of Salmonella on sprouting mung bean and alfalfa seeds by using a biocontrol preparation based on antagonistic bacteria and lytic bacteriophages. J Food Protect, 2010; 73:9–1.