1 Epidemic Intelligence Service, Division of Applied Public Health Training, Epidemiology Program Office, Centers for Disease Control and Prevention, Atlanta, GA.
2 Foodborne and Diarrheal Diseases Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA.
3 Disease Control and Environmental Epidemiology Division, Colorado Department of Public Health and Environment, Denver, CO.
4 Laboratory and Radiation Sciences Division, Colorado Department of Public Health and Environment, Denver, CO.
5 Biostatistics and Information Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA.
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ABSTRACT |
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chlorine; disease outbreaks; gastroenteritis; Salmonella typhimurium; seeds; vegetables
Abbreviations: CDC, Centers for Disease Control and Prevention; CDPHE, Colorado Department of Public Health and Environment; CI, confidence interval; FDA, Food and Drug Administration; OR, odds ratio; PFGE, pulsed-field gel electrophoresis.
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INTRODUCTION |
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Transmission is usually ascribed to the presence on seeds of pathogenic bacteria, which multiply exponentially to infectious doses during the microbiologically favorable warm, moist, and nutrient-rich conditions of sprouting (4). In response to numerous sprout-associated outbreaks, in 1998, the Food and Drug Administration (FDA) issued an interim advisory on alfalfa sprouts, noting that concentrated chlorine solutions appeared to be potentially useful for decontaminating seeds before sprouting (5
). No treatment has yet been shown to reliably prevent recovery of bacteria from sprouted seed (1
). Soaking seeds in a 20,000 mg/liter (ppm) calcium hypochlorite solution for 15 minutes is the most effective chemical seed treatment currently approved (6
9
).
In April 1999, public health officials in Colorado noted an increase in Salmonella serotype Typhimurium infections and subsequently notified the Centers for Disease Control and Prevention (CDC). For the first 16 weeks of 1999, the state reported 87 infections compared with 26, 51, and 45 infections during the same weeks in 1998, 1997, and 1996, respectively. Subtyping by pulsed-field gel electrophoresis (PFGE) demonstrated that 60 percent of these isolates shared a unique and indistinguishable PFGE pattern ("outbreak" PFGE pattern), suggesting that these infections were epidemiologically related.
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MATERIALS AND METHODS |
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To determine the source of the outbreak, we conducted a case-control study. We defined a case as a diarrheal illness (three or more bowel movements during any 24-hour period) in a Colorado resident from whom S. Typhimurium with the outbreak PFGE pattern was isolated after April 15, 1999. We matched controls with cases by age group, sex, and telephone prefix and attempted to identify three controls for each case by systematic progressive telephone digit dialing. No cases under age 6 years were interviewed. We excluded potential controls who reported having diarrhea within the 30 days preceding the onset of illness in the case to whom they were matched or who had not resided in Colorado for the 5 days preceding the date of onset of illness in that case.
We developed a written questionnaire based on face-to-face, hypothesis-generating interviews with 13 Coloradans from whom the outbreak strain of S. Typhimurium was isolated in April and May 1999. We asked cases about dairy, meat, and produce consumption and other exposures associated with salmonellosis during the 5 days before onset of diarrhea. We queried controls about the same exposures during the 5-day period preceding onset of illness in the case to whom they were matched.
Traceback and traceforward
To identify potential sources for the outbreak, we reinterviewed ill persons who had eaten statistically implicated foods and the retail suppliers of these foods to determine whether any foods shared a common source (e.g., farm, warehouse, production facility). On the basis of these interviews, we and officials from FDA investigated two sprouters on May 20 and May 25, 1999. We examined records of seed receipt and sprout sales and interviewed the owners of supplying seed companies to confirm shipping records and to establish details concerning sales of one implicated lot of clover seed.
Environmental investigation
Along with officials from the FDA, we observed growing procedures at both sprouters and interviewed employees and owners about disinfection and production methods. We collected sprout samples during production and at retail outlets, seed samples before and after presoaking in chlorine, and environmental swabs. We collected all samples in duplicate and transported them double-bagged in insulated containers on ice. All samples were cultured within 24 hours of collection.
Laboratory investigation
The Colorado Department of Public Health and Environment (CDPHE) serotyped human Salmonella isolates and subtyped human and environmental S. Typhimurium isolates by PFGE using standard PulseNet methods (10). The sprout, seed, and environmental samples were cultured at CDPHE and CDC using previously described methods (11
).
Statistical analysis
We calculated maximum likelihood estimates of matched odds ratios and their exact 95 percent confidence intervals by using Epi Info version 6.04c (12). We performed exact conditional logistic regression analyses using LogXact version 2.1.1 (13
). We calculated p values of odds ratios by the chi-square method and applied Fisher's exact test when expected values in any cell were less than five. Matched odds ratios with 95 percent confidence intervals that excluded 1.0 and p values
0.05 were considered significant.
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RESULTS |
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Cases and controls did not differ significantly according to age, sex, immunosuppressed status, treatment to reduce gastric acidity, or other preexisting conditions and behaviors that could modify risk for salmonellosis (table 1). In univariate analysis, three exposures were statistically associated with risk of illness (table 2). Eighty percent of the cases and 36 percent of the controls ate beefsteak or hothouse-style tomatoes (matched odds ratio (OR) = 5.80, 95 percent confidence interval (CI): 1.9, 21.2). Fifty-two percent of the cases and no controls ate alfalfa-style sprouts (undefined matched OR, 95 percent CI: 6.7, ). Sixty-five percent of the cases and 36 percent of the controls ate sliced turkey (matched OR = 2.81, 95 percent CI: 1.0, 8.0). In a multivariate analysis using a model containing tomatoes, sprouts, and sliced turkey, only beefsteak/hothouse-style tomatoes and alfalfa-style sprouts remained independently and significantly associated with illness, producing model-based odds ratios of 11.1 (95 percent CI: 1.4, 457) and 26.8 (95 percent CI: 4.2,
), respectively.
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Among the seven of 10 (70 percent) persons available for reinterview who ate alfalfa-style sprouts, five purchased sprouts from a single supermarket chain, supermarket chain A. Most of supermarket chain A's retail outlets (80 percent, 33 of 41) were located on the Western Slope. Supermarket chain A received the majority of its sprouts from a single supplier, sprouter X, and bought over 95 percent of sprouter X's alfalfa-style product.
Sprouter X produced alfalfa-style sprouts from alfalfa, radish, and clover seeds. In late 1998, supermarket chain A, after learning that alfalfa sprouts had been associated with outbreaks of gastroenteritis, requested that sprouter X cease using alfalfa seed in any products for their stores. Subsequently, sprouter X switched to sprouting mainly clover seeds for supermarket chain A, both alone and in mixtures with radish sprouts. Supermarket chain A carried other brands of sprouts but used sprouter X's clover sprouts exclusively at the deli counters and salad bars in their stores; three of five cases who recalled eating alfalfa-style sprouts from supermarket chain A had obtained them from the salad bar or deli.
Distribution through supermarket chain A could not have accounted for the cases along the Front Range, where roughly one third of the infections were reported. Interviews with four persons infected with the outbreak strain (not included in the case-control study) from three Front Range cities revealed that they had all eaten sprouts at unrelated retail outlets supplied by another sprouter, sprouter Y. Sprouter Y distributed sprouts through a local wholesaler to individual markets and food establishments along the Front Range.
Seed company P was the principal supplier of clover seed to sprouter X and, since December 1998, had been supplying clover seed from one lot, designated lot CC8. Seeds from lot CC8 had been sent to sprouter X in three 1,000-pound (2,202.6-kg) shipments between December 1998 and March 1999. After sprouter X exhausted his supply of lot CC8, he began purchasing clover seed from an unrelated lot supplied by another seed company and first shipped in early May 1999. At the time of our inspection, sprouter X was no longer using lot CC8 seed.
Sprouter Y also used mostly clover seed, also supplied by seed company P, and also from lot CC8. Seed company P was the sole supplier of clover seed to sprouter Y and had sent three separate 2,500-pound (5,506.6-kg) shipments between January and April 1999. Shipping invoices indicated that lot CC8 was the only seed shared by the two sprouters. The seed for lot CC8 originated in Oregon, was bagged for shipment in September or October 1998, and was most likely sold entirely to seed company P. The owner of seed company P reported that among the 13 companies producing sprouts in Colorado during late 1998 and early 1999, he had supplied lot CC8 clover seed only to sprouters X and Y. The date lot CC8 clover seed was first sent to each sprouter corresponded to the subsequent onset of illnesses in the regions that each sprouter supplied (figure 2).
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On the basis of the geographic distribution of culture-confirmed infections, we attributed 68 illnesses to sprouter X and 44 illnesses to sprouter Y. Proportionally, four times more illnesses occurred per 50-pound (110.1-kg) bag of seed shipped to sprouter X: 1.13 culture-confirmed illnesses/50-pound bag for sprouter X versus 0.29 culture-confirmed illnesses/50-pound bag for sprouter Y (figure 3).
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Environmental investigation
Sprouters X and Y used multiple seed types. At both sprouters, seeds were stored in clean, dry, rodent-free areas. For sprouting, seeds were misted and mixed at 2022°C in 6-foot (184.2-cm) diameter drums that rotated continuously. For mixed sprouts, such as clover-radish mix, seeds were combined prior to germination.
Sprouter X neither rinsed nor soaked seeds in water or a concentrated chlorine solution before sprouting. In contrast, sprouter Y reported soaking all seeds (with intermittent agitation) for 20 minutes in a 20,000 mg/liter calcium hypochlorite solution and then rinsing the seeds six times in tap water immediately before sprouting. Both sprouters used adequately chlorinated municipal water.
Sprouter X rinsed sprouting drums with tap water under normal pressure. Sprouter Y scrubbed each drum with soap, chlorine, and disinfectant and rinsed them with tap water under high pressure. Hygienic practices were otherwise comparable. Neither sprouter had evidence of rodent or insect infestation. No employees reported having a diarrheal illness between January and May 1999.
Laboratory investigation
During 1999, the CDPHE laboratory serotyped 220 Salmonella isolates as S. Typhimurium. Of these, 112 isolates produced the outbreak strain PFGE pattern. None of the sprout, seed, or environmental samples cultured by the CDPHE and CDC yielded S. Typhimurium. However, one environmental swab collected from sprouter Y by the FDA during an inspection coincident with this investigation yielded S. Typhimurium. The swab was collected from a cart used to transfer grown sprouts from the rotating bins to a rinsing device and was taken during the processing of clover sprouts grown from lot CC8 seed. The PFGE pattern of this isolate was indistinguishable from the unique outbreak strain pattern.
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DISCUSSION |
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Many consumers may not recall eating sprouts because, unlike tomatoes and sliced turkey, they are rarely the principal part of a meal, and within composite foods (e.g., deli sandwiches, mixed salads) they may go unnoticed. Previous investigations of sprout-associated outbreaks have found that the number of persons who usually recall eating sprouts is 40 percent or less (1621
). Persons who ate turkey and tomatoes in deli sandwiches (an exposure more common among cases than among controls) may have had unrecognized exposures to sprouts. If exposure to sprouts was misclassified as absent in these settings, multivariate analysis would be expected to inappropriately implicate confounding exposures (e.g., tomatoes).
Traceback demonstrated concordance in time and space between onsets of illness and the delivery of one lot of Oregon clover seed to two Colorado sprouters, which supplied the sprouts eaten by the cases. On March 31, 1999, the CDPHE recalled the implicated sprouts, and seed company P voluntarily recalled and halted further shipment of the source clover seed. Subsequently, the outbreak subsided. Isolation of the outbreak strain of S. Typhimurium from an implicated sprouter supported the conclusions of the epidemiologic investigation.
Although reliable data on sprout seed sales for human consumption are lacking, sprouters have anecdotally reported (as in this outbreak) increasingly using clover seed for the production of alfalfa-style sprouts. They perceive clover seed to be safer because the smoother seed coat may theoretically be less hospitable to bacterial con-tamination and more amenable to chemical disinfection. Earlier sprout-related outbreaks have implicated clover sprouts when these sprouts were eaten in mixtures with other sprouts (22). This investigation demonstrated that clover sprouts alone can serve as a vehicle for transmitting enteric infection. Additional outbreaks later in 1999 confirmed the risk of illness associated with eating raw clover sprouts alone (23
).
If sprouter Y consistently disinfected seeds as observed during our investigation, then this outbreak also documented that treating seeds with a 20,000 mg/liter calcium hypochlorite solution substantially reduces, but does not eliminate, the risk of illness from sprouts. Recent FDA laboratory studies with naturally contaminated seeds (24) confirm that this treatment reduces, but does not eliminate, pathogenic bacteria. These experiments were conducted using seeds from a sprout-associated Salmonella outbreak that occurred later in 1999 (25
), in which the sprouter had routinely pretreated alfalfa seed with a 20,000 mg/liter calcium hypochlorite solution as recommended (4
). Assuming that contamination of the seed implicated in the Colorado outbreak was uniform, that the seed was not subjected to environmental contamination or rodent infestation, that the sprouters grew and sold product in proportions equal to the amount of the seed they each received, and that all other biases in packaging, shipment, and storage of the seeds were equally distributed, the different attack rates associated with the two sprouters reflect differences in production. Although the production practices of sprouter X were comparatively less hygienic, there is no evidence that the level of hygiene was inadequate or permitted cross-contamination. The most notable difference was that sprouter Y soaked seeds in a concentrated calcium hypochlorite solution before sprouting them; sprouter X did not.
No S. Typhimurium was isolated from sprouter X's facility or products. Sprouter X ceased using clover seed from lot CC8 a few weeks before the unannounced inspection, reducing the likelihood that bacteria from these seeds would be present at that facility when samples were collected. Sprouter Y was using lot CC8 when environmental samples were collected.
In most other sprout-related outbreaks, infections have been ascribed to seeds contaminated with pathogenic bacteria. Salmonella experimentally inoculated on alfalfa seed increases 34 logs during sprouting (26). During growth, bacteria may become enmeshed in the tissues of the growing sprouts (27
), where they cannot be removed by methods routinely used by consumers to clean raw vegetables (e.g., washing, soaking, scrubbing). E. coli O157 has been cultured from the inner tissues of sprouts grown from seed after bacteria coating the outside of the sprouts were removed by disinfecting rinses (27
). Heating sprouts has been recommended to make them safe to eat but may render the product unappealing. Irradiation, which was recently approved for treating sprout seeds (28
), may also provide a viable option for disinfecting grown sprouts (29
).
No seed samples yielded the outbreak strain of S. Typhimurium. In most previously reported, sprout-associated outbreak investigations, the outbreak bacterial strain was not isolated from implicated seeds despite abundant epidemiologic evidence that seeds were the source. Recovery of bacteria may be complicated by uneven and low levels of seed contamination and the limited number of samples tested. Salmonella has been recovered from sprouts grown under aseptic conditions using seeds from which bacteria could not be isolated before sprouting (30). In two sprout-associated Salmonella outbreaks, the outbreak strain of bacteria was successfully recovered from implicated seeds (20
, 21
).
How seeds become contaminated is not fully understood. Sprouts for human consumption, including clover and alfalfa sprouts, come from seed grown for animal feed and are considered a raw agricultural commodity. Improperly composted manure containing pathogenic bacteria might be used to fertilize fields. Domesticated and wild animals may graze in and defecate on seed fields. Small amounts of bacteria left behind in devices used for harvesting and processing may cross-contaminate seeds. Rodents and birds carrying bacteria can infest storage facilities. In this outbreak, the FDA determined that one seed packer in Oregon produced the implicated lot by using mixed clover seed from more than 20 growers. Records of which growers' seed contributed to which lots were not maintained; further traceback to one field or farm was not feasible.
Circumstances in Colorado uniquely converged to facilitate detection of this outbreak. The majority of the implicated seed was shipped to only two sprouters in the state, who, in turn, distributed product almost exclusively within the state's boundaries, including to areas of lower population density. In other states to which the implicated seed was shipped, most sprouters received lesser quantities. In states to which comparable amounts of the seed were sent, either sprouting of the seed could not be confirmed, sprouts were distributed over multiple states within areas of greater population density, or the sprouter used methods to detect the presence of potentially pathogenic bacteria during sprout growth before the product was sold to consumers. Other possible explanations for the lack of illnesses detected outside Colorado include nonuniform contamination of the implicated seed, variable sensitivity of the surveillance systems in other jurisdictions, and more rigorous adherence by other sprouters to recommended seed disinfection protocols and hygiene.
PFGE subtyping facilitated early recognition of this outbreak and identified persons with illnesses most likely to be epidemiologically related. S. Typhimurium is the most common Salmonella serotype reported in the United States, accounting for 25 percent of Salmonella isolates reported to the CDC and the CDPHE in 1999 (31). Limiting our case definition to ill persons whose S. Typhimurium isolates produced the outbreak PFGE pattern increased the power of the case-control method.
One month after the investigation, the FDA released a consumer advisory expanding the earlier advisory that applied only to alfalfa sprouts. Citing the findings from this outbreak (32), the revised advisory noted that 1) all raw sprouts pose a potential risk of infection to all persons, 2) despite growers properly implementing approved methods to disinfect sprout seeds, illnesses continue to be associated with eating raw sprouts, and 3) consumers should cook sprouts to make them safe to eat. Three months later, the FDA released its first guidance for industry to improve the safety of raw sprouts (9
). In addition to reiterating good manufacturing practices and specifying the means for decontaminating sprout seed with chlorine solutions, this document recommended that sprouters test spent sprout irrigation water to identify bacterial contamination before releasing their product into commerce.
The number of outbreaks caused by raw sprouts may appear to be small (4), but sprouts are also consumed infrequently compared with other fruits and vegetables (2
, 3
). Considering additionally that sprouts are produced under conditions that favor rapid bacterial growth, the risk per serving of untreated raw sprouts to consumers may be substantially greater than that of other foods. Persons wishing to avoid any potential exposures to Salmonella or E. coli O157:H7 infections or those who are at risk of serious complications resulting from these bacterial infections (e.g., young children, elderly adults, and persons with weakened immune systems) should not eat raw sprouts. If consumers choose to eat them, the FDA recommends that sprouts be cooked (32
). Better methods to disinfect seeds and sprouts (including irradiation and other chemicals) and to detect contaminated product during growth deserve investigation. Designating seed fields to produce sprouts for human consumption only, fertilizing seed fields only with properly composted manure or inorganic fertilizers, and harvesting and processing seed for human consumption separately from seed for animal feed are interventions that merit consideration.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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