Outbreak of Salmonella Kottbus in Alfalfa Sprouts
Alfalfa Sprouts Arizona, California, Colorado, and NewMexico, February-April 2001
February 6, 2002
Journal of the American Medical Association
On March 12, 2001, the CaliforniaDepartment of Health Services (CDHS) identified a cluster of SalmonellaKottbus isolates with indistinguishable pulsed-field gel electrophoresis (PFGE)patterns. During February 1–May 1, CDHS identified 23 patients with S.Kottbus infections in several California counties and an additional patient fromArizona. This report summarizes the results of the investigation of thisoutbreak, which identified cases in four states and implicated alfalfa sproutsproduced at a single facility.
The median age ofcase-patients was 36 years (range: 9–72 years); 16 patients (67%) were female.Twenty-one patients developed an acute diarrheal illness, and three patients hadurinary tract infections. Three patients were hospitalized.
Using a standardizedquestionnaire, a matched case-control study was conducted. A case was defined asculture-confirmed S. Kottbus infection with onset after January 2001 in aCalifornia resident with an isolate having the outbreak PFGE pattern. The first10 reported California patients were matched with two controls by age group,sex, and city prefix code. Fifteen (63%) of 23 patients ate alfalfa sproutsduring the week before becoming ill. A significant association was found betweeneating alfalfa sprouts and illness (matched odds ratio: 5.5; 95% confidenceinterval=1.2–26.1). No other food or restaurant exposure was significantlyassociated with illness. Following the case-control study, 32 patients infectedwith the outbreak strain of S. Kottbus were identified in California(24), Arizona (six), Colorado (one), and New Mexico (one).
A tracebackinvestigation identified a single sprout producer as the source of thecontaminated sprouts. Review of the sprouter’s production records indicated thata single seed lot was temporally associated with the dates of illness onset. Aculture of a sample of this seed lot yielded S. Kottbus. These seeds wereimported from Australia in November 2000, but no further information about thedistribution of this seed lot was available. Cultures from two floor drains inthe production facility also yielded S. Kottbus. Patient, seed, andenvironmental isolates all had indistinguishable PFGE patterns.
Although the implicatedseed lot was last used on March 29, the sprouter issued a voluntary recall ofall sprout products on April 17, and ceased all sprout production pendingfurther internal review of their production processes. Review of decontaminationand distribution records indicated that at least some seeds underwent heattreatment followed by a 2,000-ppm sodium hypochlorite treatment for 15 minutes.The U.S. Food and Drug Administration (FDA) recommends decontamination of seedswith one or more treatments (e.g., soaking in a 20,000-ppm calcium hypochloritefor 15 minutes) that have been approved for reduction of pathogens in seeds (1,2).The effectiveness of alternative seed decontamination has not been established.The sprout producers subsequently agreed to use only the FDA-recommended20,000-ppm soak when sprout production resumed.
Reported by: JMohle-Boetani, MD, B Werner, MD, M Polumbo, PhD, J Farrar, DVM, D Vugia, MD,Acting State Epidemiologist, California Dept of Health Svcs. S Anderson, KKomatsu, MPH, K Tagg, N Peterson, MD, State Epidemiologist, Arizona Dept ofHealth Svcs. J Painter, DVM, S Van Dunn, MA, Div of Bacterial and MycoticDiseases, National Center for Infectious Diseases; K Winthrop, MD, M Beatty, MD,EIS officers, CDC.
S.Kottbus is a rarely reported cause of salmonellosis in the United States. During1968–1998, a median of 42 S. Kottbus isolates were reported each year toCDC through the Public Health Laboratory Information System (3). This wasthe second outbreak of S. Kottbus since 1985 and the first outbreakassociated with sprouts.
Since 1995, 15 outbreaksof Salmonella spp. and two outbreaks of Escherichia coli O157:H7infections associated with sprouts have been reported to CDC. Despite publichealth advisories about the risks for eating raw sprouts, persons at high riskfor systemic infection continue to eat sprouts (4).Two of the patients in this outbreak were immunocompromised, and one was a youngchild. In each case, persons perceived raw sprouts as a “healthy” fooditem.
Sprouts may becontaminated during seed production, germination, sprout processing, or consumerhandling and preparation (5,6). On the farm, sprouts seeds may becomecontaminated through the use of untreated agricultural water, improperlycomposted manure as fertilizer, excretion from domestic or wild animals, runofffrom domesticated animal production facilities, or improperly cleaned harvestingor processing machines (5,6). The association of specific seed lots withillness suggests that seeds are the most likely source for this and most othersprout-related outbreaks (4).Conditions suitable for seed sprouting also are ideal for increasing pathogenicbacterial counts by several logs.
The use of a 20,000-ppmcalcium hypochlorite soak before sprouting might reduce the risk forsprout-related illness (4).However, use of this high-dose soak is not completely effective, and outbreakscontinue to occur (7). Cracks and crevasses in the sprout seed may trappathogenic bacteria, making them inaccessible to lethal concentrations ofdisinfectants (5). Because >20,000-ppm calcium hypochlorite soaks canimpair seed germination (5), alternative methods are needed to reduce therisk for human disease following sprout consumption. In this outbreak, some ofthe implicated sprouts were from seeds that had undergone a combination of heattreatment and a 15-minute, low-dose calcium hypochlorite soak (2,000 ppm). Thesubsequent outbreak suggests that this hybrid technique using a heat treatmentcombined with a low-dose hypochlorite solution might not reduce adequatelypathogenic bacterial colony counts in alfalfa seeds. Reducing pathogenicbacterial counts on seed during production and harvest could improve theeffectiveness of postharvest decontamination.
Public education effortsabout the risks for eating uncooked sprouts need to be continued, particularlyamong vulnerable populations (i.e., the elderly, young children, andimmunocompromised persons). CDC and FDA recommend that persons at high risk forsystemic infections not eat raw sprouts. For persons who continue to eatsprouts, FDA recommends cooking before eating to reduce the risk for illness (8).
In response to thisoutbreak, CDHS and the California Department of Education recommend that schoolsstop serving uncooked sprouts to young children. Public health officials shouldpromote awareness of the role of raw sprout consumption in foodborne disease andconsider package labeling as a method for improving consumer awareness. Inaddition, designation of sprout seed production for human consumption at seedplanting could further reduce the risk for sprout-associated outbreaks (5).If sprout seed producers knew which sprout seed crops were dedicated for humanconsumption before harvest, producers could focus on reducing potentialcontamination in the field. Avoiding seed contamination in the field mightreduce the risk for consumer exposure to foodborne pathogens.
- U.S. 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. Federal Register 1999;64:57893–902.
- U.S. Food and Drug Administration. Irradiation in the production, processing and handling of food. Federal Register 2000;65:64605–7.
- CDC. An atlas of Salmonella in the United States: serotype-specific surveillance, 1968–98. Atlanta, Georgia: U. S. Department of Health and Human Services, CDC, 2001.
- CDC. Outbreaks of Escherichia coli O157:H7 infection associated with eating alfalfa sprouts—Michigan and Virginia. MMWR 1997;46:741–4.
- National Advisory Committee on Microbiological Criteria for Food. Microbiological safety evaluations and recommendations on sprouted seeds. Int J Food Microbiol 1999;52:123–53.
- Patterson JE, Woodburn MJ. Klebsiella and other bacteria on alfalfa and bean sprouts at the retail level. J Food Sci 1995;45:492–5.
- Brooks JT, Samantha YR, Shillam P, et al. Salmonella Typhimurium infections transmitted by chlorine-pretreated clover sprout seeds. Am J Epidemiol 2001;154:1020–8.
- U.S. Food and Drug Administration. 1999 consumer advisory. Accessed January 2002.