Outbreak of Salmonella Enteritidis Phage Type 11B in Mung Bean Sprouts in the Provinces of Alberta and Saskatchewan

Outbreak of Salmonella Enteritidis Phage Type 11B in the Provinces of Alberta and Saskatchewan, June 2000

July 15, 2003, CCDR Volume 29-14

Harb J, Isaacs S, Fyfe M, Crowe L, Slater B, Ahmed R, Rodgers F, Anderson C, Hockin J.
Field Epidemiology Training Program (Heath Canada), Vancouver, British Columbia.
 

Background

On 12 June, 2000, the Division of Enteric, Foodborne and Waterborne Diseases of Health Canada was notified of an outbreak of Salmonella Enteritidis outbreak in Alberta and Saskatchewan. By 12 June, a total of 12 cases, eight in Alberta and four in Saskatchewan, were reported by the respective health authorities. Eleven of the 12 cases had consumed meals at Vietnamese restaurants before they became ill. This report presents the findings of the outbreak investigation.

Method

For the purpose of this investigation, a case was defined as a person with laboratory confirmed S. Enteritidis phage type 11b or a dining companion of a confirmed case who experienced vomiting or diarrhea (three loose stools in a 24-hour period) beginning between 29 April and 15 June, 2000.

Environmental health officers, public health nurses and epidemiologists conducted telephone interviews with the cases and their non-ill dining companions. Standardized forms for foodborne illness investigations used by the local health authorities were modified to include menu items served at the implicated Vietnamese restaurants and used to collect the required information. Cases and their dining companions (ill and not ill) were queried about travel activities and foods consumed during the 2-week period before the onset of symptoms. If the onset of illness could not be recalled, cases were asked about the 2 weeks before specimen submission to the laboratory.

Stool specimens were requested from all cases. Various food and environmental samples were collected from the implicated restaurants and the suppliers for microbiologic analysis. Food items included mung beans, mung bean sprouts, and rice noodles. Environmental samples from the identified mung bean sprouters included floor drain swabs, sink swabs, and water used for sprouting the mung beans.

A trace-back investigation was conducted on mung bean sprouts to determine whether a common grower and/or seed supplier could be identified. Other source investigations were conducted on lettuce, shrimp, and chicken, items frequently served to the cases by the Vietnamese restaurants.

Results

Descriptive

By 12 June, 2000, 10 laboratory-confirmed cases of S. Enteritidis phage type 11b had been reported to the health authorities. Two clinical cases were also identified. Six of the confirmed cases reported Alberta as their permanent place of residence, the remaining four residing in Saskatchewan.

Cases were aged 13 to 28 years, and seven of the 12 were $20. Fifty percent (6/12) of cases were male. Onset of symptoms ranged from 29 April to 2 June, 2000, with one peak on 29 April and another on 27 May, 2000 (Figure 1). The first cluster involved mainly Alberta residents with the exception of one case (onset 1 May), a Saskatchewan resident who had eaten at a Vietnamese restaurant while visiting Calgary. The second cluster involved Saskatchewan residents only. One Saskatchewan resident reported that her symptoms began during her relocation to Alberta (onset 27 May). Her exposure to a Vietnamese restaurant was in Saskatchewan.

 Epidemiological Investigation

The 12 cases were interviewed by local public health authorities. Three non-ill dining companions of the cases were also interviewed. Useful epidemiologic analysis could not be conducted because of the limited number of the non-ill dining companions. However, all cases reported exposure to mung bean sprouts. Seven of the cases were exposed while eating at one of five Vietnamese restaurants in Calgary, four were exposed in one of two Vietnamese restaurants in Saskatchewan, and one case, a 13-year-old Vietnamese girl, was exposed at home. Of the three dining companions, two had eaten mung bean sprouts. 

Environmental Investigation

A total of seven Vietnamese restaurants were implicated in this outbreak, five in Alberta and two in Saskatchewan. Environmental health officers inspected the facilities and noted no major violations or health infractions. The menu items consumed by the cases were reviewed for their ingredients and method of preparation.

Trace-back

The trace-back investigation revealed that the mung bean sprouts sold at the implicated restaurants originated from the same bean supplier.The product was imported by a wholesaler in British Columbia and distributed to two different sprouters; one in Alberta serving the Calgary area, and the other in Saskatchewan serving the Saskatoon area. All implicated restaurants were supplied sprouts by one of the two suppliers. A common supplier was not identified for other foods investigated.

Laboratory

All food and environmental samples from the restaurants and suppliers submitted for microbiological analyses were negative for Salmonella.

Figure 1. Number of Salmonella Enteritidis phase type11b cases by date of first symptom onset

Discussion

S. Enteritidis phage type 11b is a rare strain of S. Enteritidis. Finding this rare form of Salmonella among cases during a similar period supports the hypothesis of a common source. Product trace-back information with supporting epidemiologic evidence strongly suggests that the mung bean sprouts were the most likely source of the infection in this outbreak. All seven restaurants involved in this outbreak used mung bean sprouts from one of two sprouters, one in Alberta and one in Saskatchewan. Since both growers used the same supplier, contamination of the mung beans would most likely have occurred at the source, before sprouting.

Mung beans, like other raw agricultural products, can be a source of human pathogens, including Salmonella(1). Pathogenic bacteria can be introduced to the seeds by a number of avenues, including the water used during germination and sprouting, unsanitary production practices, mishandling by the consumer or from the seed fertilizer(2). Sprouts produced from contaminated seeds are of special concern because of the potential for pathogen growth during the sprouting process, even though the pathogen may not be detected on the seed using standard sampling and laboratory methods.

If pathogens are present on or in the seeds, sprouting conditions (environmental conditions and the nutrients) may favour their proliferation.

In addition, the sprouting process does not have any inherent steps to reduce or eliminate pathogens. Previous reported outbreaks of salmonellosis associated with sprouts suggested an initial low level of contamination of the seeds, followed by growth during sprouting(3).

To reduce the risk of illnesses as a result of exposure to bean sprouts, the United States Food and Drug Administration recommends thoroughly cooking all sprouts. This advice is particularly important for children, the elderly, and people with weakened immune systems, all of whom are at high risk of developing serious illness due to foodborne disease. People in high-risk categories should avoid eating raw sprouts.

Acknowledgments

The authors would like to thank Lidia Stephaniw (Alberta Health) and Helen Bangura (Saskatchewan Health) for their participation in the outbreak investigation.

References

1.    National Advisory Committee on Microbiological Criteria for Foods. Microbiological safety evaluations and recommendations on fresh produce. Food Control 1998;10:321-47.

2.    Patterson JE, Woodburn MJ. Klebsiella and other bacteria on alfalfa and bean sprouts at the retail level. J Food Sci 1980;45:492-95.
3.    Mahon BE, Ponka A, Hall W et al. An international outbreak of Salmonella infections caused by alfalfa sprouts grown from contaminated seed. J Infect Dis 1997;175:876-82.

Source:    J Harb, MSc, CPHI(C), Field Epidemiology Training Program (Heath Canada), Vancouver, British Columbia; S Isaacs, MSc, Foodborne Waterborne and Zoonotic Diseases, Health Canada, Guelph, Ontario; M Fyfe, MD, MHSc,Communicable Disease Epidemiology, BCCDCS, Vancouver, British Columbia; L Crowe, CPHI(C), Calgary Regional Health Authority, Calgary, Alberta; B Slater, BSc, Canadian Food Inspection Agency, Burnaby, British Columbia; R Ahmed, MSc, and F Rodgers, PhD, National Laboratory for Enteric Pathogens, National Microbiology Laboratory, Winnipeg, Manitoba; C Anderson, MSc, Saskatchewan Public Health Services, Saskatchewan; J Hockin, MD, MSc, Director, PublicHealth Training and Applications, Population and Public Health Branch, Health Canada.