Multiple-Serotype Salmonella Gastroenteritis Outbreak After a Reception --- Connecticut, 2009

Multiple-Serotype Salmonella Gastroenteritis Outbreak After a Reception --- Connecticut, 2009
Weekly
September 3, 2010 / 59(34);1093-1097


In September 2009, the Connecticut Department of Public Health (DPH) identified an outbreak of Salmonella gastroenteritis among attendees at a reception. A case-control study and environmental and laboratory investigations were conducted. Nine case-patients and 14 control subjects were identified. Potato salad consumption was strongly associated with illness (odds ratio [OR] = 84.0). During the investigation, food service workers were observed to have bare-handed contact with ready-to-eat food. Five case-patients and one asymptomatic food service worker had stool samples positive for Salmonella species. Two Salmonella serotypes were identified, Salmonella enterica serovar Schwarzengrund and Salmonella enterica serovar Typhimurium variant O:5--, including coinfection in one case-patient and one food service worker. The isolates of each respective serotype (S. Schwarzengrund and S. Typhimurium variant O:5--) had indistinguishable pulsed-field gel electrophoresis (PFGE) patterns. Potato salad was the likely source of the outbreak but the contamination mechanism is unclear. Control measures included exclusion of the food service worker with Salmonella-positive stool from the restaurant until two consecutive stool samples yielded no bacterial growth. Standard public health laboratory practices in Connecticut and testing techniques used specifically during this investigation led to the rapid identification of the two serotypes. Multiple-serotype Salmonella outbreaks might occur more frequently than recognized; knowledge of all Salmonella serotypes involved in an outbreak might help implicate the outbreak source, define the scope of the outbreak, and determine the selection of appropriate control measures.

On September 18, 2009, a physician notified the DPH Epidemiology and Emerging Infections Program of a laboratory-confirmed Salmonella infection in a person who had attended a reception at a banquet hall on September 6. Preliminary information indicated that other attendees became symptomatic with gastrointestinal illness after the reception. Food served at the reception was prepared at an off-site licensed restaurant, delivered to the banquet hall by restaurant staff, and set up as a self-serve buffet. DPH and the local health department conducted an investigation to determine the source and extent of the illnesses and to recommend control measures.

A case-control study was conducted among attendees. A case was defined as diarrhea (three or more loose stools during a 24-hour period) in a reception attendee within 5 days after the reception. A control subject was defined as an attendee who did not experience gastrointestinal illness. Because no guest list existed, contact information for ill attendees was provided by the reception host; control subjects and additional case-patients were recruited by asking known attendees to identify and provide contact information for other attendees. Contact information was obtained for 25 (17%) of the approximately 150 attendees. DPH conducted telephone interviews during September 21--25 regarding illness history and food consumed at the reception; an itemized list of foods served at the reception was used to obtain food consumption histories. Of the 25 interviewed attendees, nine (36%) met the case definition, 14 qualified as control subjects, and two were excluded because they reported gastrointestinal illness that did not meet the case definition. Of the nine case-patients, eight (89%) had abdominal cramping, seven (78%) had subjective fever, six (67%) had muscle aches, and four (44%) had bloody stools (Table). Median age was 31 years (range: 25--51 years); five (56%) were male. The median incubation period* was 13.5 hours (range: 9.5--95.5 hours); median illness duration was 8.5 days (range: 0.5--14 days). A case-control analysis revealed that case-patients were significantly more likely than control subjects to have consumed potato salad (88% versus 8%, respectively; OR = 84.0; 95% confidence interval = 3.3--4,077; p<0.001).

During September 21--October 1, the local health department and the DPH Food Protection Program conducted an environmental investigation of the restaurant in which the food served at the reception had been prepared. Of the four persons who worked at the restaurant, two were directly involved in food preparation for the reception. All four were interviewed, and none reported experiencing gastrointestinal illness. During the investigation, food service workers were observed to have bare-handed contact with ready-to-eat food and did not practice adequate hand washing. Preparation procedures of items served at the reception, including the potato salad, were reviewed, and environmental samples of food contact surfaces and spices used in preparation of the reception food were collected for testing. The environmental and spice samples were obtained >3 weeks after the outbreak occurred and after the facility had been cleaned; Salmonella was not detected in these samples. No leftover potato salad was available for testing.

The stool sample from the index case-patient was collected on September 14 and processed at a private laboratory; the clinical isolate was then sent to the DPH laboratory for confirmation. Stool specimens from five additional case-patients and all four food service workers were collected during September 21--October 7 and tested at the DPH laboratory. The specimens were first plated to selective media to test for the presence of Salmonella, Shigella, Campylobacter, and Escherichia coli O157. After incubation, presumptive Salmonella colonies were serotyped† and subtyped genetically by PFGE. Serotyping and PFGE testing were not sequential.§

The isolate from the index case-patient was serotyped as S. Typhimurium variant O:5--. Initial serotyping steps performed on Salmonella isolates obtained from stool specimens revealed a preliminary antigen result consistent with the S. Typhimurium variant O:5-- already identified for the index case-patient. Consequently, investigators assumed that S. Typhimurium variant O:5-- was the only outbreak serotype. Next, while final serotyping was pending, Salmonella isolates were submitted for PFGE. Testing of the first five isolates yielded two distinct PFGE patterns (PFGE XbaI patterns JPXX01.0456 and JM6X01.0036¶). One PFGE pattern appeared to be consistent with S. Typhimurium; the other appeared to be consistent with S. Schwarzengrund. The results of serotyping verified the presence of both S. Typhimurium variant O:5-- and S. Schwarzengrund.

The identification of both S. Typhimurium variant O:5-- and S. Schwarzengrund in reception attendees raised the possibility that two different Salmonella serotypes might be involved in the outbreak. Therefore, laboratory staff systematically collected multiple single-colony picks from original media to screen for the presence of an additional Salmonella serotype. After all testing was complete, including isolation, serotyping, and PFGE, two of the six case-patients with specimens at the DPH laboratory were determined to be infected with S. Typhimurium variant O:5--, another two with S. Schwarzengrund, and one with both; no pathogens were isolated from the stool specimen of the sixth case-patient. A seventh case-patient's stool specimen was tested at a private laboratory; no Salmonella was detected. Of the four food service worker specimens tested, one yielded both S. Schwarzengrund and S. Typhimurium variant O:5-- and the other three were negative. All respective S. Schwarzengrund isolates and S. Typhimurium variant O:5-- isolates had indistinguishable PFGE patterns.

On September 25, the food service worker with positive stool findings was reinterviewed and reaffirmed the absence of recent gastrointestinal illness, including around the time of the reception. This food service worker had been responsible for transporting food to the banquet hall and ensuring that the food was maintained at the correct temperature before serving, but reported not having prepared, consumed, nor served any of the food.

Control measures implemented by the local health department included exclusion of the Salmonella-positive food service worker from the restaurant for approximately 2 weeks until two consecutive stool cultures obtained ≥24 hours apart had no bacterial growth. Health department staff members provided information about employee health policies and employee hygiene to the restaurant owners and reviewed the information with them.

Reported by
L Mank, MS, M Mandour, Connecticut Dept of Public Health Laboratory; T Rabatsky-Ehr, MPH, Q Phan, MPH, J Krasnitski, MPH, J Brockmeyer, MPH, L Bushnell, C Applewhite, M Cartter, MD, Connecticut Dept of Public Health. J Kattan, MD, EIS Officer, CDC.

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Multiple-Serotype Salmonella Gastroenteritis Outbreak After a Reception --- Connecticut, 2009