Asymptomatic foodhandlers as the source of nosocomial salmonellosis

A nosocomial outbreak of salmonellosis affected 22 patients and seven staff on 14 wards in two hospitals with shared catering facilities. The outbreak was characterized by a low level intermittent infection with Salmonella enteritidis phage type 4 over a 19-day period. The epidemiology did not suggest a common source for the outbreak and there was little evidence for person-to-person spread. Extensive food and environmental sampling failed to yield salmonella. Control measures within the kitchens and on wards early in the outbreak had no effect on the rate of infection. Faecal screening of asymptomatic people demonstrated a high carriage rate among catering staff (12.3%), compared with ward staff (2.2%) or patients (0.8%). A case-control study failed to reveal any association between illness and particular meals, food types, wards, medical attendants, medical procedures, or medication. However there was an association between illness and eating meals prepared by one carrier (P = 0.02). Transmission was believed to be via intermittent contamination of occasional meals. No further cases occurred after the exclusion of infected food handlers. The identifiable costs of the outbreak amounted to approximately pounds 33,000. These results indicate that asymptomatic food handlers may be the source of nosocomial salmonella outbreaks, and that efforts should be made to identify carriers and treat them.