An insidious nosocomial outbreak due to Salmonella enteritidis

To investigate an increase in the number of Salmonella enteritidis isolates detected in a large hospital to ascertain whether it was due to a nosocomial source, to identify the mechanisms of transmission, and to institute effective control measures to prevent future episodes.

Observational study, survey of all microbiological samples positive for S. enteritidis detected in the hospital, outbreak investigation, and review of the literature.

A tertiary-care teaching hospital for adults in Barcelona, Spain.

During a 7-month period from May to November 1998, we identified 22 inpatients with S. enteritidis infection for whom nosocomial acquisition was strongly suspected. The attack rate was 0.138 per 1,000 patient-days. All affected patients were immunosuppressed and overall mortality was 41% (9 of 22). A sample of a meal cooked in the kitchen was culture positive for S. enteritidis. All isolates shared the same antibiotic susceptibility pattern and all except one shared the same pulsed-field gel electrophoresis (PFGE) pattern, but PFGE could not differentiate between outbreak-related and control strains. After compliance with kitchen hygiene procedures was emphasized and cleansing was intensified, no more cases were detected.

Apparently, sporadic cases of S. enteritidis may be part of an outbreak with a low attack rate. A small but persistent inoculum affecting only individuals with special predisposition for Salmonella infection might account for this. Suspicion should be raised in hospitals and institutions with a highly susceptible population.