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Hospital-Associated Outbreak of Shigella dysenteriae Type 2 -- Maryland

An outbreak of severe dysentery caused by Shigella dysenteriae type 2 recently occurred at the U.S. Naval Hospital, Bethesda, Maryland. Epidemiologic investigation implicated the salad bar in the active-duty staff cafeteria as the source of infection.

In March 1983, 95 (6%) of 1,490 active-duty hospital staff members and 12 other individuals (three hospital inpatients, four visitors, and five food-service workers) became ill with acute dysentery. Onset of illness occurred over an 11-day period, and the epidemic curve was consistent with a common-source outbreak (Figure 1). Patients presented with chills, fever, abdominal cramps, and the abrupt onset of profuse watery or bloody diarrhea. Nausea, vomiting, myalgias, and dehydration were frequently noted. A case was defined as a patient with two or more of the following: 1) fever or chills, 2) nausea or vomiting, 3) watery or bloody diarrhea (more than two non-formed bowel movements per day), or 4) abdominal cramps. Twenty-four individuals required hospitalization for intravenous hydration. The duration of illness for most persons ranged from 3 to 8 days.

S. dysenteriae type 2 was cultured from stool specimens of 36 of the 80 affected individuals who were cultured. All symptomatic individuals were treated with a 5-day course of either ampicillin or trimethoprim sulfamethoxazole, both of which were effective against the organism in vitro. One hundred three of 107 known symptomatic individuals and 102 controls matched by job category were interviewed, and food-specific histories were obtained. Eating food prepared in the staff cafeteria, where 900-1,300 persons eat one or more meals daily, was significantly associated with illness (p 0.0001). Analysis of food histories from patients and controls who ate at the cafeteria at any time between February 28 and March 3 showed that patients were significantly more likely than controls to have eaten raw vegetables from the salad bar (p = 0.004). A single batch of salad vegetables prepared on February 28 was served at the salad bar through March 3. No single salad item or dressing was specifically implicated. No samples of salad from the days in question were available for culture, and no cultures taken from available food items were positive. No other outbreaks of food-related shigellosis were reported to state health authorities during the outbreak period.

Interviews and stool cultures were obtained from 63 food handlers. Five had illnesses meeting the case definition with onset concurrent with the other cases. Although no index case was identified, gastrointestinal illnesses appeared to be common causes of absenteeism among food handlers during the 3 weeks preceding the outbreak. No food handler had a positive stool culture; none reported recent emigration or foreign travel.

The scope of the outbreak was undoubtedly limited by the exclusion of civilians from eating in the cafeteria and because food for inpatients is prepared in separate areas by different personnel. Although a few staff members continued to work while ill, most did not work after onset of symptoms. Only ambulatory inpatients or visitors who ate in the staff cafeteria became ill. Two persons with presumed secondary cases who denied eating in the cafeteria were identified, including a civilian nurse who reported taking a rectal culture 12 hours before onset of her own symptoms from a hospitalized patient with a case, and a student laboratory technician who had contact with other ill technicians.

Preliminary food histories led to closing the salad bar, the cold-sandwich line, and a self-serve ice cream machine on March 7; these were reopened on March 14. Symptomatic health care workers rapidly improved with antibiotic therapy and were allowed to return to work 48 hours after symptoms had subsided. Symptomatic food handlers were required to have two negative rectal cultures, 24 hours apart, after completion of antibiotic therapy. The impact of this outbreak on the hospital operation was considerable. Well staff members volunteered for additional shifts to care for ill co-workers who were hospitalized on a separate ward. The Naval Hospital was not forced to restrict elective admissions. Reported by R Longfield, Commander MC USN, E Strohmer, R Newquist Commander MC USN, Naval Hospital, J Longfield, MD, J Coberly, Uniformed Svcs University of the Health Sciences, Bethesda, Maryland; G Howell, Captain MC USN, R Thomas, Lt MC USNR, Navy Environmental Health Center, Norfolk, Virginia; Enteric Diseases Br, Div of Bacterial Diseases, Hospital Infections Program, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: S. dysenteriae type 2 is an uncommon cause of disease in the United States; in 1982, only 0.2% (20/8,939) of the Shigella isolates of known serotype reported to CDC were type 2.

Because the infectious dose of Shigella is relatively small, it is more likely to spread person-to-person than are other enteric pathogens such as Salmonella and Vibrio cholerae 01. Although 6% of the active duty hospital staff members became ill, there was no evidence of secondary spread from staff to patients. The opportunity for spread from staff to patients was undoubtedly decreased, since

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