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Convalescent Cultures for Control of Shigellosis Outbreaks

Background: A shigellosis outbreak in the St Louis, Missouri metropolitan area. Objective: To evaluate the utility of a second convalescent stool culture following an initial negative convalescent stool culture among persons excluded from work or childcare for shigellosis. Methods: An observational study of 219 shigellosis cases. Laboratory-confirmed shigellosis patients who are required to submit 2 negative convalescent stool cultures before returning to childcare facilities or work and who submitted at least 1 culture were included in the study. Univariate and multivariable logistic regression analyses were performed to evaluate potential risk factors for a convalescent stool culture being positive. Results: Of 308 persons, 219 (71%) submitted at least 1 convalescent stool culture, and 164 (53%) submitted 2 negative convalescent stool cultures. Among 172 cases with ≥2 follow-up cultures, the probability that the second test result would agree with the first test result was 7% for a “positive” initial stool culture, and 100% for a “negative” stool culture. When adjusted for age, sex, and child care attendance, treated case-patients who had Shigella organisms in the first convalescent culture were more likely to have had stool collected <48 hours after the treatment completion and were more likely to have been treated with trimethoprim-sulfamethoxazole. Conclusions: Compliance is poor with statutes requiring serial negative stool cultures among certain populations with shigellosis. Absence of Shigella species in the first convalescent stool culture of patients recovering from shigellosis appears to be an adequate measure of bacteriologic cure; however, the health impacts of requiring any convalescent cultures during shigellosis outbreaks remain unclear.