Outbreak investigation of bacterial dysentery in two colleges in Nanjing

Abstract

Objective To investigate the source, transmission mode and infection range of the bacterial dysentery outbreak in two colleges in Nanjing. Methods Suspected cases were defined as students and staff members in these two colleges with the symptoms of diarrhea during the period from September 7 to October 8, 2015. Probable cases were defined as suspected cases with one of the three symptoms: fever(T≥37.4℃), tenesmus, or abdominal pain. Confirmed cases were defined as suspected or probable cases with PCR detection for Shigella DNA fragment positive in fecal or anal swab specimens or with isolated Shigella. The general situation of these 2 colleges was investigated. We conductedactive case searching by reporting from local medical institutions, college health center and counselors; conducted case study and describing general information, clinical manifestations, and distribution features; conducted matched case control study for suspicious meals and food; collected stools or rectal swab specimens of cases and food handlers, environmental samples and food retention samples to test Salmonella and Shigella by PCR; used PFGE for analysis of homology of the isolated strains; analyzing the distribution of cases by descriptive epidemiology method andthe difference of attack rates by Chi-square test, and calculate Odds ratios (OR) with 95 % confidence intervals (CI) of suspicious meals in case control study. Results A total of 436 and 36 cases were found out in collage A and B, leading to attack rates of 2.6% (436/15 385) and 0.23% (36/15 449) respectively and the epidemic curve indicated a point source exposure. In collage A, the attack rate of boarders was higher than that of day students (2.9%, 434/15 027 vs 0.78%, 2/258, χ2=4.09, P<0.05); the attack rate of the students lived in the south dormitory area was higher than that in the north area (4.6%, 351/7 666 vs 1.1%, 83/7 361), χ2=159.46, P<0.001). The case control study result showed that the suspicious exposure meal was dinner on September 20(OR=2.4, 95%CI: 1.2-4.7). Shigella sonnei strain I was isolated from anal swab specimens of cases and canteen employees in these two colleges, and the PFGE detection results showed a high degree of homology. Conclusions This is a bacterial dysentery outbreak caused by Shigella sonnei involving two colleges in Nanjing. We should continue to enhance the surveillance of foodborne disease and sanitary control and supervision of canteens in schools and other collective units. Key words: Bacillary dysentery; Outbreak; Shigella sonnei; Pulsed Field Gel Electrophoresis(PFGE)