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Background: On May 27, 2011, an outbreak of gastroenteritis reported in a primary school in Anhui province, China.We investigated this outbreak to identify the cause, mode of transmission and recommend control measure.Methods: A case was defined as diarrhea (≥3 liquid stools/24hours) plus ≥1 of the following: abdominal pain, fever (≥37.5℃), tenesmus, bloody or mucoid stool in a school student of staff since May 14, 2011.We collected stool samples and performed environmental inspection.We selected 61 cases and 58 controls frequency-matched on dormitories to compare their food, water exposure and personal hygiene habits.We estimated a crowding score in the dormitory using area per person as follows: 4.4 m2=1, 2.4 m2=2, 1.4 m2=3, 1.3 m2=4.Results: We identified 76 cases (Attack rate: 24%) over 8 days, all among boarding students.Boarding students had water and food source in common with other students and teachers.Stool specimens from 2 case-students and 1 healthy cafeteria worker were positive for Shigella flexneri 3c.Boarding students were temporarily housed in 9 old classrooms due to reconstruction of their dormitory.The median area per person in temporary dormitory rooms was 1.3 m2 (range: 1.3m2-4.4m2/person).The odds ratio (OR) for being a case increased by 1.5 (95%CI=1.0-2.3) for each unit increase in crowding score.Logistic regression for hand-washing score of after toilet use and before meals (always=3, sometime=2, never=l) in cases and controls showed good personal hand-washing practice was protective factor (1 increased score, OR=0.32, 95%CI=0.19-0.55).Conclusions: The most likely cause of this shigellosis outbreak was an overcrowded dormitory and poor personal hygiene in boarding students.General sanitation of living condition and personal hygiene need maintained and intensified in school setting.