Background: On 12 May 2012, over 200 college students with acute diarrhoea were reported to the Guizhou Center for Disease Control and Prevention. We conducted an investigation to identify the agent and mode of transmission and to recommend control measures.
Methods: A suspected case was a person at the college with onset of ≥ two of the following symptoms: diarrhoea (more than three loose stools in 24 hours), abdominal pain, vomiting or fever (> 37.5C) between 6 and 15 May 2012. A confirmed case also had a positive Aeromonas hydrophila culture from a stool sample. A retrospective-cohort study of 902 students compared attack rates (AR) by dining place, meals and food history. We reviewed the implicated premise, its processes and preparation of implicated food.
Results: We identified 349 suspected cases (AR = 14%) and isolated Aeromonas hydrophila from three stools of 15 cases. Students who ate in cafeteria A were more likely to be ill compared to those eating in other places (relative risk [RR]: 3.1, 95% confidence interval [CI]: 2.0-4.8). The cohort study implicated cold cucumber (RR: 2.6, 95% CI: 2.0-3.3) and houttuynia dishes (RR: 1.8, 95% CI: 1.4-2.3). Environmental investigation showed that vegetables were washed in polluted water from a tank close to the sewage ditch, then left at 30 °C for two hours before serving. The Escherichia coli count of the tank was well above the standard for drinking-water.
Conclusion: This outbreak of Aeromonas hydrophila was most probably caused by salad ingredients washed in contaminated tank water. We recommended enhancing training of foodhandlers, ensuring tanks and sewerage systems comply with appropriate standards and adequate monitoring of drinking-water sources.