What are the common causes of bacterial food poisoning? Fifty school children were admitted for diarrhoea and vomiting 3 hours after a meal at the school canteen. Describe how you would investigate the cause and source of the outbreak.

Suggested Answer:

	Bacterial food poisoning may be caused by a genuine infection of the gastrointestinal tract following consumption of heavily contaminated food, but it may also the result of preformed toxins in food. Clinical presentation depends on the pathogenic mechanisms: direct bacterial invasion of the gastrointestinal tract which results in a bloody or mucoid stools or exotoxin production which causes a nonbloody, watery diarrhea with extensive fluid loss. Invasive bacteria causing food poisoning are Salmonella, Shigella, Escherichia coli, Campylobacter, and Yersinia. Enterotoxin-mediated food poisoning are caused by Vibrio cholerae, enterotoxigenic strains of E. coli, Staphylococcus aureus, clostridium Perfringens, Vibrio parahaemolyticus and Bacillus cerus.

	Preliminary steps are first taken to care for the sick children before a thorough investigation is carried out. This involves sending the children to a nearby hospitals for treatment and rest. The symptoms of the sick children, such as diarrhea, vomiting or fever, are clearly noted down in order to establish a case definition. They are then interviewed on the food they ate before the diarrhea, the stall from which they bought the food and the time elapsed from ingestion of the meal to onset of the diarrhea. From these information, an epidemic curve can be plotted which gives the distribution of times and onset. Together with the time at which the exposure occurred, the median incubation period is calculated which will help in the identification of the causative agent. In this case, based on the incubation period of 3 hours, the most likely microbial agent is Staphylococcus aureus. A working hypothesis is formulated and interim control measures are implemented to prevent further spread of the disease. This may require temporary shutting down of the canteen, performing active case detection to search for unreported cases and informing the school children and parents to avoid eating the canteen form till  further notice. The source of the infection is determined from the information provided by the children. An active investigation is then carried out to check out the environment of the stall, the prepared food and the handling of food by the stall-holders. After the exact cause has been determined, a brief report on the investigation is written, including recommendations for measures to prevent similar outbreaks in the future. The responsible stall may be reported to the Ministry of Environment for further putative measures to be taken.