Waterborne diarrhoea outbreak in a village of 1,200: reading the attack rates before acting
Over eight days, the sub-centre covering a village of 1,200 people reports 66 cases of acute watery diarrhoea, of which 2 have died. The medical officer asks the ANM to look at the line list. Of the 380 people who draw water from the public hand pump on the north lane, 52 have been ill; of the 820 people who use the piped municipal supply or private borewells, 14 have been ill. The hand pump is 8 metres from an open soak pit serving six households and 10 metres from a cattle shed. The piped supply runs for only 45 minutes every morning. Water from the hand pump is clear and odourless. Most cases are adults aged 20 to 45, with a few children; there is no fever. Stool samples have been sent from three patients. There is one ORS corner at the sub-centre, no chlorine tablets in stock, and a village health and sanitation committee meeting scheduled in two days.
1. Calculate the attack rates for the two water-source groups, interpret them, and state what this does and does not prove.
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Calculations:
- Hand pump users: 52 ÷ 380 = 0.1368, that is 13.7%
- Piped supply or borewell users: 14 ÷ 820 = 0.0171, that is 1.7%
- Ratio of attack rates: 13.7 ÷ 1.7 = about 8.0
- Difference (attributable risk): 13.7 − 1.7 = 12 percentage points
Interpretation: people using the north-lane hand pump were roughly eight times more likely to fall ill. A ratio of this size, with a plausible route of contamination 8 metres from an open soak pit, strongly supports that water source as the vehicle. Because there is no fever and the illness is watery diarrhoea in adults as well as children, a cholera-like or enterotoxigenic organism is plausible.
What it does not prove: that the pump is definitely the source. The north-lane households may differ in crowding, sanitation, income or food sources, and the 14 cases among the other users could be secondary person-to-person spread, another exposure, or misclassification of the water source. Attack rates describe association, not causation.
Next step: an active case search with a written case definition, a line list with onset dates, a spot map and an epidemic curve; a tight single peak within one incubation period would support a point source. Send water for thermotolerant coliform counting and confirm the stool culture results.
2. Set out your prioritised control measures for the next 48 hours, including whom you would inform and why.
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First 24 hours:
- Notify immediately through IDSP to the district surveillance unit and request a rapid response team. Two deaths from acute watery diarrhoea in eight days is an outbreak, not a routine report, and the legal and ethical duty to report comes before the committee meeting.
- Case management: strengthen the ORS corner with oral rehydration salts and zinc, keep intravenous fluids available at the PHC, and brief ASHAs and Anganwadi workers on early referral - deaths in diarrhoea usually come from delayed rehydration, not from the organism.
- Stop exposure: advise the north lane not to use the hand pump until results return; arrange an alternative supply or household treatment; distribute chlorine tablets or bleaching powder, targeting free residual chlorine of about 0.5 mg/L after 30 minutes of contact.
- Sanitary survey: inspect the pump apron, drainage and the soak pit; seal or relocate the soak pit, repair the apron, and disinfect the pump and surrounding area.
Next 24 to 48 hours: health education on handwashing, boiling or filtering, and safe storage; ensure the intermittent piped supply is not being contaminated by negative pressure during shutdown; bring the VHSNC meeting forward rather than waiting; and start daily reporting of new cases so the effect of the measures can be judged.
3. Write a case definition for this outbreak, and explain why 'the water is clear and has no smell' does not reassure you.
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Case definition (person, place, time, clinical features): 'A person of any age residing in or visiting the village between [first date] and [last date] who developed three or more loose stools in 24 hours, with or without vomiting.' A suspected case meets this definition; a confirmed case has the organism isolated from stool. Sub-definitions help triage: 'severe' means any dehydration or bloody stool; 'fatal' means death with compatible illness. The definition must be written down and applied identically by everyone doing the search, otherwise the counts mean nothing.
On clear, odourless water: faecal contamination is microbiological, and pathogens are invisible, tasteless and odourless at the concentrations that cause disease. Turbidity and smell are aesthetic indicators, not safety indicators. A hand pump 8 metres from an open soak pit is a textbook hazard: shallow groundwater moves laterally, and a cracked apron or missing drainage channel lets leachate travel down the outside of the casing directly into the water. Intermittent piped supply adds a second risk - when pressure drops, contaminated groundwater can be drawn into the pipes through joints.
The only reassuring evidence would be an adequate chlorine residual at the tap and a coliform count of zero, together with a sanitary inspection showing a sound apron and a soak pit at a safe distance. Until then, treat the water as unsafe and say so plainly.