Cholera in a Peri-urban Settlement: 214 Cases and 2,000 Vaccine Doses
On 17 June the district health officer is called to a peri-urban settlement of 12,000 people, 45 km from the district headquarters. Between 3 and 17 June, 214 residents presented to the basic health unit with acute watery diarrhoea; five died, two of them children under 5 (there are 1,500 children under 5 in the settlement). There are two boreholes serving about 7,200 people (54 cases) and shallow hand-dug wells serving about 4,800 people (160 cases). Forty per cent of households use the wells, which lie within 10 metres of a leaking sewer line. Chlorination stopped three weeks ago when the dosing pump failed and has not restarted. Stool culture from 10 patients grew Vibrio cholerae O1 Ogawa in six. The BHU has 20 beds, two nurses on duty per shift, no laboratory, and one vehicle. A vaccine consignment of 2,000 doses has been offered, and the media have already reported two deaths.
1. Calculate the attack rate, case fatality and the attack rate by water source, decide whether this is an outbreak, and write a working case definition.
Read the worked answer
Attack rate = 214/12,000 = 1.8% over 14 days. Case fatality = 5/214 = 2.3%. By water source:
- Borehole users: 54/7,200 = 0.75%
- Shallow-well users: 160/4,800 = 3.3%
- Risk ratio = 3.3/0.75 = 4.4
This is an outbreak: a single case of cholera is notifiable, and here we have 214 cases of acute watery diarrhoea with a 2.3% case fatality and laboratory confirmation of V. cholerae O1 Ogawa in 6 of 10 stools. The four-fold higher attack rate among well users, together with the sewer line within 10 metres of the wells and three weeks without chlorination, points to a water-borne common-source outbreak with continuing exposure.
Case definition (suspected): any person living in the settlement who has passed three or more loose or watery stools in 24 hours, with or without vomiting, since 3 June. Confirmed: a suspected case with V. cholerae O1 isolated from stool. All ages are included because under-5s are affected; report to the district surveillance officer today and start line-listing by date of onset, age, sex, water source and outcome.
2. Rank the three control measures you would prioritise and justify the ranking. Then explain how you would use the 2,000 vaccine doses.
Read the worked answer
1. Water (first, because it stops the outbreak at source). Emergency chlorination of both boreholes and the shallow wells to give 0.2–0.5 mg/L free residual chlorine at the point of use, using chlorine tablets or dilute sodium hypochlorite; repair or bypass the failed dosing pump; stop use of the wells nearest the sewer leak until they test clear; repair the sewer line and seal the well heads. Verify with chlorine spot testing at well and household level.
2. Case management (second, because nearly all cholera deaths are preventable dehydration). Oral rehydration corners in the BHU and in each affected neighbourhood, open 24 hours, with oral rehydration salts plus zinc for children, IV Ringer's lactate for severe dehydration, a designated cholera treatment area, and a written referral protocol. With two nurses per shift and 20 beds, this means training and deploying community volunteers now.
3. Sanitation and hygiene (third, because it interrupts household transmission). Safe disposal of faeces, latrine provision, soap and handwashing promotion, safe household storage, and food-handler education.
Vaccine: 2,000 doses cannot cover 12,000 people, and oral cholera vaccine is an adjunct - it does not treat and takes about 7–10 days to protect. Use it as a single-dose reactive campaign targeted at the highest-risk groups: household contacts and immediate neighbours of the 214 cases (ring vaccination), residents of the shallow-well areas in the two worst-affected neighbourhoods, and carers of under-5s and food handlers. Do not spread 2,000 doses thinly across the whole settlement.
3. What single indicator would tell you whether your response is working, and what findings would make you change course?
Read the worked answer
Single best process indicator: the proportion of households in the affected areas with free residual chlorine of at least 0.2 mg/L at the point of use, measured by spot testing a random sample of about 30 households per neighbourhood each week. It tells you whether chlorination is actually reaching people rather than sitting in a tank.
Supporting outcome indicators:
- weekly new cases plotted by date of onset - the epidemic curve should start falling within one incubation period, about five days;
- proportion of patients receiving oral rehydration within one hour of arrival;
- proportion presenting with severe dehydration;
- case fatality, with a target below 1%.
What would make me change course:
- Cases still rising seven days after chlorination began - look for an unchlorinated source, a contaminated storage container, or a second transmission route.
- Cases clustering among borehole users, which would mean contamination is in the distribution system rather than the wells, pushing me towards tanker water and a full pipeline inspection.
- An unexpected clinical picture, such as a high proportion of patients presenting with vomiting or collapse and little diarrhoea, which should prompt re-examination of the diagnosis (for example, another pathogen or a chemical contamination).
- A rising case fatality, which would indicate a failure of case management rather than of water control.