Dengue outbreak: containers, not the pond
During a dengue outbreak in a town of 40,000, a nursing student joins the vector-control team. In a survey of 120 houses, 30 have at least one container with Aedes larvae. The positive containers are mostly overhead tanks, plastic drums and flower pots holding clear stored water; the town's large pond and open storm drains show no Aedes larvae. The team's current plan is to fog the streets at 6 a.m. and to spend most of its budget on draining the pond. The district hospital has reported 18 suspected dengue cases in a week, including a 9-year-old with persistent vomiting and abdominal pain. The student has three days, a team of eight workers and a limited supply of larvicide approved for drinking-water containers. The municipal chairperson wants 'visible action' before the next inspection. Last year's survey in the same wards found a container index of 4%, and the team assumed this year would be similar.
1. Calculate the container index and interpret it for this town.
Read the worked answer
Container index (CI) = houses with at least one positive container ÷ houses inspected × 100 = 30/120 × 100 = 25%. For Aedes, a CI above about 5% (with a Breteau index above 20) signals high transmission risk; 25% is five times that threshold.
The distribution matters as much as the number: breeding is concentrated in domestic clear-water storage - tanks, drums, flower pots - not in the pond. That fits Aedes aegypti, which prefers clean, peridomestic water and lays eggs just above the water line. So a large adult mosquito population is emerging from houses, and the jump from 4% last year to 25% now predicts further cases unless the containers are dealt with.
2. Explain why fogging at 6 a.m. and pond draining are weak priorities, using vector biology.
Read the worked answer
Fogging kills flying adults only; it does not touch eggs, larvae or pupae, and its effect lasts days. Aedes aegypti is a day-biter with peaks in early morning and late afternoon, so 6 a.m. is not the worst timing, but fogging without source reduction is a repeated cost with no lasting effect - adults re-emerge from the same containers.
Draining the pond targets the wrong species: Aedes rarely breeds in ponds or drains; Anopheles (malaria) and Culex use such water. Spending most of the budget there gives the chairperson 'visible action' but little dengue control. The high-yield measure is source reduction: empty, scrub and dry or cover containers weekly, because Aedes eggs survive desiccation on the container wall and hatch when water returns. Larvicide approved for drinking water goes into tanks that cannot be covered, and the 25% CI shows which houses to revisit.
3. Draft a three-day plan for eight workers and the larvicide, including what to do about the 9-year-old.
Read the worked answer
Day 1: split the eight into four pairs; each pair covers 30 houses, emptying and scrubbing every container, covering tanks, and recording a repeat index. Treat only uncovrable drinking-water tanks with the approved larvicide at label dose, and tell residents why. Day 2: repeat the search for containers missed or refilled, add schools and temples, and teach a weekly 'dry day'. Day 3: resurvey a sample of about 40 houses to estimate the new CI and target the worst wards. Fogging, if used at all, is timed for early morning or late afternoon during the outbreak and never replaces source reduction.
The 9-year-old with persistent vomiting and abdominal pain has warning signs of dengue and needs same-day assessment at a facility that can monitor platelets and give fluids - not a home visit. That referral belongs in the team's work plan, because severe dengue can progress within hours.