Malaria epidemic definition — DTM&H MCQ

Correct answer: E — Incidence exceeding the upper third quartile of previous 5 years for the same period

The correct answer is E, incidence exceeding the upper third quartile of the previous 5 years for the same period. WHO epidemic detection methods define a normal seasonal pattern (the normal epidemic channel) using historical monthly or weekly case counts over a consecutive 5 to 10 year series, with the upper limit of this channel set at the third quartile for that specific calendar period. This corrects for seasonal and year-to-year variation, so a genuine excess above expected baseline, rather than an arbitrary absolute number, defines an epidemic threshold. The stem describes flooding driving a three-fold rise compared with the same period last year, which is exactly the scenario this method is designed to flag once historical quartile data confirm the rise is outside the expected range. This approach underpins early warning and epidemic preparedness systems used across malaria-endemic districts, including in Zambia. Why the other options are wrong: D. More than 100 cases in one week: this is an arbitrary fixed number with no adjustment for population size, baseline transmission intensity, or season, so it cannot function as a universal epidemic definition. C. CFR exceeding 5 percent: case fatality rate measures severity of established disease, not epidemic onset, and is unrelated to how a rise in incidence is defined. A. Any case in a previously malaria-free area: this describes an outbreak in a non-endemic or elimination setting, not the standard epidemic threshold used in endemic districts with seasonal transmission. B. Doubling of cases compared to the previous month: month-to-month comparison ignores normal seasonal surges (for example after rains) and would generate false alarms every transmission season. Key point: WHO epidemic thresholds compare current incidence against the historical third quartile for the same calendar period over the preceding 5 years, not against arbitrary counts, case fatality, or short-term month-on-month change.

Reference: WHO, Malaria Surveillance, Monitoring and Evaluation: A Reference Manual (2nd edition, 2024), Section on epidemic detection and the normal epidemic channel, https://www.who.int/publications/i/item/9789240112476