Cholera Epidemics of the Past Offer New Insights Into an Old Enemy.

Background: Although cholera is considered the quintessential long-cycle waterborne disease, studies have emphasized the existence of short-cycle (food, household) transmission. We investigated singular Danish cholera epidemics (in 1853) to elucidate epidemiological parameters and modes of spread.

Methods: Using time series data from cities with different water systems, we estimated the intrinsic transmissibility (R0). Accessing cause-specific mortality data, we studied clinical severity and age-specific impact. From physicians' narratives we established transmission chains and estimated serial intervals.

Results: Epidemics were seeded by travelers from cholera-affected cities; initial transmission chains involving household members and caretakers ensued. Cholera killed 3.4%-8.9% of the populations, with highest mortality among seniors (16%) and lowest in children (2.7%). Transmissibility (R0) was 1.7-2.6 and the serial interval was estimated at 3.7 days (95% confidence interval, 2.9-4.7 days). The case fatality ratio (CFR) was high (54%-68%); using R0 we computed an adjusted CFR of 4%-5%.

Conclusions: Short-cycle transmission was likely critical to early secondary transmission in historic Danish towns. The outbreaks resembled the contemporary Haiti outbreak with respect to transmissibility, age patterns, and CFR, suggesting a role for broader hygiene/sanitation interventions to control contemporary outbreaks.