Small outbreaks of cholera frequently come prior to epidemics

A mathematical biologist examined past cholera epidemics in London and discovered a trend in how cholera epidemics strike.

Cholera epidemics frequently come with a warning sign – in other words, severe outbreaks are often preceded by smaller outbreaks – according to a study released in late November by the Royal Society in London.

Map of cholera deaths near London’s Broad Street. Image credit:

According to the study in the mid-19th century, London experienced four major years of cholera epidemics, with one year killing more than 13,000 Londoners. Looking at London’s cholera records, mathematical biologist Joseph Tien noticed a trend.

The severe cholera outbreaks always struck in summer, with the exception of smaller outbreaks during the spring of 1832, autumn of 1848, and winter of 1853. Those outbreaks are what Tien calls “heralds” or warning signs, of a bigger epidemic coming during the peak season for cholera.

In London’s case, the peak season for cholera is summer. Tien and his colleagues theorize that the arrival of a new strain of cholera triggered the smaller, off-season outbreaks.  But climate conditions would limit cholera’s transmission, until warmer temperatures allowed the strain to resurface and spread.

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