Cholera first infected humans in the early 19th century in Bengal, a region that straddles what is now the border between Bangladesh and India, and the bacterial disease still sweeps through the area regularly. After sifting through historical records of cholera deaths in Bengal, a team of scientists in the United States and Europe proposes a new explanation for these repeated outbreaks, suggesting that immunity to cholera wanes more rapidly than thought and that many more people than believed become infected without exhibiting symptoms.
Cholera is usually caused by bacteria in water contaminated by feces from infected people. The disease can kill within hours of the first symptoms. For reasons no one quite understands, cholera epidemics seem to follow the seasons, surging and then receding in predictable patterns. Researchers have offered various explanations for the recurring outbreaks, including temperature shifts and the changing ecology of local water bodies, but they haven't had enough evidence to nail down the cause.
So Aaron King, a theoretical ecologist at the University of Michigan, Ann Arbor, teamed up with a biostatistician and cholera experts to map the pattern of cholera epidemics across decades. They obtained death records from 26 districts in Bengal from 1891 to 1940, when the region was a British colony. Although the records did not include information on survivors, the mortality data are unlikely to be wrong because severe disease is distinctive, King says.
Using estimates of variables such as population size and the chance of dying from cholera, the researchers gauged how closely the mortality data hewed to different transmission models. To their surprise, the model that worked best predicted that cholera survivors were only immune to the disease for a few months rather than for a few years, as researchers previously believed. The model also suggested that a very high proportion of people infected by cholera had no symptoms, putting the ratio of asymptomatic to symptomatic people as high as 250-to-1, the group reports in tomorrow's issue of Nature. Previous estimates have ranged between 3-to-1 and 100-to-1. The model could not answer whether these asymptomatic individuals could transmit disease, something King hopes others will pursue.
These results could help explain why cholera epidemics follow the paths they do, King says. The seasonal outbreaks might occur because cholera simply "spreads until there's essentially no one left to spread to." At that point--with most of the population infected but without symptoms and others having died--cholera fades away because there are few susceptible hosts. The disease then remains in hiding until rapidly waning immunity allows a new outbreak.
The only way to know if the findings are right is to locate people with asymptomatic infections during epidemics, says Ira Longini, a biostatistician at the University of Washington, Seattle. Cholera expert David Sack of Johns Hopkins University in Baltimore, Maryland, doubts that immunity wanes as quickly as the data suggest--vaccine trials indicate it may last several years. However, Sack acknowledges that no one really knows how long immunity from asymptomatic infections persists, because those people are difficult to identify and haven't been studied.