Scientists are closing in on a forecast model that may soon be able to predict future cholera outbreaks based on increases in temperature and rainfall, according to a study published Tuesday.
An analysis of several years of past data in cholera-prone parts of Zanzibar, Tanzania, showed that when temperatures rose one degree Celsius, cholera cases were likely to double within four months, said scientists at the International Vaccine Institute in Seoul, South Korea.
A small increase in rainfall (200-milliliter or 6.7 ounces per month) also forecast that a "substantial increase could be expected within two months," said the research in the June issue of the American Journal of Tropical Medicine and Hygiene.
Cholera, a diarrhea-causing bacteria that can be lethal in vulnerable populations, often strikes without warning, and by the time symptoms arise it is often too late for vaccines to be effective.
The disease is primarily spread through fecal contamination of food and water. The bacteria can live in the environment, so rising water levels may also make it easier for the illness to spread, particularly in impoverished areas with poor sanitation.
The researchers applied the same statistical model "used to study seasonal trends for other infectious diseases, such as malaria and dengue fever, to retrospectively predict the cholera case-load in the region for 2003 to 2008," said the study.
"The predicted levels based on climate conditions closely matched actual cholera cases and outbreaks recorded in surveillance reports over the same time period."
The ability to predict cholera outbreaks could save lives and cut back on cases, said lead author Rita Reyburn, a research associate at IVI.
"We are getting very close to developing a reliable forecasting system that would monitor temperatures and rainfall patterns to trigger pre-emptive measures -- like mobilizing public health teams or emergency vaccination efforts -- to prepare for an outbreak before it arrives."
Cholera typically strikes in sub-Saharan Africa and parts of southeast Asia. An epidemic in Haiti has killed nearly 5,000 people and experts say 800,000 will be sickened by it this year.
The study called it "particularly troubling" that the doubling of cases coincided with such a small boost in average minimum temperature -- from 23 Celsius (73 Fahrenheit) to 24 Celsius (75 degrees Fahrenheit).
According to the Intergovernmental Panel on Climate Change's third assessment report, average temperatures could rise globally from between 1.4 degrees Celsius and 5.8 degrees Celsius by the end of the century.
"Based on the results of this analysis we would expect a very high cholera caseload in Asia and Africa if the temperatures hit the higher end of that range," said Mohammad Ali, a senior scientist at IVI.