-As disease surges in Haiti and Africa—and climate change threatens new wave of epidemics—new early warning system would buy time for interventions
Seoul, KOREA - With recent deadly cholera outbreaks in Haiti and Cameroon providing the latest indication of a menacingly resurgent disease, scientists have discovered rain and temperature fluctuations in at-risk areas could predict epidemics months in advance.
After analyzing several years of disease and environmental data from cholera-endemic areas of Zanzibar, Tanzania, scientists from the International Vaccine Institute (IVI) in Seoul, Korea found that a mere one degree Celsius increase in the average monthly minimum temperature was a warning sign that cholera cases were likely to double within four months. Similarly, a 200-millimeter increase in monthly rainfall totals indicated a slightly lower but still substantial increase could be expected within two months. The study is published on June 1 in the American Journal of Tropical Medicine and Hygiene.
The authors note their work eventually could allow public health authorities in areas where cholera is common to anticipate outbreaks and move to intervene, given that such measures as vaccines are far less effective once an epidemic is in full swing. Moreover, cholera outbreaks are on the rise while in the coming decades climate experts predict hotter and wetter weather in many cholera-endemic areas that could add fuel to the fire.
The ability to predict an epidemic could be vital to fighting a disease that, according to the World Health Organization (WHO), kills 100,000 to 130,000 people every year and sickens millions more. Cholera outbreaks are increasing, with 24 percent more cases reported worldwide between 2004 and 2008 compared to the previous four years, a trend that is expected to accelerate in the coming decades of global warming.
“Predicting outbreaks is crucial because right now public health officials only know for sure that an outbreak is underway when people start getting sick, which is too late for things like vaccines to have maximum effectiveness,” said Dr. Mohammad Ali, a senior scientist at IVI. “If we wait for clinical signs of the disease to emerge, that means a large portion of the population is already carrying the cholera bacteria and they just are not yet exhibiting clinical symptoms.”
In Cameroon, an outbreak of cholera now underway in the capital city of Yaounde is being blamed on the unprecedented arrival of heavy rains in February, months before the rainy season usually commences—a phenomenon some speculate is illustrative of climate change. In Haiti, an outbreak aggravated by unsanitary conditions caused by last year’s earthquake has killed 4,600 people. Now, the advent of the regular rainy season in Haiti has sparked a new wave of cholera illnesses some experts believe could kill twice that amount or more by the end of the year.
Cholera is endemic to impoverished, tropical areas—mainly in sub-Saharan Africa and South and Southeast Asia—where poor sanitation and lack of clean water facilitate the spread of the disease, which is mainly through fecal contamination of food and water. Cholera is particularly feared among infectious diseases for its ability to cause such a sudden and intense onset diarrhea that a victim can go from seemingly healthy to dead from dehydration in only 24 hours. Also, when outbreaks occur, the number of people infected increases dramatically and the case fatality rate can skyrocket from less than 5 percent to almost 50 percent.
Public health interventions can include mass vaccination (which can have a “herd immunity” effect that offers protection against cholera to neighbors of those who receive the shot), widespread distribution of antibiotics, and preemptive public health campaigns focused on simple but highly effective measures like frequent hand washing and better access to clean water.
In their efforts to develop a cholera prediction tool, the IVI scientists—along with a cholera expert from the University of North Carolina at Chapel Hill in the U.S.—looked at routine monthly cholera disease surveillance reports from several years in Zanzibar that allowed them to see when the disease was at a relatively normal level and when it spiked to epidemic proportions. They then looked at a wide variety of monthly environmental data for the same period, including rainfall totals, high and low temperatures, humidity, and sea surface temperatures. When they compared the data, they found that cholera outbreaks were most closely associated with a rise in minimum average temperatures and average rainfall levels.
They employed a statistical model that has been 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. The predicted levels based on climate conditions matched very closely to the actual cholera cases and outbreaks recorded in the surveillance reports over the same time period.
The researchers note that their study represents an advance in developing a forecasting system for cholera outbreaks because there are many environmental factors known to contribute to cholera infections, but it has been difficult to single out which ones are the most important to monitor.
Also, the fact that cholera cases doubled following only a small increase in the average minimum temperature could be a particularly troubling sign. In the study, the doubling occurred when the average minimum temperature rose from 23º C/73ºF to 24º/75º. The study notes that climatologists predict that the build-up of greenhouse gases in the atmosphere could cause average temperatures globally to increase from 1.4ºC to 5.8º C over the next 100 years.
“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,” Dr. Ali said.
About the IVI
The International Vaccine Institute (IVI) is the world’s only international organization devoted exclusively to developing and introducing new and improved vaccines for the world’s poorest people, especially children in developing countries. Established as an initiative of the United Nations Development Program in 1997, the IVI operates under a treaty signed by 40 countries and the World Health Organization. The IVI conducts research in more than 30 countries in Asia, Africa and Latin America on vaccines against diarrheal infections, bacterial meningitis and pneumonia, Japanese encephalitis, and dengue fever. For further information, please visit: www.ivi.int