By Joel N. Shurkin, ISNS Contributor
(ISNS)—Scientists working with data from the cholera-plagued Tanzanian archipelago of Zanzibar have found that even a slight variation in temperature or rainfall could herald an epidemic.
That’s both good news and bad news.
The good news is that by monitoring the weather and changes in the climate, epidemiologists may be able to predict the arrival of a disease epidemic up to four months in advance, early enough to make maximum use of vaccines.
The bad news is that with global warming, it is likely cholera epidemics will increase in frequency, and with the climate and rain patterns changing, epidemics could be even more frequent. It’s happening already—recent epidemics in Haiti and Cameroon indicate a resurgent disease.
Cholera is a particularly ghastly disorder caused by the Vibrio cholerae bacteria and is transmitted through drinking water contaminated by human feces. Its main symptom is violent, severe diarrhea, followed by dehydration. In some recorded epidemics, the death rate for infected individuals can be as high as 50 percent. For some patients, the time between feeling healthy and death can be as little as 24 hours.
Cholera originated in the Indian subcontinent and is a disease usually found in developing areas of the world. It arrived in Europe in the early 19th century, reaching pandemic proportions several times.
The relationship between weather, seasons and cholera is long-established. Sea surface height, sea surface temperature and the concentration of chlorophyll in the ocean have already been shown to be predictive in earlier studies in India and Bangladesh.
Researchers from the International Vaccine Institute in Seoul, South Korea, went back to disease and environmental records in Zanzibar between 2003-08. Reporting in the June issue of the American Journal of Tropical Medicine and Hygiene they studied rainfall totals in Zanzibar, high and low temperatures, humidity, and sea surface temperatures. Similar techniques have been successfully used to plot malaria and dengue fever.
They found that a one degree Celsius increase in the average monthly minimum temperature was a sign that the number of cholera cases would double within four months. Further, a 7.8 inch increase in monthly rainfall totals was predictive of a substantial increase in cases within two months.
Giving vaccines to a population that may already have been infected is less effective than vaccinating them before infection, so being able to get ready for an outbreak would save lives.
An epidemic now underway in Yaounde, the capital city of Cameroon, has been blamed on unusual heavy rains coming well before their normal time. The researchers think that is a good example of environmentally driven disease.
The 2010 earthquake in Haiti also triggered cholera. More than 300,000 people have been sickened and 5,000 died. The rainy season is about to start, and the researchers fear an explosion of the disease.
While the study from Zanzibar is useful it would be more useful if it could be extrapolated to other areas, and so far it cannot, said David Sack, professor of international health at the Johns Hopkins Bloomberg School of Public Health in Baltimore.
Sack studies cholera in Bangladesh. He said that the disease comes in March in the south of that country, October and November in the north, and year round in Dacca, the capital city—all this in a country the size of Florida.
So what is true in Zanzibar may not be true in Nigeria.
But the seasonality of the disease is well-established and more extreme summers may cause more grief.
"Climatologists predict a 1.4-5.8 C (2.5-10.435-42 F) degree rise in mean temperature over the next 100 years," the vaccine researchers wrote. "Increased sea temperatures and levels associated with global warming intuitively suggest the possibility of increased cholera incidence in many resource-poor regions of the world."