A new study into the linkages between rain, temperature and cholera shows scientists may be able to predict epidemics in time to save people from the life-threatening disease.
After analysing several years of disease and weather data from cholera-endemic areas of Zanzibar, Tanzania, scientists from the International Vaccine Institute (IVI) in Seoul, Korea, found that if a more than one degree Celsius increase in the average monthly minimum temperature and a 200mm increase in monthly rainfall were recorded in a month, a cholera outbreak was imminent in the following month.
“A mere one degree Celsius increase in the average monthly minimum temperatures was a warning sign that cholera cases were likely to double within four months,” said Mohammad Ali, a senior scientist at IVI, and one of the authors of the study published in the June 2011 issue of the American Journal of Tropical Medicine and Hygiene.
Rita Reyburn, a research associate at IVI and the study’s lead author, said in a statement: “Our work validates the notion that rainfall and temperature increases are often precursors to cholera outbreaks in vulnerable areas.”
Climate experts have predicted that hotter and wetter climate in many cholera-endemic areas could see higher caseloads.
“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,” she added.
Every year mostly African and Asian countries record 3-5 million cholera cases and 100, 000-130,000 deaths because of the disease, according to the World Health Organization (WHO).
The IVI study supports a growing number of findings that establish a link between cholera incidence and climatic factors, though research does not discount the significance of factors such as poverty and access to clean drinking, Ali said.
Peter J. Hotez, president of the American Society of Tropical Medicine and Hygiene, pointed out, however: “Cholera outbreaks are occurring with increasing frequency and severity. We are seeing month-long outbreaks now.”
The Zanzibar study was an “innovative approach that if used in conjunction with other preventive measures could significantly reduce the needless suffering and deaths of thousands of people,” he said.
Deadly outbreaks in Cameroon, and in Haiti after the 2010 earthquake, have claimed thousands of lives. “We could have saved these lives if we had vaccinated vulnerable people,” said Hotez.
An epidemic in Zimbabwe, which began in August 2008, lasted almost a year and spread throughout the country as well as to neighbouring Zambia and South Africa, said WHO in its position paper on vaccines in 2010. At the end of July 2009, more than 98,000 cases and 4,000 deaths had been reported in the region, it said. The number of cases reported could drop because of poor surveillance, WHO warns.
The mounting evidence of links between higher temperatures and cholera incidence should add a sense of urgency to efforts to make cheap cholera vaccines available to poor communities in cholera-endemic countries, he said.
Hotez pointed out that there are only two oral vaccines available in the world: Dukoral manufactured in Sweden, which costs US$60-80 (for the required two doses) and the much cheaper and very new alternative Shanchol or mORCVAX, manufactured in India, which costs around $2.
An injectable vaccine is manufactured in some countries, but is not recommended by WHO because of its limited efficacy.
Shanchol was developed in collaboration with IVI, with funding from the Bill & Melinda Gates Foundation by modifying a vaccine used in Vietnam, said Ali. After trials in Vietnam and India, the vaccine was approved in India in 2009. “We are awaiting approval from WHO to allow its purchase by UN agencies and internationally.”
Hotez said the world needed to enhance production of the vaccines to maintain a global stockpile as cholera cases mount.
Cholera is endemic in poor, tropical areas mostly in sub-Saharan Africa and South and Southeast Asia – where poor sanitation and lack of clean water help the spread of the disease, mainly through faecal contamination of food and water.
Cholera is particularly feared for its ability to cause such a sudden and intense onset of diarrhoea that a victim can go from seemingly healthy to death in 24 hours. Also, when outbreaks occur, the number of people infected increases dramatically and the case fatality rate can skyrocket; rates of up to 50 percent are being reported in complex emergencies with limited resources, according to the researchers.
In its last assessment, the Intergovernmental Panel on Climate Change (IPCC), an authoritative global scientific body, cited research in Bangladesh, led by US scientist Rita Colwell in the late 1990s, that established the link between the cholera bacterium, sea surface temperature and phytoplankton (microscopic plant-like organisms that live in the ocean).
Warmer surface temperatures increase the abundance of phytoplankton, which support a large population of zooplankton (animal-like micro-organisms), which serves as a reservoir for cholera bacteria, a waterborne disease.
Colwell and her colleagues also traced the source of the cholera bacterium to the plankton in rivers and estuaries.
”Our study also followed Dr Colwell’s work, but we were unable to pick up the phytoplankton off the Zanzibar coast, but we were able to establish the link between higher temperatures and rainfall,” said Ali.
Researchers in Africa, led by Miguel Ángel Luque Fernández from the Institute of Health Carlos III, based in Madrid, were the first to show a link between higher temperature and rainfall and the incidence of cholera over a three-year period from 2003 in Zambia, in a study published in the Transactions of the Royal Society of Tropical Medicine and Hygiene, in the UK.