Satellite Images May Help Predict The Next Cholera Outbreak
As cholera continues to ravage parts of Sub-Saharan Africa, South Asia and Latin America -- reportedly reaching Puerto Rico and Hong Kong this week -- public health researchers are looking to the skies in hopes of anticipating future outbreaks.
Satellite images of the oceans, researchers say, could soon forecast where and when cholera is most likely to strike.
Certain developing countries, such as Bangladesh and Mozambique, already know to expect the unwelcome visitor almost every year and typically have measures in place to minimize its impact. Cholera's recent arrival in Haiti and Pakistan, however, caught the nations by surprise. It had been a century since either faced an outbreak of the disease, which causes severe diarrhea and a 50 percent chance of death due to dehydration if not treated quickly.
According to Shafiqul Islam, an expert in environmental engineering and water diplomacy at Tufts University in Medford, Mass., this left the Haitian people vulnerable and their health officials unprepared. Over 6 percent of Haitians initially infected succumbed to cholera, compared to just 0.1 percent of victims in Bangladesh.
"That's amazing, and extremely troubling," said Islam, highlighting the similarly poor economic conditions in the two nations and the fact that cholera has such an easy and cheap cure: clean water with some sugar and salt.
If Haiti had been warned a couple of months in advance to prepare large quantities of this simple solution, along with other treatments and vaccines, it might have been a different story, Islam said.
Ever since John Snow first identified cholera in London 150 years ago, researchers have focused primarily on understanding the microbiology of the bacteria, Vibrio cholera, and how to help the human body combat it.
Despite substantial progress made on this front, the disease continues to be a global threat, affecting 3 to 5 million people annually and killing more than 100,000 of its victims, according to the World Health Organization. Experts don't expect it to go away any time soon.
But if science takes a step back to evaluate the timing and places that can set the stage for a cholera epidemic, suggested Islam, we might better coexist with the stubborn strains.
"If you can use this information to make a prediction, then you can mobilize the necessary resources," he said.
In a study published in the the May issue of Water Resources Research, Islam and his colleagues describe how large-scale environmental conditions can be conducive to the initiation, transmission and propagation of cholera.
The team looked at data from Bengal Delta in Bangladesh, identifying two annual peaks for cholera cases: one in the spring and one in the fall.
The first peak appeared to be triggered by a "low flow," in which long-term drought conditions resulted in a mix of salt and fresh water off the Bangladesh coast. Cholera thrives in such brackish conditions, where it hitches rides on tiny marine mammals called zooplankton. These hosts can multiply rapidly over a period of a couple of months -- especially when stimulated by an algal bloom -- and eventually introduce cholera to coastal cities via seafood or drinking water.
A few months later, just as an affected region is sighing a breath of relief for a waning outbreak, the heavy rains and flooding of monsoon season can revive and spread cholera bacteria inland. This second peak is most common in regions with poor water and sewer systems. (A total of 44 cases of cholera have been reported in the U.S. over the last five years, but good water infrastructure continues to keep the disease in check.)
Although the timing and number of peaks can vary between regions, the components that lead to a cholera outbreak can likely be generalized beyond Bangladesh, suggested Islam.
NASA satellites could identify the chlorophyll abundant in phytoplankton within the Earth's oceans, he explained. Since zooplankton feed on phytoplankton and also carry the toxic bacteria, satellites could be used to develop prediction models that forecast cholera outbreaks two to three months in advance.
"If you want to make predictions, three days or even three weeks in advance is not enough," said Islam. "You need at least two to three months in order to warn the public and allow professionals enough time to get ready."
Satellite monitoring could be even more crucial in the years ahead as current climate models point to both increased drought and severe flooding. "If these models are correct," said Islam, "then cholera will get more intense."
A separate study published in the June issue of the American Journal of Tropical Medicine and Hygiene linked a 1 degree Celsius (1.8 degrees Fahrenheit) rise in the average monthly minimum temperature to a doubling in the number of cholera cases within four months in Zanzibar, Tanzania.
A substantial increase in cases was also seen two months after a 200-millimeter (7.9-inch) rise in monthly rainfall.
Downpours may not only affect the spread of the disease, but could also help initiate its growth. "In general, warmer sea surface temperatures and a warmer atmosphere lead to increasingly frequent and heavy rain," said Dr. Paul Epstein, associate director of the Center for Health and Global Environment at Harvard Medical School. "These intense rains can flush nutrients, organisms and chemicals into coastal marine habitat and trigger an algal bloom."
Non-environmental factors may play a role, too. A catastrophic earthquake hit Haiti in January 2010, damaging already poor sanitary systems. That, along with the possible introduction of the bacteria by United Nations peacekeepers, are hypothesized to have played a significant role in the country's outbreak, which first manifested in October.
But Islam doesn't think those factors tell the whole story. A very large earthquake struck Pakistan in 2005, and cholera was not one of the consequences. Meanwhile, when floods struck the same country this year, a massive cholera outbreak did result.
"The right environmental conditions must be present for the disease to spread," explained Islam.
Once a cholera outbreak is predicted for a region, he added, "a multi-pronged approach" should be initiated.
It's not feasible to vaccinate everyone given the $10-to-$15 price tag and limited production. What's more, a dose is only effective for one or two seasons. Antibiotics can help in the fight, noted Epstein, but he highlighted the greater importance of stocking up on clean water, salt and sugar.
Other preparations include protecting and treating the water supply -- often a more long-term solution requiring improvements in the division between water and sewer infrastructure.
Of course, it takes time for any measures to be mobilized. And once the first cases appear, it's often too late. Fortunately, cholera is particularly "amenable to early warning," noted Epstein.
Over the next several years, Islam expects that potential to be realized with the widespread use of satellite-based prediction.
"We hope this will change the game," he said.