Watching the climate-cholera connection from orbit
Dr. Kiki Sanford on how scientists are predicting outbreaks from an unusual new vantage point.
Researchers reported at the American Geophysical Union annual meeting in San Francisco, CA on a satellite-based cholera outbreak prediction model in which specific environmental factors are correlated with epidemic cholera outbreaks. That's right, scientists are predicting disease from space.
Cholera is a devastating diarrheal disease indicative of inadequate sanitation and health infrastructure. It is caused by a bacterium called Vibrio cholerae that exists naturally in the environment. That means we can't get rid of it. We are just unlucky that it affects us the way that it does, so we have to learn how to deal with it, which basically involves cleaning up water supply and infrastructure.
Cholera infects some 3-5 million people a year globally, but is really only a concern in developing countries where water sanitation is not sufficient to remove or kill the bacteria. The dehydrating effects of cholera lead to death in 20% of those infected, and are the result of toxins released by the organism once it begins to colonize the small intestine.The disease doesn't really spread person-to-person, but outbreaks result from a complex combination of hydrological, climate, and societal factors.
"Cholera" by Unknown - Le Petit Journal, Bibliothèque nationale de France. Licensed under Public Domain via Wikimedia Commons.
The organism can cause disease on an endemic and epidemic basis. In some regions of the world, usually coastal areas, it exists chronically in the population and environment with disease outbreaks occurring on a fairly regular basis. For example, in Bangladesh, the disease experiences a resurgence twice a year; once in the spring as a result of cholera-containing coastal waters intruding into the river system during the dry season when flows are low, and again in the fall after the monsoons when flooding causes contamination of water resources.
Epidemics, however, are much less predictable, occurring in a sudden, sporadic manner, and very often inland. Above average temperatures with above average precipitation combined with poor sanitation is the necessary pattern for epidemic outbreaks to get a start. In addition to the environmental parameters, any societal condition that devastates the water and community structure puts society at risk, but these are currently much more difficult to monitor.
The Indian sub-continent is where cholera is thought to have originated. But, it has been re-emerging in Africa over the past 10-15 years, especially in coastal Africa. So, researchers are using over 40 years of data to get an idea of its potential development in various African regions.
In India, 30 years of data show an increasing trend in both incidence and prevalence of the disease, which means that fatalities are down even as the number infected goes up. Prevalence doubled over the 30 year time period, and within just the past 10 years increased even faster. This is especially interesting as the past 10 years have been among the hottest on record. The models produced by the researchers based on the historical data and predicted hydroclimatic conditions suggest that by the 2050’s the entirety of central Africa will be at a substantially increased risk of epidemic cholera outbreaks. They didn't make any predictions for the Americas.
Rita Colwell, the lead researcher on this work, said over the phone that there is further environmental warming expected with projected climate change, and mentioned that in lab experiments with plankton, one of cholera's host species, increasing water temperatures leads to the production of larger numbers of infectious bacteria. Other experiments in the Gulf of Alaska have also revealed evidence that cholera concentration in water increases with increasing temperatures.
It's not a stretch to say that predictions of increased ocean warming and storms in areas of poor sanitation world-wide will lead to more frequent and intensive epidemics due to higher cholera numbers existing in the water. Yet, we have a chance of preventing these from terrible occurrences with predictive models like those described today. With enough lead time, health organizations will be able to distribute vaccines and oral rehydration treatments to areas before devastation takes place.
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