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Will climate change worsen Ebola outbreaks?

Percent change in average precipitation as forecast by the RCP 8.5 climate change scenario, in which greenhouse gas emissions continue to rise through the 21st century. (IPCC)

The Ebola outbreak is “out of control” in parts of West Africa, says the head of the Centers for Disease Control and Prevention.  Could climate change hasten the spread of the deadly virus?  Perhaps, but the linkages are complicated, according to limited scientific literature on the topic.

Ebola is a disease caused by the Ebola virus. Scientists don’t currently know how the disease originates, nor how to cure it. They know the virus is transmitted via contact with an infected animal. It appears that in most cases, Ebola outbreaks begin when humans eat infected bush meat — wild animals like bats and monkeys — which is cooked and used for sustenance in regions where food is scarce.

Scientific and health organizations have long held that climate change poses risks to human health, that warming may expand the range of certain water- and air-borne diseases. For example, given that it thrives in warm water, Cholera has been found to correlate with rising sea surface temperature.  And while adaptive measures like improved medical care and vaccines could control their spread, Malaria, Dengue fever, and Lyme disease are all expected to expand in geographical area as global temperature increases, according to the Intergovernmental Panel on Climate Change.

However, the connection between climate and Ebola is more complex since it’s related to changes in precipitation characteristics, rather than directly linked to temperature.

A 2002 study published in the journal of Photogrammetric Engineering and Remote Sensing found that sudden shifts from dry to wet conditions were associated with Ebola outbreaks from 1994 to 1996 in tropical Africa.

As the globe warms, precipitation extremes are projected to increase.  Periods of drought are expected to become more frequent in some areas while heavy rain events, when the occur, are forecast to become more intense.  Presumably, those areas which see precipitation variability increases – with abrupt shifts from extremely dry to extremely wet periods –  would be most vulnerable to Ebola outbreaks.

But projecting future changes in precipitation on the regional scale is difficult.  Over Africa, the forecast is complex. Rainfall is projected to decrease over northern Africa, but remains uncertain over much of the rest of the continent.

Given the mixed-bag forecast, it remains unclear whether we will see an increase or decrease in Ebola outbreaks from precipitation patterns alone. But given that periods of drought are expected to increase, and more water vapor is available for rainfall in a warmer atmosphere, one can imagine that the climate change-Ebola link is non-zero.

Further, the interplay between climate change and deforestation potentially positions humans closer to infected animals. As deforestation stuns the habitat, people are often left to hunt “survivor species,” like bats, which are one of the most common natural reservoir of the Ebola virus. Yale’s Environment 360 writes:

The transmission of the filovirus Ebola from bats to humans illustrates the complexity of the spread of these diseases and its relation to climate change and land disturbance. Simple bat-human contact isn’t sufficient for the filovirus Ebola to erupt.
“It’s a cascade of events” that bring bats, apes, and humans together in unusual ways, aggravated in part by “unique climatic conditions,” says World Health Organization zoonoses expert Pierre Formenty. According to a NASA analysis of meteorologic satellite data, Ebola outbreaks correlate with heavy rains at the end of a period of intense aridity. Extremely dry conditions force some fruit trees to defer fruiting. When the rains come and the stricken trees put out fruit, all manner of fruit-starved species, including Pteropus bats and apes, gather to feast. Large numbers of creatures concentrated under newly fruit-heavy trees provide microbes such as Ebola a prime opportunity to jump from one species to another. And once Ebola starts circulating heavily in a new species such as apes or bats, it can readily be transmitted through infected blood and other fluids to humans.
Whether the warming planet will aggravate the unusual rainfall patterns that set the scene for Ebola outbreaks remains unclear. While experts in conservation medicine have made great strides delineating the links between climate and disease, projections about how disease patterns will shift as the climate changes are still nascent. Worryingly, however, a trend toward aridity has already been noted across African rainforests.

Kris Ebi, an expert on climate change and health at the University of Washington, is concerned that climate change stresses on agriculture may force more people to eat bats and other animals that carry the disease.

“We already know climate change is weakening crop yields,” Ebi told Al-Jazeera America. “When there’s high food insecurity, how will people go about making sure that they have enough food for their families?”

In its latest assessment report, the IPCC emphasized that adaptive measures can reduce human vulnerability to changes in disease incidence caused by climate change:

The most effective vulnerability reduction measures for health in the near term are programs that implement and improve basic public health measures such as provision of clean water and sanitation, secure essential health care including vaccination and child health services, increase capacity for disaster preparedness and response, and alleviate poverty.

While Ebola is a serious challenge the international health community and Africa must confront, U.S. health officials emphasize the disease is not a risk in the United States.

From Lenny Bernstein’s To Your Health blog:

Transmission of Ebola requires direct contact with an infected person’s blood, vomit or feces during the period that he or she is contagious, something that is extremely unlikely for anyone but health-care workers. The virus is not spread by coughing or sneezing. Nor do Americans bury their own dead family members or friends, as some residents of Sierra Leone, Liberia and Guinea must do with Ebola victims.
“There is zero danger to the U.S. public from these [two] cases or the Ebola outbreak in general,” said Amesh Adalja, a member of the public health committee of the Infectious Disease Society of America and an infectious disease doctor at the University of Pittsburgh.