How Climate Change Is Exacerbating the Spread of Disease

by Renee Cho | September 4, 2014

The accelerating Ebola epidemic in West Africa, which the World Health Organization (WHO) has called “unprecedented,” has so far killed more than half the 3,069 people who contracted the disease in Liberia, Sierra Leone, Guinea and Nigeria. The Centers for Disease Control fears that a worst case scenario could mean 1.4 million cases by January.

Ebola can live in animals for years without making them sick; it is transmitted to humans through contact with an infected animal. Once in a human, the disease is spread by direct contact with the bodily fluids of the infected person, and as yet there is no vaccine.

Some scientists think that climate change, with its increase in sudden and extreme weather events, plays a role in Ebola outbreaks: dry seasons followed by heavy rainfalls that produce an abundance of fruit have coincided with outbreaks. When fruit is plentiful, bats (the suspected carriers of the recent Ebola outbreak) and apes may gather together to eat, providing opportunities for the disease to jump between species. Humans can contract the disease by eating or handling an infected animal.

According to Kris Murray, senior research scientist at EcoHealth Alliance, an organization that researches and educates about the relationships between wildlife, ecosystems and human health, climate change has strong potential to play a role in increasing the risk for Ebola.

“With climate change expected to put increasing pressure on food security in Africa, food shortages will push more people to alternative food sources and consumption of bushmeat, like bats, will likely increase,” said Murray. Almost 50 percent of Ebola outbreaks have been directly linked to bushmeat consumption and handling (the origin of the current outbreak, however, has still not been determined).

Murray added, “Also, some of our computer modeling suggests that with climate change, in parts of central and western Africa, the range of some bat species could expand…this means increased contact between bats and humans.”

According to the United States Agency for International Development, “nearly 75 percent of all new, emerging, or re-emerging diseases affecting humans at the beginning of the 21st century are zoonotic” — meaning they originate in animals. These include AIDS, SARS, H5N1 avian flu and the H1N1 flu. More and more wild animals, which may have carried diseases without effect for years, are coming into contact with humans, often because of deforestation.

Sierra Leone, where Ebola is currently raging, lost 96 percent of its forest by the 1920s and may lose the rest by 2018, according to the U.N. Guinea, where the 2014 Ebola outbreak began, has lost 20 percent of its forests since 1990. The human activity that drives deforestation—logging, mining, slash and burn agriculture, demand for firewood and road building—means more and more people are entering the forest, and thus forcing animals like bats to find new habitats closer to human civilization.

The EcoHealth Alliance works globally to save endangered species and their habitats and to protect ecosystems. “A really important message that we advocate is that protecting the environment will help protect ourselves from a range of natural disasters, including disease,” said Murray.

The WHO has warned that contagious diseases are on the increase as a result of “the combined impacts of rapid demographic, environmental, social, technological and other changes in our ways-of-living. Climate change will also affect infectious disease occurrence.” A number of diseases well known to be climate-sensitive, such as malaria, dengue fever, West Nile virus, cholera and Lyme disease, are expected to worsen as climate change results in higher temperatures and more extreme weather events.

Malaria killed 627,000 in 2012 alone. According to the Intergovernmental Panel on Climate Change (IPCC), climate change will be associated with longer transmission seasons for malaria in some regions of Africa and an extension of the disease’s geographic range. As temperatures warm, the Plasmodium parasite in the mosquito that causes malaria reproduces faster and the vector (the organism that transmits a disease), i.e. the mosquito, takes blood meals more often. Rain and humidity also provide favorable conditions for young mosquitoes to develop and adult mosquitoes to survive.

Dengue fever infects about 400 million people each year, and is one of the primary causes of illness and death in the tropics and subtropics. The IPCC projects that the rise in temperatures along with projected increases in population could put 5 to 6 billion people at risk of contracting dengue fever in the 2080s. The reproductive, survival and biting rates of the Aedes aegypti mosquitoes that carry dengue and yellow fever are strongly influenced by temperature, precipitation and humidity. In the summer of 2013, Aedes aegypti, usually found in Texas or the southeastern U.S., suddenly appeared in California as far north as San Francisco, though fortunately, none of the mosquitoes tested carried dengue or yellow fever.

The hantavirus broke out in the southwest U.S. in 1993 after a six-year drought ended with heavy snows and rainfall. The precipitation allowed plants and animals to grow prolifically, which resulted in an explosion of the deer mice population. The mice may have carried hantavirus for years, but suddenly many more mice were coming into contact with humans.

People became infected through contact with infected mice or their droppings. Hantavirus Pulmonary Syndrome has now been reported in 34 states. Through 2013, 637 cases were reported in the U.S., and approximately 230 people have died.

In 1999, the West Nile virus, transmitted to humans by mosquitoes, made its first appearance in the Western Hemisphere in New York, after a drought followed by heavy rains. Since then, over 1,600 people have died of the disease. This month, the number of reported cases of West Nile virus—1,993 including 87 deaths— is the highest year-to-date total since it arrived in the U.S., with Texas being hardest hit. A recent study suggests that in the future, higher temperatures and lower precipitation will lead to a higher probability of West Nile cases in humans, birds and mosquitoes.

Extreme weather events can produce a cascade of other effects that influence disease. Heat and droughts create dry conditions, providing fuel for forest fires that end up fragmenting forests and driving wildlife closer to humans. Droughts and floods affect crop yield, sometimes resulting in malnutrition, which makes people more vulnerable to disease while forcing them to find other food sources. Flooding can provide breeding grounds for insects and cause water contamination, leading to the spread of diarrheal diseases like cholera. Moreover, extreme weather can disrupt the finely tuned relationships between predators and prey, and competitors that keep pathogen-carrying pests like mice and mosquitoes in check.

Climate-sensitive diseases are also affected by the shorter-range climate impacts arising from El Niño, which occur when unusually warm sea surface temperatures develop off the Pacific coast of South America. Historically, El Niño events have resulted in drier and hotter than normal conditions in some areas and wetter and cooler than normal conditions in others, effects felt mostly in the tropics. An El Niño is predicted to occur this fall and to last until February 2015, likely bringing with it more rainfall and higher temperatures.

The Earth Institute’s International Research Institute for Climate and Society (IRI) develops tools to monitor and predict epidemics, and uses high-level climate information to help societies, particularly in developing countries, understand and manage the impacts of both short and long-term climate. Madeleine Thomson, director of the IRI/PAHO-WHO Collaborating Centre for Early Warning Systems for Malaria and Other Climate Sensitive Diseases, oversees the public health activities at IRI.

One climate contribution to this work is Enhancing National Climate Services (ENACTS), an initiative developed by IRI and its partners. It is making high-quality climate data more available to decision makers in Africa, including those who are dealing with malaria. ENACTS reviews historical climate data to understand natural climate variability over time and monitors current climate, so that decision makers get an “early warning” about when climate effects could trigger malaria outbreaks and when malaria interventions should be implemented.

With a potential El Niño developing, there is an increased likelihood of higher than normal rainfall in East Africa in the next rainy season and malaria control agencies are already getting prepared. Some have shifted stocks of diagnostic tools and medicines out of reserves and made them more readily available, explained Thomson. “Once they get the early warning they can reach out to their donors and the government for more money, they can communicate with the community to use bed nets, to attend clinics if they are sick, and to get their children drugs on time. If there are concerns about an impending epidemic, they can also use insecticide spraying.”

When discussing the likely impact of climate change, Thomson explained that the time frame for decision makers who have to determine budgets and make plans is relatively short, maybe four or five years. Even for global policy, the time frame is at maximum maybe 10 to 20 years. “These are not climate change time frames,” said Thomson. “In terms of decision-making, they have to deal with short-term climate variability—especially for rainfall. The shift to warmer temperatures in highland areas, however, is linked to the larger climate system. It will impact vector-borne diseases, because when it’s warmer, the disease transmission system speeds up.”

Scientists recognize that climate is only one factor in the spread of disease. “Underdeveloped countries are so vulnerable to infectious disease because they are not able to protect themselves with good housing, good infrastructure, education, a sound health system, access to vaccines, and surveillance systems that track cases of disease,” said Thomson. “These vulnerabilities are the first problem—climate impacts on top of them just compound the problems.” In the U.S., we are better protected against the spread of disease by our high standard of living and strong surveillance systems.

The good news, said Thomson, is that there is a shift in awareness at the global level about the need to understand how climate change is affecting disease, and it is filtering down to local levels. The WHO’s first international conference on health and climate, which took place in Geneva, Switzerland, at the end of August, is evidence of this growing awareness.