Climate Change and the Spread of Infectious Diseases

When most people think of global climate change, they imagine melting ice caps, dying polar bears, hotter summers, and more erratic weather patterns.

Here's another one to add to your list: A higher risk of certain infectious diseases.

Case in point: West Nile virus, which is spread to humans by mosquitoes that have bitten infected birds. First introduced into the United States in 1999, it can lead to high fevers, meningitis, encephalitis, permanent disability and even death.

In a recent study published in the journal PLoS Pathogens, a publication of the Public Library of Science, a team of public health researchers and ecologists showed that warming temperatures helped enable a new strain of West Nile virus to displace the original strain.

The new strain of West Nile virus replicates faster inside mosquitoes than the older strain, increasing the likelihood that mosquitoes will transmit the disease to humans. Disturbingly, the newer strain can also reproduce more easily at even higher temperatures. These two characteristics suggest it could lead to an increase in the number of cases of West Nile, especially in the northern parts of the United States.

Although West Nile has been in this country for less than a decade, it has already killed more than 1,000 people, including a large outbreak from 2002 to 2003. During those two years, there were almost 14,000 reported cases of West Nile in the United States, including 548 deaths , with a concentration of cases in the Gulf Coast states like Louisiana and Mississippi, as well as Ohio, Michigan, Indiana and Illinois, North and South Dakota, Nebraska, Colorado and Wyoming.

Experts are concerned that West Nile is just one of many infectious diseases that could increase or spread its range in a warmer world.

Why climate matters

The World Health Organization, the International Panel on Climate Change, and the Environmental Protection Agency have all issued reports on how climate change could impact human health. All specifically warn that climate change could impact infectious diseases.

There are several ways this could happen. One of the main concerns centers on the so-called "vector-borne" diseases—that is, diseases like West Nile virus and malaria that are spread not directly by humans, but rather are transmitted to humans by other species, such as mosquitoes, ticks and rats.

Changing weather patterns due to climate change will provide an opportunity for the insects and animals that transmit infectious diseases to change their geographic range, says Kristie Ebi, the lead author for the chapters on human in recent reports by the U.S. Climate Change Science Program and the International Panel on Climate Change. As these creatures change their locations, they may introduce the diseases into areas where they aren't currently found.

Climate change can also amplify the prevalence of a disease in places where it already exists. Warmer weather and longer, frost-free seasons can expand both the sheer number of individual disease carriers, as well as the time period during which people are vulnerable to bites.

In addition, warmer weather may help the pathogens within infected carriers multiply more quickly, says Ebi.

"Climate change also brings new challenges to the control of infectious diseases," the World Health Organization stated in a recent report, adding ominously: "Many of the major killers are highly climate sensitive [to] temperature and rainfall, including cholera and the diarrheal diseases, as well as [vector-borne] diseases [such as] malaria [and] dengue. In sum, climate change threatens to slow, halt or reverse the progress that the global public health community is now making against many of these diseases."