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Add an increase in infectious diseases to your list of worries related to climate change. Vector-borne diseases—fungus-, virus- and bacteria-based afflictions spread to humans by insects, rodents and other organisms, collectively known as vectors—are found in distinct regions of the world based on the habitat of the offending organism. Yet a change in global temperatures could result in their movement to new, previously uninhabitable areas. In addition, illnesses caused by pathogens that spread through improper sanitation or contaminated food and water could increase with drought or flooding conditions, exacerbated by climate change.

Malaria
Malaria is caused by the microorganism, Plasmodium, which is transmitted to humans through the bite of the Anopheles mosquito. The incidence of malaria is limited to the tropical habitat of this mosquito where higher temperatures allow for survival of both the mosquito and the Plasmodium parasite.

With 350-500 million malaria cases each year, outbreak depends on temperature, rainfall and humidity—all of which can be impacted by climate change. According to statements by the Centers for Disease Control (CDC), the Intergovernmental Panel on Climate Change (IPCC) and the World Health Organization (WHO), the area of malaria transmission is expected to increase as global temperatures rise, resulting in several hundred million additional cases each year.

West Nile Virus
West Nile virus originated in the West Nile District of Uganda in the 1930s, and reached the United States in 1999. Carried by birds, this virus is transmitted to humans by mosquitoes.

Hot, dry summers could lead to increased outbreaks, but it has been difficult to predict exactly how weather will affect the spread of the virus. A June 27, 2008, article in the journal, PLoS Pathogens, discusses a new, possibly more virulent strain of the virus detected in the US in 2001, It is transmitted much more efficiently than the 1999 virus, and enhanced with increased temperatures.

Dengue and Dengue Hemorrhagic Fever
Dengue is a viral disease spread by mosquitoes found mainly in tropical locations with an estimated 100 million cases a year. It is a relatively mild disease, but can result in the more severe form, dengue hemorrhagic fever. An increase in global temperatures could expand the range of the dengue-carrying mosquitoes, resulting in the spread of the illness to new areas.

Hantavirus
Several outbreaks of Hantavirus have occurred in the southwestern United States. The virus is carried by mice and other small rodents, and transmitted to humans through the vermin’s urine or droppings. According to the CDC, there is a possible link between El Niño weather patterns and increased outbreaks of Hantavirus in the US. The increase in rainfall due to El Niño leads to an increase in the number of rodents that spread the virus. And climate change can directly affect El Niño weather patterns.

Tick-Borne Illnesses
Ticks harbor a number of microorganisms that can cause disease; the most well-known being Lyme disease and Rocky Mountain Spotted Fever, both caused by bacteria. As seen with mosquitoes, higher global temperatures could expand the range of the disease-carrying ticks, as well as increase their reproduction rates—and their likelihood to bite humans.

Cholera
The bacteria that cause cholera are spread through contaminated food and water. Outbreaks are often seen in areas with poor sanitation, water shortages or flooding following a natural disaster. As climate change impacts water supplies and the incidence and severity of storms, there will likely be a rise in the number of cholera cases. While easily treated by rehydration, this basic remedy may not be available following a natural disaster or in poorer areas.

Asthma
While seemingly not as alarming as infectious diseases, it is possible that cases of pollen-induced asthma could increase. As the planet gets warmer, more plants will grow, have longer growing seasons, and produce more pollen. While just an inconvenience for many, asthma attacks can be fatal in the sick, the young and the elderly.

Ample evidence indicates that changes in temperature, rainfall and humidity affect the organisms that carry and transmit pathogens to humans. Possible results include facilitating the spread of the disease to new areas, multiplying the number of outbreaks, and increasing the severity of the symptoms. In addition, climate changes could create conditions allowing for the transmission of infectious agents through unclean water and food.

For these reasons and many others, we must do all we can to reverse human affects on climate. In the meantime, it behooves us to monitor related health issues and take the necessary defensive actions.

Additional resources:
Centers for Disease Control and Prevention
World Health Organization
Intergovernmental Panel on Climate Change

Updated 9/3/10; originally posted 6/25/09.

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