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Climate Change May Make Insect-Borne Diseases Harder to Control

Warmer temperatures will combine with numerous other factors to make diseases like malaria and West Nile virus harder to control



Centers for Disease Control and Prevention

Climate change can influence how infectious diseases affect the world, particularly illnesses spread by vectors like mosquitoes. Now scientists have developed some understanding about how rainfall and temperature can influence malaria, dengue and West Nile virus infections as well as ways to combat them.

Vector-borne diseases are among the most complex and vexing illnesses to manage, since so many elements are at play, like host resistance, the environment, urbanization and the pathogens themselves. As a result, it's difficult to tease out any one factor to measure, and with the added effects of warming weather and shifting precipitation, ongoing disease-management efforts are becoming more complicated.

"Climate change is not going to invent any new diseases; it's going to make controlling existing diseases harder," said Diarmid Campbell-Lendrum, head of the climate change team at the World Health Organization's headquarters. "We've been describing the links between climate change and health for quite a long time."

Diseases spread by mosquitoes are particularly affected by climate change, since the insects lay their eggs in standing water, be it in puddles, ponds, lakes or tide pools. Standing water varies with rainfall, humidity and temperature, with wetter weather typically showing a greater number of mosquitoes. When a female mosquito matures, she can spread infections by drawing blood from an infected host and transmitting the illness to a different host with another bite.

Heat can also influence the how a pathogen is spread. In the case of the West Nile virus, rising temperatures work both in favor of and against the spread of the disease.

"The [West Nile] pathogen -- the warmer the temperature, the faster it moves from the blood to being transmitted. It usually takes a while for the virus to get into the mosquito's salivary glands," said Marm Kilpatrick, an assistant professor in ecology and evolutionary biology at the University of California, Santa Cruz. "The biting rate also gets faster. So those things are all going to give you more transmission."

On the other hand, hotter weather shortens the mosquito's life span. "What you basically have going on is three factors going in one direction and one factor in the [other] direction," said Kilpatrick, who published a paper on West Nile virus last month in the journal Science. "It's a little bit tricky to make a solid prediction."

West Nile virus is an interesting case study because the disease originated in Africa and emerged in North America in 1999, relatively recently as far as diseases go. Because of this, researchers have tracked where the disease has spread over time. Kilpatrick found that the virus is most abundant in human-developed areas like cities and farms. He also found that the virus quickly adapted to use local mosquitoes in the United States, and hosts like robins played an important role in spreading the virus over long distances.

However, Kilpatrick hesitates to link climate change directly to yearly changes in West Nile infection rates, since land use changes and infected hosts move around. Nonetheless, lessons from climate change and West Nile can be applied to more prevalent diseases. "The same questions we're wondering about West Nile apply to malaria and dengue," said Kilpatrick.

A killer with no cure spreads
Dengue fever is one of the world's most common diseases, with one-third of the world living in endemic areas, according to the U.S. Centers for Disease Control and Prevention. It afflicts 900 million people worldwide, and the number is growing, according to Khoa T.D. Thai, a researcher at the Academic Medical Center in Amsterdam and at the Oxford University Clinical Research Unit in Ho Chi Minh City in Vietnam.

Thai, who co-authored a paper on dengue this past summer in Experimental Biology and Medicine, said that the number of people afflicted by the disease is increasing, but part of the rise comes from more awareness and better diagnostics as health care reaches impoverished and rural areas.

The distribution of the disease is also growing, some of which Thai attributes to climate change as regions with tropical climates expand. Since dengue has no cure or vaccine, fighting the disease must focus mainly on prevention, whether it's draining water to prevent mosquitoes from breeding or wearing long sleeves and using mosquito repellents.

Anticipating climate trends is also important in fighting mosquito-borne diseases, particularly with malaria. "The key step to all of this is to understand what happens with climate variability," said Andy Dobson, a professor of ecology and evolutionary biology at Princeton University. "Although we've got some fantastic weather data, it's always being cleaned up. In general we need much better models for how climate variability affects malaria."

Getting ahead of malaria shifts from climate change is important because people who have been exposed to the parasite the most have the strongest resistance to it, said Dobson, who wrote about this topic in Trends in Ecology and Evolution in June. As the disease moves to new areas, previously unexposed populations may experience an epidemic.

"Malaria will expand particularly in the mountain regions. We've seen increasingly strong evidence of that in the east African highlands," he said, noting how drought and rainfall have shifted and warming weather has made mountains more welcoming to mosquitoes.

"Malaria will always come from an interaction between temperature and rainfall," added Dobson. "The rainfall certainly determines the mosquito abundance, and temperature affects the parasites."

Dobson said it is more efficient to combat malaria by using mosquito nets and managing the environment rather than some of the more exotic proposed control measures like lasers to shoot mosquitoes. "Understanding the ecological dynamics of the disease will get you much more bang for your buck," he said. With better climate predictions and land management, Dobson said malaria and other vector-borne diseases can be curtailed significantly.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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