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.