El Nino Makes Mosquitoes Thrive

USA Today (Society for the Advancement of Education), June, 1999

As goes El Nino, so goes Aedes aegypti, some public health officials fear. As climatic events become more pronounced, the range and prevalence of a mosquito whose disease-transmitting ways already put half the world's population at risk might expand even more.

While that is a very real concern, predicting the ebb and flow of populations of the mosquito that transmits dengue, a family of debilitating and sometimes fatal viral diseases, has been more art than science until now. A computer model being honed by scientists at the University of Wisconsin-Madison may help predict when and where in the world the mosquito might show up in response to large-scale climate events like El Nino. Developed by graduate student Marianne Hopp and climatologist Jonathan Foley, it uses climate data such as precipitation, temperature, humidity, and cloud cover to predict the weather's influence on the mosquito during its four life stages.

Aedes aegypti, the principal carrier of the dengue virus, has expanded its range from its primordial home in Africa to most of the tropical and subtropical world. It is especially prevalent in urban areas, where it breeds in rainwater that accumulates in discarded tires and containers. Although dengue is little known in the U.S., it is characterized by the World Health Organization as the most common mosquito-borne viral disease, putting as many as 2,500,000,000 people at risk.

Also known as breakbone fever, dengue is characterized by headache, fever, sore muscles, and extreme pain and stiffness of the joints. It can be completely incapacitating and is sometimes fatal, especially to children in its more serious form--dengue hemorrhagic fever. There is no vaccine to prevent the disease, and it is prevalent in nearly 100 countries.

Aedes aegypti has expanded its range in recent years to reinfest many parts of the Americas, areas where it was thought to have been eradicated by intensive post-World War II mosquito-control programs. Its range includes the southeast U.S. and southern Texas.

Could the mosquito's range and populations change even more in response to large climate events like El Nino? According to Hopp, it probably has. "Because El Nino events result in some regions being warmer and wetter, it's reasonable to believe that mosquito populations will be affected," she says. "[In 1998], regions of South America, Southeast Asia, and the Western Pacific had very large numbers of dengue cases compared to previous years, likely as a result of our recent El Nino."

In addition to El Nino-fueled epidemics of dengue that may occur in regions of the world already infested with Aedes aegypti, another fear is that climate events like El Nino may help the spread of the mosquito from the tropics and subtropics to more temperate regions of the world. Results of the model suggest that Aedes aegypti could survive as far north as the mid-western U.S. during the warmer months of the year.