Forecasting The El Nino-Driven Ebb And Flow Of A Rogue Mosquito

Or that, at least, is what some public health officials fear, that as climate events such as El Nino become more pronounced, the range and prevalence of a mosquito whose disease-transmitting ways already puts half the world's population at risk might expand even more.

And 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.

But now a computer model being honed by scientists at the University of Wisconsin-Madison may help predict population booms, and 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, the model is built on an earlier model developed by the U.S. Department of Agriculture. It was described to scientists here today (Aug. 4) at a meeting of the Ecological Society of America.

"It's a mosquito model, not a climate model," said Hopp, but 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 mosquito that is 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.

And although dengue is little known in the United States, it is characterized by the World Health Organization as the world's most common mosquito-borne viral disease, putting as many as 2.5 billion people at risk. Also known as breakbone fever, dengue is a 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 southern United States, mostly in the southeast and southern Texas.

But 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 said. "This year, 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."

Hopp's model depicts the worldwide population dynamics of the Aedes aegypti mosquito based on climatic variables, looking at the development at each life stage of the mosquito all over the globe.

Because the habits and preferences of Aedes aegypti are well known, Hopp is confident that it is possible to predict new areas where the mosquito could survive should it be introduced.

"In addition to modeling where the mosquito occurs, we model when the mosquito populations will be peaking during the year, and how large the population is compared to previous months and years," Hopp said. "Potentially, with the use of long-lead climate forecasts, we may model future populations and potential population explosions that might result in outbreaks of dengue."

In addition to El Nino-fueled epidemics of dengue that may occur in regions of the world already infested with Aedes aegypti, another fear, Hopp said, 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 midwestern United States during the warmer months of the year.

Hopp said it is important to note that while climate plays a role in the abundance and distribution of the mosquito, local public health and education measures will play a crucial role in limiting the spread of the disease. "Those parts of the world that lack the infrastructure - public education and mosquito eradication programs, for example - are more at risk," she said.