Drought-induced amplification key to West Nile transmission

May 9 2006

The spread of West Nile Virus appears to be triggered by a complex interaction of mosquitoes, nesting birds and specific weather patterns, scientists say, which leads to "amplification" of the virus within mosquito populations.

Researchers from Oregon State University and the University of Florida have identified how those factors mesh to create heightened risk of the West Nile Virus in southern Florida, and they hope to expand their studies to the rest of the nation.

Results of the research have been published by the Centers for Disease Control.

Many early hydrologic models predicting the transmission of West Nile Virus and other mosquito-borne diseases may have been a bit too simplistic, relying on factors such as total rainfall to estimate disease risk, said Jeffrey Shaman, an assistant professor of atmospheric sciences at Oregon State University. The situation, he adds, is much more complex.

"In some cases, rain can actually help control mosquitoes by flushing away larval habitats," Shaman said. "And simply having more mosquitoes doesn't necessarily mean that we'll experience a greater incidence of West Nile Virus. The mosquitoes themselves must first be infected with the virus. Researchers call the process through which more mosquitoes become infected 'amplification,' and there are a number of factors that lead to that stage.

"By identifying these factors in the wild, it will enhance our ability to create control strategies."

In their studies, Shaman and colleague Jonathan F. Day from the University of Florida found that spring drought followed by continual summer rainfall is critical for the amplification and transmission of West Nile Virus and a similar disease, St. Louis Encephalitis Virus, in southern Florida. When drought occurs early in the year, the limited water resources confine mosquito populations to selected habitats - specifically isolated, densely vegetated hammocks where conditions remain humid.

These moist hammocks also happen to be the spring nesting and roosting sites of many species of wild birds, which act as hosts and carriers for the diseases. While confined in the hammocks, the mosquitoes feed almost exclusively on the nesting birds and as a result, each bird is bitten by numerous mosquitoes. A single infected bird can thus infect many more mosquitoes than if conditions were wet and the mosquitoes were more broadly dispersed, Shaman said.

"This phenomenon, called 'drought-induced amplification,' is a key to transmission," he said.

When summer rainfall increases, surface humidity levels rise and the mosquitoes are able to disperse and initiate secondary transmission away from the original amplification sites, the researchers pointed out. With this dispersal, the mosquitoes are more likely to come into contact with humans - elevating the risk of human incidence of the diseases.

"Drought-induced amplification may be somewhat unique to southern Florida, where drought tends to occur in the spring and coincides with the birds' nesting season," Shaman said. "The mosquito situation itself also is somewhat unusual. In most areas of the country, one species of mosquito infects the birds and another species then passes the disease along to humans.

"Florida has one species of mosquito that routinely bites both," he added.

Not all of the world's more than 3,600 species of mosquitoes transmit diseases to humans. The mosquito must be sufficiently competent to act as a carrier, thus some species can act as hosts for certain diseases, while others are more "refractive," - not carrying enough of the disease to transmit it.

West Nile Virus transmission requires mosquito species that prefer feeding on birds, but like mosquitoes, not all birds are good carriers. Some are ineffective hosts, Shaman said, while others - like crows - are very susceptible and may die from the virus. Birds that are effective hosts may carry the virus and infect biting mosquitoes for 4-5 days before recovering from the illness.

"It is this coming together of factors that leads to the spread of the disease," Shaman said. "But because the amplification is concentrated - in time and space - it does make it easier to devise control strategies. Chemical application is the most likely scenario, but because it could be applied in selected areas, it would be more cost-effective and potentially less environmentally threatening."

The spread of West Nile Virus through the U.S. has been sporadic, the researchers say, with hotspots arising one year in Colorado, and other regions during other years. The key to understanding the spread of the disease is to investigate the local conditions that may lead to amplification.

"It is a localized phenomenon," Shaman said. "We have to understand what goes on at the local level, at the appropriate scale, before we can reach the same conclusions that we found in southern Florida. But in almost all cases, the amplification of West Nile Virus will start with mosquitoes that carry the disease mingling with birds that are good carriers.

"How fast and far it spreads from there depends on weather, terrain, vegetation, humidity, the types of birds that live in the region and even the number of housing developments in a given area," he added. "These are the variables that need to be studied across the country."

Shaman and Day hope to expand their studies to analyze different regions of the country and create models similar to that of southern Florida, where certain weather patterns set off the chain of events that leads to amplification.