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Mosquitoes Infect Each Other; Researchers Find That West Nile Virus Can Spread Among Pests Merely Close to Each Other

Posted on: Monday, 4 July 2005, 15:00 CDT

Scientists have discovered a new method by which the West Nile virus can pass from one host to another.

Though the findings challenge conventional wisdom of how viruses can spread, scientists emphasize that the new route does not put humans at greater risk.

According to traditional understanding, the West Nile virus life cycle works like this: A mosquito infected with the virus bites a bird, passing the virus into the avian host.

Four to six days later, enough virus circulates in the bird's bloodstream that an uninfected mosquito that bites the bird could acquire the virus, become infected itself, and pass the virus to another bird.

Now researchers at the University of Texas Medical Branch in Galveston have found that a mosquito can acquire the virus simply by feeding next to an infected mosquito, even if the host animal doesn't have virus in its bloodstream.

"This was quite surprising," said Stephen Higgs, lead author of the study published in the Proceedings of the National Academy of Sciences. "We didn't expect this at all."

Higgs' team studied the virus transmission by placing two coffee mug-sized cans side by side.

One contained infected mosquitoes and the other uninfected mosquitoes.

The cans were covered by a fine mesh screen, across which the team laid a sedated mouse.

This allowed the mosquitoes to feed at the same time even though the two sets of mosquitoes could not physically interact.

The experiment, repeated five times, consistently showed that 2% to 6% of the uninfected mosquitoes ultimately acquired the virus.

In one case, only a single infected mosquito was used, but that was enough to pass the virus to two of 87 uninfected mosquitoes. An infection rate of 2% to 6% might not seem like much.

"Yes, but we were expecting a transmission rate of zero," Higgs said. "We knew this phenomenon occurs in ticks, but we had no idea it also worked in mosquitoes."

"This is huge," agreed Thomas German, professor of entomology at the University of Wisconsin-Madison who was not involved in the research. "If someone had come in and asked me if this could happen, I would have said no. But this changes everything. Even though it's happening at low percentages, it's still significant."

Though the experiment was simple, scientists say the results are revolutionary.

"We all had one understanding of how viruses spread. It looks now like we were a little nave," said Geoffrey Letchworth, a veterinary medical officer with the U.S. Department of Agriculture not involved in the research. "I expect people are gearing up to do a lot more research on this, to see if this also applies to the many other pathogens mosquitoes carry."

The research might explain how the virus proliferated so rapidly.

"West Nile virus spread farther and more quickly in the U.S. than anyone expected," Higgs said. "It's possible this method is one reason why." Perhaps several mosquitoes fed on cows or horses at the same time, close enough to each other to pass along the virus even if the host animal never got infected.

The virus first appeared in the U.S. in New York in 1999. Since then, it has spread to every state in the continental U.S.

There have been seven reported cases of human infection in the U.S. this year four in South Dakota and one each in Kansas, Missouri and Texas.

In 2004, 98 people in the U.S. died from West Nile, down from 264 deaths in 2003. Wisconsin has had two deaths in 29 reported cases since 2003. Higgs said the team's results should not be cause for alarm because this new method of transmission will not be relevant to humans.

The next research step is to examine how close together the mosquitoes have to be when they feed.

As Letchworth put it, "if an infected mosquito is on your right foot and an uninfected one on your left foot, would the virus still get passed on? My impression is no, but that's what we have to find out."

The Milwaukee Health Department tries to track the virus by trapping mosquitoes. There have been no West Nile-related deaths in Milwaukee.

"There's always concern where there's any type of virus that might infect our citizens," said Health Commissioner Bevan Baker.

The city also tests dead birds for evidence of West Nile infection. Residents who find dead birds are encouraged to report them to the city at (414) 286-5488.

About West Nile virus

Most West Nile virus infections produce no symptoms or a mild flu- like illness. In less than 1% of cases, infected people develop an inflammation of the brain or spinal cord. Severe cases can be fatal.

You can take simple precautions to reduce your exposure to West Nile:

-- Wear long-sleeved shirts, long pants and socks outdoors.

-- Use insect repellent that contains DEET.

-- Limit outdoor activity during dawn and dusk, the hours during which mosquitoes typically feed.

-- Install and maintain screens on doors and windows.

-- Remove sources of standing water, such as birdbaths and gutters, where mosquitoes lay their eggs.

Elderly people and those with weak immune systems are urged to be particularly careful. Peak mosquito season is in August and September.

Source: U.S. Centers for Disease Control

Copyright 2005, Journal Sentinel Inc. All rights reserved. (Note: This notice does not apply to those news items already copyrighted and received through wire services or other media.)

Source: Milwaukee Journal Sentinel

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