Human infection by West Nile virus may be more common in North America than in Europe because of a hybrid mosquito that bites both birds and humans, carrying the virus from one to the other.
European mosquito species tend to bite either birds or humans, but not both, a researcher says.
A genetic study of the Culex pipiens mosquito in 33 locations in Europe has found that there are two different forms of the same species with different behaviors. One type lives underground, such as in a subway or in weather-protected areas, and one lives above ground, in the open. The underground type will bite humans, but not birds, while the above ground type favors birds.
Infection spread from birds to humans
Dina M. Fonseca of the Smithsonian Institution, lead author of the study this week in the journal Science, said 40 percent of the mosquito population genetically analyzed in the United States are a type of hybrid Culex that will bite both humans and birds.
Since birds are the primary reservoir for West Nile, the hybrid Culex forms an infection bridge between humans and birds that may not exist to the same extent in Europe, said Fonseca.
“If you have a mosquito that only bites mammals, there is no transmission to humans. If you have a mosquito that only bites birds, there will be a transmission to other birds, but there will be no transmission to people,” said Fonseca. “You must have a mosquito that will bite the bird today and the human tomorrow for a transmission to occur.”
She said the West Nile virus is not transmitted between humans or other mammals. It happens only when the mosquito, which in this case is called a “bridge vector,” first bites a bird and then a mammal, passing the West Nile virus from an infected bird into an animal or human. The disease transmission will not go the other way, mammal to bird, said Fonseca.
'Just one of the factors'
The federal Centers for Disease Control last year received reports of 9,186 cases of West Nile in 45 states. About 80 percent of people infected by the virus will have no symptoms at all. For others, the effects will be flu-like symptoms — fever, headache, swollen lymph glands and nausea — that go away in a few days.
About one in every 150 people infected with West Nile will develop a serious illness, sometimes with convulsions, high fever, disorientation, numbness, coma and even paralysis. These symptoms can last weeks and, for some, there can be permanent neurological damage.
Fonseca said the unusual behavior difference between European and American Culex mosquitoes is not the final answer explaining the difference in rates of West Nile virus infection.
“This could be just one of the factors in the unprecedented epidemic of West Nile here,” she said. “We’re not saying this is the only reason ... but we’re saying this could be a component of this difference.”
Larry Kramer, a lab director in the New York State Department of Health in Albany, said in Science that Fonseca’s suggestion “is a very enticing theory” that “fits with what we’ve been seeing.”
However, Andrew Spielman of the Harvard School of Public Health in Boston said in Science that the evidence supporting the theory “is anything but solid.” He and others believe West Nile became a menace in the United States because the virus is new to the people and birds in the Americas and they have not evolved immunity.
Europeans, he said, have been exposed to the virus for centuries.
