Love stings: A hybrid of two native strains of C. pipiens could bring West Nile virus to the UK. Credit: US Geological Survey

Last year, West Nile virus (WNV) claimed the lives of 277 Americans. But while sporadic outbreaks have occurred in southern Europe, no one has yet fallen sick in the UK. One group of British scientists says the key lies in two indigenous mosquito variants and is investigating whether a hybrid of the two could trigger a US-style epidemic.

Although more than 20 species of mosquitoes in the US are infected with WNV, Culex pipiens has historically been the primary culprit. C. pipiens has two known physiological variants—C. pipiens molestus, which bites only people, and C. pipiens pipiens, which bites only birds. For the virus to be transmitted from bird to human—the usual path of infection—the vector must bite both.

Dina Fonseca, an entomologist at the Philadelphia Academy of Sciences, uses rapidly mutating genetic markers called microsatellites to try to differentiate the two C. pipiens variants. C. pipiens molestus is a recently domesticated version of C. pipiens pipiens, Fonseca says. “One possibility is that while in the old world you have this domesticated type and this feral type, both of them got introduced into the US and mated, and you got a hybrid that may behave differently.”

Most researchers outside Britain have had no difficulty in mating the two variants in the lab. But in Britain, the two remain isolated in the wild, says Colin Malcolm, a researcher at the University of London. He plans to collaborate with Fonseca to compare British and American C. pipiens using microsatellites.

Malcolm has been investigating reports of a five-year-long biting nuisance in a village in Clackmannanshire, Scotland. His team found the culprit to be a population of C. pipiens molestus breeding in a semi-underground factory. When Malcolm tested the genotype of the mosquitoes, he found, to his surprise, that about half the population tested positive for an insecticide-resistant haplotype or gene set, A2B2, that originated in Africa. Because none of the C. pipiens pipiens they tested within a five-kilometer radius of the factory carried the same haplotype, they concluded that the two variants were not interbreeding.

“We are probably benefiting from the fact that in Britain—so far at least—our C. pipiens pipiens is very bird-specific,” says Malcolm.

In the insecticide-resistant Scottish population—the first of its kind to be described in Britain—entomologists are dealing with an unknown quantity, says Malcolm. “[The A2B2 haplotype] arrived in Marseilles and we watched it travel north,” he says. “That means that we are dealing with a [C. pipiens] molestus that isn't necessarily conforming to characteristics that we might have in Britain.”

The continued absence of WNV-related encephalitis or meningitis in Britain is reassuring, Malcolm says. That could be down to the genetic isolation of C. pipiens pipiens, or it could be because the birds they are feeding on are not infected. Last October, rumors appeared in the British press that scientists had found antibodies against WNV in dead birds, but the claims have yet to be supported by a paper published in a peer-reviewed journal.

“In a country like this there is relatively little exposure to humans by mosquitoes,” says Ernie Gould of the Oxford Centre for Ecology and Hydrology. Gould is one of many researchers monitoring birds for new viruses. “The virus may be circulating in the wild and may be virulent,” Gould says, “but it still need not be causing disease.”

Most people in Britain have C. pipiens pipiens breeding in their back garden, Malcolm points out. If the virus is in birds, and a hybrid pipiens-molestus subspecies emerges that bites both man and bird, Britain will be as vulnerable as the US.