Cross-bred mosquitoes spread West Nile in US: study

Cross-bred mosquitoes that feed on humans and birds could be helping the West Nile virus sweep across North America, researchers report.

Cross-bred mosquitoes that feed on humans and birds could be helping the West Nile virus sweep across North America, researchers report. In Europe, two distinct sub-species of Culex pipiens mosquito harbour the virus – one that feeds on humans and one that feeds on birds. This difference may explain why European West Nile outbreaks have been relatively small, Dina Fonseca of the National Museum of Natural History in Washington and colleagues claim. In contrast, West Nile has swept across North America, wiping out populations of crows and jays and killing hundreds of people. West Nile virus was first seen in New York City in 1999, infecting 62 people. By 2002 it affected more than 4,000 people throughout the country, killing 284.Last year West Nile infected 9,300 people and killed 240 in the United States. It also infected an estimated 1,300 people in Canada, killing 10, and was reported in Mexico. The virus has a complicated life cycle. It first infects birds and must build up to a certain level in their bodies before it can infect mosquitoes. It then must also build up to a certain level in the mosquitoes before they can infect humans with their bites. Researchers until recently believed there was another barrier, because they believed that mosquitoes most likely to be infected by birds were less likely to bite and thus infect people. “Unlike European Culex pipiens, US Cx pipiens appears to quite readily bite both avian hosts and humans,” Ms Fonseca’s team wrote in their report, published in Friday’s issue of the journal Science. The cross-bred mosquitoes, combined with the infection of migrating birds and dense urban populations, could have created conditions for the fast spread of the virus, the researchers said. (Source: Reuters Health, ABC Health News, March 2004)