West Nile Virus Transmission Cycle Research Articles

Detection of West Nile virus-infected mosquitoes and seropositive juvenile birds in the vicinity of virus-positive dead birds.

Mosquitoes and wild birds were collected from three sites near locations in the New York City metropolitan area where single, West Nile (WN) virus-positive dead birds were found early in the 2000 transmission season. The mosquitoes were tested for the presence of infectious virus with a Vero cell culture assay and for WN viral RNA by using reverse transcriptase-polymerase chain reaction (RT-PCR) protocols. Serum samples from wild birds were tested for the presence of neutralizing antibodies against WN virus. Infectious WN virus and WN viral RNA were found in Culex species adult mosquitoes from each of the three sites, and a seropositive hatch-year house sparrow (Passer domesticus) was found in one of the three sites. Molecular techniques used to identify the species in the positive mosquito pools found that most of the pools contained a combination of Culex pipiens and Cx. restuans. The minimum infection rate in Culex species mosquitoes from the sites ranged from 0.2 to 6.0 per 1,000 specimens tested. The results demonstrated that, at least early in the transmission season, detection of a WN virus-positive dead bird indicates a local WN virus transmission cycle. This information is valuable in focusing subsequent surveillance and vector management programs. In addition, the RT-PCR procedure for detecting WN viral RNA in mosquito pools detected more positive pools than did the Vero cell plaque assay.

Rôle des oiseaux migrateurs dans l'épidémiologie du virus West Nile

West Nile (WN) virus, first isolated from a human in Uganda, is a common arboviral disease in sub-Saharan Africa and India. The viral cycle of transmission, which involves mosquitoes and birds, usually inducing a non-symptomatic disease, was extensively studied in Egypt 50 years ago. Humans and horses are sensitive to WN virus infection. Most cases are non-symptomatic, but encephalitis cases are also reported as well as some fatalities, mostly in old patients. For a few years, West Nile has appeared as a reemerging virus with several outbreaks recorded in the Mediterranean region and Southern and Eastern Europe (Algeria 1994, Romania 1996, Israel and France 2000). West Nile anti-bodies were detected in many bird species, and the virus was isolated from birds like a turtle dove in Slovakia, a barred warbler in Cyprus, a rock pigeon in Egypt or a dead white stork in Israel. Birds are suspected for multiple introductions of WN virus in Europe from endemic countries in tropical Africa. Usually outbreaks occurred in temperate countries during the summer period, and mostly in humid regions (deltas, etc.) where large colonies of migrating and resident birds and populations of potential vectors like Culex modestus are present. Several factors are necessary for an outbreak to occur: the arrival of migrating infected birds presenting a high viremia susceptible of infecting potential vectors, replication in mosquitoes and amplification of the virus within susceptible birds and mosquitoes, and transmission to humans and/or horses. West Nile virus emergence for the first time in the Americas in 1999 in New York was revealed by the death of several thousand native birds, mostly crows and blue jays. However, bird migrations do not seem to be involved in the recent WN introduction in the United States. A better knowledge of the different factors involved in the WN virus transmission cycle are needed, especially the possible persistence of viral infection in birds and the role of ticks in WN virus dissemination. Combinations of surveys of potential regions of reemergence of WN virus in Europe in connexion with other places in the Middle East, North Africa and tropical Africa would be important for a better understanding of the epidemiology of this arbovirosis.