American Robins are the Primary Host of West Nile Virus in the Northeast
Definitive Paper on Breakthrough Slated for Publication
NEW YORK - June 1, 2006 - Scientists at the Consortium for Conservation Medicine (CCM) at Wildlife Trust New York, the New York State Dept. of Health and the Smithsonian Migratory Bird Center have shown that American robins are the primary host of West Nile virus in the Northeast USA.
Their study, funded by the US Government National Institutes of Allergies and Infectious Diseases (NIAID) was based at sites in Washington DC and Maryland. The research was recently published in the leading British journal Proceedings of the Royal Society B: Biological Sciences. The authors show that mosquitoes that carry West Nile virus have an apparent preference for American robins despite the presence of other, more abundant species such as house sparrows. They also show that because American robins produce high levels of virus in their blood when infected, the majority of West Nile virus-infected mosquitoes at the study sites became infected from feeding on robins.
West Nile virus was introduced into the western hemisphere in New York in 1999. It has since spread across the USA, into Canada, Mexico and some South American countries causing over 800 deaths. West Nile virus (WNV) infects a range of bird species and causes high mortality rates in some, such as American crows. There has been a debate among scientists over which bird species are the main 'amplifying' hosts - i.e. those responsible for infecting most mosquitoes in a region. Previously, scientists hypothesized that crows, jays, or the very abundant house sparrows are the key amplifying hosts. The current study used state-of-the-art techniques to show that American robins are the most important amplifying hosts at the sites studied.
First, scientists captured mosquitoes that had recently fed and still contained blood at a series of sites in Washington (including the Smithsonian Mall) and Maryland. They used the powerful technique of PCR to amplify DNA from the blood in the mosquitoes and determined the species that the mosquitoes fed on. "This allowed us to understand the feeding patterns of the mosquitoes that carry West Nile virus - very powerful information" says Dr Laura D. Kramer, Director of the Arbovirus Laboratories, Wadsworth Center, NY State Dept of Health.
The DNA analyses showed that American robins were the most commonly fed upon host species, a finding that agreed with recently published findings from Connecticut. But the research team took the study a step further. Using censuses of the different bird species at their sites, the researchers were able to show that mosquitoes fed on robins over 40% of the time, even though they were only 4% of the bird community. The team then incorporated data on the levels of virus produced in the blood of different bird species when they are infected. Birds, such as American robins, that have a high level of virus in their blood when they're infected are more likely to infect mosquitoes that bite them.
The teams' analyses show that American robins are responsible for the majority of infected mosquitoes at their sites - 16 times more than expected based on their abundance alone. Lead author, Dr Marm. Kilpatrick of the Consortium for Conservation Medicine commented "This is a dramatic result - our data clearly show that American robins are the focus of West Nile virus transmission at our sites, even though they are only a small part of the bird community". He adds: "Even more importantly, our research shows that sites with the most intense feeding on American robins have earlier West Nile virus epidemics - data that can be used to help manage outbreaks".
But the team emphasized that removing American Robins from backyards will not remove the threat of West Nile virus. Previous work published by the group in the journal Public Library of Science Biology in February showed that when robins depart in the early fall, the next favorite host for these West Nile virus-carrying mosquitoes are people.