West Nile Virus is Tough on Bird Population

West Nile Virus is Tough on Bird Population
West Nile Virus is Tough on Bird Population

May 20, 2003

URBANA - For over 130 species of birds, getting bitten by a mosquito carrying the West Nile virus can be fatal. And, although crows and jays appear to be hardy, aggressive birds, there is something about their immune system that makes them "differentially susceptible to the virus" said Jeff Brawn, a University of Illinois researcher. Brawn has been working with medical entomologists for the last year to study the effects of the virus on bird populations. He hopes to study how the virus will cause an evolutionary change in the crow’s immune system.

"Let's say, 100 crows are infected and 90 of them die," said Brawn, an ecologist in the College of Agricultural, Consumer and Environmental Sciences at U of I. "There may be something about the 10 who survive that could be passed on to subsequent generations, ultimately increasing their resistance to the virus." Those birds who are most immune to the virus would survive, passing on that characteristic to their young.

If crow populations do decrease it could have some interesting effects on the birds since crows eat other birds’ eggs, said Brawn. "And, if we were free of them it could affect the reproduction of songbirds."

If crows are unable to develop immunity to West Nile virus and the crow population drops dramatically, what effect will it have on the environment as a whole? Brawn used the analogy of removing the rivets one at a time from the wing of an airplane to describe the ecological future. "The first few rivets removed may not pose a problem, but take enough out of the wing of the plane, and there are fatal consequences."

In another research project at the U of I conducted by Sarah Yaremych, crows have been captured and tagged. Blood samples are taken to test for the virus as well as viral antibodies. Tiny radio transmitters, that do not harm or hinder the birds, have been attached to some of the captured crows. Preliminary findings report that approximately half of the tagged crows died of West Nile virus over the summer of 2002.

Crows and jays are in the family of birds known as Corvidae. Crows are native to America, scavengers, adaptable to the environment and smart. "They don't harm people and there are no known cases of a human getting West Nile Virus from a bird," said Brawn. "In fact, you'd have to be nose to nose with a crow to get the virus -- exposed to the breath from its nostrils or get the infected bird's feces on an open cut."

Because people get the virus from mosquitoes that are carrying it, Brawn said that infected birds pose a zero health risk as far as contracting West Nile virus. He cautioned though that people who handle live birds might be more susceptible.

Most humans bitten by a mosquito carrying the West Nile virus don't get sick and in fact don't even know that they have the virus. Seniors, young children, and people with compromised immune systems are more susceptible to getting sick after being bitten by an infected mosquito.

It is still a mystery as to what's causing the spread, said Brawn. Other birds carry the virus but don't die from it. The virus lives in the blood stream and can be transmitted for six to 10 days, depending on the bird. So, one answer may be that infected birds fly to another state, a mosquito bites it and carries the virus to another bird or animal. But it could be transported unintentionally by moving an infected mosquito inside a car, train or airplane.

In 2002, it was late May when the first dead birds were reported in Illinois. Although people are highly unlikely to contract West Nile virus from a dead bird, it is advisable to call local authorities if you discover a dead bird in the wild or in your yard.