Wayne Kramer and Jessica Brauch

View more photos of the birds used in this study.

West Nile virus is a mosquito-transmitted virus that cycles in nature primarily between mosquitoes and birds. It was first detected in the United States in 1999 and in Louisiana in 2001. The virus can cause severe clinical disease in humans and horses. While human cases continue every year across the country, horse cases have dramatically declined because of the development and widespread use of an effective equine vaccine for West Nile virus.

After the virus was first detected in the United States, it quickly spread from coast to coast in only four years and has now been found in every state except Alaska and Hawaii. It is important to point out that the virus is transmitted by different species of mosquitoes in different parts of the country. The birds that serve as the most important reservoirs (amplifying hosts) differ by region of the country as well. Although West Nile virus has adapted to various combinations of mosquitoes and birds across the country, these variations account for some of the major differences observed in the levels of activity in different areas of the country and the number of human cases from year to year.

Since its detection in Louisiana in 2001, more than 1,000 cases and 63 human deaths have been reported to the Louisiana Department of Health and Hospitals as of the end of 2008. Many human cases result in a mild fever and flu-like illness and are never reported. Severe cases can result in encephalitis or meningitis, and deaths attributed to West Nile virus have been recorded in Louisiana every year since 2002. The cases occur across the state and, on a yearly basis, typically peak in July and August when vector mosquito populations are most abundant.

The strain of West Nile virus that arrived in the United States was unusual in that it was observed to cause significant mortality to some bird species that became infected with the virus. Bird groups especially susceptible to death from the disease were the corvids (jays and crows) and raptors (hawks and owls). This information helped document the activity and movement of the virus across the country.

From 2006 to 2008, LSU AgCenter scientists conducted a survey of wild birds and their involvement with West Nile virus. Birds were trapped weekly at two sites in East Baton Rouge Parish, using mesh mist nets and one-way door traps. Records were kept of the birds' identification, age and sex, if it could be determined. A blood sample was taken from each bird and tested for West Nile virus RNA and antibodies to West Nile virus. Birds infected with West Nile virus by the bite of an infected mosquito later develop antibodies. These birds are important in the West Nile virus cycle because they serve as amplifying hosts or reservoirs of the virus. This means that uninfected vector mosquitoes that feed on these birds become infected and further the cycle. The vast majority of these infected birds have not been observed to show any negative effects of West Nile virus.

More than 2,400 wild birds were sampled. West Nile virus was detected in nearly 4 percent of wild bird blood samples, and antibodies to West Nile virus were detected in about 12 percent of samples. The species with the most infected individuals were Northern cardinal, house sparrow, American goldfinch, white-throated sparrow, yellow-rumped warbler, brown thrasher, Northern mockingbird, Carolina wren, tufted titmouse and mourning dove.

Positive West Nile virus bird species from this study can be placed in different categories based on the time of the year they spend in the state. The most important bird species relative to the summer West Nile virus transmission cycle are the year-round residents. Birds from our study in this group include Northern cardinal, house sparrow, brown thrasher, Carolina wren, mourning dove and tufted titmouse. These birds breed in Louisiana and are abundant and available as hosts during the periods of the spring and summer when vector mosquitoes are abundant and most likely to transmit West Nile virus. Summer-breeding birds present in Louisiana may also have a role in virus transmission. Birds in this category include the summer tanager, orchard oriole, prothonotary warbler, Northern parula and white-eyed vireo.

LSU AgCenter researchers also trapped Louisiana birds that can be classified as winter residents. Bird species trapped in significant numbers from this group included white-throated sparrow, yellow-rumped warbler, American goldfinch, savannah sparrow, American robin and chipping sparrow. Although they spend only the winter months in the state, they can play a role in West Nile virus maintenance and transmission because they can become infected just prior to northward migration and, thus, help disperse the disease to other Northern states during early spring. We also trapped birds migrating through Louisiana on their way northward. Birds in this group would include the indigo bunting, the magnolia warbler, ovenbird and gray catbird.

It has been reported that West Nile virus has had a significant negative impact on the populations of many bird species in North America following its introduction some 10 years ago. This impact, however, seems to have gradually diminished because the virus has started to reach an equilibrium with the most sensitive bird species. Blue jays, for example, died in large numbers at first as a result of West Nile virus, but now these birds appear to be healthy even when antibodies to the virus are detected in their bodies.

Wayne Kramer, Associate Professor, and Jessica Brauch, former Graduate Student, Department of Entomology, LSU AgCenter, Baton Rouge, La.