National Wildlife Health Center

...advancing wildlife and ecosystem health

West Nile Virus Research

Much work has been done to understand the public health effects of this disease since its arrival in New York City in 1999, but less understood are the ways West Nile Virus (WNV) is affecting wildlife populations, particularly wild birds, a major reservoir of the virus. The NWHC is working on a number of research studies to investigate WNV in wild birds.

Greater Sage-Grouse
Greater sage-grouse currently occupy about half of their historic range in the West. Sage brush habitat has decreased because of a variety of factors, including livestock grazing and other agricultural practices, energy development, urban development, mining, drought and invasive plants. Because of shrinking habitat, the effects of emerging diseases on greater sage-grouse populations are of particular concern.

In 2003, WNV was the confirmed cause of death in 18 of 22 sage grouse found dead in Wyoming, Montana, and Alberta, Canada (Naugle et al., 2004). This was the first reported WNV-related mortality in sage grouse and there is concern that WNV may contribute to sage grouse population declines as the disease continues to spread.

West Nile virus was confirmed in greater sage-grouse found dead in Malheur County, Oregon, in August 2006 and again in 2007. In 2006, scientists first isolated the virus in greater sage-grouse in Oregon. Three fresh greater sage-grouse carcasses, 1 sick northern harrier, and more than 60 decomposed remains of greater sage-grouse were found in this mortality event. A greater sage-grouse from Harney County, Oregon, was also confirmed positive for WNV; however, extensive searches in the surrounding area did not reveal any significant mortality.

NWHC is investigating the effects of WNV on greater sage-grouse. One project addresses the amount of exposure to WNV in greater sage-grouse populations, as well as to passerines (song birds) and wild horses within the sage-grouse habitat. Blood samples are collected from these animals to determine if they have been exposed to WNV and to evaluate their potential role in the continued transmission of the disease within the sagebrush habitat.

Preliminary results from another ongoing experimental study related to sage-grouse indicates that vesper sparrows, a common sagebrush-associated bird, are likely relatively resistant to severe disease caused by WNV. They may serve as an amplifying host of WNV, meaning that they can survive infection, transmit the virus to mosquitoes and keep the virus circulating.

Another species that share similar habitat with sage grouse is the chukar partridge. NWHC scientists have done WNV experimental infection studies on these birds and have demonstrated that they also may serve as amplifying hosts and as potential sentinels for WNV in sagebrush habitat. Chukar partridge likely share the same water sources; therefore, they have the same potential exposure to WNV as greater sage-grouse where they overlap in range. Preliminary results from susceptibility studies show that chukar partridge appear to be moderately susceptible to WNV when challenged at 6 weeks of age. They also appear to develop sufficient viremia—the presence of viruses in the bloodstream—to theoretically infect mosquitoes. The ability of chukar partridge to serve as amplification hosts of WNV was tested by allowing a particular species of mosquito (Culex tarsalis) to feed on WNV-infected chukar partridge at the height of viremia. Analysis of these samples is underway.

In 2004, NWHC scientists, in collaboration with the USGS Fort Collins Science Center, began a study to test wild populations of American kestrel, a common North American raptor, for evidence of WNV. Scientists trapped kestrels within their nest boxes and marked the birds with USGS identification bands. Blood samples, and oral swabs were taken to test for WNV. The goal was to capture and test breeding adult kestrels, and also collect data on chick and brood survival, nest success, and overall bird survival. The data are being used to compare the survival of WNV-infected birds with the survival of uninfected birds in the same population. Data are also being used to test antibody passing from kestrel hens to chicks.

Currently we are completing analysis of laboratory samples and serological data to determine the prevalence and impact of WNV on kestrel populations in the Colorado Front Range. In contrast to some observations of high mortality among raptors from WNV, kestrel populations do not appear to be heavily impacted by WNV in Colorado, even though exposure rates were high.

White Pelicans
American white pelicans have been found to be especially susceptible to WNV. NWHC is collaborating with Northern Prairie Wildlife Research Center and the U.S. Fish and Wildlife Service to investigate evidence of flock immunity to WNV in white pelicans at two breeding sites in North Dakota and South Dakota. We are testing for evidence of transfer of WNV antibody from female to chick. Also, we are testing fledgling healthy pelicans late in the season to detect those that have antibody to WNV, meaning that they were exposed at some point to WNV, but survived to the fledgling stage.

Impact of Multiple Stressors on Avian Immunity to WNV
During the course of daily life, wild birds are periodically exposed to environmental stressors such as breeding activities, migration, food-shortages and predation. When a wild bird is reacting to one or more of these conditions, its ability to fight viral infection such as WNV may be reduced. We are investigating the effects of certain stressors to better define the relationship between stress and a bird's response to WNV because stressed birds may be immune suppressed, carry more virus, and thus more likely infect mosquito vectors. We have developed a non-invasive test to determine if certain environmental conditions have led to chronic stress and increased the susceptibility of birds to WNV infection. The information we find while performing these studies may inform wildlife managers of potential threats to the health of wildlife. This work may also help public health agencies to identify stressed bird communities that are more likely to transmit virus to human-biting mosquitoes.

Surveillance for WNV in Hawaii
The NWHC's Honolulu Field Station was established to serve state and federal agencies with wildlife health related issues in Hawaii and the Pacific. The Honolulu Field Station (HFS) collaborates with the U.S. Fish and Wildlife Service and the State of Hawaii in surveillance and interdiction efforts to detect and prevent the establishment of new diseases into the state, including surveillance for WNV, which has not been documented in Hawaii.

Scientists from the HFS sample wild birds around the Honolulu International Airport weekly to detect the potential introduction of WNV into Hawaii. Samples are tested for antibodies to WNV at the State of Hawaii Department of health. Serology of wild birds has been shown to be the most sensitive method to detect WNV activity. Results of WNV surveillance in wild birds are provided to the Hawaii Department of Health, Hawaii Department of Land & Natural Resources, and the U.S. Fish and Wildlife Service to guide further surveillance and preventive measures. In 2007, the HFS collected blood samples from 1,660 birds at Honolulu International Airport for WNV testing. Since 2002, the HFS has bled 7,600 birds and thus far, none of these birds tested positive for WNV. Additionally, HFS scientists have provided technical training to state agencies in neighboring islands (Kauai, Hawaii, Maui) in methods of sample collection for WNV surveillance.

A recent NWHC study tested the susceptibility of seven non-native Hawaiian bird species and found that four of the seven may serve as amplification hosts of WNV if the virus is introduced to Hawaii. These species may also be useful in a dead or diseased bird-based WNV surveillance program within the Hawaii Department of Health's WNV surveillance program. Dead bird collection and testing for WNV, especially crows, has served as a major form of WNV surveillance in the contiguous U.S. However, while Hawaii has the Hawaiian crow, most of the endangered population is protected from WNV in captive breeding centers in Hawaii and cannot serve to alert the Department of Health as to the presence of WNV.

Through experimental laboratory work following inoculation and oral exposure to WNV, NWHC scientists tested the relative susceptibility and immune response of several non-native birds that are abundant around airports in Hawaii including lace neck doves, common mynahs, Japanese white eyes, zebra doves, house finches, and house sparrows. These non-native species are common in the ports, airports, and urban areas where WNV is likely to enter Hawaii. The results from these WNV-challenged birds are being compared to the response of known resistant and highly susceptible birds. Preliminary results indicate that java sparrows, house finches, house sparrows, and Japanese white-eyes may be at least moderately susceptible to WNV, and may serve as amplification hosts of the virus and possible surveillance species through their mortality. In contrast, lace neck doves, zebra doves, and common mynahs, are susceptible to WNV, but these species do not develop morbidity or mortality and may be less useful for surveillance for WNV in Hawaii.

Another study involves the Hawaiian Amakihi. Amakihi are part of a family of birds known as Hawaiian Honeycreepers, which were a wildly diverse and successful avian species before the arrival of humans on the islands exposed them to invasive diseases and species. Today, at least 12 different species of Honeycreeper are listed as endangered. The Center's Amakihi study aims to help assess the potential effect WNV could have if it reaches Hawaii. Scientists are testing the birds' susceptibility to WNV infection, while investigating how well the virus is transmitted naturally via mosquitoes.

These studies, together with the collaborative programs and monitoring USGS is conducting with the CDC and other local public health and vector control agencies, paint a picture of a comprehensive research strategy to better understand WNV in wildlife and ecosystems.

CDC Map of 2008 WNV activity

USGS 2008 WNV Map (bird, human, mosquito, sentinel, veterinary)

West Nile Virus Publications related to Sage Grouse
Outbreak of West Nile Virus in Greater Sage-Grouse and Guidelines for Monitoring, Handling, and Submitting Dead Birds. Brett L. Walker, David E. Naugle, Kevin E. Doherty and Todd E. Cornish Wildlife Society Bulletin, Vol. 32, No. 3 (Autumn, 2004), pp. 1000-1006 Published by: Allen Press

David E. Naugle, Cameron L. Aldridge, Brett L. Walker, Todd E. Cornish, Brendan J. Moynahan, Matt J. Holloran, Kimberly Brown, Gregory D. Johnson, Edward T. Schmidtmann, Richard T. Mayer, Cecilia Y. Kato, Marc R. Matchett, Thomas J. Christiansen, Walter E. Cook, Terry Creekmore, Roxanne D. Falise, E. Thomas Rinkes, Mark S. Boyce (2004) West Nile virus: pending crisis for greater sage-grouse Ecology Letters 7 (8) , 704-713 doi:10.1111/j.1461-0248.2004.00631.x.


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