Abstract

Since its introduction to North America in 1999, West Nile virus (WNV) has impacted a broad range of animals and humans.⁶ Reports of raptors infected with WNV are limited in the scientific literature; however, there is evidence of regional increases in raptor mortality.³ Although impact on avian populations secondary to WNV infection is unknown, anecdotal reports suggest a likely negative effect in the forthcoming years.^1,3

As part of avian surveillance for WNV in Virginia (2003), cloacal and/or oropharyngeal swabs collected from 61 live raptors admitted to the Wildlife Center of Virginia (WCV) were tested at Virginia Department of Consolidated Laboratory Services (DCLS) for WNV by real time reverse-transcriptase polymerase chain reaction (RT-PCR) using FAM- and TAMRA-labeled probes and primers that previously have been reported.^2,5 Forty raptors, including nine species, were positive for WNV by RT-PCR on oropharyngeal and/or cloacal swabs (Table 1) with red-tailed hawks (Buteo jamaicensis) (RTH) and great-horned owls (Bubo virginianus) (GHO) over-represented (15/40; 37.5% and 16/40; 40%, respectively). Seventeen of 32 birds (53%) tested only with oropharyngeal swabs were positive. In addition, 23 of 29 birds (75.8%) tested positive with combined oropharyngeal and cloacal swabs. Four birds (two GHOs and two RTHs) (4/29; 13.8%) were positive on oropharyngeal swabs but negative on cloacal swabs. Two RTHs (2/29; 6.9%) were positive on cloacal swabs but negative on oropharyngeal swabs.

Table 1. Raptors from the Wildlife Center of Virginia positive for West Nile virus by RT-PCR using oropharyngeal and/or cloacal swabs during 2003

Physical examination, hematology, serum chemistry profile, and radiographs were performed on WNV-infected birds. Clinical presentation varied between species. The most common findings on physical examination in all species were nonspecific signs of illness including depression, dehydration, and emaciation. The main presenting signs in GHOs included head bobbling, head tremors, and ataxia. Hematology (n=10) showed a moderate anemia, marked leukocytosis, heterophilia with left shift and a monocytosis. Chemistry results (n=4) suggested dehydration. On radiography (n=7), GHOs had a mild to moderate interstitial lung pattern (4/7). One GHO had mild splenomegaly and one had hepatomegaly. In contrast, RTHs presented with nonspecific signs of illness with minimal neurologic signs. Hematology (n=9) showed a moderate to marked anemia, moderate leukocytosis and heterophilia with left shift. No consistent chemistry (n=3) findings were noted. On radiography, RTHs were emaciated (4/5) with decreased splenic mass (3/5). Two of five RTHs also had fractured long bones.

The mean monthly numbers of raptors admitted to WCV for the previous 10 yr were compared to the monthly number of raptor admissions for 2003. There was a decrease in the number of nestlings received during May and June 2003. The number of cases in 2003 showed a marked increase during August and September, followed by a marked decrease in admissions for October to December, compared with the previous 10 yr. Retrospective review of medical records from 2002 suggested a similar epidemiologic pattern and clinical presentation, although less marked; however, no WNV cases were confirmed.

Of great concern is the impact of West Nile virus infection on threatened species and those of ecological importance. For example, two bald eagles (Haliaeetus leucocephalus) were identified as WNV positive by RT-PCR, which were euthanatized due to poor prognosis for recovery. In addition, four juvenile peregrine falcons (Falco peregrinus) were positive for WNV detected by RT-PCR, including three juveniles that were submitted directly to DCLS by Virginia Department of Game and Inland Fisheries (VDGIF). These falcons were part of the VDGIF Peregrine Falcon Restoration Project: Falcon Trak (http://www.dgif.state.va.us/wildlife/falcontrak/index.html) (VIN editor: link could not be accessed and was modified on 2/4/21 https://dwr.virginia.gov/wildlife/birds/peregrine-falcon/falcontrak/).

Early detection of clinical cases with accurate and rapid diagnosis will aid in monitoring the spread of WNV. This is the first clinical description of WNV in red-tailed hawks and will assist in recognition of this disease. The clinical presentation of great-horned owls is consistent with previous reports of WNV infection in owls.³ Oropharyngeal and cloacal swabs for WNV RT-PCR provided a reliable antemortem diagnosis of current infection in field samples and is consistent with the findings of Komar et al. (2002).⁴ Due to the difference in RT-PCR results in 6 birds, testing of both oropharyngeal and cloacal swabs is recommended. RT-PCR of oropharyngeal and cloacal swabs correlated well with clinical presentation of WNV in great-horned owls and red-tailed hawks. The epidemiologic findings are consistent with outbreaks of WNV infection in raptors from Virginia during 2002 and 2003. In addition, the change in the monthly distribution of raptor admissions may indicate declines in local populations and provides evidence to support that WNV is having a negative impact on local raptor populations. The apparent increased number of WNV-infected raptors during 2003 is consistent with the generalized spread of WNV in Virginia, compared with previous years (D.N. Gaines pers. communication). Studies are urgently needed to determine if the decline in the number of raptor admissions during spring and winter represents more widespread raptor population declines.

Acknowledgments

We thank David N. Gaines, Ph.D., and Suzanne R. Jenkins, V.M.D., M.P.H., of the Virginia Department of Health, for their assistance. In addition, we thank the Virginia Department of Game and Inland Fisheries for their cooperation and the wildlife rehabilitation staff of the Wildlife Center of Virginia for their expertise and animal care.

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