West Nile Virus: Is It Coming Our Way?
In New York City and surrounding counties during the summer and fall of 1999, West Nile Virus (WNV), an arthropod-borne disease, caused 62 cases of encephalitis in humans, including 7 deaths. It also caused the death of hundreds of birds and 9 horses. Prior to this outbreak, the virus had never before been identified in the Western Hemisphere. In February 2000, WNV was identified in a bird in Baltimore, MD. In May, it was confirmed in birds in New York and New Jersey, indicating that the virus had survived the winter.
WNV is transmitted by the bite of a mosquito. In the United States, Culex pipens, a common household mosquito that feeds on birds, humans, and animals, is the primary vector. Mosquitoes become infected after feeding on infected birds and then spread WNV to humans, animals and other birds by their bites. WNV is not spread person-to-person/animal/bird.
As with other mosquito-borne viruses, many vertebrate species show evidence of natural infection with WNV. Wild and domestic birds show the most consistent evidence of infection. While some species do not become ill after becoming infected, other species have a high mortality rate. In the New York outbreak, crows, sparrows, blue jays, and raptors were primarily affected.
In domestic animals, clinical signs of WNV infection have primarily been observed in horses. Although most horses are asymptomatic, those that develop encephalitis due to West Nile have a 25-35% mortality rate. Dogs, cats, cattle, and sheep can also be infected with WNV but clinical symptoms have rarely been reported.
Due to the migratory nature of birds, it is likely that sometime in the future the virus will appear in our area. Therefore, the Virginia Beach Department of Public Health has instituted enhanced surveillance for West Nile virus. This includes collecting and testing mosquitoes and dead birds, especially crows. We have also added West Nile virus to the tests that are routinely done on the sentinel chicken flocks.
As a veterinarian, your part in this surveillance is having a heightened suspicion for WNV infection in the horses and birds that you treat. Clinical signs of West Nile virus infection in horses include listlessness, stumbling and incoordination, weakness of limbs, ataxia, partial paralysis, and death. Symptoms in birds includes excessive wing flapping, difficulty flying and other signs of central nervous system disorder.
If you observe the above signs and symptoms in any of your clients or if you have questions, please call the Environmental Health Office of the Virginia Beach Department of Public Health at 518-2646.
We will discuss with you obtaining the appropriate specimens for diagnostic tests.