The West Nile Virus

The West Nile Virus outbreak last summer was a very hot topic in the media. It had a devastating effect on many free range birds and many birds exhibited outdoors at the Bronx Wildlife Park (Bronx Zoo). The zoo lost around 30% of their outdoor birds. Species included Chilean Flamingos (6% of population lost), American Bald Eagle, Snowy Owl, Mauritius Pink Pigeons, and a King Parrot. The keepers at the zoo had noticed an increased number of dead birds found in the park starting as early as May. The CDC was contacted and insisted there was not an endemic problem in the area. Tracy McNamarra, DVM, pathologist for WCS (Wildlife Conservation Society, a.k.a. Bronx Zoo) performed necropsies on these exotic birds and crows, and recognized various degrees of encephalitis (inflammation of the brain) and severe myocarditis (inflammation of the heart muscles. Tissues were sent to a colleague of Dr. McNamara at USAMRIID (United States Army Medical Research Institution of Infectious Diseases). At this time, the Health Department and CDC were reporting the virus as Saint Louis encephalitis. Dr. McNamara knew that Saint Louis encephalitis did not kill birds. USAMRIID had the ability to test for diseases unknown in the United State and identified the virus as West-Nile Fever.

West Nile Fever or Virus is called an arbovirus, one that can only be transmitted in nature by insects. West Nile belongs to the Flaviviridae family, which includes many viruses that can cause encephalitis in humans and mammals. Flaviviridae are enveloped viruses with a lipoprotein (lipid or fat and protein) covering. Viruses with envelops are generally more unstable and easier to inactivate with disinfectants. West Nile is also an RNA replicating virus; they typically cause the most severe outbreaks or pandemics (affecting a great number of different species at the same time). USGS National Wildlife Health Center in Madison Wis. believes that the West Nile strain in the New York area is more virulent (deadly) to our native birds or represents a new, more virulent strain of the virus. Branson Ritchie' s Avian Virus: Function and Control explains that viruses can evolve and adapt by changing the envelop to allow the virus to infect new species of birds.

In Wildlife Health Alert #99-02B (updated Jan. 12, 2000), the American Crow appears to be the most susceptible species to West Nile Fever. CDC (Center for Disease Control) in Fort Collins Co. have confirmed 17 native species have tested virus positive. Connecticut has added the 18th. The species include: American Crow, Rock Dove (pigeon), Bald Eagle*, Mallard*, Red-tailed Hawk, Belted Kingfisher, Sandhill Crane*, Laughing Gull*, American Robin, Broad-winged Hawk, American Kestrel, Blue Jay, Black-crowned Heron*, Fish Crow, Copper' s Hawk and Herring Gull. (* captive birds). West Nile has been diagnosed in birds from New York, New Jersey and Connecticut. The virus has been detected in a single crow in Saratoga County, NY, western New Jersey (Hunterdon & Warren Counties), south-central New Jersey (Burlington County), and east to Sullfolk County on Long Island, and the East Haven area of Connecticut. USGS reports that the magnitude of mortality is difficult to assess. Of the 255 specimens tested by CDC, only about 55% have tested positive for West Nile. The earliest confirmed isolate was from a specimen collected August 9, 1999.

West Nile Fever is closely related to Saint Louis encephalitis virus, but generally causes milder disease in humans, according to the CDC Mortality and Morbidity Weekly Review October 1, 1999/48(38);845-9. The West Nile virus is transmitted by the female mosquito, which acquires the virus when it bites an infected bird for the blood meal required to produce mosquito eggs. If the same mosquito bites a human, it can pass the virus to a human. Like St. Louis encephalitis, the West Nile is not transmitted directly from person to person or from birds to people. West Nile virus was first recorded in the Province of Uganda in1937. The first reported epidemics occurred in Israel during 1950-1954 and in 1957. Epidemics have been reported in Europe in the Rhone Delta of France in 1962 and in Romania in 1996. The largest reported epidemic was reported in South Africa in 1974. It is unclear whether this current outbreak is a previously identified strain or a new variant. This is the reason it is sometimes called the West Nile-like virus.

CDC reports West Nile can infect a wide variety of vertebrates, but in humans it usually produces either asymptomatic (no symptoms) infection or mild febrile (fever) disease. Symptoms of viral encephalitis include: high fever, headache, skin rash, swollen lymph glands, neck stiffness, disorientation and convulsions. The incubation period is generally 5-15 days following the bite of the infected mosquito.

Within its normal geographic distribution of Africa, the Middle East, western Asia and Europe, West Nile has not been documented to cause epizootics (the sudden occurrence and rapid spread of disease in many animals in one area) in birds. Crows in these areas with antibodies to West Nile are common, suggesting that asymptomatic or mild infection occurs. Therefore, an outbreak causing high mortality in Crows and other species is usual in both Saint Louis encephalitis or West Nile virus and may represent introduction to a naive (never exposed) bird population or a new more virulent (capable of causing disease) strain. Like Saint Louis encephalitis, West Nile is transmitted by Culex species mosquitoes, but also can be transmitted by Aedes, Anopheles, and other species. Most of the mosquitoes trapped in this outbreak were Culex, suggesting an important role of this species.

Dr. Nick Komar, CDC Labs, Fort Collins Colorado, began testing in September 1999. Using polymerase chain reaction (PCR) and DNA sequencing isolates, indicated they were closely related to West Nile Virus. Serum titers for West Nile test for the presents of antibodies for West Nile virus. Veterinarians draw 3 cc (about one fifth of a teaspoon) of blood from the birds being tested. The blood is centrifuged and the serum is separated and frozen. The serum is shipped overnight express on dry ice. A positive result means the bird has been exposed to West Nile and has produced antibodies. Dr. Komar believes these birds are immune to West Nile virus. Birds infected by mosquitoes either become ill and die within 5 -7 days or are asymptomatic and produce antibodies. It is believed that during this 5-7 day period, the virus amplifies in the blood and becomes a reservoir for the virus. After this period, the bird no longer has the virus in its blood and can not spread West Nile to other species.

Dr. Komar recommends controlling the mosquito population by removing or reducing breeding areas and avoiding exposure to mosquito as the best means of dealing with this disease.