Last month the Washington Department of Health announced the state’s first apparent human case of West Nile virus (WNV). The flavivirus, which can cause encephalitis in birds, horses and humans, was first identified by a veterinary pathologist at the Bronx zoo in 1999. Since then the disease has spread across the continental United States much faster than expected by public health officials. Though extensive work is being done nationwide to identify the exact nature of this disease, much about the West Nile virus is unknown.
What is known is how the disease is spread. It involves a bird-mosquito-bird cycle where birds, bitten by infected mosquitoes, are able to develop a high enough viremia (amount of virus in the blood) to transmit the virus to other mosquitoes. People and horses are incidental hosts and not able to transmit the virus to mosquitoes. In temperate climates such as Washington, WNV occurs most commonly in late summer or early fall.
In 2002, the Washington Animal Disease Diagnostic Laboratory (WADDL) at Washington State University was appointed by the State Department of Health to be the designated diagnostic lab for West Nile virus in the state.
Dr. Tom Besser, professor and director of services at WADDL, helped implement the program and said an avian surveillance study is under way to discover where the virus is circulating within Washington. Researchers are primarily studying the infection rate of regional bird species and expect to examine 500-1000 birds between April and October.
More than 100 species of North American birds have been infected so far, but only a few of those go on to develop clinical signs of disease and death. Corvids, a group of birds including magpies, crows, ravens and jays, can develop high viremia and are most likely to die. House sparrows are also very susceptible. Apparently chickens and ducks are not. Research is under way to identify which types of healthy birds serve as an active reservoir of the disease through incubation and spread of the virus.
Tom Gibbs, public health adviser for the Washington State Department of Health, stresses that the general public should report sightings of dead birds to their county health departments as they may be important indicators of possible outbreaks of WNV in people and horses. These sightings also provide information when it is difficult to obtain enough fresh bird specimens for actual testing.
WADDL uses two methods of testing for WNV. Dan Bradway, associate in molecular diagnostics, uses the CPR (polymerase chain reaction) test for the avian surveillance study. It involves taking oral swabs or tissue samples from birds and running them through electrophoresis, which can detect DNA of the West Nile virus. Results of the test can be obtained within 1-2 days.
Down the hall, microbiologist John VanderSchalie runs the IgM Capture ELISA test for suspected cases of equine infection. This test is unique in that it tests for one specific class of antibody — the IgM antibody. When a horse is infected with the West Nile virus, it will typically mount an immune response within 4-12 days. Therefore, this is the most effective time to test the blood. The IgM Capture ELISA test also is able to distinguish between vaccinated horses and natural infection. Test results are available by the next day.
As for prevention of WNV, the state veterinarian recommends all horses be immunized with the highly effective vaccine West Nile-Innovator by Ft. Dodge. There are, as yet, no approved vaccines for humans or pet birds so prevention focuses primarily on control of mosquito larvae by removing mosquito breeding locations such as standing water. Use of repellents, screens, long-sleeved clothing and fluorescent lights at night, which do not attract mosquitoes, is also recommended.
Dr. Diana Stone, who teaches veterinary public health and zoonotic disease at the College of Veterinary Medicine, recommends that anyone who finds a dead bird in their backyard pick it up with a shovel or gloves rather than touching it with bare hands. They should then call their local health department for further information on WNV testing and sending samples to WADDL. A small number of squirrels have also tested positive in other areas, so it would be wise to handle dead squirrels with similar caution.
The same goes for pet birds and equine. If a parakeet or Shetland pony shows signs of possible encephalitis, such as stumbling/weakness (inability to fly in birds), depression, paralysis or inability to swallow, owners should first contact their local veterinarian who can decide if diagnostic blood tests or postmortem examination at WADDL are warranted.
Besser says it is important to remember that a very low percentage of mosquitoes are actually infected with the virus. However, more than 200 people died in the United States as a result of the virus in 2002.
Most people who are infected show no sign of disease, but 20% may exhibit flu-like symptoms within 3-14 days of a mosquito bite. Signs can include fever, headache, body aches, skin rash and swollen lymph glands.
A smaller percentage develop the severe encephalitis/meningitis syndrome with headache, high fever, stiff neck, confusion, coma, tremors, muscle weakness, convulsions and paralysis.
As for the future of WNV in Washington, Besser sides with the optimistic view that in a few years, as the virus reservoir in birds decreases, the number of human and equine cases will also decrease. With widespread vaccination, the number of horse infections should drop dramatically. But both Besser and Stone agree that this year the incidence of West Nile Virus infection will probably be going up in Washington.
