Calaveras Humane Society

West Nile virus causes an infectious disease in horses, birds, and humans typically. The condition is an encephalitis, or inflammation of the brain and brainstem. West Nile virus, as the name suggests, was originally endemic to the Middle East and the African continent. It first appeared in the United States 1999. An unexplained die-off of blackbirds and crows in Central Park was traced to the virus, which was also identified in several human patients suffering from a previously unexplained neurological condition in New York area hospitals. Over the next several years, the infection was reported in both humans and horses as it spread rather rapidly across the United States . For a time, the Rocky Mountains delayed its spread farther west. However, it reached California by 2002. At that time, more than 15,000 cases in horses had been reported from 41 states.

Birds act as the reservoir host with the mosquito responsible for transmission of the disease from bird to bird, or bird to horse, or worse bird to human. Many species of mosquito can transmit the virus, some common to Calaveras County . A wide variety of birds can harbor the virus but blackbirds and crows are most susceptible to the virus’ ill effects.

In horses, 10-40% will show signs of illness, and of these about 30% will die. Horses that become ill have a variety of symptoms but most will show various neurological signs including incoordination, lameness, head pressing, depression and weakness. No treatment except for supportive care exists, and eventually most horses are euthanized due to recumbancy, paralysis and humane reasons. In horses, diagnosis is based on clinical signs and blood tests. Analysis of spinal fluid can be helpful, but this disease in horses can be confused with other neurologic diseases like rabies, protozoal myelitis, and herpes virus infection among others and can be frustrating to diagnosis.

In humans, infection is often sub clinical, with no symptoms at all. However, a certain portion of those people infected will show flu-like symptoms and a small portion of these will develop a neurological disease and die. There is no specific treatment, only supportive care and the hope that the immune system will throw off the infection.

The most effective way of dealing with this epidemic disease is prevention. Very effective protection can be provided by once or twice yearly administration of West Nile Virus vaccine in horses. Additional steps in prevention involve control of bird populations, but more importantly control of mosquitoes. In Calaveras County , mosquito abatement is an active program during mosquito season and residents can do their part by destroying mosquito habitat, especially any locations where standing water exists.