West Nile-Like Virus Confirmed In NY, Suspected in NJ, Conn.

Determined To Be The Cause Of Human And Equine Illnesses In New York City

For the first time, West Nile virus--or a new subtype of that virus--has been confirmed in humans and horses in the United States. The outbreak of encephalitis-like sickness in humans in New York City now has been verified as caused by West Nile virus. This is the first confirmed evidence of West Nile virus infection in the Western Hemisphere. There have been 39 cases of West Nile virus confirmed in humans in New York City, and six deaths attributed to the disease (mostly in older persons). There are more than 168 cases under investigation.

The virus is suspected in wild bird populations in New Jersey and Connecticut. In New Jersey, cases of a similar virus, Saint Louis encephalitis, have been confirmed in both dead crows and homing pigeons. The birds have been sent to the national Centers for Disease Control (CDC) and are under testing for West Nile virus. West Nile is a mosquito-borne (possibly arthropod-borne) virus that belongs to the Flavivirus family. The virus is included in the Saint Louis encephalitis subgroup of viruses that also includes Japanese encephalitis, Murray Valley encephalitis, and Kunjin encephalitis. (The latter two viruses are found in Australia.) Initially, doctors in New York thought the outbreak of illness was due to Saint Louis encephalitis virus.

The principal species involved in the maintenance of West Nile virus is wild birds--which act as hosts for the virus--and domestic quadrupeds, such as horses.

It should be noted that horses are considered tangential or dead-end hosts of the virus and are very unlikely to play a role in spread of the disease. Horses can become infected with the virus and develop neurologic signs since the virus can invade the spinal cord and brain.

Probably the most significant known pathogen of this subgroup of viruses is the Japanese encephalitis virus, which is widely distributed in many countries in Asia. It has been estimated that this virus is responsible for 50,000 human cases and 10,000 human deaths a year in those areas of the world in which it occurs.

Previously, West Nile had only been identified in Africa, the Middle East,southwest of Asia, and occasionally in Europe, including parts of the former USSR. Within the last two months, an outbreak of West Nile virus was reported in the Volgograd region of Russia that involved approximately 559 human cases and 33 deaths. In 1996, an outbreak in Budapest's capital city of Bucharest (thought to have been carried into the city by migrating birds) caused 393 human cases and 17 deaths.

On Long Island, a group of 17 riding horses experienced an outbreak of EPM-like neurologic problems. Two of the horses were euthanized and found to have West Nile virus in their brain tissue.

It is not known which wild birds are hosting the West Nile virus. Birds infrequently become sick or die from the virus. However, crows and other birds found dead in the Bronx zoo initially were diagnosed with the virus,which caused an intensive testing of other birds and uncovered the probability of virus in the dead crows in New Jersey and Connecticut.

Health officials are concerned that migrating birds will carry the West Nile virus south this fall. There have been no killing frosts in the Northeast that would kill off the adult mosquito population and stop the spread of the virus. If birds carry this virus to southern states, the West Nile virus infection could recur next year when the mosquito populations emerge in the warmer, wet spring weather. There is question whether this virus could be carried to southern states where mosquito-borne illnesses are a problem year-around.

Mosquito species belonging to the Culex, Aedes,and Anopheles genera can carry the virus, with Culex pipiens being implicated in virus transmission in New York City. This virus was first isolated in Uganda in 1935. Extensive studies on the epidemiology of the virus were carried out in Egypt in the late 1940s and early 1950s because the virus was endemic there--thus the name West Nile.

An outbreak in the early to mid-1960s in the Rhone delta of southern France caused the first recorded deaths of horses due to this virus. The disease was reproduced and first proven to cause illness and death in horses following that outbreak. In the late 1970s, there was a report of a horse in Portugal that died of West Nile.

Then nothing was heard until August of 1998, when from August to October an outbreak observed in Tuscany, Italy, that caused the deaths of nine horses was attributed to West Nile virus.

The question of how this virus was introduced into the United States has been bandied about. It is hard to imagine how a migratory bird could have covered the distance across the ocean to bring the virus to this continent. There is no known carrier state in man, but not every exposure of a virus results in death or disease.

NOTE: This web site will have updates on the West Nile virus story as developments reach our offices. According to sources late Tuesday afternoon, Oct. 19, as many as 23 horses have displayed symptoms of the disease in New York--13 of which have died or have been euthanized. Blood tests are being performed on horses with symptoms. Bookmark our site to find out the latest on WNV. We will add links to web sites about the virus on Wed., October 20. --SC

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More