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 In 1999, West Nile Virus (WNV) appeared for the first time in the state of New York. Until that time, the West Nile virus had never been seen in the Western Hemisphere. Even though virus outbreaks flared up sporadically in Africa, Asia, Europe and other parts of the world, the combination of vast ocean protection and strict livestock entry enforcement had stopped its invasion into this hemisphere.

Since the virus was first identified in New York, WNV has continued to spread across the United States. By the end of 1999, the virus was verified in four states. By the end of 2000, WNV had been identified in twelve states. In 2001, the West Nile virus was continuing to spread, and as of Sept. 4, 2001, 21 states had reported "verified" cases of West Nile Virus.

At an Oct. 9, 2001 meeting of the California Equine Medication Monitoring Advisory Committee, Dr. Boone of the University of California at Davis (UC-Davis) stated. "We had expected the West Nile virus to be seen in California in the year 2005. But now, we anticipate seeing cases of it in 2002."

According to the Center for Disease Control's Vector-Borne Infectious Diseases section, "The continued expansion of West Nile virus in the United States indicates that it is permanently established in the Western Hemisphere."

West Nile Virus is probably one of the least-known diseases among horse owners, yet one of the most dangerous because not only do horses develop the disease...but also humans and other mammals. Generally causing flu-like symptoms, it can lead, in its most severe cases, to central nervous system infections, causing encephalitis and meningitis.

As with all equine and human diseases, the first line of defense is with the horse owner and individual himself. Knowing and education about and understanding what the disease is...its symptoms and its "course"...can head off more severe and even deadly infections. In western states, we have the advantage of knowle4dge learned from pervious years and cases in the eastern states.

The outbreak of West Nile Virus in New York was considered "an unprecedented event". After the initial outbreak in 1999, it was not certain if the virus had run its course or if the cycle of infection would begin again when mosquitoes re-emerged. It was hoped that the cold winter months would destroy the virus and mosquitoes carrying it. But not only did the West Nile virus persist through the 1999-2000 winter, but it spread rapidly throughout New York State and the northeastern and Mid-Atlantic states.

By the end of summer and fall 2000, all but one county in New York State had documented, confirmed cases. West Nile Virus was diagnosed in 14 New York City later died as also 1,263 birds. Also, 400 mosquito pools were confirmed with WNV and ten live wild birds, eight sentinel chickens, two bats, 28 horses, one domestic rabbit, one squirrel and one chipmunk, plus six other individuals in New Jersey were hospitalized in 2000 due to West Nile Virus infection.

Mosquitoes transmit the disease, and mosquitoes are common to every state in the nation. Although not every mosquito transmits WNV, mosquitoes are considered the "bridge vector" since they are the carriers of the virus from the infected to the non-infected. Since the West Nile virus typically circulates between birds and bird-feeding mosquitoes, the mosquito is referred to as a "bridge vector" because it carries the diseases outside its usual cycle, from bird to bird and into an unexpected or "peripheral" species...humans and horses.

Transmission diagram

So far, WNV has been detected in1 4 different mosquito species. But the good news is that even in areas where mosquitoes do carry the virus, less than one percent are considered infected. If the mosquito is infected, less than one percent of people who get bitten and become infected will get severely ill. People 50 and older are at the highest risk of becoming seriously ill of they are infected. Also, West Nile Virus cannot be transmitted from person to person.

According to the Center for Disease Control, symptoms of WNV in humans include flu-like symptoms. But a person who develops a high fever, confusion, muscle weakness or severe headaches should see a doctor immediately. Once infected, WNV multiplies in the person's' blood system and crosses the blood-brain barrier to reach the brain, which can be fatal. There is no current human vaccine for WNV, but several companies are working toward developing a vaccine.

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Signs of West Nile Encephalitis

It is important not to presume that a horse with clinical signs of encephalitis has West Nile Encephalitis (WNE). A definitive diagnosis requires ruling out other important diseases with similar neurological signs. Rabies, botulism, Equine Protozoal Myeloencephalitis (EPM), Eastern (EEE)  Western (WEE) and Venezuelan (VEE) Encephalitis are all examples of other diseases with neurological signs similar to WNE. A positive diagnosis can only be made by examining blood from an infected horse, a task a veterinarian has to perform.

The WNV can affect the central nervous system and lead to WNE. Although approximately half of the horses identified with WNV in 2000 recovered from the disease, horses with severe clinical signs can die as a result of infection. Clinical signs may include one or more of the following:

  • Loss of appetite and depression
  • Fever
  • Weakness of hind legs
  • Paralysis of hind legs
  • Impaired vision
  • Head pressing
  • Head tilt
  • Aimless wandering
  • Convultions
  • Inability to swallow
  • Circling
  • Hyperexcitability
  • Coma

Remember - Don't Jump to Conclusions!

If you arrive at a barn one morning and find your horse "wandering around in a circle", don't jump to the conclusion that he has WNV. Or, if the horse is seen standing in a corner of his stall, head down, "depressed" with hay still in the manger ("loss of appetite"), don't jump to conclusions! Knowing how your horse acts and reacts, his attitude and "body English" will help you determine if the animal has a touch of something, is just anxiously awaiting a feeding of grain, or is snoozing in a corner. But, if you get that little "gut" feeling that something isn't quite right, contact the veterinarian. Just discussing the virus in your area can put one's mind at ease and answer those nagging questions. No one knows your horse better than you, and your observations will often detect the first signs of a potential illness, injury or disease. So, if in doubt, don't jump to conclusions...walk to the phone and call your vet.

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Bonnie Davis is a Bay Area resident, free lance writer and horsecamping/trail riding advocate with over 30 years experience. Her stories, articles, and columns have been published in national and international publications such as Western Horseman, Paint Horse Journal, Horse & Horseman, Quarter Horse Journal, Western Side (Italy), Cascade Horseman, California Horse Review, Performance Horse Review, and San Jose Mercury News.  Bonnie was a featured speaker at Horsexpo in Sacramento in '99, '00 and '01.

? 2002 Bonnie Davis and The Bay Area Equestrian Network.