West Nile Vet and Horse Owners Info

West Nile Encephalitis is a viral disease of both humans and horses transmitted by infected mosquitoes. It was first detected in the United States in 1999. Since its initial discovery in New York City, the virus has spread westward across the United States. The onset of clinical disease has been noted from mid-June through mid-December; the disease tends to peak in August and September.

How does a horse become infected with the virus?

Infected mosquitoes can transmit WNV to horses and people. The virus is maintained in a transmission cycle between birds and mosquitoes. There is no documented evidence of natural transmission from person-to-person or birds/mammals to humans.       

What are the clinical signs of disease in the horse?

Horsesmay develop lethargy, fever, hindquarter weakness, involuntary muscle contractions, excitability, loss of coordination, head tilt, impaired vision, inability to swallow, circling, staggering gait, convulsions, paralysis, and coma.  From a survey of Minnesota horse owners who had horses ill with WNV, these were common signs of illness: ataxia (50%), muscle tremors (24%), depression (20%), fever (18%), lethargy (12%), and anorexia (9%).  Sixteen percent of horses were recumbent. Differential diagnoses include: rabies, eastern equine encephalitis, western equine encephalitis, equine herpesvirus-1, equine protozoal myeloencephalitis, wobbler syndrome, botulism, and other toxicoses.

Do horses that develop illness due to West Nile Virus recover?

Early estimates from a Minnesota survey reveal that 40% of horses surviving West Nile Virus illness will exhibit residual effects from the disease.  These clinical signs typically include gait and behavioral abnormalities.

Can the disease affect other animals?

WNV is rarely found in animals other than horses and birds. While cases have been reported in the following animals--cats, dogs, camelids (alpacas and llamas), reindeer, seals, sheep, and squirrels --these animals generally appear to be resistant.

How and where should veterinarians report suspected case information?

Remember, Equine Encephalitis is a reportable condition to the Minnesota Board of Animal Health. Contact the Board at (651) 296-2942.

 What diagnostic tests are available for horses?

 Blood, tissue, and cerebral-spinal fluid can be collected for testing. Samples can be sent to the Minnesota Veterinary Diagnostic Laboratory for testing. The Minnesota Veterinary Diagnostic Laboratory cannot test blood samples from other animals. 

Is there a specific treatment for West Nile Encephalitis in the horse?

Currently, only supportive care can be offered to infected horses. In 2002, 38 percent of Minnesota horse cases died or were euthanized. Several plasma products are available to boost antibodies against WNV.

Is there a vaccine available for West Nile Virus?

There are two West Nile Virus vaccines for horses, which have been granted licensure by the USDA. The Center for Veterinary Biologics assures the safety and efficacy of the vaccine. The recommended protocol for vaccination is two (1 ml) doses given intramuscularly three weeks apart. It is recommended that vaccination be completed at least three weeks before the mosquito season, and repeated annually. A fall booster may be recommended if weather conditions promote high mosquito populations. Annual re-vaccination is recommended for previously vaccinated horses. Horse vaccines such as Western or Eastern Equine Encephalitis vaccine will not protect against WNV. 

In Minnesota, the majority of WNV cases occurred in unvaccinated horses or horses not adequately vaccinated. A 2002 survey of veterinarians and horse owners found that vaccinated horses were more likely to survive WNV infection than unvaccinated horses. Therefore, it is important to vaccinate early to stimulate adequate immunity.

How can horses be protected aside from vaccination?

Prevention measures include using mosquito repellents and avoiding outdoor activities at dawn or dusk.  Horses should be housed in screened areas during these peak hours of mosquito activity. Steps should also be taken to eliminate mosquito breeding habitat by paying special attention to natural or man-made places where water can collect and mosquitoes can lay their eggs – old tires, birdbaths, and clogged rain gutters.

What should individuals/veterinarians do with dead birds?

West Nile Virus has been identified in at least 140 species of birds.  American crows and blue jays seem to be especially susceptible. To report ill or dead birds, see the Minnesota Department of Health website at www.health.state.mn.us.

 What type of West Nile Virus surveillance program does Minnesota have in place?

Disease surveillance activities are being jointly coordinated with the Minnesota Department of Health, the Minnesota Board of Animal Health, the Department of Natural Resources, the Metropolitan Mosquito Control District, and the Minnesota Veterinary Diagnostic Laboratory. 

How does the disease affect humans?

Most peoplebitten by an infected mosquito will not get sick. Those who do develop illness usually have mild, flu-like symptoms such as fever, headache, and body aches. Rarely, severe illness develops and is characterized by high fever, neck stiffness, muscle weakness, convulsions, and paralysis. Ten to fifteen percent of people with severe illness may die. Elderly people have the highest risk of developing severe disease.

Where can veterinarians and horse owners get more information about West Nile Virus?

College of Veterinary Medicine http://www.umn.cvm.edu/
Centers for Disease Control and Prevention www.cdc.gov/ncidod/dvbid/westnile/q&a.htm
United States Department of Agriculture www.aphis.usda.gov
Minnesota Department of Health www.health.state.mn.us
Minnesota Board of Animal Health www.bah.state.mn.us