For Immediate Release
June 18, 2009
Public Information Officer
Montana Department ofLivestock
Vaccination Reduces Potential for West Nile Virus in Horses
Cases of West Nile Virus (WNV) in horses are down 80-plus percent from peak highs as equine owners learn to combat the disease, and animal health officials are encouraging horse owners to vaccinate their animals against the mosquito-borne disease.
"The number of cases is declining, so we're making progress, but we need to make sure horse owners know how preventable West Nile Virus is," said Dr. Tahnee Szymanski, a veterinarian with the Montana Department of Livestock. "Using a vaccine, or better yet, a vaccine in combination with measures to control the vector, will significantly reduce the potential for horses to contract the virus."
Seven cases of WNV in horses were reported in the state last year. Those numbers are down dramatically from the previous two years, when Montana had 36 (second most in the nation) and 24 cases, respectively. Nationally, the number of reported cases fell 84 percent in just two years, from 1,086 reported cases in 2006 to 178 in 2008.
During the past seven years, 94 percent of the state's reported equine WNV cases involved unvaccinated horses, Szymanski said, with roughly one-third of those resulting in the death or euthanasia of the infected animal.
WNV was first found in the U.S. on the east coast in 1999. Since then, the disease has spread steadily westward, arriving in Montana in 2002. The disease has broad distribution in Montana, having been found in 34 counties.
Clinical signs of the disease in horses include loss of appetite and depression, weakness or paralysis of hind limbs, muzzle twitching, impaired vision, loss of coordination, head pressing, aimless wandering, convulsions, inability to swallow, hyper-excitability and coma. WNV mimics other serious neurological diseases like sleeping sickness, equine encephalitis and rabies, and should be immediately reported so that a licensed veterinarian can make a diagnosis.
Szymanski said it's not too late to have horses vaccinated or get booster shots for previous vaccinations.
"The WNV season can run as long as late October, so it's not too late to get your horses vaccinated," Szymanski said. "Historically, this is about the time of year when WNV begins to appear."
A vaccine for WNV in horses was first introduced six years ago, with three currently approved for equine use. Vaccination does not guarantee protection against infection, and horses vaccinated against eastern equine encephalitis, western equine encephalitis and Venezuelan equine encephalitis are not protected against WNV. Horse owners should consult with their veterinarian to determine which vaccine best suits their needs.
There is no specific treatment for WNV in horses, although supportive care consistent with standard veterinary practice for animals with a viral infection is recommended. Full recovery from the disease is likely, although roughly 30 percent of infected horses will die or be euthanized.
Effective mosquito control also helps decrease the potential for spreading the disease. Watering troughs should be cleaned regularly, and standing water where mosquitoes breed should be managed if possible. A variety of water treatment solutions that kill fly and mosquito larvae but are nontoxic to animals are commercially available. For information on controlling mosquitoes to protect livestock, contact Greg Johnson, veterinary entomology for the Department of Animal and Range Sciences at Montana State University (406/ 994-3875; email@example.com ).
WNV has not yet been found in Montana this year, with only one case (Washington County, Texas) reported nationally.
West Nile Virus is a reportable disease. Any confirmed or suspected case should be immediately reported to the Montana state veterinarian at 406/444-2043 and/or USDA-APHIS Veterinary Services 406/449-2220.
Additional information about WNV in horses can be found at:
- MDOL's web site, http://liv.mt.gov/liv/ah/diseases/wnv/general.asp;
- USDA-APHIS web site, http://www.aphis.usda.gov/vs/nahss/equine/wnv/;
- Centers for Disease Control web site, http://www.cdc.gov/ncidod/dvbid/westnile/qa/wnv_horses.htm
Additional information about human health aspects of WNV can be found on the Centers for Disease Control web site at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm, and the Montana Department of Human Health & Public Services web site at http://www.dphhs.mt.gov/PHSD/epidemiology/commun-disease-epi-index.shtml