West Nile Virus Affecting Horses Throughout USA

Stagnant water - Source of disease-carrying mosquitoes
Stagnant water - Source of disease-carrying mosquitoes Shutterstock

Newsdate: Friday August 10, 2018, 9:00 am
Location: MORGAN Hill, California

According to the Equine Disease Communications Center, new cases of West Nile virus had been reported in New York, Montana, Minnesota, Idaho, and Wisconsin in recent days. In New York the State Dept of Agriculture and Markets confirmed the state’s first 2018 West Nile Virus case.

Mosquito - Source of West Nile virus.

Mosquito - Source of West Nile virus

Simple insect control measures should be utilized and insect repellents used when horses are in areas exposed to mosquitoes to prevent West Nile virus disease.
© 2013 by PD-USGOV-HHS-CDC

The horse, from Addison, Steuben County N.Y., a 15-year old gelding, started to show signs on Aug. 4. The owner noticed ataxia on all four limbs, generalized muscle fasciculations, decreased tail tone, but no fever. The gelding was hospitalized the following say for testing and further treatment. Blood sample results for West Nile Virus along with concurrent signs of illness confirmed active WNV infection. The horse had not been vaccinated for West Nile Virus in recent years. The horse had no recent travel history. As of Aug. 8, the horse is recovering from infection.

In Great Falls, Montana, the Montana Department of Livestock has confirmed the first reported cases of equine West Nile Virus in Montana in 2018. The two cases are in Musselshell County and Lake County, according to a press release from the agency.

The announcement follows detection of the virus in mosquito surveillance pools from Cascade, Hill, and Lewis & Clark counties. 

The Minnesota Board of Animal Health confirmed the first of equine West Nile Virus for 2018 on July 27th, 2018 in a 3-year-old Miniature Horse in Pine City, Minnesota. The stallion presented with acute neurologic signs, exhibiting profound weakness in his front limbs, tripping and falling and requiring assistance to stand. The horse has no documented history of vaccination against West Nile Virus and is currently receiving supportive care.

The Idaho Department of Agriculture has confirmed a case of West Nile Virus in a 2 year old Quarter Horse mare located at a private facility in Owyhee County. The horse showed an onset of clinical signs on July 19th, 2018 including ataxia and incoordination. Testing was confirmed positive for WNV at the ISDA Animal Health Laboratory. The horse had been vaccinated as a filly but had not received a booster vaccination. The horse is improving with supportive care.

West Nile Virus affects humans, equines, and birds. It is spread through the bites of infected mosquitos; horses cannot transmit the virus directly to people. Detection of the disease in horses and mosquitoes in Montana serve as an important reminder for people to take steps to prevent West Nile Virus infection.

“There is no direct treatment for the virus in horses, but vaccination is highly effective in preventing disease. Horses that are vaccinated rarely die or are euthanized because of the disease,” said Dr. Tahnee Szymanski, Assistant State Veterinarian of Montana. 

She continued, “Vaccination is typically administered in the spring, but may offer some protection even this late in the season. Work with your veterinarian to determine if your horse could still benefit from vaccination.

Horse owners should be aware of the typical signs of West Nile Virus which include:

  • Fever, loss of appetite and depression
  • Incoordination or weakness of the hind limbs
  • Muscle or muzzle twitching, drooling

Vaccination against the West Nile virus is the best method of prevention. A USDA licensed vaccine is recommended as a core vaccine and is an essential standard of care for all horses in North America.

Four USDA licensed vaccines are currently available (two are inactivated whole WN virus vaccines; one is a non-replicating live canary pox recombinant vector vaccine and one is an inactivated flavivirus chimera vaccine):

All of the current WN vaccine products carry one year duration of immunity, with challenge, consistent with their respective label claims.

The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%. Data have supported that 40% of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities, 6-months post-diagnosis.

In addition to a sound vaccination protocol recommended by a veterinarian, simple insect control measures should be utilized and insect repellents used when horses are in areas exposed to mosquitoes. Horses should be stabled during dusk and dawn hours and other times when mosquitoes are present and good use of fan to insure insects don't invade barn and stable areas is important.

In addition, horse owners should eliminate opportunities for mosquito breeding by draining wet areas of pasture, filling puddles, repairing eve troughs, gutters, and clearing any containers that might hold even small pools of water.

Draining water tanks once or twice weekly should be a priority during mosquito season. Additionally, controlling mosquitoes in ponds and large water containers through the use of larvacides and fish helps keep the mosquito population down.

About the Author

Flossie Sellers

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As an animal lover since childhood, Flossie was delighted when Mark, the CEO and developer of EquiMed asked her to join his team of contributors.

She enrolled in My Horse University at Michigan State and completed a number of courses in everything related to horse health, nutrition, diseases and conditions, medications, hoof and dental care, barn safety, and first aid.

Staying  up-to-date on the latest developments in horse care and equine health is now a habit, and she enjoys sharing a wealth of information with horse owners everywhere..