Be Proactive to Prevent This Disease

Stagnant water

Stagnant water is a breeding ground for mosquitoes, and removal is part of an insect control management plan. Photo courtesy Purdue Extension.

KNOXVILLE, Tenn. — A yearling filly in Bourbon County, Kentucky, near Lexington, recently tested positive for West Nile Virus. While this area of Kentucky may seem far from some parts of Tennessee, the movement of migratory fowl could introduce an infected bird to your area within a matter of hours. Now is the time to consider your disease management protocol for prevention of West Nile Virus (WNV).

Dr. Lew Strickland, University of Tennessee Extension Veterinarian, and Dr. Jennie Ivey, University of Tennessee Equine Extension Specialist, encourage Tennessee equine owners to be proactive in preventing WNV from affecting their horses. “WNV is the leading cause of encephalitis in horses in the United States. This virus is present in all of the continental United States, along with most of Canada and Mexico,” Strickland states. “It is important for owners to be educated on transmission, vaccination and management strategies.” 

Transmission of the virus occurs from bird reservoir hosts by mosquitoes to horses. This means a mosquito must feed from an infected bird, then it can transmit the virus to a horse or human during another blood meal.  Horses and humans are dead-end hosts for WNV, meaning the virus is not directly contagious from horse to horse, or from horse to humans. In addition, indirect transmission via mosquitoes from infected horses is highly unlikely because these horses do not circulate a significant amount of virus in their blood. As a result, horses with the disease do not need to be isolated from other horses or from humans. The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33 percent and horses that survive the acute illness caused by WNV have the potential to exhibit residual effects up to 6 months post-diagnosis.

Affected horses may be depressed and off-feed. Horses with WNV often have muscle tremors and may have convulsions. Some affected horses experience hypersensitivity and maybe easily startled by noise or touch, while others seem drowsy and less reactive than normal. Other signs include walking continuously without purpose or control, circling, exhibiting lack of coordination, and experiencing paralysis, especially in the hind legs. 

Most veterinarians currently recommend that horses in Tennessee receive Spring and Fall booster vaccinations after the initial two-dose vaccine series. This procedure enables horses to maintain a high level of antibodies against WNV. It is extremely important that horses initially receive two doses of the WNV vaccine at the time interval recommended by the vaccine manufacturer. Maximal protection does not occur until several weeks after the second, initial vaccination. If you are in a high-risk area, an additional booster maybe recommended by your veterinarian if this is the first time a horse is receiving the vaccine. Due to its prevalence, the American Association of Equine Practitioners considers the WNV a core vaccine, and recommends vaccination for all horses in North America independent of location, travel and management practices.

Vaccination alone may not give your horse full protection, and a variety of management strategies can be helpful in disease prevention. Mild winters and plentiful rain in the Southeast create ideal conditions for WNV to become a year-round problem with more intense outbreaks in the summer and fall due to mosquito population. Since mosquitoes are the vector for WNV, insect control is key in prevention of this disease. Ivey states, “Mosquitoes require stagnant water to reproduce. Inspect areas around your barn for potential sources of standing water and take action to remove if possible.” 

Any object that can hold water can become a breeding ground, including buckets, old tires, puddles or flooded pastures and unused water troughs. Create drainage for areas that tend to collect water after rain showers. Bring horses inside during peak insect feeding times (early morning and dusk). In addition, topical insect control applied to your horse will repel mosquitoes feeding. 

WNV can certainly be a devastating disease for your horse; however, a prevention protocol will reduce the risk of this disease on your farm. If you have any questions, contact your local veterinarian, your county Extension agent, Dr. Jennie Ivey ( or Dr. Lew Strickland  

Through its mission of research, teaching and extension, the University of Tennessee Institute of Agriculture touches lives and provides Real. Life. Solutions.



Dr. Jennie Ivey,, or Dr. Lew Strickland,


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