West Nile Disease (WND) occurs everywhere in North America and is a preventable neurological disease in horses. It is spread by the bite of an infected mosquito to horses, people, birds, and other susceptible species. Some consider horses, humans, and birds of the crow family to be more at risk.
Horses are at the highest risk for contracting West Nile virus during peak mosquito season, which starts in the summer and continues through fall. Cases of the disease are being reported in all of the western states this summer.
Mosquitoes (carriers of the virus) become infected while taking a “blood meal” from West Nile virus (WNV)-infected birds; reservoirs of the disease organism. Crows seem to be particularly susceptible.
WNV cannot spread from horses to other horses nor horses to people or people to horses. However, if you are in the company of an infected horse and there are mosquitoes around, there’s always the chance that infected mosquitos could be reasonably close.
This virus was first diagnosed in dying birds in the U.S. in 1999. This then-unknown neurologic disease soon spread to humans, horses and other birds. WNV was reported in 25 horses in that year with cases peaking at 15,257 horses in 2002. A conditional license for vaccination against WND was approved in 2001. Many believe the rapid development of the vaccine played a key role in the control of WNV in those early years.
Thousands of horses in the U.S. were dying from this new and fatal disease. Less than four years after its North American introduction, it posed the biggest health threat the horse industry had seen in the 20th century. Even today, many believe that West Nile virus in horses has been largely underreported in North America in the past and present.
Diagnosis
WND is now an important consideration in the differential diagnosis of horses presenting with signs of neurologic disease in all areas of North America. Other equine neurologic diseases presenting with many of the same signs include rabies, Eastern and Western encephalitis, and the Western encephalitis, and the neurologic form of equine herpes virus.
West Nile does not always lead to signs of illness in horses. The incubation period of West Nile virus in horses is three to 15 days. Clinical signs of mild disease may last only a few days. For horses that do become clinically ill, symptoms are flu-like; off feed, depression, lethargy and fever.
The virus typically infects the central nervous system and changes in mentation including apprehension, hypersensitivity to touch, and sound disorientation. Hyper-excitability or aggression can also be associated with the disease.
More severe neurological symptoms include muscle and skin fasciculation, impaired vision, ataxia (stumbling and incoordination), aimless wandering, and circling. Signs can also vary from drowsiness to driving or pushing forward to asymmetrical weakness and imbalance to convulsions, seizures, recumbency, coma and death.
About a 30-40 percent of unvaccinated horses that are symptomatic with the mosquito-borne virus die or must be euthanized. In the opinion of some owners, horses surviving the acute illness exhibit residual effects six months following diagnosis, such as gait and behavioral abnormalities that were attributed to the illness.
Diagnosis of WND is based on clinical signs, vaccination history, and environmental exposure to mosquitoes. Observations of dead birds may be an extra clue as to presence of the virus. If horse owners notice symptoms of WND in their horses, they should contact a veterinarian immediately. A diagnostic blood test is available and recommended to help rule out other severe neurologic disease plus indicate mosquitoes carrying the virus are in the area and need to be eliminated.
Treatment
Treatment is symptomatic; goals for WND are minimizing fever and inflammation in the nervous tissue using appropriate anti-inflammatory medications. Supportive care is often difficult especially when these animals are recumbent. Wobbly and downer horses, especially if their mentation is altered, can be dangerous to be around.
It’s important to talk to the attending veterinarian about the cost of treating these cases. The cost of supportive care for an infected horse can exceed $10,000, and varies depending on severity of clinical signs, prognosis, location or veterinary hospitals with capabilities of intensive care for these patients, which often includes slings and availability of spare personnel.
Vaccination remains the most effective way to help protect horses against WND, although the clinical disease cannot be completely prevented. There are several licensed vaccines currently available. Consultation with a veterinarian is advised as there are different vaccination protocols depending on age, use, location of horse, etc. Because of the success of vaccination, WNV is highly preventable in the fully and repeatedly vaccinated horse.
In addition to vaccinations, preventive management practices may minimize the risk of the spread and transmission of WNV from infected mosquitoes to horses. Reduction of mosquito numbers and exposure can be achieved by reducing or eliminating stagnant or standing water in your area, keeping horses in the barns from dusk to dawn (prime mosquito feeding times), setting out mosquito traps, keeping air moving with fans, use of mosquito fish in water troughs, and removing organic debris promptly.
The use of mosquito dunks in areas of standing water is an option.
Also, don’t forget the “four Ds” for human personal protection against skeeters; these can also be applied to our equine comrades:
• Dusk and Dawn—These are the periods of greatest mosquito activity. This can be a problem since these periods fall within cool temperatures to begin or finish working cattle, training horses, or taking a pleasure ride for some folks.
• Dress—Wear long sleeves and pants that mosquitoes can’t bite through. Horses can also be protected with mosquito sheets.
• Drain—Get rid of standing water. This can be difficult since things like old tires, unused feed troughs, and 50-gallon barrels are frequent sites on the ranch, but collect water that often goes unnoticed. Additionally, actively used water troughs for livestock can be a risk. If dumping water out your 200-gallon water troughs every 10 days doesn’t fit your ranch management plan, keep in mind there are other possible economically-friendly and safe options to “doctor” your drinkers, ponds, small tanks, and outdoor Jacuzzis.
• DEET—DEET is the most effective mosquito repellent ingredient commercially available in products safe for use on both horse and riders. Keep in mind children should use products with lower concentrations.
The best protections are prevention. Get rid of skeeters and their nesting grounds as best you can and vaccinate your horses. Don’t say goodbye to those special horses, foal crops, or the remuda; it is cheaper to vaccinate than treat.
