West Nile virus reaches horses through mosquito bites. Most infected horses never get sick, but in the ones that do, the virus targets the brain and spinal cord and can make them seriously ill.1PubMed. Incidence and effects of West Nile virus infection in vaccinated and unvaccinated horses in California2CDC. West Nile: Causes and How It Spreads No drug kills the virus, so prevention does most of the work, and a yearly vaccine is highly protective.3Total Equine Veterinary Associates. West Nile Virus | Total Equine Veterinary Associates4PubMed Central. Animal and Human Vaccines against West Nile Virus A case in your barn means infected mosquitoes are active nearby, so it is a warning for your household too.
Symptoms of West Nile Virus in Horses
The most common signs are neurological. A horse may stumble or move with poor coordination, along with weakness in the hind legs and muscle tremors or twitching.5PubMed. Investigation of an outbreak of encephalomyelitis caused by West Nile virus in 136 horses Fine twitching under the skin, often over the face, neck or shoulders, is a common sign.6PubMed. West Nile virus encephalomyelitis in horses: 46 cases (2001)7Equine Veterinary Education. Clinical screening of horses and early warning for West Nile virus Many sick horses also run a fever.8PubMed. West Nile virus encephalomyelitis in horses: 46 cases (2001) Weakness and stumbling can be lopsided, worse on one side or in one leg.9PubMed. West Nile virus encephalomyelitis in horses: 46 cases (2001) Some horses act strangely, grind their teeth, or seem dull or overly jumpy.10Equine Veterinary Education. Clinical screening of horses and early warning for West Nile virus
Early signs are subtle. The horse may trip or drag a toe, especially behind, or sway or lean when you turn it tightly or ask it to back up. Signs often come on over a day or two in late summer or fall.
If a horse is still on its feet but you suspect a problem, walk it in a straight line and then in a tight turn. Mild stumbling often shows only when the hind end has to coordinate. Watch for a hind toe that drags, a fetlock that knuckles over, or legs that cross.
Any new stumbling or twitching in mosquito season warrants a same-day call to your vet. An uncoordinated horse can fall onto handlers, so keep yourself out of its path. Take its temperature before the vet arrives, and note when signs began and when the horse was last vaccinated. Rabies can look similar, so wear gloves around the horse’s mouth and avoid handling saliva until the vet has ruled rabies out.
In severe cases a horse may go down and be unable to rise. Some horses survive long stretches lying down if they can stay upright on their chest and keep eating.11PubMed Central. Epidemiology and Clinical Manifestation of West Nile Virus Infections of Equines in Hungary, 2007–2020
How Horses Catch West Nile Virus
The virus normally circulates between mosquitoes, mainly Culex species, and birds.12CDC. West Nile: Causes and How It Spreads Common birds like robins and house sparrows carry high virus levels while staying mostly healthy, so mosquitoes pick it up easily.13extension.rutgers.edu. West Nile Virus in Horses: Symptoms, Treatment & Prevention About a week after biting an infected bird, a mosquito can pass the virus on.14CDC. West Nile: Causes and How It Spreads Horses get infected when those same mosquitoes bite them.15extension.rutgers.edu. West Nile Virus in Horses: Symptoms, Treatment & Prevention
Horses are dead-end hosts: they never carry enough virus to infect mosquitoes that bite them.16PubMed Central. Comparison of West Nile Virus Disease in Humans and Horses: Exploiting Similarities for Enhancing Syndromic Surveillance That means a sick horse can’t infect barn mates or you, directly or through mosquitoes.17CDC. West Nile: Causes and How It Spreads18PubMed Central. Comparison of West Nile Virus Disease in Humans and Horses: Exploiting Similarities for Enhancing Syndromic Surveillance Separating a sick horse isn’t needed to protect others, though herdmates share the same mosquito exposure, so check their vaccines.
Risk climbs through late summer and fall. Each round of mosquitoes biting infected birds produces more infected mosquitoes, so the chance of a bite from a carrier is highest late in the season.
Treatment and Recovery
No medicine kills the virus, so treatment supports the horse while its immune system fights.19Total Equine Veterinary Associates. West Nile Virus | Total Equine Veterinary Associates Anti-inflammatory drugs reduce swelling in the nervous system.20Total Equine Veterinary Associates. West Nile Virus | Total Equine Veterinary Associates Plasma containing West Nile antibodies is sometimes given during treatment.21Total Equine Veterinary Associates. West Nile Virus | Total Equine Veterinary Associates During the critical phase, keeping the horse eating and upright on its chest matters a great deal.22PubMed Central. Epidemiology and Clinical Manifestation of West Nile Virus Infections of Equines in Hungary, 2007–2020
Nursing care does much of the work. A weak, wobbly horse needs deep bedding and often padding to protect it when it stumbles or falls. A horse that is down may be helped to stand or supported in a sling, which takes pressure off and lets it try to get up. Fluids and assisted feeding keep strength up if the horse can’t or won’t eat and drink enough. Recovery is usually measured in weeks, so expect a long stretch of care rather than a quick turnaround.
Many horses recover fully.23UC Davis Center for Equine Health. West Nile Virus But as many as 40% may have lasting effects such as behavior changes, muscle wasting, limb weakness and lower stamina.24New Mexico State University Extension. West Nile Virus in Horses: Symptoms, Treatment & Prevention
Horses with lasting effects need a gradual, vet-guided return to work. Some regain soundness over months. Residual weakness can make riding unsafe, so have the horse reassessed before you climb on.
How Vets Confirm West Nile and What Else It Could Be
The preferred test in a living horse is a blood test for the early antibodies made soon after infection, which your vet may call an IgM test.25Washington Animal Disease Diagnostic Laboratory. West Nile Virus in Horses: Symptoms, Treatment & Prevention A positive result plus matching signs strongly supports the diagnosis.26Washington Animal Disease Diagnostic Laboratory. West Nile Virus in Horses: Symptoms, Treatment & Prevention Vets rely on antibodies because looking for the virus itself in blood often misses it.27PubMed. Diagnosis of West Nile virus infection in horses After death, testing brain tissue for the virus is preferred.28Washington Animal Disease Diagnostic Laboratory. West Nile Virus in Horses: Symptoms, Treatment & Prevention
A recent vaccine can also trigger a positive early-antibody blood test.29PubMed. Comparison of assays for the detection of West Nile virus antibodies in equine serum after natural infection or vaccination So the vet needs your horse’s vaccine dates and may test twice, a few weeks apart.30Cornell University College of Veterinary Medicine. West Nile Virus Diagnostic Testing
West Nile can mimic EPM, rabies, eastern equine encephalitis, equine herpesvirus-1 and botulism, and you can’t tell them apart by looking.31Conley and Koontz Equine Hospital. EPM vs West Nile Virus Tell the vet about recent travel, shows or new horses on the property, and how quickly the signs came on.
The look-alikes matter. Rabies is a danger to people, so ruling it out protects you as well as the horse. Equine herpesvirus-1 spreads from horse to horse, unlike West Nile, so that diagnosis means barn mates may need to be separated and monitored.
Vaccinating Your Horse Against West Nile
In one study, no vaccinated horse developed West Nile disease, while about 5% of unvaccinated horses did, even though two-thirds became infected.32PubMed. Incidence and effects of West Nile virus infection in vaccinated and unvaccinated horses in California In a severe challenge test, all vaccinated horses survived while all unvaccinated ones died.33PubMed Central. Comparative efficacies of three commercially available vaccines against West Nile Virus (WNV) in a short-duration challenge trial involving an equine WNV encephalitis model Protection fades, so yearly boosters are needed.34PubMed Central. Animal and Human Vaccines against West Nile Virus
- Previously vaccinated adults: once a year in spring, before mosquito season.35American Association of Equine Practitioners. West Nile Virus Vaccination Guidelines
- Long or intense mosquito seasons: more often than yearly.36CDFA. West Nile Virus Vaccination
- Pregnant mares: 4 to 6 weeks before foaling, which can protect foals for up to 4 months.37American Association of Equine Practitioners. West Nile Virus Vaccination Guidelines38CDFA. West Nile Virus Vaccination
- Foals of vaccinated mares: a 3-dose series starting at 4 to 6 months, second dose 4 to 6 weeks later, third at 10 to 12 months.39American Association of Equine Practitioners. West Nile Virus Vaccination Guidelines
- Earlier starts: some guidance starts foals at 3 to 4 months.40CDFA. West Nile Virus Vaccination
- Never-vaccinated adults: need a starting series of more than one dose.
A single shot in a never-vaccinated horse is not full protection, because the first vaccination needs more than one dose to build immunity. Waiting until cases are reported nearby is risky, since immunity takes weeks to build and infected mosquitoes may already be biting. Skipping a year lets protection fade, and the horse may then be treated almost like one never vaccinated, so ask your vet what is needed to get back on track.
Several licensed vaccines exist and all have been shown to protect. Your vet may choose based on age, pregnancy status and history. Record the brand and date every time, because that record helps the vet interpret an early-antibody test if the horse ever gets sick.
Cutting Mosquitoes Around the Barn
Water troughs, ponds and liquid manure are the top mosquito breeding spots on horse farms.41PubMed. Diversity and ecology survey of mosquitoes potential vectors in Belgian equestrian farms: A threat prevention of mosquito-borne equine arboviruses Dump tubs and buckets at least every 3 days.42horsehealthproducts.com. Mosquito Control: A Multi-Pronged Battle Discard tires and junk that hold water, and fix drainage.43horsehealthproducts.com. Mosquito Control: A Multi-Pronged Battle A fountain, filter or larva-eating fish keep ponds from breeding mosquitoes, and local mosquito programs sometimes offer fish free or cheap.44horsehealthproducts.com. Mosquito Control: A Multi-Pronged Battle
Scrub troughs as well as dumping them, since scrubbing removes the slime and any eggs or larvae that dumping alone can leave behind. Check gutters, tarps and low spots after rain, because water that sits in them can breed mosquitoes within days. Move manure piles away from water and keep runoff from pooling.
Repellents, insecticides, stabling and screens all help protect the horse.45EDIS. Protecting Florida Horses from Mosquitoes Few owners limit mosquito habitat, though it adds to vaccine protection.46TopSCHOLAR. Strategies for Reducing Mosquito-Borne Disease Vulnerability in Equine Populations: A Kentucky Case Study Culex bite most at dusk and through the night, so stable horses then where practical. Reapply repellent as the label directs, and use fly sheets and masks to reduce exposed skin. Wear repellent at the barn yourself, since the same mosquitoes bite people.