West Nile Virus in Horses: Symptoms, Treatment, Vaccination, and Prevention
- Horse Education Online
- 4 days ago
- 12 min read
“West Nile in horses” spikes every late summer for a reason: mosquitoes. Birds carry the virus; mosquitoes spread it; horses (and humans) are dead-end hosts—we don’t pass it on to each other, but we can get seriously ill. Most exposed horses show no signs at all, some get a short fever, and a smaller group develop neurologic disease (wobbly gait, tremors, trouble swallowing). Outcomes range from full recovery to prolonged rehab; a fraction become recumbent and require intensive nursing.
What owners need right now: recognize early red flags, collect objective data (temperature, pulse, respiratory rate), and phone your veterinarian promptly if neurologic signs appear. Vaccination schedules and a few barn-level mosquito steps make the biggest difference year over year.
Quick refresher before you take temps and pulse? Review normal ranges here: The Horse’s Vital Signs and Average Heart Rate for a Horse—What’s Normal and How to Check It.
At-a-Glance
West Nile Virus (WNV) is a mosquito-borne neurologic disease. Call your vet fast if you see fever, hind-end weakness/ataxia, muscle tremors, hypersensitivity to touch, facial droop, or if your horse goes down. Care is supportive (anti-inflammatories, fluids, nursing). Vaccination + mosquito control are the proven preventions. This guide includes a symptom timeline and what to measure at home—with a printable checklist in the full article.
Want structured learning and printable cheat-sheets? Explore our Memberships and our vetted Study Materials.
What is West Nile Virus in horses
Cause & transmission.
West Nile Virus is a flavivirus spread by mosquito bites. Wild birds are the main reservoir; mosquitoes feed on infected birds, then can transmit WNV to horses (and people). Horses do not spread WNV to other horses.
Seasonality.
Risk is highest late summer through fall, especially after warm, wet periods that boost mosquito populations. Regions with standing water (pond edges, clogged gutters, tire piles, low-lying pastures) see more activity.
Why disease severity varies.
Exposure dose & mosquito density (more bites, higher risk).
Immune status & vaccine history (up-to-date vaccination is protective).
Age/comorbidities (very young/older horses or those with metabolic stress may cope worse).
Timeliness of care (early anti-inflammatories, fluids, and nursing can improve comfort and outcomes).
Owner tip: Treat WNV like a time-sensitive neurologic emergency. If you ever see ataxia (wobbliness) or your horse goes down, call your veterinarian immediately and keep the horse quiet, padded, and safe until help arrives.
Early symptoms and red flags (timeline)
You’ll often see a two-phase picture: a short fever/lethargy window, then neurologic signs in some horses. Use the table below to track what you see and what to do.
Day 0–2: early systemic signs
Fever (often ≥ 38.6–39.5 °C / 101.5–103.1 °F), lethargy, reduced appetite, mild stiffness.
Behavior may feel “off”: quieter than usual, less interested in grain, reluctance to work.
Day 2–5: neurologic signs (seek veterinary care)
Ataxia (hind-end wobble, wide-based stance, dragging toes)
Muscle fasciculations (twitching of muzzle, neck, or shoulders)
Hyperesthesia (over-reactive to touch/sound)
Cranial nerve signs (facial asymmetry, droopy lip/ear, difficulty swallowing)
Weakness → recumbency (horse lies down and struggles to rise) in severe cases; rarely seizures
What to measure at home (and why it matters)
Temperature: normal ~37.2–38.3 °C (99–101 °F); > 39.2 °C (102.5 °F) is a red flag.
Heart rate: normal 28–44 bpm; persistent > 60 bpm at rest suggests pain/stress.
Respiratory rate: normal 8–16/min; > 24/min plus fever warrants a call.
Mucous membranes/CRT: pale, brick-red, or tacky gums, or capillary refill > 2 sec = concern.
Refresh the full technique and ranges in The Horse’s Vital Signs and the heart-rate guide here.
Early signs & actions (quick table)
Day window | Sign you might see | How to check (owner) | Action |
0–2 | Fever, dullness, reduced appetite | Digital thermometer; observe feed intake | Monitor closely, record temp q12h; reduce work; call vet if ≥ 39.2 °C (102.5 °F) or if other signs appear |
0–2 | Stiffness, reluctance to move | Walk on straight/turn; note reluctance | Light rest; phone vet if stiffness escalates or fever persists >24h |
2–5 | Ataxia (wobbly/hind-end weakness) | Tail pull test (gently); observe corners/backing | Call vet immediately; confine to deep-bedded stall to prevent falls |
2–5 | Muscle tremors/fasciculations | Watch muzzle/neck/shoulders at rest | Keep environment quiet/dim; vet exam same day |
2–5 | Hyperesthesia (over-reactive to touch/sound) | Lightly stroke neck/flank | Minimize stimuli; vet exam same day |
2–5 | Facial droop, difficulty swallowing | Check symmetry, observe drinking | Remove hay nets, offer small sips; vet now |
Any | Recumbency (can’t rise), seizures | Visual | Emergency: keep clear space, soft bedding; urgent vet |
Download this West Nile Virus in horses owner pack—Daily Symptom & Vital Sign Log plus a When to Call Your Veterinarian guide—to track vitals consistently and act fast on red flags.
Real-world example:
A 12-year-old gelding in late August shows 39.4 °C (103 °F) fever and goes off feed for a day. By day 3 he’s hind-end wobbly and his muzzle twitches. Owner moves him to a padded, quiet stall, calls the vet, and starts logging temps/heart rate every 4–6 hours. This is classic west nile virus in horses and needs prompt supportive care.
When to call your veterinarian (decision points)
If you remember one rule for west nile in horses, make it this: any neurologic sign = call your vet now. That includes a wobbly hind end, muscle tremors, hypersensitivity to touch or sound, facial droop, trouble swallowing, or a horse that suddenly prefers a wide-based stance to keep balance.
A fever that pushes past 39.2 °C (102.5 °F), especially when paired with dullness or stiffness, also warrants a same-day exam. Horses with West Nile can slide from “a bit off” to “unsafe to stand” quickly; the sooner your veterinarian is looped in, the better your horse’s comfort and safety.
Use your basic health checks to guide the call: temperature, heart rate, respiratory rate, gum color and capillary refill time. If numbers are abnormal or climbing over a few hours, don’t wait for “one more check.” For a quick refresher on early illness cues, see How to Tell if Your Horse is Sick—Early Signs Every Owner Should Know.
Call immediately if you see any of the following:
A horse that can’t rise, is falling, or looks progressively weaker
Ataxia (hind-end wobble), muscle fasciculations, or difficulty swallowing
Fever > 39.2 °C (102.5 °F) that doesn’t ease within 12–24 hours, or returns with new neurologic signs
Keep the horse quiet in a deep-bedded stall, remove hay nets and high buckets (aspiration risk), and turn off loud fans/music until your vet arrives.
Diagnosis: what your vet may do
Your veterinarian will start with a calm, systematic physical and neurologic exam: mentation, cranial nerves (eye/ear symmetry, lip tone, swallow), gait and coordination, and a check for pain or trauma. The goal is to confirm neurologic dysfunction, ensure the horse is safe to handle, and rule out other causes that can mimic west nile virus in horses.
Common rule-outs include EEE/WEE, EHV-1 neurologic disease, head or spinal trauma, toxicities, and—if fever, diarrhea, or laminitis are present—Potomac Horse Fever (PHF), which can rarely appear with neurologic-seeming weakness. For context on PHF patterns and vital-sign shifts, review Potomac Horse Fever—Symptoms, Vital Sign Changes, and When to Act.
Testing you might hear about:
IgM capture ELISA on serum: detects recent/active West Nile exposure.
Paired titers (two blood samples, 10–14 days apart): rising antibodies support a new infection.
CSF tap or PCR in select cases: used when results would change biosafety or case management.
Your vet will also weigh in on barn-level risk (recent mosquito blooms, nearby positives) and vaccination status. While results may take time, supportive care often begins immediately based on the exam, because comfort and safety can’t wait on a lab report.
Treatment and nursing care
There’s no antiviral that “kills” West Nile disease in horses; care is supportive and focuses on reducing inflammation, stabilizing hydration and electrolytes, and protecting a neurologically unstable horse from injury. Most horses are treated on farm; a small subset need hospitalization for fluids, sling support, or intensive nursing.
What treatment often looks like (guided by your vet):
Anti-inflammatories for fever and comfort, judicious sedation if hyperesthesia makes the horse unsafe, and IV or oral fluids/electrolytes if intake is poor. If weakness is significant, your vet may recommend a sling or padded stall management to prevent pressure sores and falls. Because swallowing can be affected, feeding plans shift to soaked, soft rations offered low and in small, frequent meals.
Owner nursing that makes a real difference
Environment: Quiet, dim, non-slippery footing; deep bedding; wide doorway. Remove high hay nets or buckets to reduce choking/aspiration risk.
Hydration: Aim for typical intake of ~25–55 mL/kg/day (about 12–30 L/day for a 500 kg horse). Offer fresh water at ground level and consider soaked hay pellets or mashes. If drinking lags or gums feel tacky, alert your vet promptly; see How to Tell if a Horse is Dehydrated—Simple Checks Every Owner Should Know.
Skin & comfort: Turn the horse or help it to shift weight every 2–4 hours if weak; cushion hips, shoulders, and hocks; keep the tail clean and wrap only if you can monitor.
Feeding: Start with small, easy-to-swallow meals (soaked hay pellets, well-moistened senior feeds). Watch for coughing or nasal feed material—both signs to stop and call the vet.
Monitoring: Log temperature, heart/respiratory rate, appetite, manure/urine, and coordination twice daily at minimum. Share trends with your veterinarian.
Most horses that remain standing and can eat/drink improve over days to a few weeks, though some need several weeks for neurologic signs to resolve, and a minority have residual deficits (subtle toe dragging, mild asymmetry). Set expectations early with your vet, and plan rechecks to adjust medications and exercise as recovery unfolds.
Want structured care logs and printable barn checklists? They’re included with our Memberships and in our Study Materials library.
Vaccination: timing and options
West Nile vaccination is core for horses in North America. You can buy WNV as a stand-alone shot or inside a combo (your common “5-way” becomes a “6-way” when WNV is added). Either way, the goal is the same—prime a naïve horse, then booster before mosquito season every year. In higher-risk barns (long warm seasons, wetlands, frequent positives), many vets step up to semi-annual boosters.
Two details matter more than brand names: timing and completing the primary series. Most adult horses need two doses for their first series (intervals vary by product; your veterinarian will follow label/AAEP guidance), then an annual pre-season booster. Pregnant mares are typically boosted 4–6 weeks pre-foaling so foals receive antibodies in colostrum. Foals themselves start later (to avoid maternal antibody interference) and usually receive a three-dose series.
Quick reference (confirm with your veterinarian/label):
Horse | Primary series | Booster | Notes |
Adult, never vaccinated | 2 doses, 3–6 weeks apart | 1 dose annually before mosquito season | Consider semi-annual in high-risk areas |
Adult, lapses > 12 months | Restart with 2-dose series | Annual thereafter | Treat like naïve if history is unclear |
Pregnant mare | Keep current per adult schedule | Booster 4–6 weeks pre-foaling | Optimizes colostral antibodies |
Foal of vaccinated mare | Begin at 4–6 months, 3 doses q4–6 weeks | Booster at 10–12 months | Vet may shift earlier/later by risk |
Foal of unvaccinated mare | Begin 3–4 months, 3 doses q4–6 weeks | Booster at 10–12 months | Earlier start due to low passive transfer |
If you already give a 5-way (EEE/WEE, tetanus, influenza, EHV-1/4), adding WNV turns it into a 6-way—handy for compliance, but it’s still your calendar that protects you. Put your WNV booster on the calendar 3–4 weeks before mosquitoes surge.
For a deeper look at multi-valent products and scheduling trade-offs, see 5-Way Equine Vaccine—What It Covers and Why It Matters.
Owner tip: if your horse had confirmed WNV in past years, don’t assume “lifetime” protection. Many clinicians still vaccinate annually because field exposure, age, and immune variability differ horse to horse.
Mosquito control that actually lowers risk (with checklist)
West Nile prevention lives or dies on how well you break the mosquito life cycle (7–10 days in warm, still water) and how consistently you block bites during peak activity (dusk–dawn, late summer–fall). Think in layers—fix water first, then make horses harder to bite, and finally clean up the property edges so new mosquitoes aren’t constantly drifting in.
1) Kill the nurseries (this does the most good).
Walk the barn with a bucket in hand and tip anything that holds water: tires, feed pans, muck tubs, tarps, wheelbarrows, gutter elbows, tractor buckets. Scrub the scum ring on troughs; that biofilm shelters larvae. Repair downspouts and slow leaks. Low, muddy gateways? Add rock or re-grade so puddles don’t linger.
2) Make bites harder.
Run box fans in stalls/run-ins; airflow makes landing difficult. Use tight screens on windows and well-fitting door strips. During heavy weeks, shift turnout away from dusk and dawn. Consider well-fitted fly sheets/hoods for sensitive horses.
3) Use products correctly—not guesses.
Choose equine-labeled repellents (pyrethrin/permethrin classes) and apply to a clean, dry coat, reapplying on the label’s schedule. For ponds/swales you can’t drain, use Bti larvicide dunks only in non-drinking water. Never add oil, bleach, or dish soap to troughs—unsafe and illegal in many regions.
4) Mind the perimeter.
Mow tall weeds, trim brush near water, stack rails and tarps so they shed rain, and keep manure areas tidy (Culex prefer still water over manure, but wet edges around piles can breed them).
5) Keep score.
After a storm, do a 10-minute “water patrol.” Fewer larvae and fewer adults inside the barn within a week means your plan is working.
Barn mosquito-control checklist
Task | How to do it well | When |
Dump & scrub small containers (pans, muck tubs, tires) | Empty completely and scrub the ring; store upside-down | Every 2–3 days; after rain |
Trough maintenance | Brush the scum line, refill; place in sunny, breezy spot | Weekly (more in heat) |
Fix drainage & leaks | Clear gutters/downspouts; extend splash blocks; mend hydrants | Weekly check; post-storm |
Fans & screens | Run fans in stalls; repair screens/door strips that gap | Set up for season; monthly check |
Turnout timing | Avoid dusk–dawn during peak weeks | Peak weeks only |
Repellent use | Equine-labeled; apply to clean, dry coat; follow re-apply interval | As labeled |
Standing water you can’t drain | Bti dunks in non-potable water (pond edges, swales) | Per label (often monthly) |
Vegetation control | Mow weedy margins; trim around water/ditches | Biweekly |
Recovery and return-to-work plan
Think “neurologic injury first, fitness second.” Inflammation around the brain and spinal cord needs time to quiet down, and coordination must be stable before you add work. Your vet’s timeline wins, but here’s a practical owner roadmap you can adapt.
Phase 1 — Stabilize (Days 0–7).
Quiet stall rest with deep bedding and good lighting so the horse can place feet safely. Keep the environment calm (fans help with mosquitoes and noise), feed soaked, easy-to-swallow meals at ground level, and log vitals and coordination twice daily. If the horse was recumbent or severely ataxic, expect this phase to last longer and plan rechecks with your vet.
Phase 2 — Controlled motion (Week 2).
If the horse is eating, drinking, standing, and neurologic signs are improving, begin 5–10 minutes of hand-walking once or twice daily on straight, level footing. No lunging, small circles, or tight turns. Increase walking time by 5 minutes every 2–3 days as long as coordination remains steady and there’s no fever or new tremors.
Phase 3 — Rebuild basics (Weeks 3–4).
Progress to 15–30 minutes of walking under saddle (or in-hand if safer). Add very short trot sets (30–60 seconds) on straight lines only, watching for toe-dragging or loss of balance. Keep sessions short and end on success. Many horses live comfortably here for a week or two while the nervous system catches up.
Phase 4 — Conditioning (Weeks 5–6+).
If coordination is normal and appetite/attitude are good, extend to 30–45 minutes of walk/trot work, sprinkle in large, shallow figures, and only then consider canter. Save lateral work and jumping for much later. Schedule a veterinary re-check before returning to full training, competitions, or trail miles.
Pause and call your vet if any of the following pop up:
New or worsening ataxia, stumbling, facial asymmetry, or muscle tremors
Fever ≥ 38.6 °C (101.5 °F) after exercise or at rest
Resting heart rate > 60 bpm or respiratory rate > 24/min that doesn’t settle within 10–15 minutes post-work
Reduced appetite, attitude change, or coughing while eating (possible swallow issues)
Download this West Nile Virus in horses owner pack—Daily Symptom & Vital Sign Log plus a When to Call Your Veterinarian guide—to track vitals consistently and act fast on red flags.
For a refresher on normal ranges and how to measure them, see our guides to The Horse’s Vital Signs and Average Heart Rate for a Horse—What’s Normal and How to Check It. If you’d like printable tracking sheets, they’re included in our Memberships and Study Materials.
FAQs
Is West Nile contagious horse-to-horse?
No. Horses (and humans) are dead-end hosts for West Nile virus in horses. The virus spreads by mosquito bites, not by contact with stall neighbors, waterers, or tack.
Can a vaccinated horse still get West Nile?
Breakthroughs are uncommon but possible. Vaccination significantly lowers the risk and severity—vaccinated horses are far less likely to become recumbent and more likely to recover with supportive care.
How long does immunity last after vaccination?
Plan on an annual pre-season booster; some high-risk barns use semi-annual boosters. If your horse is naïve or lapsed, complete the primary series first. For combo scheduling details, see 5-Way Equine Vaccine—What It Covers and Why It Matters.
What’s the survival rate and typical recovery time?
Most standing, eating horses recover over 2–6 weeks with good nursing and veterinary care. Horses that become recumbent have a more guarded outlook and may need intensive support; some horses retain mild, long-term coordination quirks.
Is there a specific antiviral for West Nile in horses?
No. Treatment is supportive: anti-inflammatories, fluids/electrolytes, a safe, quiet environment, and meticulous nursing to prevent falls, sores, and aspiration.
My horse had West Nile years ago—do we still vaccinate?
Yes. Natural infection doesn’t guarantee durable protection. Most veterinarians recommend continuing annual boosters based on field risk and individual factors.
Should I separate horses or move barns during an outbreak?
Isolation doesn’t change risk the way it does for contagious respiratory disease. Focus on mosquito control (standing water, fans, screens, turnout timing, labeled repellents) and keep all horses current on vaccination. For printable barn checklists and logs, explore our Study Materials or join our Memberships.
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