Eastern & Western Equine Encephalitis in Horses: Signs, Vaccination & Mosquito Control

EEE and WEE signs that belong in the emergency lane

These diseases can progress quickly. Do not wait for every sign on the list before calling your veterinarian.

• Fever, marked depression, unusual quietness, or loss of appetite.
• Sudden behavior change, impaired vision, circling, or head pressing.
• Muscle twitching, tremors, weakness, stumbling, crossing the limbs, or loss of coordination.
• Difficulty swallowing, facial weakness, paralysis, or an inability to stand normally.
• Seizures, collapse, or recumbency.

What to do in the first hour

1. Stop work and call your veterinarian. Give the horse's temperature, vaccination history, travel history, mosquito exposure, and the exact time the change was first noticed.
2. Reduce injury risk. Move only when it can be done safely. Use a quiet, low-traffic area with secure footing and remove buckets, sharp edges, and obstacles that could injure a disoriented horse.
3. Do not force movement, feed, or water. A weak or uncoordinated horse can fall, and a horse with swallowing difficulty can aspirate.
4. Limit shared equipment and unnecessary traffic. EEE and WEE do not spread directly from horse to horse, but other neurologic diseases can initially look similar. Follow your veterinarian's biosecurity instructions until the cause is clearer.
5. Document the change. Record temperature, appetite, manure, drinking, posture, coordination, facial symmetry, and any progression while help is on the way.

How EEE and WEE spread

EEE and WEE circulate primarily between mosquitoes and birds. Horses become infected through the bite of an infected mosquito. Horses are considered dead-end hosts, meaning they do not develop enough virus in the bloodstream to infect mosquitoes and continue the transmission cycle.

That distinction matters at the barn: an affected horse is not the source of EEE or WEE infection for barnmates. The shared exposure is mosquito activity in the area. Veterinary evaluation still matters because several contagious and noncontagious conditions can cause similar neurologic signs.

Vaccination is the foundation of prevention

The American Association of Equine Practitioners classifies Eastern and Western equine encephalomyelitis vaccination as core protection for horses. Build the schedule with your veterinarian and follow the vaccine label. Timing can differ for previously unvaccinated horses, foals, broodmares, horses with incomplete records, and horses living or traveling where mosquito seasons are long.

• Review records before mosquito season rather than waiting for a local case.
• Allow enough time for a primary series or booster to produce an immune response before likely exposure.
• Ask whether local risk, travel, age, health status, or an extended mosquito season changes the booster plan.
• Keep the date, product, lot number, and veterinarian record where they can travel with the horse.

Vaccination lowers risk but does not make mosquito management optional. No vaccine or repellent creates a zero-risk barn.

Reduce mosquito pressure around the horse

The strongest barn plan removes breeding sites, reduces mosquito access, and adjusts routines when local pressure rises.

• Dump standing water from buckets, tires, wheelbarrows, tarps, flowerpots, and unused equipment.
• Scrub and refill troughs and water containers on a consistent schedule.
• Clear clogged gutters, drains, and low spots that hold water after irrigation or rain.
• Use fans and intact screens where practical; mosquitoes are weak fliers in moving air.
• Manage vegetation, manure, and wet organic debris near stalls and turnout areas.
• Adjust turnout around local mosquito activity when practical, especially during peak pressure.
• Use only an equine-labeled repellent, exactly as directed, when repellent is part of the plan.

Repellent is one layer. It does not replace vaccination, habitat control, veterinary guidance, or current local disease information.

Before hauling to a show, clinic, trailhead, or new barn

• Confirm vaccination records and destination requirements before loading.
• Check current state-animal-health and equine-disease alerts for the origin, route, and destination.
• Inspect the trailer and destination stall for standing water, damaged screens, and heavy mosquito activity.
• Pack the veterinarian's number, horse identification, health records, thermometer, and a basic emergency kit.
• Take a baseline temperature and note appetite, attitude, and coordination before departure and after arrival.

For the separate West Nile virus intent, use the West Nile Virus in Horses guide. For broader emergency and seasonal education, visit the Horse Health Library.

EEE and WEE in horses: frequently asked questions

Sources reviewed

American Association of Equine Practitioners vaccination guidance; USDA APHIS equine encephalitides guidance; Equine Disease Communication Center disease information and July 2026 alerts; and CDC Eastern equine encephalitis transmission and surveillance guidance.