Equine Influenza in Horses: Signs, Spread and Fall Vaccine Timing

What Is Equine Influenza, and What Just Happened in Wisconsin?

Equine influenza is a highly contagious respiratory disease of horses caused by an influenza A virus. It shows up as fever, a dry cough and nasal discharge, and it spreads quickly wherever horses share air or equipment.

On Sept. 30, 2026, The Horse and the Equine Disease Communication Center (EDCC Alert 5278) reported a confirmed case in a 10-year-old mare in Polk County, Wisconsin. The Horse describes her as a Quarter Horse; the EDCC alert lists her as a Quarter Horse cross. She developed fever, cough and bilateral nasal discharge on Sept. 22 and tested positive on Sept. 25. She is recovering, and four horses were exposed.

The EDCC lists the case as confirmed under voluntary quarantine, at a private facility. The EDCC alert lists the mare as unvaccinated. The Horse's write-up does not mention her vaccination history.

One case on one farm is not a regional outbreak. It is a useful nudge, because the same virus shows up every fall and winter as horses move between barns and shows.

Horse Flu Symptoms: What to Look For

The classic picture is sudden and obvious. Merck Veterinary Manual describes high fever (up to 106°F), a dry, harsh cough, depression, loss of appetite, weakness, watery nasal discharge and slightly enlarged lymph nodes.

The signs, in the order owners usually notice them

• Dry, hacking cough. Usually the first sign, according to the AAEP, though not every infected horse coughs. Deep and repeated, sometimes set off by exercise or hay dust.
• Fever. Usually follows the cough. A resting adult is normally about 99 to 101°F.
• Nasal discharge. Clear and watery at first. It can turn thick and pus-like, as it was in the Wisconsin mare.
• Lethargy and poor appetite. A horse that stands with its head low and ignores its grain is telling you something.
• Swollen legs or lymph nodes. Less common, but reported.

Signs tend to appear within a few days of exposure. Vaccinated horses can have milder illness, and some infected horses show very little while still shedding virus.

Horse cough and nasal discharge: how flu differs from strangles and EHV-1

No owner can sort these out by eye. Even vets need a nasal swab or blood test. But the patterns below help you describe what you see on the phone.

Merck Veterinary Manual notes that flu can be hard to tell apart from herpesvirus infections, though cough is more typical of influenza. Read our guides to strangles and EHV-1 for their own warning signs.

How Equine Influenza Spreads: Air, Hands and Shared Gear

The virus moves in respiratory secretions. A coughing horse throws out droplets, and the AAEP says the virus spreads rapidly through groups of horses that way, as well as on fomites (objects that carry it).

That means a horse can catch it from another horse in the next stall, from a wash-rack hose, from a shared bucket or from a person who touched a sick horse and then a healthy one. Merck notes the virus can survive a couple of days on objects and in water.

It is also why it thrives at shows and boarding barns: many horses, shared air in stabling tents and trailers, and shared water. A horse with only mild signs can still shed virus.

Why exposed horses get isolated and temperature-checked

The Wisconsin report lists four exposed horses. That is standard outbreak handling: separate them, then check temperatures often.

A new cough or a rising temperature can show up before a horse looks sick. Catching fever early lets your vet test quickly and keeps the rest of the barn from being exposed for days.

Prevention Checklist for Barn Owners and Show Riders

Most of this is free and works against many respiratory diseases, not only flu.

• Quarantine new arrivals. Keep them separate, with no nose-to-nose contact, for the period your vet recommends. Two to three weeks is common.
• No nose-to-nose contact. That includes over fences, at the trailer and at the in-gate.
• No shared buckets, hoses or tack. Bring your own water bucket and clean it. Do not dunk a common sponge.
• Wash hands and change clothes between a sick or new horse and the rest of the herd. Do the sick horse last.
• Take temperatures twice a day during an outbreak for every horse exposed, and write them down.
• Disinfect shared equipment, trailers and stalls, and let them dry.
• Stay home if your horse is coughing or has a fever, even if a show is on the calendar.

Our horse first aid kit guide shows how to take temperature, pulse and respiration, and the health basics section covers normal vital signs.

Equine Influenza Vaccine: The Six-Month Show Rule

Here is the lasting takeaway. The Horse notes that US Equestrian requires proof of an equine influenza vaccination within the six months before a sanctioned competition or event.

If your last flu shot was in March or April, you may already be outside that window for a late-fall or winter show. Check the date on your vet record, and check the specific prize list, because some shows and barns ask for more.

Timing: what the AAEP says, and what your vet decides

The AAEP treats flu as a risk-based vaccine, meaning the schedule depends on the horse's age, exposure and location. Horses at higher risk, such as those who travel or live in busy barns, may be boosted every six months instead of yearly.

The AAEP also notes the intranasal vaccine gives protection in around a week, so "a few days before the show" is too late for full protection. Your vet will pick the product and date, and whether a horse with an unknown history needs a different starting plan.

Vaccination does not make a horse immune to everything. It lowers the odds and severity of illness, and helps limit spread.

When to Call the Vet, and What to Watch Next

Call your veterinarian the same day if you see any of these:

• Temperature over about 101.5°F, especially with a cough or dull behavior
• A new cough, or a cough that is getting worse
• Thick, yellow or green nasal discharge
• Fast, labored or noisy breathing
• Stopped eating, or standing off by himself

Trouble breathing is an emergency. Do not wait for the morning.

Ask your vet about isolation, testing and how much rest to give. Merck notes the cough can last for weeks and that airways need time to heal, so do not rush a horse back to work because the fever has passed.

Over the next few weeks, watch the EDCC alert page and your state veterinarian's site for new cases, and check with your show manager about health rules. See our care guide for routine barn management, and the Coggins test explainer for the other paperwork you need to travel.