Vet Tips 2025: What Horse Owners Need to Know About Equine Viral Arteritis (EVA) 🐴🦠

By Dr Duncan Houston BVSc

Equine Viral Arteritis (EVA) may not always be deadly, but it can wreak havoc on breeding farms, cause abortion in mares, and create life-long carriers in stallions. If you’re involved with breeding, showing, or transporting horses, understanding EVA is essential for protecting your herd. 🧠🐎

What Is EVA? 🧬

EVA is a contagious viral disease of horses caused by the equine arteritis virus (EAV). It spreads through respiratory secretions and semen, making both direct contact and breeding practices key transmission routes. 🧪

Species Most Affected:

• 🐎 Standardbreds
• 🏇 Warmbloods
• 🐴 Quarter horses (notably involved in a 2006 multi-state outbreak)

How Is EVA Transmitted? 🔄

There are two main routes of infection:

• 💨 Respiratory spread via nasal secretions (especially at shows, sales, or transport hubs)
• 💉 Venereal spread via semen—including fresh, chilled, and frozen

Stallions can become long-term carriers of the virus in their reproductive tract, shedding the virus through semen without showing any signs. ⚠️

Clinical Signs of EVA 🩺

Symptoms vary by horse and may be mistaken for other conditions. Watch for:

• 🌡️ Fever
• 🥱 Depression, poor appetite
• 💧 Nasal discharge
• 🦵 Swelling of limbs, scrotum, or mammary gland
• 👁️ Inflammation or redness around the eyes
• 🌿 Hives or skin bumps

In pregnant mares:

• 🤰 Abortion risk (especially if infection occurs mid-gestation)
• 🍼 Foals born from late-term exposure may appear healthy but can carry or spread the virus

Why Is EVA a Concern in Foals? 🐣

• 🧒 Young foals may develop severe pneumonia or intestinal illness if infected early
• ⚰️ Mortality can occur in very young or immune-compromised foals

Prevention in pregnant mares is key to avoiding reproductive loss and neonatal illness. 🧪👶

Testing for EVA 🧬🩸

Since both natural infection and vaccination produce antibodies, EVA testing is important before vaccinating:

• 🧪 A simple blood test checks for EAV antibodies
• ✅ Negative test: safe to vaccinate, and results can be recorded
• ❌ Positive test: may indicate prior infection or prior vaccination (not distinguishable)

All breeding stallions should be tested prior to vaccination or breeding season. 📋

Should I Vaccinate for EVA? 💉🤔

There is a safe and effective vaccine for EVA. However, timing is critical and depends on:

• 📅 Stallion’s breeding schedule
• 🐴 Mare’s pregnancy status
• 📊 Serology (antibody status)

Why testing first matters: Vaccinated horses will test positive for EVA, and there's no way to distinguish between infection and vaccine-induced antibodies. 🧠

Vaccination Protocol Tips:

• 🧬 Test before giving the first vaccine dose
• 🧾 Keep written documentation of negative status pre-vaccination
• 📈 Consider booster doses if breeding regularly

Always consult your vet before initiating an EVA vaccine plan—especially in breeding programs. 🩺

EVA on Breeding Farms 🐎🏠

Because EVA spreads through semen, prevention on breeding farms includes:

• 🧪 Testing all stallions and mares before breeding
• 📉 Avoiding use of semen from unknown sources or untested stallions
• 💉 Vaccinating stallions with documented negative status
• 📖 Keeping detailed breeding records and vaccination dates

Is EVA a Reportable Disease? ⚠️📋

In some jurisdictions, EVA is a reportable disease, especially if associated with an abortion storm or outbreak. Always consult your veterinarian and local regulatory authorities for guidance. 🧾

Need Help Building an EVA Protocol? Ask A Vet 📱🐴

Not sure if your horses should be vaccinated for EVA? Need help preparing for breeding season or interpreting test results? Connect with a certified equine veterinarian through Ask A Vet.

Download the Ask A Vet app for 24/7 advice on virus prevention, breeding protocols, vaccination strategies, and travel requirements. 🐎💬

Final Thoughts: EVA Requires Smart Breeding Choices 🧠💉

Equine Viral Arteritis isn’t always deadly—but it can cause major disruptions on a farm. With testing, vaccination, and careful record-keeping, you can keep your horses and foals safe. Knowledge is your best defense. 🧬

Need help planning your EVA vaccine schedule or getting your horses tested? Visit Ask A Vet and speak with an equine expert today. 🐴📋