Equine Viral Arteritis (EVA): Symptoms, Transmission, and Prevention in Horses

Equine Viral Arteritis (EVA) is an infectious viral disease that affects horses around the world. It is caused by the Equine Arteritis Virus (EAV), an RNA virus belonging to the Arterivirus family. Although many infected horses recover fully, the disease can have serious consequences, especially in breeding operations, where it can cause abortions in pregnant mares and long-term carrier states in stallions.

Because EVA can spread quickly and remain hidden in carrier horses, understanding its transmission, signs, and prevention is essential for horse owners, breeders, and veterinarians.

What Is Equine Viral Arteritis

EVA is primarily a respiratory and reproductive disease of horses. It can infect all breeds and ages and is most significant for its impact on pregnant mares and breeding stallions. While many horses show only mild flu-like symptoms or no symptoms at all, the virus can cause abortion, illness in foals, and temporary infertility in stallions.

Once a horse recovers from EVA, it develops lasting immunity. However, some stallions may become long-term carriers, shedding the virus in their semen and posing a continuous risk to uninfected mares.

How EVA Spreads

EVA spreads through both respiratory and reproductive routes. The virus is present in nasal secretions, semen, urine, and other bodily fluids.

Transmission Routes

• Respiratory Transmission: Horses can inhale the virus through droplets from coughing or sneezing. This is common during outbreaks where horses are kept in close quarters.
• Venereal Transmission: Stallions infected with EAV can shed the virus in their semen for months or even years, infecting mares during natural breeding or artificial insemination.
• Indirect Transmission: Contaminated equipment, clothing, or hands can spread the virus between horses.
• Transplacental Transmission: Pregnant mares can pass the virus to their foals through the placenta, which may lead to abortion or the birth of weak foals.

Carrier stallions are the primary long-term source of infection, and managing them is critical for controlling the disease in breeding programs.

Symptoms of Equine Viral Arteritis

The severity of symptoms depends on the horse’s age, immune status, and the strain of the virus. Some horses show no signs at all, while others develop noticeable illness.

Common Symptoms

• Fever (lasting 2 to 9 days)
• Nasal discharge and conjunctivitis (inflammation of the eyes)
• Swelling of the eyelids, limbs, or scrotum
• Loss of appetite and depression
• Urticaria (hives) or skin rash in some cases
• Abortion in pregnant mares
• Weak or stillborn foals

In stallions, EVA may cause temporary infertility due to inflammation of the reproductive tract, though most recover within weeks.

In Foals

Foals infected in utero or shortly after birth may develop severe respiratory distress and, in some cases, die of pneumonia or systemic infection.

Carrier Stallions and Persistent Infection

One of the most challenging aspects of EVA control is the ability of stallions to become long-term carriers. After infection, some stallions continue to shed the virus in their semen indefinitely, even though they appear completely healthy.

The carrier state is testosterone-dependent, meaning geldings and mares do not become carriers. Carrier stallions are a major concern in breeding programs, as they can infect mares during natural service or artificial insemination.

Testing stallions for EVA before breeding is therefore an essential biosecurity measure.

Diagnosis

Veterinary diagnosis of EVA involves a combination of clinical observation and laboratory testing. Because many of the symptoms resemble other viral diseases, such as equine influenza or equine herpesvirus, testing is necessary for confirmation.

Diagnostic Methods

• Serology (blood test): Detects antibodies against EAV, indicating current or past infection.
• Virus isolation or PCR: Identifies the virus in nasal swabs, semen, or tissue samples.
• Semen testing: Determines whether a stallion is shedding the virus and acting as a carrier.

Mares that abort should have both the foetus and placenta tested to confirm whether EVA was the cause.

Treatment

There is no specific antiviral treatment for EVA. Management focuses on supportive care and preventing secondary infections.

• Provide rest and supportive care for horses with fever or respiratory signs.
• Administer anti-inflammatory drugs as prescribed by a veterinarian to reduce swelling and fever.
• Ensure adequate hydration and nutrition.
• Isolate affected horses to prevent spread.

In stallions with temporary infertility, rest and supportive care usually lead to full recovery within one to three months. Carrier stallions cannot be cured, though their infection can be managed safely within a controlled breeding program.

Prevention and Control

Prevention is the most effective way to protect horses from EVA, especially in breeding facilities.

Vaccination

Vaccination is highly effective at preventing EVA and controlling outbreaks.

• Stallions and Colts: All breeding stallions should be vaccinated before entering stud duties and tested beforehand to ensure they are not already carriers.
• Mares: Vaccinate mares before breeding with a known carrier stallion or before artificial insemination with semen from a seropositive stallion.
• Foals: Vaccinate after weaning, following veterinary recommendations.

Vaccinated horses should be clearly documented, as vaccination causes them to test seropositive (showing antibodies), which can be confused with natural infection.

Biosecurity Measures

• Isolate new horses for at least three weeks before introducing them to the herd.
• Use separate equipment and handlers for infected or exposed horses.
• Disinfect stalls, transport vehicles, and breeding equipment regularly.
• Avoid direct contact between pregnant mares and horses of unknown health status.
• Dispose of aborted material safely and disinfect the area thoroughly.

Managing Carrier Stallions

• Identify carriers through semen testing.
• Breed only to vaccinated or seropositive mares to prevent infection.
• Store and label semen from carrier stallions separately and handle it under strict biosecurity conditions.

Outbreak Response

If an EVA outbreak is suspected:

1. Notify your veterinarian immediately for confirmation and guidance.
2. Quarantine affected horses and stop all breeding and movement.
3. Test all horses on the premises to identify carriers and exposed animals.
4. Disinfect all areas and equipment.
5. Work with veterinary authorities to manage and contain the outbreak.

Movement restrictions and breeding bans may be temporarily imposed to control disease spread.

Prognosis and Long-Term Impact

Most horses recover completely from EVA without lasting health effects. However, abortions in mares, neonatal deaths, and the establishment of carrier states in stallions can have significant economic and breeding consequences.

Carrier stallions can continue to breed safely within managed programs, provided mares are vaccinated and biosecurity measures are maintained.

Conclusion

Equine Viral Arteritis is a contagious but manageable disease. Its greatest risks lie in its impact on reproduction and its ability to persist unnoticed in carrier stallions. With vigilant biosecurity, responsible vaccination, and regular testing, breeders can prevent outbreaks and protect both their horses and their breeding investments.

By understanding the nature of the virus and adhering to sound management practices, horse owners can greatly reduce the impact of EVA and ensure the health and safety of their equine populations.