Equine Herpesvirus (EHV-1 and EHV-4) in Horses: Respiratory, Abortion, and Neurologic Disease

Equine herpesvirus is a common, contagious viral infection in horses. The two strains most often discussed are EHV-1 and EHV-4. Both can infect the respiratory tract and spread through nasal secretions, close contact, and contaminated hands, tack, buckets, or stall surfaces. Many horses are exposed early in life, and the virus can become latent, meaning it stays hidden in the body and may reactivate later during stress.

EHV-4 is most strongly linked to respiratory disease, especially in foals, weanlings, yearlings, and young horses in training. EHV-1 can also cause respiratory illness, but it is more concerning because it may trigger late-term abortion, weak newborn foals, or a neurologic form called equine herpes myeloencephalopathy (EHM).

Signs can range from mild fever and nasal discharge to severe weakness, incoordination, urine retention, or inability to stand. That wide range is one reason early veterinary involvement matters. A horse with a mild fever today may need close monitoring tomorrow, especially if there has been recent travel, a new arrival to the barn, or known exposure.

For pet parents, the biggest takeaway is that EHV is both a medical and a biosecurity problem. Your horse may need treatment, but the rest of the barn may also need temperature checks, movement restrictions, and a plan with your vet to reduce spread.

EHV-1 and EHV-4 are caused by contagious equine herpesviruses that spread mainly through respiratory droplets and direct horse-to-horse contact. They can also spread indirectly on shared water buckets, feed tubs, lead ropes, grooming tools, tack, clothing, and hands. In practical terms, a horse does not need nose-to-nose contact with every horse in the barn for the virus to move.

One of the challenging features of herpesviruses is latency. After infection, the virus can remain dormant in the horse and reactivate later. Stressors such as transport, showing, weaning, illness, crowding, or other management changes may contribute to viral shedding. That means a horse may appear healthy and still become a source of infection later.

EHV-1 is the strain most associated with abortion and neurologic disease, while EHV-4 is more commonly linked to upper respiratory illness. Young horses are often more affected by respiratory disease, while pregnant mares and horses in high-traffic facilities face additional risk from EHV-1.

Outbreaks are more likely when horses from different sources mix together, especially at shows, sales, training barns, breeding farms, and layup facilities. Poor isolation practices, shared equipment, and delayed recognition of fever can all increase spread.

Your vet usually starts with a physical exam, temperature history, and questions about travel, new horse arrivals, pregnancy status, and other sick horses on the property. Because EHV can look like other respiratory or neurologic diseases, testing is important. The most common first-line tests are a nasal or nasopharyngeal swab and a blood sample for PCR.

PCR testing helps identify viral genetic material and is widely used for horses with fever, respiratory signs, abortion, or suspected EHM. In neurologic cases, your vet may also recommend a full neurologic exam, complete bloodwork, and sometimes cerebrospinal fluid (CSF) analysis. If a mare aborts, the fetus and placenta may be submitted for laboratory testing to confirm EHV-1.

Diagnosis is not only about the individual horse. In a barn outbreak, your vet may advise testing multiple horses, twice-daily temperature monitoring, and movement restrictions while results are pending. State reporting requirements can vary, especially for neurologic EHV-1 cases, so your vet may coordinate with animal health officials.

A negative test does not always end the conversation. Timing of sampling, stage of infection, and whether the horse is actively shedding can affect results. Your vet may recommend repeat testing if suspicion remains high.

Prevention starts with biosecurity. New horses and horses returning from shows, sales, clinics, or other facilities should be kept separate before joining the resident group. Many equine references recommend an isolation period of about 21 to 28 days, along with daily temperature checks and watching for cough, nasal discharge, or reduced appetite.

Do not share buckets, bits, tack, grooming tools, towels, or lead ropes between horses unless they have been cleaned and disinfected. Hand hygiene matters. So do dedicated boots and clothing for sick or exposed horses. In outbreaks, your vet may recommend dividing horses into groups based on exposure and limiting all movement on and off the property.

Vaccination is an important tool, but it is not a complete shield, especially against neurologic disease. Your vet can help tailor a vaccine schedule based on age, travel, breeding status, and barn risk. Foals, young performance horses, breeding farms, and pregnant mares often need more structured planning. Pregnant mares are commonly vaccinated during pregnancy with products labeled for abortion risk reduction.

Stress reduction also plays a role. Long transport, crowding, abrupt management changes, and mixing unfamiliar horses can increase risk. A practical prevention plan combines vaccination, quarantine, temperature monitoring, and fast action when any horse spikes a fever.