EHV-1 & EHV-4: Risks for Breeding Mares & Biosecurity

EHV-1 & EHV-4: Risks for Breeding Operations, Abortion Storms & Biosecurity Measures

Equine Herpesvirus-1 (EHV-1) and EHV-4 are among the most consequential pathogens in equine breeding — EHV-1 alone is responsible for late-term abortion storms, the potentially fatal neurological syndrome known as EHM, and ongoing economic losses at stud farms worldwide. Implementing rigorous, evidence-based biosecurity before an outbreak occurs is the single most effective strategy to protect your mares, your foals, and your operation.

What Are EHV-1 and EHV-4, and Why Do They Matter to Breeders?

Both EHV-1 and EHV-4 are DNA herpesviruses that infect horses globally. They share the capacity to cause respiratory disease, but their reproductive and neurological consequences diverge sharply — a distinction that has profound implications for breeding farm management.

According to the College of Veterinary Medicine at Michigan State University:

• EHV-1 causes late-term abortions (typically in the third trimester), equine herpes myeloencephalopathy (EHM), and respiratory disease
• EHV-4 primarily causes respiratory disease and only rarely triggers abortion or neurological complications
• Up to 70% of infected horses develop lifelong latency following either infection, meaning clinically healthy horses in your herd may be silent reservoirs

The D752 neurotropic genotype of EHV-1 is identified in approximately 75% of EHM cases and is associated with prolonged viremia and higher neuropathogenic potential, making accurate strain identification critical when an outbreak is suspected.

How Does EHV-1 Cause Abortion Storms?

Abortion storms — multiple mares aborting in rapid succession — are among the most economically devastating events a breeding operation can face. EHV-1 reaches the placenta via cell-associated viremia, triggering acute placentitis and fetal death. Characteristic features include:

• Abortions occur without prior warning signs in the mare
• Fetuses are typically expelled in the eighth to eleventh month of gestation (third trimester)
• The aborted fetus and placenta carry extremely high viral loads and are the primary source of horizontal transmission to other mares on the property
• One index case can rapidly cascade into multiple abortions within days if biosecurity is not immediately enacted

Mares that abort due to EHV-1 usually recover without complications and can be rebred, but the loss of late-term foals, quarantine costs, and reputational damage can be severe.

What Is the Neurological Form (EHM) and How Dangerous Is It?

Equine Herpes Myeloencephalopathy (EHM) is the most feared manifestation of EHV-1 infection. Unlike the common respiratory form, the neuropathogenic strain carries a 30–50% mortality rate in affected horses (UC Davis School of Veterinary Medicine).

The 2011 outbreak at the Western National Cutting Horse Event in Ogden, Utah, illustrates the scale of risk at congregate events:

Critically for breeders, research revealed two unexpected risk factors:

• Horses vaccinated against EHV-1 within five weeks of exposure were 3.3× more likely to contract EHM
• Female horses were 2.8× more likely than males to develop EHM

These findings underscore that vaccination, while still recommended as part of a comprehensive program, does not confer complete protection and must be combined with strict biosecurity.

What Are the Clinical Signs Breeders Should Recognize?

Respiratory Signs (EHV-1 and EHV-4)

• Fever (≥101.5°F / 38.6°C) — often the first observable sign
• Nasal discharge (serous to mucopurulent)
• Submandibular lymphadenopathy
• Depression and anorexia

Reproductive Signs (EHV-1)

• Sudden abortion without prior illness in the mare, typically in the last trimester
• Stillbirths or weak, rapidly dying neonates (“EHV-1 foals”)
• No outward signs of placentitis in most cases

Neurological Signs — EHM (EHV-1)

• Ataxia (incoordination), often beginning in the hindquarters
• Urine dribbling and bladder dysfunction
• Hindlimb weakness progressing to recumbency
• Fever often precedes neurological signs by 24–48 hours

Fever is closely correlated with EHV-1 shedding — twice-daily temperature monitoring during any suspected outbreak is the most critical early-detection measure available on the farm.

How Is EHV-1 Diagnosed?

Accurate diagnosis requires submitting both nasopharyngeal swabs and whole blood (EDTA) samples simultaneously. PCR is the most common and practical detection method, while virus isolation remains the gold standard for definitive confirmation.

If an aborted fetus is found on the property, it should be treated as a biohazard: wear gloves and protective clothing, bag the fetus and placenta separately, and contact your veterinarian immediately for diagnostic submission.

What Biosecurity Measures Are Most Effective?

At the Farm Level

1. Isolate new arrivals for a minimum of 21 days before contact with resident horses
2. Monitor temperatures twice daily on all horses during any suspected outbreak — fever is your earliest reliable indicator
3. Designate a biosecurity officer responsible for enforcing protocols, especially during high-traffic breeding seasons
4. Eliminate communal water sources — automatic water troughs shared between paddocks are a primary transmission vector
5. Avoid nose-to-nose contact between horses from different premises
6. Dedicate equipment (halters, lead ropes, grooming tools) to individual horses; disinfect shared equipment between animals
7. Implement traffic control — visitors, farriers, and veterinarians should follow a clean entry protocol

Specific Protocols for Pregnant Mares

Breeding mares warrant an elevated level of protection:

• House pregnant mares separately from sport horses, young stock, and recently arrived horses
• If a mare aborts, immediately isolate her in a separate stall, restrict access to that area, and notify your veterinarian
• Disinfect the foaling stall thoroughly after any abortion — EHV-1 is susceptible to most standard disinfectants (bleach 1:10, quaternary ammonium compounds)
• Keep detailed records of any fever episodes, reproductive losses, or neurological events using a tool like Breedio to flag health deviations across your herd

At Congregate Events

Given the scale of the 2011 Ogden outbreak, any movement of breeding stock to sales, shows, or stallion stations requires:

• Pre-event health certificates including EHV-1 status
• Avoiding shared equipment or water with horses from other farms
• Temperature monitoring before, during, and for 21 days after return
• Immediate isolation of any horse returning from a venue with a reported EHV case

Does Vaccination Protect Against EHV-1 Abortion?

This is one of the most nuanced questions in equine reproductive medicine. Current EHV-1 vaccines are conditionally effective:

• No currently licensed vaccine in North America or Europe provides complete protection against abortion or EHM
• Vaccines may reduce the severity and duration of viremia and clinical signs
• The AAEP (American Association of Equine Practitioners) recommends EHV-1/EHV-4 vaccination of pregnant mares at months 5, 7, and 9 of gestation as a standard protocol
• Vaccination within five weeks of high-exposure events may paradoxically increase EHM risk — timing matters

Always consult with your veterinarian to design a vaccination schedule appropriate to your farm’s risk profile, geographic region, and event calendar.

How Can Breeders Track and Reduce Risk Across the Herd?

Early detection depends entirely on systematic monitoring. With large breeding operations managing multiple mares at different gestation stages, manual tracking is error-prone and inconsistent.

Breedio was designed specifically for this workflow — allowing you to log daily temperature readings, flag health events, and correlate reproductive data with health history across your entire mare band. The Features include gestation stage tracking, which helps you identify which mares are entering the highest-risk third trimester at any given time. Track Your Mares to maintain the kind of real-time visibility that makes early detection of an EHV-1 cluster possible before an abortion storm develops.

Key Takeaways for Breeding Farm Managers

• EHV-1 is the primary reproductive threat — EHV-4 rarely causes abortion or EHM
• Up to 70% of horses carry lifelong latent infection, meaning any herd stress, transport, or immunosuppression can trigger viral reactivation
• Abortion storms begin with a single index case — isolate any aborted mare immediately and treat all reproductive material as infectious
• Twice-daily temperature monitoring is the most practical early-warning system available
• Vaccination reduces risk but does not eliminate it — biosecurity protocols are non-negotiable regardless of vaccination status
• EHM carries 30–50% mortality and has become a notifiable/regulatory condition in multiple jurisdictions
• Digital health tracking across your mare band is the difference between catching an outbreak early and managing a crisis

In 2026, with EHM outbreak alerts continuing to be recorded across North America each quarter, complacency is not an option. The combination of sound biosecurity, systematic health monitoring, appropriate vaccination timing, and rapid veterinary response gives your breeding operation the best possible defense against equine herpesvirus.