Equine Herpesvirus

Equine herpesvirus is one of those things that sounds scarier than it usually is, though the neurological form can be genuinely terrifying when it hits. Most horses get exposed to herpesvirus at some point in their lives, and for most of them it causes respiratory symptoms that look like a bad cold and then they recover. The problem is that the virus never truly leaves once they've been infected - it hides out in nerve cells and can reactivate when the horse gets stressed, creating the potential for them to shed virus and infect other horses even when they're not showing symptoms.

There are several strains of equine herpesvirus, but the ones horse owners need to worry about are EHV-1 and EHV-4. EHV-4 is the more common one and mostly causes respiratory disease - coughing, runny nose, fever, general malaise. EHV-1 can cause respiratory disease too, but it's the one that can also cause neurological disease and abortion storms in pregnant mares. When you hear about a barn going on lockdown because of EHV-1, it's usually because they've had a neuro case or multiple mares aborting. That's the nightmare scenario that makes everyone lose sleep.

The neurological form of EHV-1, called equine herpesvirus myeloencephalopathy or EHM, is thankfully rare but devastating when it occurs. Horses can go from looking fine to being unable to stand within hours as the virus attacks blood vessels in the spinal cord. Some horses recover with intensive care, others end up paralyzed, and some don't make it at all. There's no way to predict which horses will develop the neurological form when exposed to EHV-1, which is part of what makes it so scary.

Because herpesviruses establish latent infections, there's no getting rid of them once a horse is infected. The virus integrates into nerve cells and sits there quietly most of the time, but stress can cause it to reactivate. Travel, competition, changes in management, or any significant stressor can trigger viral shedding, potentially spreading infection to other horses. This is why show barns and breeding farms have strict biosecurity protocols, and why you're not supposed to share water buckets or tack between horses when there's been a respiratory outbreak.

This guide will help you understand how equine herpesvirus spreads, recognize the signs of infection, know when to call the vet immediately, and implement biosecurity measures that reduce your horse's risk of exposure. You'll learn about vaccination strategies, quarantine protocols, and what to do if your barn has a confirmed case.

Equine herpesvirus spreads through direct contact between horses and through contaminated equipment, hands, and clothing. When an infected horse coughs or has nasal discharge, they're shedding virus particles that can land on other horses or contaminate surfaces. Horses touching noses over fences, sharing water buckets, or being handled by someone who just touched an infected horse can all result in transmission. The virus can survive in the environment for several days on surfaces like buckets and halters, though it doesn't last as long outside a host as some other pathogens.

The biggest risk factor for EHV infection is simply being a horse in contact with other horses. Any time you bring new horses onto your property, take your horse to a show or clinic, or have people handling multiple horses moving between yours and others, you're creating opportunities for viral transmission. Young horses seem more susceptible to respiratory disease from EHV, while pregnant mares face the additional risk of virus-induced abortion. Older horses that were infected years ago can still harbor latent virus that reactivates under stress.

Stress is a major trigger for viral reactivation in horses that are latently infected. When horses travel long distances, compete intensively, undergo training pressure, or experience significant management changes, their immune systems become compromised and latent herpesvirus can wake up and start replicating. This is why respiratory outbreaks often follow big shows or sales where horses from multiple locations come together, get stressed, and start shedding virus to each other.

Crowded conditions amplify transmission risk substantially. Horses stabled in close quarters, sharing airspace and equipment, pass infections much more readily than horses living in spread-out situations. Show barns, breeding farms, and training facilities with constantly changing populations face ongoing challenges maintaining biosecurity against respiratory viruses. The more horses your horse is exposed to, especially horses of unknown health status, the higher the risk.

Immunity from vaccination or previous infection provides some protection but doesn't guarantee a horse won't get infected or become seriously ill. Vaccines for EHV reduce disease severity and may reduce viral shedding, but they don't prevent infection entirely and don't provide reliable protection against the neurological form. Previous infection provides some immunity but doesn't prevent reinfection or reactivation.

Pregnant mares face unique risks because EHV-1 can cross the placenta and cause late-term abortion. Most EHV-related abortions happen between months seven and eleven of pregnancy, and often the mare shows no signs of illness before losing the foal. This is why breeding farms maintain such strict isolation protocols for pregnant mares and why new horses coming onto breeding properties go through extended quarantine periods.

The respiratory form of EHV typically starts with a fever, often spiking to 102-106 degrees. Horses look dull and depressed, standing around with their heads down rather than showing interest in what's going on around them. Within a day or two of the fever starting, you'll see nasal discharge that starts clear and watery but can turn thick and cloudy as secondary bacterial infections set in. Coughing develops as the respiratory tract gets inflamed, and horses may have enlarged lymph nodes under their jaws that you can feel when you run your hands along their throats.

Young horses with respiratory EHV often look pretty sick for several days but usually recover with supportive care. They'll run fevers for three to seven days, have significant nasal discharge and coughing, and just feel generally miserable. Some horses go off their feed completely while they're feeling their worst, while others keep eating but without their usual enthusiasm. The cough can linger for weeks even after the fever resolves and the horse seems to feel better overall.

The neurological form announces itself very differently and much more urgently. Horses may start with mild incoordination or weakness in the hindquarters that progresses rapidly over hours. Some horses develop a strange gait where their hind legs seem disconnected from what their front end is doing. Urine dribbling or loss of tail tone can be early signs that the virus is affecting the spinal cord. As things progress, horses may go down and be unable to get back up, or they may stand but with such severe ataxia that they're dangerous to themselves and anyone near them.

In pregnant mares, EHV-1 abortion often happens with no warning. The mare seems fine one day and the next day you find an aborted fetus in the stall or paddock. Sometimes multiple mares on a farm will abort within days of each other if they were all exposed to the same source of infection. The aborted fetuses typically look fresh and well-developed, not decomposed or abnormal, which is part of how vets distinguish EHV abortions from those caused by other problems.

Some horses infected with EHV show minimal symptoms or none at all, which is part of what makes controlling outbreaks so difficult. A horse can be shedding significant amounts of virus while appearing completely healthy, infecting other horses before anyone realizes there's a problem. This is especially common in horses that have been previously exposed and have some immunity - they fight off clinical disease but still shed virus for a period of time.

The timing of symptom onset after exposure varies but typically falls in the four to ten day range. This incubation period means a horse could pick up infection at a show on Saturday and not show signs until the following week after they're back home. By then they may have infected their barnmates, which is why post-show quarantine protocols exist even though most people don't follow them as strictly as they should.

Diagnosing EHV requires lab work because the symptoms look like a dozen other respiratory viruses and you can't tell from clinical signs alone which virus you're dealing with. Your vet will typically take a nasal swab to test for viral shedding using PCR, which can detect viral DNA even in small amounts. Blood tests can show whether a horse has antibodies to EHV and whether those antibody levels are rising, which indicates active infection rather than just old exposure. For neurological cases, spinal fluid analysis may be needed to rule out other causes of ataxia and confirm EHV as the culprit.

There's no specific antiviral treatment that kills equine herpesvirus, so treatment focuses on supportive care and managing symptoms while the horse's immune system fights off the infection. For respiratory cases, this means rest, plenty of water, good ventilation, and sometimes anti-inflammatory drugs to reduce fever and make the horse more comfortable. Antibiotics don't do anything against viruses but may be prescribed if secondary bacterial infections develop. Most horses with uncomplicated respiratory EHV recover fine with basic supportive care and time.

Neurological cases require much more intensive management. Horses that go down need deep bedding, frequent turning to prevent pressure sores, catheterization if they can't urinate normally, and sometimes sling support to help them stand. Anti-inflammatory drugs at high doses may help reduce spinal cord inflammation. Some vets use antiviral drugs like valacyclovir, though the evidence for their effectiveness in treating active EHV is mixed. Horses with mild neurological signs that can still walk may recover completely, while those that are down and can't stand have a much more guarded prognosis.

The decision about whether to treat aggressively or euthanize a horse with severe neurological EHV is gut-wrenching. Some horses do recover from being completely down and go on to be useful riding horses again, but the recovery takes months of intensive care and there's no guarantee of success. Other horses suffer through weeks of treatment and never regain enough function to have quality of life. Your vet can give you a realistic assessment of your horse's chances based on how severe their signs are and how quickly they're progressing.

Prevention through vaccination is imperfect but still worthwhile. The EHV vaccines on the market reduce disease severity and may reduce viral shedding, but they don't prevent infection and they don't reliably prevent the neurological form. Pregnant mares should be vaccinated at specific intervals during pregnancy to reduce abortion risk. Performance horses benefit from vaccination before travel and competition. Even with vaccination, strict biosecurity practices remain essential because the vaccines only provide partial protection.

Quarantine protocols for new arrivals and horses returning from shows or other facilities help limit disease spread. Ideally new horses should be isolated for at least two weeks with separate equipment, separate handlers, and temperature monitoring twice daily. In reality most barns don't maintain this level of separation, which is why respiratory outbreaks remain common despite everyone's best intentions.

During an outbreak, aggressive biosecurity becomes non-negotiable. Infected horses need complete isolation from healthy ones, meaning separate stalls in a different barn if possible, or at minimum at the opposite end of the barn with nobody walking between sick and healthy horses. All equipment used on sick horses stays with those horses - no sharing buckets, halters, grooming tools, or anything else. People handling sick horses should do so last in their work routine and change clothes and wash thoroughly before handling healthy horses.

Temperature monitoring helps catch infections early before horses are shedding maximum virus. Taking rectal temperatures twice daily on all horses in a facility where there's been exposure lets you identify febrile horses quickly and isolate them before they infect more of the herd. This is tedious and time-consuming but can significantly reduce how many horses ultimately get sick during an outbreak.

Cleaning and disinfection of contaminated areas and equipment should happen after the outbreak resolves but also during active infection to reduce environmental viral load. EHV is relatively easy to kill with proper disinfectants - bleach solutions, phenolic disinfectants, and many commercial barn disinfectants work fine. Wash everything with soap and water first to remove organic material, then apply disinfectant and let it sit for the contact time specified on the label. Don't rush this process because surface contamination absolutely can spread infection.

Decisions about moving forward with shows, clinics, or bringing in new horses should factor in outbreak status. Many show organizations and clinics have policies about horses from affected barns being excluded for specific time periods after the last case. Even if there's no official policy, consider whether it's ethical to potentially spread infection to other people's horses. Your reputation in the horse community matters, and being the barn that brought EHV to a show doesn't do you any favors.

For horses recovering from respiratory EHV, gradual return to work is important. Just because the fever is gone and the nasal discharge has cleared up doesn't mean the horse is ready to go back to full work immediately. Their respiratory system needs time to heal, and pushing too hard too soon can lead to secondary complications or prolonged coughing issues. Plan on at least two to three weeks of light work after symptoms resolve before asking for serious effort.

The reality is that you probably can't keep your horse from ever being exposed to EHV unless you maintain a completely closed herd with no new horses ever coming on the property and your horse never leaving. For most people that's not realistic or desirable. What you can do is reduce exposure risk through smart management and minimize disease severity through vaccination and good general health practices.

Vaccination should be part of your routine health program even though the protection isn't perfect. Most performance horses benefit from EHV vaccination every six months, timed before travel or show season if possible. Pregnant mares need a specific vaccination schedule during pregnancy. Your vet can recommend the best protocol for your situation based on your horse's risk level and what's happening with EHV in your area.

Biosecurity practices you can control include not sharing equipment between horses at shows, avoiding nose-to-nose contact with strange horses, and maintaining quarantine for horses returning from off-site events. Washing your hands between horses and not wearing the same clothes to multiple barns in one day are simple habits that reduce disease transmission. These seem like small things but they add up when everyone in a barn is being careful.

The prognosis for horses with respiratory EHV is generally good - most recover completely within a couple weeks with supportive care. The prognosis for neurological cases is much more variable, ranging from complete recovery to euthanasia depending on severity. Mares that abort due to EHV can usually be bred successfully in subsequent years once they've recovered, though some breeding farms will test them to make sure they're not still shedding virus before putting them back with a stallion.

Staying informed about outbreaks in your area helps you make risk-based decisions about where you take your horse and what level of biosecurity you need to maintain. Many state veterinary organizations track and report significant EHV outbreaks, and keeping an ear to the ground in your local horse community gives you early warning when something is circulating nearby.