Equine herpesvirus (EHV) is a family of highly common viruses that infect horses worldwide, causing illness ranging from mild respiratory infections to fatal neurological disease. There are five recognized types, but EHV-1 and EHV-4 are the ones horse owners encounter most often and need to understand. Both are so widespread that most horses will be exposed at some point in their lives.
The Five Types of EHV
Each type of equine herpesvirus targets different body systems and carries different risks:
- EHV-1 is the most dangerous. It primarily causes upper respiratory disease and late-term abortion in pregnant mares, but it can also trigger a neurological condition called equine herpes myeloencephalopathy (EHM) that is sometimes fatal.
- EHV-4 mainly causes respiratory disease, especially in foals. It can occasionally cause abortion but does so rarely compared to EHV-1.
- EHV-2 is found in the respiratory lining and white blood cells of healthy horses of all ages. Its role in disease is still unclear, though it has been linked to eye infections in foals and mild asthma in young athletic horses.
- EHV-3 causes coital exanthema, a generally mild venereal disease that produces sores on the genitals. It does not cause respiratory illness.
- EHV-5 is associated with a serious lung condition called equine multinodular pulmonary fibrosis, where scar tissue builds up in the lungs. It has also been linked to certain cancers of the lymphatic system.
How EHV Spreads
EHV-1 and EHV-4 spread primarily through respiratory droplets when an infected horse coughs, snorts, or shares air space with other horses. Direct nose-to-nose contact is the most efficient route, but the virus also survives on surfaces. Shared water buckets, feed troughs, tack, grooming tools, and even handlers’ hands and clothing can carry the virus between horses.
After exposure, the incubation period can be as short as 24 hours but typically runs 4 to 6 days. An infected horse sheds the virus through nasal secretions for 7 to 10 days, though shedding can last longer in some cases. This means a horse can be contagious before it shows obvious signs of illness.
Latency: The Virus Never Truly Leaves
Like human cold sores (caused by a related herpesvirus), EHV establishes lifelong latency. After the initial infection resolves, the virus hides in nerve tissue and white blood cells, where the horse’s immune system cannot eliminate it. The horse appears completely healthy but still carries the virus.
Stress can reactivate a latent infection and cause the horse to start shedding virus again without any warning signs. Common triggers include long-distance transport, competition, changes in herd dynamics, weaning, surgery, and other illnesses. This silent reactivation is one of the main reasons outbreaks seem to appear out of nowhere at shows, sales, and training facilities.
Respiratory Disease
The most common form of EHV illness looks a lot like a cold. Horses develop a fever (often 102°F or higher), nasal discharge, cough, and general lethargy. Younger horses and foals tend to show more pronounced symptoms, while older horses with some immunity may have only a mild fever that goes unnoticed. Most horses recover from respiratory EHV within one to three weeks with rest and supportive care.
Abortion and Neonatal Death
EHV-1 is one of the leading infectious causes of late-term abortion in mares. Abortion typically occurs between 8 and 11 months of gestation, often weeks to months after the initial respiratory infection has cleared. The virus travels through the bloodstream in infected white blood cells and reaches the blood vessels of the uterus. There, it damages tiny blood vessels, cuts off oxygen supply to the fetus, and triggers abortion.
Because of the delay between respiratory infection and abortion, the connection is not always obvious. In some outbreaks, multiple mares on the same farm abort within a short window, a pattern known as an “abortion storm.” Foals born alive from infected mares are often weak and immunocompromised, with high mortality rates in the first days of life.
The Neurological Form (EHM)
The most feared complication of EHV-1 is equine herpes myeloencephalopathy. This neurological form develops when the virus damages blood vessels in the brain and spinal cord, cutting off blood flow and causing inflammation and tissue death.
Early signs often start with a fever and may progress rapidly. Horses can become uncoordinated and weak, particularly in the hind legs. You may notice toe-dragging, a wobbly gait, or a characteristic “dog-sitting” posture where the horse drops its hindquarters. Difficulty urinating and defecating are also common. In severe cases, horses become unable to stand, develop extreme lethargy, or enter a coma-like state.
The neurological form carries a mortality rate of 30 to 50%. Horses that cannot rise on their own have the worst prognosis. Those that remain standing and receive aggressive supportive care have a better chance of recovery, though some are left with lasting neurological deficits.
How EHV Is Diagnosed
Diagnosis relies on PCR testing, which detects the virus’s genetic material. The most accurate approach is to submit both a nasal swab and a blood sample at the same time. The nasal swab picks up viral shedding from the respiratory tract, which is most significant during the first 10 days of infection. The blood sample detects the virus circulating in white blood cells, which peaks between days 5 and 10 after infection.
Modern PCR tests can also distinguish between neuropathogenic strains of EHV-1 (those more likely to cause EHM) and standard respiratory strains. This distinction helps veterinarians assess the risk level during an outbreak and guide quarantine decisions.
Treatment Options
There is no cure for EHV. Treatment is supportive, meaning veterinarians manage symptoms while the horse’s immune system fights the virus. For respiratory disease, this typically means rest, anti-inflammatory medications to control fever, and monitoring for complications.
For the neurological form, treatment is more intensive and may include IV fluids, anti-inflammatory drugs to reduce swelling around the spinal cord, and physical support for horses that are struggling to stand. Antiviral medications like valacyclovir (similar to drugs used for human herpes infections) have been tried, but research results have been disappointing. In one controlled study, ponies treated with valacyclovir showed no difference in clinical signs, viral shedding, or the amount of virus in their blood compared to untreated animals, even though the drug reached adequate concentrations in the bloodstream. Some veterinarians still use antivirals in outbreak situations, but their effectiveness remains unproven.
Vaccination and Its Limits
Vaccines against EHV-1 and EHV-4 are available and widely used. They can reduce the severity of respiratory disease and may lower the risk of abortion, which is why many breeding farms require vaccination of pregnant mares during specific months of gestation. However, none of the currently approved EHV vaccines claim to prevent the neurological form, EHM. Vaccination may reduce the amount of virus a horse sheds during an outbreak, which helps slow transmission, but it does not guarantee protection against the most serious complications.
Biosecurity During an Outbreak
Because EHV spreads so easily and silently, biosecurity is the single most important tool for preventing outbreaks. If EHV is confirmed or suspected at a facility, the standard response includes isolating sick horses, halting all horse movement on and off the property, and monitoring every horse’s temperature twice daily. A fever is often the first detectable sign, appearing before nasal discharge or neurological symptoms.
The virus is relatively fragile in the environment and can be killed by common disinfectants. Thorough cleaning of stalls, equipment, and any shared surfaces is essential. Handlers should change clothes and wash hands between working with isolated horses and healthy ones. Quarantine periods typically last at least 21 days after the last horse shows clinical signs, though some facilities extend this to 28 days for extra caution.
At events and competitions, practical precautions include avoiding shared water sources, bringing your own buckets and equipment, minimizing nose-to-nose contact between unfamiliar horses, and isolating any horse that develops a fever. New arrivals to a barn should ideally be quarantined for two to three weeks before mixing with resident horses, a simple step that can prevent a single carrier from sparking a facility-wide outbreak.