22/11/2025
Equine Herpes Viruses
Brian S. Burks DVM
Diplomate, ABVP
Board-Certified in Equine Practice
Herpesviruses are enveloped, double-stranded DNA virus ubiquitous in nature, affecting many hosts. Herpes viruses are well known to cause latent infections and the virus is maintained in the host. A continued cycle of viral latency and recrudescence plays an important role in viral maintenance. Immunosuppression, associated with attenuated cell-mediated immunity, leads to viral shedding in latently infected individuals. Horses are viral carriers for life.
There are at least 11 different herpes viruses that affect equids, including five that affect donkeys. Alphaherpes viruses include EHV-1, EHV-3, and EHV-4. In 1932, the association between EHV-1 and abortion was first noted, although it was called influenza virus at first due to the similarity of symptoms with that virus. By 1954, it was recognized that EHV-1 caused both respiratory disease and abortion. Later, these were separated into EHV-1 and EHV-4, although the separation is not complete, and each virus may cause both respiratory disease and abortion. Some strains of EHV-1 cause neurologic disease with ataxia and quadriplegia, called Equine Herpes Myeloencephalopathy (EHM).
EHV-1 is associated with both neurologic and respiratory disease. Neurological signs appear as a result of damage to blood vessels in the brain and spinal cord associated with EHV infection. Interference with the blood supply leads to tissue damage and a subsequent loss in normal function of areas in the brain and spinal cord. It also causes abortion and neonatal illness. EHV-4 is restricted to the respiratory tract. Abortion and myelitis are due to vasculitis induced by the virus.
Clinical signs of EHV1/4 typically include pyrexia, serous nasal discharge, and coughing. Secondary bacterial infections may cause purulent nasal discharge. There may also be serous ocular discharge and lymphadenopathy. Morbidity rates can be quite high. Clinical manifestations of respiratory disease are more severe in younger animals.
Infected pregnant mares abort 2-12 weeks post infection, usually in the latter part of gestation. There are few signs beyond abortion, and future fertility is not affected. The aborted fetus is fresh. Abortions can occur individually, or as an abortion storm within a herd.
When a live foal is delivered by an infected mare, there is severe pneumonitis, icterus from hepatic involvement, and marked neutropenia due to bone marrow destruction.
Signs of EHM include:
⢠Fever preceding neurologic signs (either in a horse diagnosed with EHM or in horses that have been exposed to a horse diagnosed with EHM)
⢠Decreased coordination
⢠Urine dribbling
⢠Loss of tail tone
⢠Hind limb weakness
⢠Leaning against a wall or fence to maintain balance
⢠Lethargy
⢠Inability to rise.
Diagnosis is based upon history and clinical signs, along with laboratory testing. The âgold standardâ is virus isolation, but this takes time and viral shedding may not be high enough for detection. Likewise, acute and convalescent serum samples take time (4 weeks) to look for a four-fold increase in neutralizing antibodies. Polymerase Chain Reaction (PCR) testing is currently the diagnostic test of choice due to its high sensitivity and specificity. Samples include whole blood, serum, nasopharyngeal swabs, and BAL fluid. Testing of healthy horses is not recommended due to the ubiquity of the virus and its latency in most horses. Interpretation of results should be done with caution, testing only horses that have appropriate clinical signs.
Treatment of EHV-1/4 is largely supportive, although antiviral medications can be used for those with signs of neurologic disease (myeloencephalopathy, EHM). Horses with secondary bacterial infections of the respiratory or urinary tracts benefit from antibiotic therapy. Antibiotics do not treat viral infections. Anti-inflammatory therapy is beneficial for those with pyrexia and neurologic deficits. Urinary catheterization and re**al emptying may be required due to the inability to eliminate urine and f***s.
Foals infected with EHV-1 in utero are unlikely to survive despite intensive medical treatment. Antiviral medication such as acyclovir or valacyclovir can be considered.
Clinical signs may improve over 6-12 months and may persist in mild form. Many affected horses can return to athleticism. Horses that become recumbent have a guarded to poor prognosis for survival.
The horse also is the host of EHV-3, an alphaherpes virus (like EHV-1 and EHV-4) that causes equine co**al exanthema but is not typically associated with systemic illness. Co**al exanthema is a very contagious, self-limiting venereal disease. It is transmitted by coitus, insects, fomites, and inhalation. It is endemic in most equine populations. It may be transmitted in fresh or frozen semen
The external genitalia of stallions and mares have painful pustular lesions. Clinical signs develop within one week of infections, consisting of multiple red nodules on the v***a, vaginal mucosa, cl****al sinus, and perineum of mares and the p***s of stallions. The vesicles that form eventually rupture and become coalescing ulcerations. Lesions may also form in the mouth, nostrils, or lips.
Diagnosis is by clinical appearance, skin biopsy and virus isolation.
If the ulcers become secondarily infected, antibiotics may be required, but generally topical wound care is sufficient. Lesions typically heal by 14 days after the onset of clinical signs, leaving depigmented circular areas in the affected skin.
Like other EHVs, infection is latent and lifelong. Recrudescence can occur in stressful situations.
Equine herpes viruses 2 and 5 are gammaherpes viruses that are prevalent in the equine population. As with other herpes viruses, they remain latent and may cause disease following stressful situations, including hauling and showing.
EHV-2 is ubiquitous among horses. It may cause respiratory disease, but its importance as a respiratory pathogen has not been fully elucidated. The virus has been associated with pulmonary inflammation, keratoconjunctivitis, fever, pharyngitis, anorexia, immunosuppression, lymphoma, and lymphadenopathy. EHV-2 has been found in circulating white blood cells, nasal secretions and nasopharyngeal swabs, kidney, bone marrow, spleen, and reproductive and ocular tissues. It is possible that this virus may cause immunosuppression that is important in the pathogenesis of other respiratory viruses.
Horses with keratitis caused by EHV-2 can present with painful superficial punctate lesions. Affected horses will exhibit blepharospasm, chemosis, and conjunctivitis, along with serous ocular discharge. Visualization of these lesions may require Rose Bengal staining. Cytology shows lymphoplasmacytic inflammation without bacterial or fungal agents. Although EHV keratitis can be treated with antiviral medications, horses with nonulcerative keratitis respond to corticosteroids and cyclosporine treatment, suggesting that pathology is immune-mediated, rather than directly virus induced.
Foals are exposed to equine herpesvirus-5 within the first ten days of life. They may develop a mild fever and mild upper respiratory disease. Most horses develop and recover from infection, after which the virus becomes latent in immune cells. The gammaviruses are readily found in many areas of the body, making it difficult to determine if the virus is causing disease or is simply present in a latent state.
Equine herpesvirus- 5 has been found in nearly all reported cases of equine multinodular pulmonary fibrosis, a condition of nodules in the lung that cause extensive, irreversible scarring of the alveoli, the air sacs in the lungs where gas exchange occurs. The virus is closely related to the EpsteinâBarr virus, the agent responsible for glandular fever in humans.
Clinical signs of EMPF include weight loss, poor condition, and increased respiratory rate and effort. Affected horses are febrile and abnormal lung and tracheal sounds. Additional clinical signs include lymphadenopathy, painful ambulation, oral cavity ulcerations, and keratoconjunctivitis. Horses with EMPF will have an elevated white blood cell count, loss of albumin in the blood, and increased inflammatory proteins.
Diagnosis is by clinical findings, thoracic radiography, lung biopsy, and thoracic ultrasound. EHV-5 is found in lung fluid of affected horses. There can be complications with lung biopsy in horses with dyspnea/tachypnea. The pleural surface is thickened and irregular. Multiple pulmonary nodules are present. Radiography may show characteristic nodular lesions in addition to a diffuse interstitial pattern. Bronchoalveolar lavage may have similar findings to horses with recurrent airway obstruction, and horses with heaves that do not respond to therapy and environmental management should be tested for EMPF. Definitive diagnosis of EMPF is based upon histopathology of lung tissue, supported by DNA (PCR) testing of BAL fluid for EHV-5. In donkeys, the asinine herpesvirus-5 causes a similar condition.
How can I prevent EHV-1, the primary cause of EHM, from spreading to other horses?
There are many steps you can take to help prevent the spread of EHV-1.
⢠Stop horse movement if your animals may be infected with EHV- 1. This is the most important first step horse owners can take. Horses should neither enter nor leave a premises where EHM has been diagnosed until cleared by the veterinarian.
⢠Do not allow horses exposed to EHM case(s) to have contact with unexposed horses on the premises.
⢠Isolate sick horses. Horses that have aborted or shown signs of fever, respiratory disease, or neurologic disease should be separated from healthy horses. Ideally, the sick horse(s) should be moved into a separate building or paddock on the premises, or be transported to a veterinary hospital with an isolation facility.
⢠Do not share equipment among horses on the facility. Since this virus can be spread from horse to horse via contaminated objects such as water/feed buckets or bridles, equipment should not be shared among horses.
⢠Practice proper biosecurity measures to prevent people from spreading the virus. Since people can transfer this virus from horse to horse via their hands and clothing, personnel should wash their hands after handling one horse and before handling another. They should also change their clothes and footwear after working with a sick horse. Optimally, a person who takes care of a sick horse should not work with healthy horses. When this is not practical, healthy horses should be handled first and sick horses last. Wearing gloves and using disinfectant to sanitize footwear can also help minimize the risk of people spreading the virus between animals.
Individual farms and showgrounds may be quarantined by the Department of Agriculture, but there is generally no restriction of movement across state lines with a valid Coggins test and Certificate of Veterinary Inspection (within 30 days) that states the horse(s) have not been on grounds where EHV-1 cases have been identified.
Vaccines exist to control the respiratory and abortion manifestations of EHV-1; however, the currently licensed vaccines are not labeled for the prevention of EHM. University and private researchers are looking into several existing vaccines to determine if they protect against EHM. New vaccines for EHM are also being studied. It is generally felt that by lowering the numbers of shedding viral particles by vaccination, the less likely another horse will show clinical signs. It is not known if the latently infected horse will still become ill with EHM under the right circumstances, even if vaccinated. Apparently healthy horse can shed virus, especially when stressed and spread the virus to other horses.
Practicing biosecurity remains the best form of prevention.
Fox Run Equine Center
www.foxrunequine.com
(724) 727-3481
Fox Run Equine Center is your local Veterinarian in Apollo serving all of your needs. Call us today at 724-727-3481 for an appointment.
