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What are Herpesvirus?

There are five herpesviruses (from the family Alpha-herpesviridae) that infect horses; EHV-1, 2, 3, 4 and 5; however, it is EHV-1 and EHV-4 that cause serious illness in horses. EHV-1 is found often in horses worldwide and while it is commonly known to cause reproductive issues, it has also been found to lead to respiratory and neurological illness. EHV-1, also called equine rhinopneumonitis virus, is more likely to be found in very young horses, causing respiratory disease and in some cases abortion and neurological illness. Both EHV-1 and EHV-4, which are carried in an inactive state by most horses, are a significant cause of respiratory illness in horses. Transmission of the virus is due to either direct or indirect contact with a horse’s fetus that had been miscarried, a placenta (or fluids from the placenta) or any discharge from the horse’s nose.

While there are five herpesviruses, two in particular cause serious illness in horses, including respiratory and possibly reproductive and neurological disease.

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Symptoms of Herpesvirus in Horses

Equine herpesvirus has an incubation period of 2-10 days. Horses that develop the infection may experience:

  • A temperature of 102-107 degrees Fahrenheit
  • A significant amount of nasal discharge
  • A sore throat
  • Cough
  • A very low white blood cell count
  • Enlarged lymph nodes
  • A lack of appetite
  • A lack of energy
  • It is common to develop a secondary bacterial infection.  

Should a horse be pregnant, abortion can occur between 7 and 11 months of pregnancy (usually between 2-12 weeks from time of infection). If the horse is infected late in their pregnancy, they may carry the foal to term, however the foal may have fulminating viral pneumonitis and be sensitive to secondary bacterial infections. In most cases, the foal will pass away shortly after birth.

Neurological symptoms that may be seen include:

  • Lack of coordination
  • Paralysis of your horse’s hind limbs
  • Difficulty getting up from lying down
  • Inability to control their bladder
  • Loss of feeling in the skin near his tail and back limbs

Should your horse be sensitized to the virus, you will notice mild or minimal symptoms.


There are five herpesviruses that infect horses (EHV-1, 2, 3, 4, 5). EHV-1 and EHV-4 are the two that can lead to serious disease in your horse. 

EHV-1 is known for causing abortion as well as respiratory and neurological disease. EHV-4, also known as equine rhinopneumonitis virus, most often causes respiratory illness, but like EHV-1 can cause neurological disease and abortion (rarely) in pregnant horses.

Causes of Herpesvirus in Horses

The equine herpesvirus can be transmitted when an infected horse and an uninfected horse come in contact; either directly or indirectly. The disease can be spread through nasal discharge, by contaminated buckets or blankets, or can travel in the air for short distances. Should a horse have contact with an aborted fetus, or the placenta or placental fluids of an infected horse, the virus can be transmitted. 

The way disease occurs in EHV-1 and EHV-4 is very different. EHV-4 infection is limited to the respiratory tract epithelium and lymph nodes, while strains of EHV-1 have an affinity for the vascular endothelium. The EHV-1 virus will secure entrance to peripheral tissues through the bloodstream, which can lead to abortion or neurologic disease.

Once a horse has been infected with the virus, he will become a carrier of the virus. Should your horse be a carrier, the virus might reactivate if your horse experiences a period of significant stress or takes a high dose of steroids.

How likely it is that your horse will develop an infection is dependent on the strain of the virus, the state of his immune system, his age and if female, whether she is pregnant. Should your horse have previously had the infection, if he came in contact with the virus he would develop an infection with mild or no symptoms.

Diagnosis of Herpesvirus in Horses

While your veterinarian will conduct a physical examination of your horse, EHV-4, also known as equine rhinopneumonitis and equine influenza, equine arteritis or other equine respiratory infections can be hard to tell apart when considering the symptoms your horse is displaying. Should the exam suggest the possibility of equine herpesvirus infection, your veterinarian will look to confirm the diagnosis through PCR or seek to isolate the virus from a nasal or throat swab or through a blood sample. Your veterinarian may also look at blood samples to see if there are antibodies present.

Should it be thought that an aborted fetus may be the result of an EHV-1 infection, diagnosis will be made through the presence of microscopic lesions on the fetus, isolating the virus and confirming the presence of antibodies in the fetal tissue. Good places to find the virus are the lung, liver, adrenal, and lymphoreticular tissues.

Treatment of Herpesvirus in Horses

There is no particular treatment for equine herpesvirus infection. Rest and supportive care are important to avoid secondary bacterial complications. Should your horse have a fever above 104 degrees Fahrenheit, antipyretics may be recommended. If your veterinarian suspects that your horse has a secondary bacterial infection due to nasal discharge or lung disease, antibiotics may be prescribed. Non-steroidal anti-inflammatory medication may be recommended to help with fever, pain and inflammation in your horse. If a foal is infected with EHV-1 while in the womb, the animal will likely die shortly after birth even with intense treatment.

Recovery of Herpesvirus in Horses

The outcome will be determined by the particular strain of the virus, your horse’s immune system, whether or not your horse is pregnant, and possibly the age. In the neurologic form of EHV-1, recovery will vary based on how severe the symptoms. If your horse experiences an uncomplicated infection he should recover completely in a few weeks. 

It is important to have your horse rest until he is fully recovered and have him slowly return to full activity. You will want to follow the recommendations of your veterinarian and attend follow up appointments to ensure the best outcome for your horse.