Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Equine Herpesvirus 1 & 4, Rhinopneumonitis

Synonyms: EHV-1, EHV-4


A group of Herpes viruses affect horses. The most common of these affect the respiratory tract, Type 1 (EHV-1), and Type 4 (EHV-4).

These viruses are transmitted by direct and indirect contact with nasal secretions from infected animals and air-borne secretions from coughing horses. They can also be spread by handlers carrying the virus on their hands, clothes and equipment.

These viruses infect the respiratory tract (rhinopneumonitis) and respiratory illness ranges from mild to severe. They are analogous to viruses that cause cold-like symptoms in people. Like humans and cold viruses, most horses have been exposed to these viruses at some point in their lives.

Herpes viruses in people are known to hide within the body ("going into latency") under the radar of the immune system for long periods and later activate under periods of stress. The situation is similar in horses, and some horses are "silent", chronic carriers of the virus.

Common signs of these respiratory viruses include fever, lethargy, weight loss, runny nose and cough. Signs of infection from these viruses are usually self-limiting, meaning that they resolve on their own or with little treatment. Horses are ill for a few days to a week, and then improve in most cases. Young horses are more frequently affected. In rare cases, brain or spinal cord paralysis results within weeks of recovering from a respiratory infection of EHV-1.

EHV-1 is the most important of the Herpes viruses because it can cause several different, severe syndromes. It can cause abortion in mares. It can also cause the birth of weak foals that only survive a short time. Importantly, in some cases EHV-1 can cause very severe neurologic disease that results in weakness and paralysis that can result in down horses that cannot rise. Many severely affected horses do not survive. This syndrome is called EHM (Equine Herpes Myelitis or Myeloencephalitis)

It is believed by some that EHM results from a misdirected immune response to the virus, that causes severe inflammation in the spinal cord. There are also some genetic differences in EHV-1 viruses that are associated with EHM.

EHV-1 and EHV-4 are reportable diseases, meaning that if a horse has or is suspected of having either of these diseases, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other states and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

Diagnosis is by laboratory testing:looking for viral DNA in nasal swabs or blood using PCR, or looking for blood antibodies that suggest infection.

Treatment is mostly nursing care and supportive. Horses with the respiratory viruses show a course similar to the common cold. Horses with EHM require intensive management.

my vet's role


The prognosis is good in most horses infected with these diseases.

It is poor for horses that develop EHM and especially for horses that are down and cannot rise.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
more observations

Questions To Ask Your Vet:
  • How can I manage my horse to reduce the risk of exposure at shows or events?
  • How will I know if my horse will have the more serious, neurologic signs?

Vaccines for these diseases may be recommended in certain circumstances, but are not completely protective against infection with the viruses or against the neurologic disease EHM.

Vaccination may be recommended to prevent EHV-1-induced abortion and to reduce its spread and exposure in show horses.
The most important aspect of prevention is good biosecurity. Prevent close contact with outside horses, especially those that may have been exposed to the virus. Use of a quarantine facility for new horses helps to reduce spread of these diseases within a facility.

further reading & resources

Author: Doug Thal DVM Dipl. ABVP