Merck Manual

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Pet Owner Version

Equine Herpesvirus Infection (Equine Viral Rhinopneumonitis)


Bonnie R. Rush

, DVM, MS, DACVIM, Equine Internal Medicine, College of Veterinary Medicine, Kansas State University

Reviewed/Revised May 2019 | Modified Oct 2022

Equine herpesvirus 1 (EHV-1) and equine herpesvirus-4 (EHV-4) make up 2 distinct groups of viruses. Both are widespread in horse populations worldwide and are a major cause of respiratory disease. Transmission occurs by direct or indirect contact with infectious nasal secretions, miscarried fetuses, placentas, or fluids from the placenta. A horse’s susceptibility to infection depends on the strain of virus, the immune status of the animal, pregnancy status, and possibly age. The infection is mild or unseen in horses immunologically sensitized to the virus by a previous infection. Most horses carry the EHV-1 and EHV-4 viruses in an inactive state. The infection remains dormant for most of the horse’s life, although stress or immune system problems may result in “reawakening” of disease and shedding of infectious virus.

Outbreaks of respiratory disease occur annually among foals in areas with concentrated horse populations. Most of these outbreaks are caused by strains of EHV-4. Infection of pregnant mares with EHV-1 may result in abortion 2 to 12 weeks after infection. Nervous system disease is another possible outcome of a specific strain of EHV-1 infection.

Signs of infection include fever, nasal discharge, depression, throat inflammation (pharyngitis), cough, poor appetite, and enlarged lymph nodes. Horses infected with EHV-1 strains often develop a fever that rises, falls, and then rises again. Infected horses may also develop bacterial infections that cause nasal discharge and lung disease. If nervous system disease develops, signs may vary from mild incoordination to paralysis of the hind end (forcing the horse to be unable to rise or stand), loss of bladder and tail function, and loss of sensation to the skin around the rectal, genital, and groin areas. In rare cases, the paralysis can progress to all 4 legs (quadriplegia) and death.

Equine herpesvirus infection cannot be differentiated from other causes of equine respiratory disease, such as equine influenza Equine Influenza Equine influenza is highly contagious and spreads rapidly among susceptible horses. Horses 1-5 years old are the most susceptible to infection. Disease varies from a mild, undetected infection... read more , solely on the basis of signs. The definitive diagnosis is determined by identifying the virus in samples obtained from the nose and throat and from blood testing early in the course of the infection.

There is no specific treatment for this infection. Rest and good nursing care may minimize the chance of secondary bacterial infection. Fever-reducing medications may be recommended in some cases, such as for horses with a fever greater than 104°F (40°C). Antibiotic therapy is usually initiated when there is suspicion of secondary bacterial infection, such as pus‑containing nasal discharge or lung disease. If horses with EHV-1-associated nervous system disease remain able to walk, or are down for only 2 to 3 days, the outlook is usually favorable. Intensive nursing care is necessary to avoid lung congestion, pneumonia, ruptured bladder, or intestinal problems. Recovery may be complete, but a small percentage of cases have nervous system damage.

For prevention and control of EHV-4- and EHV-1-related diseases, management practices that reduce viral spread are recommended. New horses (or those returning from other premises) should be isolated for 3 to 4 weeks before mixing with resident horses, especially pregnant mares. Pregnant mares should be maintained in a group away from the weanlings, yearlings, and horses out of training. In an outbreak of respiratory disease or miscarriage, affected horses should be isolated and appropriate measures taken for disinfection of contaminated premises.

Several types of vaccines have been developed but are not available in all countries. Your veterinarian can provide appropriate information about types of vaccines and the need for vaccination in your area. The immunity induced by vaccination against EHV-1 and EHV-4 generally lasts for only 2 to 4 months, and available vaccines do not cover all strains to which horses can be exposed. Vaccination usually begins when foals are 3 to 4 months old and, depending on the vaccine used, a second and third dose may be given. Periodic booster vaccinations may be given until the horse reaches maturity. Vaccination programs against EHV-1 should include all horses on the premises.

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