Common Equine Diseases

The following list summarizes several common equine diseases and indicates whether an effective vaccine exists to protect your horse.  Also see our recommended vaccinations for more information.

Equine Influenza (“Flu”)

This highly contagious viral disease appears suddenly, is costly to treat, and can leave your horse in a weakened condition.  Equine Influenza can not be passed to or from horse and human. Symptoms include fever, coughing, nasal discharge, and loss of appetite.  Infected horses must be kept at complete rest to avoid complications from secondary bacterial infections such as pneumonia.  The virus mutates into new strains over time, and several distinct strains are now circulating in the U.S.  All horses should receive a vaccine containing the most current influenza strains available.

Rhinopneumonitis/Equine Herpesvirus (EHV)

Of the several equine herpes viruses identified, Type 4 (EHV-4) and Type 1 (EHV-1) are the most clinically important.  The disease is characterized by respiratory infections, paralysis, abortions, and occasionally death in young horses.  EHV is highly contagious spreading through aerosolized secretions, contact with infected horses, and contaminated feed and water utensils.  Every horse should be vaccinated for EHV-4 and EHV-1.  For abortion protection in the pregnant mare, vaccinate at 3, 5, 7 and 9 months of pregnancy for EVH-1 with Pneumobort-K.

Equine Encephalomyelitis (“Sleeping Sickness”)

Transmitted by mosquitoes, Equine Encephalomyelitis is an often fatal disease that attacks the brain and central nervous system.  Stricken horses may lose their appetite, display excitable or compulsive behavior, and walk blindly into objects.  Surviving horses may suffer permanent damage.  There are three strains of the virus.  Eastern (EEE) is the most deadly, killing 75-100% of infected horses.  Western (WEE) is more common and somewhat less virulent.  Both EEE and WEE can occur throughout the U.S., and horses should be vaccinated for both.  Vaccination advisories against the Venezuelan strain (VEE) have been issued for horses in southern U.S. states, especially those bordering Mexico.

Equine Infectious Anemia Virus (EIA)

Equine infectious anemia is a potentially fatal viral disease.  There is no cure or effective treatment for EIA.  Most infected horses show no symptoms but remain contagious for life, endangering the health of other horses. EIA is transmitted by blood or by passage across the placenta in the pregnant mare directly to the foal.  Blood transmission can occur via blood-sucking insects such as horse flies, deer flies and mosquitoes.  Symptoms may include one or more of the following:  Fever, depression, decreased appetite, fatigue, reduced stamina or weakness, rapid weight loss.  This is a reportable disease.  The Coggins test is the only way to accurately determine whether a horse is infected.  All positive cases must be filed with the state veterinarians and the federal Animal and Plant Health Inspection Service.  If your horse tests positive for EIA, your options are extremely limited.

West Nile Virus

West Nile Virus (WNV) is a virus that is transmitted principally by various species of mosquitoes that can cause inflammation of the brain and spinal cord (encephalomyelitis).  WNV circulates in nature between birds and mosquitoes.  Mosquitoes are unable to transmit the virus from horses to horse or from horse to human.  While the clinical signs of WNV can vary in range and severity, the most frequently observed include in coordination, especially of the hind limbs.  Depression or heightened sensitivity to external stimuli, stumbling, toe dragging, leaning to one side, and in severe cases, paralysis of the hindquarters, recumbency, coma, and death.  Other clinical signs that may be noted include fever, generalized weakness, impaired vision, inability to swallow, and aimless wandering.

Streptococcus equi (“Strangles”)

“Strangles” is a contagious bacterial disease seen most often in young horses.  Clinical signs include fever, nasal discharge, cough, loss of appetite, trouble swallowing and swollen lymph nodes in the head.  Enlarged inner lymph nodes may impair breathing or even asphyxiate the horse.  Strangles is transmitted via nasal secretions, pus from draining abscesses, dirty hands, flies, and contaminated feed buckets and grooming tools.  Vaccination can significantly decrease severity of disease.

Tetanus (“Lockjaw”)

Tetanus is caused by Clostridium tetani bacteria entering wounds from barbed wire, dropped nails, or even surgical procedures such as castration.  Afflicted horses can suffer muscle spasms, high fever, violent reactions to sudden movement or noise, and even death from asphyxiation.  Vaccination is the best prevention.

 Equine Rabies

Rabies is an invariably fatal viral disease contracted from the bite of an infected animal. It is spread primarily by raccoons, bats, skunks, foxes and coyotes, and is most common in the northeast U.S. and Texas.  In one nine-year period, over 500 cases of equine rabies were reported in the U.S.  If rabies is suspected, call a veterinarian immediately.  Vaccination is recommended.

Potomac Horse Fever

Named after the region where the disease was first diagnosed in 1979, Potomac Horse Fever (PHF) is characterized by fever, lameness, diarrhea and occasionally death.  It occurs most often during summer months in the northern U.S. and Canada.  Scientists still don’t know exactly how the disease is transmitted.  Vaccination is strongly recommended for horses in areas where Potomac Horse Fever has been diagnoses, and for horses traveling to such areas. 

Equine Protozoal Myeloencephalitis

EPM is an infection of the central nervous system of horses.  The neurologic signs that it causes are most commonly asymmetric incoordination (ataxia), weakness and spasticity and may mimic other neurologic conditions.  The opossum has been implicated as the definitive host of the EPM organism.  Cerebrospinal fluid (CSF) is traditionally tested for diagnosis.  However, advances in EPM research have provided blood tests that quantify the level of antibodies in the blood and when combined with clinical signs allow for confident diagnosis without the more invasive spinal tap.  Falsely positive or negative tests are
common when only the blood is tested.  Any horse that is suspected to have EPM should have CSF tested for the presence of
antibodies to this parasite.  There is no effective vaccine at this time.