Definitions of Equine Diseases

Anthrax is a bacterial disease caused by the organism Bacillus anthracis. Horses most commonly show septicemia with enteritis and colic. The disease is known worldwide, but is not common in the United States. Most cases are reported curing the warm summer months. Vaccination is only recommended in enzootic areas of the country (i.e. South Dakota, Louisiana, Texas, Missouri, California, and Arkansas). In the case of an outbreak, only those animals not exhibiting clinical signs should be vaccinated.

Botulism is a neuroparalytic disease caused by the organism Clostridium botulism. In North America, type B is most commonly found in the Mid-Atlantic States and Kentucky, type A is prevalent west of the Rocky Mountains, and type C is typical in Florida. Most foals are infected by growth of the bacterium within the gastrointestinal tract between 2 weeks and 8 months of age and are referred to as having “shaker foal syndrome”. Adult horses are most commonly infected via ingestion of preformed toxins referred to as “forage poisoning”. Clinical symptoms include progressive muscle paralysis affecting the limbs, jaw, and throat muscles. Frequent paralysis of the upper eyelid, tongue, and tail are also noted.

Encephalomyelitis is a viral disease also known as “sleeping sickness”. The three types are Eastern, Western and Venezuelan. The disease is characterized by various neurological signs, including excitement, dementia, head pressing, and circling. Transmission occurs by mosquitoes, and the disease is associated with areas inhabited by them and where winter temperatures are not severe. Vaccination before mosquito season and biannually in humid areas of the country is recommended. Initial dose should be boostered in 3-4 weeks. Horses should be re-vaccinated 2-3 times yearly thereafter.

Equine viral arteritis is a viral disease that can cause respiratory disease and abortion in horses. The vaccine is only recommended for use in stallions, open mares, and young horses 6 weeks of age or older. Pregnant mares and open mares or stallions that are within 3 weeks of being bred should not be vaccinated.

Influenza is an acute viral disease that affects the upper respiratory tract of horses. The disease spreads rapidly and outbreaks can occur any time of the year – commonly among young horses moved into larger populations at training barns, racetracks, and show grounds. Symptoms include a dry cough, fever, nasal discharge, loss of appetite, and muscle soreness. Coughing is the most common sign, often lasting 1-3 weeks. Vaccination is by a killed-virus vaccine, which requires frequent administration every 2-6 months to provide appropriate protection to susceptible horses. The vaccine is available in monovalent or polyvalent forms. A post vaccination fever is sometimes detected in some horses, while others may show a loss of appetite and depression. Once a horse is infected, the upper respiratory tract takes three weeks after remission of the disease to renew itself. It is important to vaccinate the competitive and/or show horse(s) traveling to and from show, sales and breeding farms. Current recommended vaccination schedules are very 6 months for adult horses at low to moderate risk and every 1-3 months for high-risk animals. Vaccinations should be administered 2-3 weeks before a stressful event because of the increase chance of the previously mentioned side effects.

Potomac horse fever (equine monocytic ehrlichiosis) is an acute enteric disease characterized by diarrhea, depression, reduce gut motility, and often laminitis due to infection by Ehrlichia risticii. PHF is a seasonal disease, occurring mostly in late spring to early fall and has been reported mainly in the northeastern United States and Canada. As soon as PHF occurs on a farm, there is a high risk of recurrence. It is believed an insect vector is responsible for transmission of the disease. If an infected horse survives, it will develop immunity for about 2 years. Vaccines offer short-term protection and revaccination every 4-6 months is recommended.

Rabies is a neurological rhabdovirus disease with clinical signs that range from lethargy to bizarre, frightening behavior. Symptoms may include colic, lameness, muscle in-coordination, incontinence, muscle spasms, paralysis, blindness, depression, and aggression. Horses in enzootic areas should be vaccinated; especially those exposed to wildlife. Rabies is invariably fatal in unvaccinated animals and is zoonotic; therefore, exposed horses should be euthanized or isolated and closely observed by a veterinarian for 6 months. Vaccination of unvaccinated horses after exposure to the disease is not recommended. Rabies vaccine should be administered in the lower thigh, not in the neck, due to a local tissue reaction.

Rhinopneumonitis is a viral disease caused by a herpesvirus. This highly infectious disease can cause respiratory infection, paralysis, and death in horses. EHV-1 causes abortions in pregnant mares, usually in the last three months of pregnancy. WHV-4 is prevalent among young horses. Symptoms include fever, depression, loss of appetite, swollen lymph nodes, and profuse nasal discharge. Rhino vaccines offer short-term immunity and require frequent administration. Vaccination every 2-3 months is recommended, depending on the environmental risk. Preventative medicine is best when combined product is used (EHV-1 & EHV-1b). Pregnant mares should be vaccinated at 5, 7, and 9 months of gestation with approved vaccines. Initial dose must be boostered in four to six weeks with annual revaccination required.

Strangles (distemper, barn fever) is an infectious, highly contagious bacterial disease mostly seen in young horses and caused by Streptococcus equi. The bacteria invade the mucous membranes of the nose and throat causing severe inflammation and pain. The extreme swelling of the lymph nodes causes the horse to sound as if it is being strangled, thus the origin of the name “strangles”. Abscesses mainly occur in the throatlatch region or between the jaws and can cause immense distress to infected horses. Horses presently in, or going to be in, a high-risk environment should be vaccinated. Broodmares should be vaccinated 3 to 6 weeks prior to foaling and foals should be vaccinated at 2-3 months of age and again at 6 months of age. Adult horses that are not at a high risk are usually not vaccinated for this disease. Depending on the environment, either annual or biannual vaccinating can be practiced.

Tetanus is a non-infectious disease caused by a bacterium and is characterized by a general increase in muscle stiffness, tremors, lockjaw, and prolapsed third eyelid. Horses display a “sawhorse” stance when exhibiting clinical signs of the disease. Clostridium tetani, the causative organism, is found in the soil worldwide, and infection is associated with high mortality. Horses with no history of immunization that are injured or undergo surgery are at a high risk for tetanus. Animals should receive tetanus toxoid and tetanus antitoxin administered at two separate locations on the body. This gives the horse active (tetanus toxoid) and passive (tetanus antitoxin) immunity for protection. A booster should be given annually or at any time of exposure.