Eastern/Western Equine Encephalomyelitis (EEE/WEE) are viral infections of the horse’s brain and spinal cord. The virus is maintained in reservoirs (primarily birds and rodents) and transmitted to the horse by the bite of an infected mosquito. The disease is fatal in 50-90 percent of cases.

Surviving horses often have residual mental dullness. Treatment is generally supportive because antibiotics are ineffective.

There are two types of equine encephalomyelitis commonly seen in North America—Eastern Equine Encephalomyelitis (EEE) and Western Equine Encephalomyelitis (WEE). EEE is the most prevalent and deadly. A third type, Venezuelan Equine Encephalomyelitis (VEE), is seen in Central America and northern South America but has not been reported in the United States since 1971. Eastern/Western Equine Encephalomyelitis (EEE/WEE) are considered core equine vaccinations and are indicated in the immunization program for all horses.

All horses are at risk of developing tetanus, an often-fatal disease caused by the neurotoxin produced by the Clostridium tetani (Cl. tetani) bacteria.

Spores of Cl. tetani survive in the environment for many years, resulting in an ever-present risk of exposure to horses and people in equine facilities. Tetanus is not a contagious disease but is the result of Cl. tetani infection of puncture wounds (particularly those involving the foot or muscle), open lacerations, surgical incisions, exposed tissues such as the umbilicus of foals and reproductive tract of the postpartum mare (especially in the event of trauma or retained placenta).

Rabies is an infrequently encountered neurologic disease. While the incidence of rabies in horses is low, once signs of clinical disease appears, the fatality rate is 100%. There is no treatment for the disease, and it has considerable public health significance.

Exposure occurs through the bite of an infected (rabid) animal, typically a wildlife source such as raccoon, fox, skunk or bat. Bites to horses occur most often on the muzzle, face and lower limbs. The virus migrates via nerves to the brain, where it initiates rapidly progressive, invariably fatal encephalitis.

West Nile Virus (WNV) is the leading cause of arbovirus encephalitis in horses and humans in the United States. According to AAEP Vaccination Guidelines, more than 25,000 cases of equine WNV encephalitis have been reported in the United States since 1999.

This virus has been identified in the entire continental United States, most of Canada and Mexico. The virus is transmitted from avian reservoir hosts by mosquitoes to horses, humans and a number of other mammals. Infected horses are not contagious.

The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33 percent. According to the AAEP, there is data showing that 40 percent of horses that survive the acute illness caused by WNV still exhibit residual effects, such as gait and behavioral abnormalities, six months post-diagnosis². Vaccination for WNV is recommended as an essential standard of care for North American horses.

Equine Influenza Virus (EIV) is one of the most common and contagious infectious diseases in the horse. It spreads rapidly through groups of horses in aerosolized droplets dispersed by coughing. It is endemic in the equine population of the United States and throughout much of the world. Equine influenza can be particularly challenging because it is a unique virus that changes over time. Just like human flu vaccines, equine flu vaccines must be periodically updated to protect against the strains of influenza currently circulating and threatening horses.

Because EIV is considered the most economically important respiratory disease in the horse, all horses should be vaccinated against EIV unless they live in a closed and isolated facility.

Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) can each infect the respiratory tract, causing disease that varies in severity from subclinical (not apparent) to severe. EHV-4 is the most common infectious upper respiratory disease in the horse³ and is typically associated with upper respiratory disease in younger horses. EHV-1 can cause respiratory disease, late-term abortions, early foal deaths and neurologic disease. Infection of the respiratory tract with EHV-1 and EHV-4 typically first occurs in foals early in life, but recurrent infections are seen in weanlings, yearlings and young horses entering training—especially when horses from different sources are commingled.

Both EHV-1 and EHV-4 spread via coughing horses, by direct and indirect contact with nasal secretions and, in the case of EHV-1 abortion, contact with aborted fetuses, fetal/placental fluids and the placenta. Horses can have latent infections and may not show clinical signs but may experience reactivation of infection and shed the virus when stressed. Those factors compromise efforts to control these diseases and explain why outbreaks of EHV-1 or EHV-4 can occur in closed populations.

Streptococcus equi subspecies equi (S. equi var. equi) is the bacterium that causes the highly contagious disease strangles (also known as “distemper”). Strangles commonly affects young horses (weanlings and yearlings), but horses of any age can be infected. It is the most commonly diagnosed upper respiratory disease in horses 6-10 years of age⁵, though clinical signs may not be as severe in adult horses as they are in young horses. Infected horses can spread disease without showing clinical signs and can be a source of infection for an undetermined period of time. Proper biosecurity is key to preventing and managing disease spread.

The organism is transmitted by direct contact with infected horses or subclinical shedders, or indirectly by contact with people, water troughs, hoses, feed bunks, pastures, grooming equipment and even insects contaminated with nasal discharge from infected horses. S. equi has demonstrated environmental survivability, particularly in water sources and when protected from exposure to direct sunlight and disinfectants.

Caused by Neorickettsia risticii (formerly Ehrlichia risticii) and originally described in 1979 as a sporadic disease affecting horses residing in the eastern United States near the Potomac River. Potomac Horse Fever (PHF) has since been identified in various other geographic locations in the United States and Canada. The disease is seasonal, occurring between late spring and early fall in temperate areas, with most cases in July, August and September at the onset of hot weather.

If PHF has been confirmed on a farm or in a particular geographic area, it is likely that additional cases will occur in future years. Foals appear to have a low risk of contracting the disease.

Equine Viral Arteritis (EVA) is a contagious disease caused by equine arteritis virus, an RNA virus that is found in horse populations in many countries. While typically not life-threatening to otherwise healthy adult horses, EVA can cause abortion in pregnant mares and, uncommonly, death in young foals; it can also establish a long-term carrier state in breeding stallions. While various horse breeds appear equally susceptible to EVA, the prevalence of infection can vary widely, with higher seropositivity rates occurring in Standardbreds and Warmbloods.

Abortion is a frequent sequel to infection in the unprotected pregnant mare. Young foals exposed to EVA can develop a life-threatening pneumonia or enteritis.

Rotavirus is a major cause of infectious foal diarrhea and has been documented to cause 50 percent or more of foal diarrhea cases in some areas. While rotavirus diarrhea morbidity can be high (50 percent of susceptible foals), mortality is low (<1 percent) with veterinary intervention. Equine rotavirus is transmitted via the fecal-oral route and damages the small intestinal villi, resulting in cellular destruction, maldigestion, malabsorption and diarrhea.

Vaccination of mares results in a significant increase in foals’ rotavirus antibody titers. Field trials of rotavirus vaccination in pregnant mares have shown a decrease in incidence and severity of foal diarrhea on farms that historically had annual rotaviral diarrhea cases. According to AAEP, other studies have shown increased rotavirus antibody in vaccinated mares’ colostrum.

Anthrax is a serious and rapidly fatal septicemic disease caused by proliferation and spread of the vegetative form of Bacillus anthracis in the body. Infection is acquired through ingestion, inhalation or contamination of wounds by soil-borne spores of the organism. Anthrax is encountered only in limited geographic areas where alkaline soil conditions favor its survival. Vaccination is indicated only for horses pastured in endemic areas.

Botulism is caused by the exotoxin of Clostridium botulinum and typically occurs when horses eat decayed vegetable matter, animal carcasses or improperly prepared haylage or silage. Horses are extremely susceptible to the toxin, which is the most potent biological toxin known. It causes neuromuscular weakness that progresses to paralysis.

Death is due to respiratory paralysis. Treatment with antibiotics or antitoxin is helpful if given early in the course of the disease.