Diseases of Concern
The diseases to which horses are vulnerable vary widely. Some are obvious, others pose hidden threats. Some target younger horses while others affect all ages. The key to good health for your horse is in knowing when, how and where these enemies might strike — and how to best defeat them. The following recommendations are for adult horses. Check with your veterinarian on proper vaccination for foals and previously unvaccinated horses.
Overview
The American Association of Equine Practitioners (AAEP) has created a category of vaccinations called core vaccinations.
Core vaccines have clearly demonstrated efficacy and safety, with a high-enough level of patient benefit – and low-enough level of risk — to justify their use in all horses.
Eastern/Western Equine Encephalomyelitis (EEE/WEE)
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). 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 vaccines and are indicated in the immunization program for all horses.
- Clinicial signs can include:
- Behavioral changes
- Loss of appetite and fever
- These can progress in 12 to 24 hours to:
- Dementia with head-pressing
- Teeth Grinding
- Circling
- Blindness
- Ataxia (incoordination)
All horses are at risk of developing tetanus, an often fatal disease caused by the neurotoxin produced by the Clostridium tetani (Cl. tetani) bacteria. Tetanus toxoid is a core equine vaccine and is indicated in the immunization program for all horses.
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).
- Tetanus toxin affects the horse by causing:
- Rigid paralysis
- Spasms of the muscles (often the jaw muscles are affected first, hence the name "lockjaw")
- Anxious expression
- Reaction to noises or movements with spasms or convulsions
- Respiratory paralysis and dehydration that can lead to death
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 a considerable public health significance. AAEP recommends that rabies vaccine be a core vaccine for all horses.
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.
- Rabies can look like many diseases. Clinical signs vary widely and can include:
- Colic
- Ataxia (incoordination)
- Muscle spasms
- Blindness
- Possible aggression
- Lameness
- Incontinence
- Paralysis
- Depression
West Nile Virus (WNV) is the leading cause of arbovirus encephalitis in horses and humans in the United States. According to AAEP Vaccination Guidelines, nearly 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.
The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33 percent. According to 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-diagnosis. Vaccination for WNV is recommended as an essential standard of care for North American horses and is a core vaccination according to the AAEP.
- Clincial signs include:
- Ataxia (incoordination)
- Loss of appetite
- Muscle-twitching
- Stumbling
- Fever
- Partial paralysis
- Neurologic signs may include head-pressing, inability to stand up, convulsions and possibly death
Overview
According to the AAEP, risk-based vaccines are administered on the basis of a risk assessment performed by your veterinarian. Criteria can include your horse's age, exposure level and geography. Use of these vaccines may vary among individuals, populations and/or geographic regions.
Equine Influenza Virus (EIV) is one of the most common infectious diseases of the respiratory tract of horses. It is endemic in the equine population of the United States and throughout much of the world. EIV does not constantly circulate, even in large groups of horses, but is sporadically introduced by an infected horse. Because EIV is considered the most economically important respiratory disease in the horse, all horses should be vaccinated against Equine Influenza (EIV) unless they live in a closed and isolated facility.
EIV is highly contagious, and the virus spreads rapidly through groups of horses in aerosolized droplets dispersed by coughing.
- Clincial signs include:
- Fever (102.5° to 106.5°F)
- Frequent dry cough
- Nasal discharge
- Lethargy
- Anorexia
- Possible secondary bacterial pneumonia
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. 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.
- Clinical signs of the respiratory form of EHV-1/EHV-4 include:
- Fever
- Lethargy
- Anorexia
- Nasal discharge
- Cough
- EHV-1 also causes:
- Epidemic abortion in mares
- The birth of weak non-viable foals
- Sporadic paralytic neurologic disease (equine herpesvirus myeloencephalopathy — EHM) secondary to vasculitis of the spinal cord and brain
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.
- Clinical signs are variable but may include:
- Fever
- Mild to severe diarrhea
- Laminitis
- Mild colic
- Decreased abdominal sounds
- Uncommonly, pregnant mares can abort
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 pneumoenteritis.
- Clinical signs, if they occur, typically develop 3-7 days post-infection and are variable but may include:
- Fever, depression
- Anorexia
- Dependent edema (lower limbs, scrotum and prepuce or mammary glands)
- Localized or generalized urticaria (severe itching or hives)
- Swelling above or around the eye
- Conjunctivitis
- Serous to mucoid nasal discharge
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 though 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
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 though 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.
- Clinical signs include:
- Weakness
- Muscle tremors
- Protruding tongue
- Stumbling and collapse
- Frequent lying down
- Drooping tail
- Constipation
- Paralysis