January 2012

Infectious Equine Diseases

Equine Encephalomyelitis (Sleeping Sickness): Encephalomyelitis is caused by a virus, which is transmitted by mosquitos. The virus causes inflammation of the brain and spinal cord and is often fatal.  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 for the Venezuelan strain (VEE) have been issued for horses in the southern states, especially those bordering Mexico.  The vaccine is very effective against the disease. We typically recommend administering the vaccine annually.

Equine Herpesvirus (EHV)/Rhinopneumonitis: Of the several equine herpes viruses identified, Type 1 (EHV1) and Type 4 (EHV4) are the most clinically important.  EHV is characterized by respiratory infections, paralysis, abortions, inflammation of the spinal cord and occasionally death in young horses. EHV is highly contagious, spreading through aerosolized secretions, contact with infected horses, and contaminated feed and water utensils.  Though the vaccine is not 100% effective and the protection only lasts 10-12 weeks, every horse should be vaccinated.  Pregnant mares should be vaccinated at 5, 7 and 9 months from their breeding date.  Horses that are travelling to shows, races, sales, etc. should be vaccinated every 6 months. Pleasure horses that do not travel should be vaccinated annually. The vaccine does not protect against the neurologic form of the disease.

Equine Influenza (Flu): Influenza is a highly contagious virus that appears suddenly and causes high fever, coughing, nasal discharge and loss of appetite.  Infected horses should be kept at complete rest to avoid complications 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.  The vaccine is not 100% effective, and the protection lasts only 10-12 weeks. Horses travelling to shows, sales, racing events, etc. should be vaccinated every 6 months. Horses that do not travel should be vaccinated annually.

Equine Piroplasmosis: Equine piroplasmosis is caused by two main organisms, Theileria equi and Babesia caballi.  These organisms are mainly transmitted by ticks, but can also be spread between horses by contaminated needles, transfusions, etc.  Clinical signs include: weakness, loss of appetite, fever, anemia, weight loss, jaundice and edema.  Horses who survive this condition may carry and shed the parasite for long periods of time and can infect other horses mainly via tick exposure.  Diagnosis of this condition is through ELISA blood testing.  Due to the seriousness of this condition, infected animals must be reported and mandatory testing is required for events such as the AQHA World Show and Quarterhorse Congress.  No vaccine is currently available.  Though treatment is possible, it requires special regulatory permission.  

Equine Protozoal Myeloencephalitis (EPM): EPM is a disease that attacks the central nervous system of the horse.  It is causes by the parasite, Sarcocystis neurona, and sometimes Neospora hughesi.  Transmission is by ingestion of infective sporocysts that are found in the feces of opossums.  Signs of EPM may include seizures, depression, muscle atrophy, incoordination, lameness and paralysis. 

Equine Viral Arteritis (EVA): EVA is a highly contagious disease often occurring in epidemic form on breeding farms, at shows and at race tracks.  It is most severe in very young and very old horses, as well as those under stress.  Signs include fever, coughing, depression, leg and facial swelling, high rates of abortion and occasionally death.

Potomac Horse Fever (PHF): Named after the region where the disease was first diagnosed in 1979, PHF is characterized by fever, laminitis, edema, severe diarrhea and sometimes death.  Potomac Horse Fever is caused by the parasite Neorickettsia risticii.  Horses are infected through small land snails and mayflies that carry the parasite.  It occurs most often during summer months in the northeast U.S.  It is not contagious and occurs more commonly in wet areas.  The vaccine is fairly effective and is administered once a year depending on exposure risk. It is administered 2-4 x yearly in problem areas.

Rabies: Rabies is an invariably fatal viral disease that infects the nervous system of mammals.  All mammals, including humans, are susceptible.  It is contracted from the saliva or bite of an infected animal.  It is spread primarily by raccoons, bats, skunks, foxes and coyotes.  Rabies has been found in both wild and domestic animals in northeast Ohio.  The vaccine is given once yearly and is very effective.

Rhodococcus equi: Rhodococcus equi causes pneumonia in young foals, usually up to 4 months of age.  It is characterized by large abscesses that form in the lung, making treatment difficult.  It is caused by a soil-borne bacteria.  After the bacteria multiplies in the respiratory tract, it is coughed up and swallowed resulting in soil contamination through infected feces.  Clinical signs may include: fever, cough and labored breathing.  Though less common, abscesses may form outside of the lungs in areas such as the spine or under the skin.  Though there is no vaccine available for this condition, high risk foals may receive ultrasound screenings and be given hyperimmune plasma to minimize their risk.  A combination of antibiotics are used for treatment of this condition.  The prognosis for this condition ranges from good (if caught early) to poor. 

Streptococcus equi (Strangles): Strangles is a highly contagious bacterial disease caused by Streptococcus equi. It is most often seen in young horses, though horses of all ages can be affected.  Symptoms include: high fever, nasal discharge, abscessed lymph nodes, cough, loss of appetite, trouble swallowing and swollen lymph nodes in the throat.  Enlarged inner lymph nodes may impair breathing or even asphyxiate the horse.  Horses may also develop guttural pouch infections, sinus infections, purpura hemorrhagic, laryngeal paralysis and bastard strangles. Strangles is transmitted via nasal secretions, pus from draining abscesses, dirty hands, flies and contaminated feed buckets and grooming tools.  There is an intranasal vaccine which is more effective than the intramuscular vaccine. The vaccine is given every 6 to 12 months for horses considered at high risk.   

Tetanus (Lockjaw): Tetanus is a disease caused by a bacterial toxin (Clostridium tetani) which usually enters the body through wounds from barbed wire, dropped nails or even surgical procedures such as castration.  The bacteria is commonly found in horse manure.  Tetanus is characterized by spasmodic contractions and rigidity of some or all of the voluntary muscles (especially of the jaw, face and neck).  Affected horses can also suffer from high fever, violent reactions to sudden movement or noise and even death from asphyxiation.  The disease is almost always fatal.  The vaccine is very effective and administered once yearly. The vaccine is also boostered in case of laceration, surgery or penetrating wounds.

West Nile Virus: Since its discovery in New York in 1999, this mosquito-borne disease has spread quickly throughout the U.S.  West Nile concerns equine veterinarians because it can cause encephalitis (inflammation of the brain and spinal cord), muscle weakness, partial paralysis, fever, convulsions, coma and death.  Once horses show clinical signs of illness, the disease is fatal nearly 40% of the time.  Horses that travel to risk areas such as southern states should be boostered 2 weeks before travelling.  In our region, vaccination is recommended annually in the spring.  

Vaccinations are available for many of the conditions listed above.  Vaccines are preparations of killed microorganisms, living weakened microorganisms, etc. introduced into the body to produce immunity to a specific disease by causing the formation of antibodies.  Vaccines are very delicate compounds, which if handled or administered incorrectly will be ineffective or neutralized.  Please discuss vaccination protocols and other prevention strategies with your veterinarian.