Equine Disease Identification


Strangles is the common name for a bacterial infection of Streptococcus equi ssp. equi affecting horses.  The organism is found throughout the entire United States.  It is spread through nasal discharge of infected horses.  It is an infectious, highly-contagious disease characterized by abscesses of the lymphoid tissue, usually under the jaw, but Bastard Strangles can affect the horse on many other areas of the body.  Symptoms usually appear 3-14 days after exposure.  Other symptoms include fever, nasal discharge, depression, difficulty swallowing, respiratory noise, and abnormally extended head and neck.  Young horses who contract strangles usually have a harder time recovering so we recommend administering a yearly strangles vaccination for all at-risk horses under the age of ten with a booster two to three weeks after the initial dose.


‚ÄčTetanus is usually fatal in horses and is caused by the bacteria Clostridium tetani.  This bacteria is a primary contaminant of soil with an increased amount on farms due to the fact that it is very common to find this organism in the manure of horses and cows.  Horses are extremely sensitive to tetanus, however it is very easily prevented with a yearly vaccination.  The toxin produced by Clostridium tetani is one of the most potent toxins currently known, the quantity the size of a pinhead can cause death.  The neurotoxin enters the bloodstream from the site of infection and irreversibly binds to the body's nerve endings.  Clinical signs may include colic, stiffness, lameness, difficulty in opening the jaw, standing in a "sawhorse" stance with very rigid legs, elevated tailed, spasms in the muscles, hypersensitivity, spasms of the eye and third eyelid, head pressing, and respiratory failure.  Prevention requires an initial vaccination and a booster 3-4 weeks later, and thereafter, an annual booster.

At Triple Oaks, we feel it is important for you to recognize possible disease your horse could contract.  Below are pictures and descriptions of several contagious diseases.  If you have any questions, please feel free to contact our office at (763) 219-4984.  The more you know, the healthier your horse will be.


VS is a contagious disease that afflicts livestock, wildlife, and even humans.  The disease is caused by a virus which can have significant financial impact on the horse industry.  There are still some questions regarding how VS is transmitted, it is believed insects such as biting flies and gnats transmit the virus.  VS also seems to be passed from horse to horse by contact with saliva or fluid from ruptured blisters.  Clinical signs include blister-like lesions developed on the tongue, mouth lining, nose, and lips.  Lesions can also develop on the coronary bands of hooves or on the udder or sheath.  Humans can contract VS from infected horses therefore proper biosafety measures are important.  Prevention include effective insect control programs and biosecurity diligence.  There is currently no vaccine for VS.  Supportive treatments will help the horse during the period of its own natural healing process.

States with positive cases of VS in 2015 include: Arizona, New Mexico, Texas, Utah, South Dakota, and Colorado.

West Nile Virus

WNV is a disease of wild birds that is spread by blood feeding insects, mainly mosquitoes.  WNV is transmitted by mosquitoes who bite an infected bird and later feeds on humans and horses.  Birds circulate high levels of the virus in their blood and serve as the sole source of the virus for mosquitoes.  Neither horses nor humans circulate enough virus in their blood to pass the virus back to mosquitoes.  WNV cannot be spread from horse to horse or horse to human.  Clinical signs of an infected horse include: lack of coordination or stumbling, depression, anorexia, hind-end weakness, falling, inability to rise, paralysis of the lower lip, muscle twitching, grinding teeth, inability to swallow, head pressing, colicky appearance, hypersensitivity to noise and light, excessive sweating, disorientation, convulsions, and possible total paralysis. Medicine has come a long way in treating and curing horses infected with the West Nile Virus.  Although it is still considered a serious threat to horse's lives it is no longer the deadly killer it once was in Minnesota. It is preventable.  Prevention requires an initial vaccination and a booster 3-4 weeks later, and thereafter, an annual booster.

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‚ÄčRabies is an acute viral infection of the nervous system caused by the rabies virus.  This disease is a fatal disease, there is no cure or treatment.  Transmission is almost always by the bite of an infected animal when the saliva containing the rabies virus is introduced into the body.  In horses, the virus can live in the body for up to a year before symptoms develop.  There are two forms of the disease.  The classic, aggressive "mad-dog" syndrome, known as the furious form, is not common in horses.  If it does appear, the animal becomes irritable or vicious, the pupils dilate, and seizures and lack of muscle coordination are common.  The paralytic syndrome, known as the "dumb" form of rabies, usually involves paralysis of the throat and jaw muscles, often with excessive salivation and inability to swallow.  Paralysis progresses throughout the body resulting in death.  People can be infected by horses when examining the horse's mouth, putting a bit in its mouth, or giving it medication with bare hands.  The only way to verify the diagnosis of rabies is lab tests of the animal's brain after the animal is deceased.  It is preventable by the use of a once yearly vaccination.

Lyme Disease

Lyme Disease is considered to be one of the most prevalent insect-borne bacterial infections in North America.  Though this is a disease that affects many mammals, it is only spread through deer ticks (not wood ticks!), not horse to horse or horse to human.  The most common clinical signs include shifting leg lameness, behavioral changes, swollen joints, unwillingness to move and death; however, Lymes can present itself through neurological symptoms with head pressing and incoordination.  Some horses will show a 'bullseye' tick bite, however most owners may never find the tick.  Consistent removal of ticks can help reduce your risk.  There is currently no vaccine for Lyme Disease in horses, early diagnosis and lengthy, aggressive treatment are the best chance for recovery.

Potomac Horse Fever

Potomac horse fever (PHF) is an acute enterocolitis syndrome producing depression, anorexia, colic, fever, and diarrhea in horses of all ages, as well as abortion in pregnant mares.  Laminitis is a major concern for horses infected with PHF.  The causative agent of PHF is a bacteria called Neorickettsia risticii often infecting aquatic insects, such as the Mayfly, during their larval stage of life.  Accidental ingestion of Mayflies are a primary cause of PHF.  The disease is seen in Spring, Summer, and early Fall and is associated with pastures bordering standing water as well as creeks or rivers (basically everywhere in Minnesota).  Prevention requires an initial vaccination and a booster 3-4 weeks later, and thereafter, an annual booster.  While the vaccination is not always effective in preventing the disease, it is able to greatly reduce symptoms resulting in a much higher rate of survival.  Treatments consists of hospitalization, fluid therapies, antibiotics, and heavy monitoring to bring PHF under control.

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Rhinopneumonitis a.k.a. Equine Herpesvirus (EHV-1; EHV-3 and EHV-4)

As of 2015, nine EHV's have been identified worldwide.  Three of these pose the most serious health risks and economic impacts for horses in the United States: EHV-1; EHV-3; and EHV-4.  

EHV-1 can cause four presentations in horses: neurological, respiratory, abortion, and neonatal death.  Respiratory symptoms for EHV-1 include: fever, lethargy, anorexia, nasal discharge, and cough.  EHV-1 also has a neurological mutation form, called Equine herpesvirus myeloencephalopathy (EHM).  With EHM, neurological signs manifest as a result from damage associated with EHV-1 to brain blood vessels and the spinal cord.  Neurological symptoms include: hind-end weakness, toe-dragging, incoordination, uninary/fecal incontinence, 'urine dribbling,' and 'dog-sitting.'  EHV-1 and EHM are spread through direct and indirect contact with nasal secretions, aborted fetuses, fetal fluids, and placentas.

EHV-3 causes a venereal disease that affects the external genitalia causing lesions, called equine coital exanthema.  It is spread primarily through venereal contact, although it is able to spread through sharing contaminated equipment.

EHV-4 causes upper respiratory tract disease in foals which is usually nonfatal.  Respiratory symptoms for EHV-1 and EHV-4 include: fever, lethargy, anorexia, nasal discharge, and cough.  EHV-4 is spread through secretions from infected coughing horses, by direct or indirect contact with nasal secretions.  

Prevention for EHV-1 and EHV-4 exist for the respiratory and abortion manifestations and requires an initial vaccination and a booster 3-4 weeks later, and thereafter, an annual booster.  There is no vaccination for the prevention of EHM or EHV-3, however.  Biosecurity is the best prevention for these diseases.

Even though the virus is never completely abated, supportive treatments are available.

Eastern Equine Encephalitis/Western Equine Encephalitis

Encephalitis basically means 'swelling on the brain.'  EEE and WEE can both be fatal neurologic diseases transmitted by blood feeding insects.  EEE has a fatality rate of 75-90% while WEE has a fatality rate of about 40%.  This disease is important as it can affect people as well.  The disease is spread from birds to animals and humans by insects.  It cannot be passed from horse to horse or horse to human.  Infected horses mainly serve as a marker to show it is present in an area.  Symptoms include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures.  Prevention requires an initial vaccination and a booster 3-4 weeks later, and thereafter, an annual booster.

Equine Infectious Anemia

EIA is an infectious disease caused by a RNA virus.  It is spread by the transfer of virus-infective blood via blood-feeding insects (mainly deer flies and horseflies) traveling between horses in close proximity.  It may also be spread to a foal through the placenta of infected pregnant mares.  After a horse is infected it can take anywhere from 15-45 days for clinical signs to appear.  Classic cases progress through clinical episodes, i.e., an initial episode lasting 1-3 days of fever, depression, and thrombocytopenia (deficiency of platelets in the blood causing bleeding, bruising and slow blood clotting) usually occurs.  However these symptoms, depending on severity, can easily be overlooked.  Following this initial phase is usually a prolonged period of recurring fever, thromboctopenia, anemia, edema, muscle weakness, and loss of condition.  The interval between episodes can range anywhere from days to weeks to months.  In most cases the episodes subside within a year and infected horses become carriers of the EIA virus.  Once a horse is infected they will always have the disease.  Because no specific treatment or effective vaccine is available it is recommended that infective horses be euthanized.  An annual Coggins test can be preformed by your veterinarian to confirm your horse is free of EIA.


Anaplasmosis is a tick-borne disease caused by the bacterium Anaplasma phagocytophilum that infects white blood cells.  Anaplasmosis is spread only by the bite of an infected deer tick.  It cannot be spread from horse to horse or horse to human.  Clinical signs can appear in an infected horse one to three weeks after exposure to a tick carrying A. phagocytophilum.  Clinical signs include fever, depression, anorexia, swelling of the limbs and under the chest or abdomen, small purple spots on mucous membranes, yellow mucous membranes, inability to rise, and muscle stiffness.  This is a common disease in our area (East Central Minnesota) and prevention is difficult in highly tick-infested areas, but daily vigilance to remove ticks is recommended.  There is no vaccine for anaplasmosis, however there is a 5 day highly successful treatment available.

Equine Influenza

Equine Influenza is a highly contagious disease that spreads rapidly among horses. Horses who travel frequently, in a show circuit, boarded with other horses, or under the age of 5 are the most susceptible.  However, influenza is usually not fatal.  The clinical signs typically show after an incubation period of 1-3 days.  Symptoms include a high fever (up to 106 degrees F); serous nasal discharge: dry, harsh coughing; depression, anorexia, and weakness.  Some horses can recover in 2-3 weeks while others can take up to 6 months.  Secondary bacterial infections can easily occur during this time of suppressed immunity.  Prevention requires an initial vaccineationand a booster 3-4 weeks later, and thereafter, an annual booster.  Boosters are often given to show horses during periods of high stress and greater exposure to outside horses.

Equine Protozoal Myeloencephalitis (EPM)

EPM is a debilitating neurologic disease in horses caused by a protozoa (Sarcocystis neurona) shed by the scat of opossums.  The protozoa infects and invades the central nervous system of the horse.  An estimated 50% of horses are exposed to EPM every year with about 1% developing clinical signs.  A large variation of neurologic signs are possible with EPM which can make this disease confusing.  Depending on the extent of central nervous system damage horses may show signs suddenly or progressively.  Symptoms include atrophy of gluteal muscles, gait abnormality, spinning circles, functional impairment of one or more limbs, weakness, asymmetric muscle atrophy, incoordination, leaning to one side, head tilting, stiffness, and falling over.  There is no vaccine at this time for EPM, however there are now effective medicines to combat this disease.