Equine Ehrlichiosis

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Also Known As

Equine granulocytic ehrlichiosis, Anaplasmosis


Equine ehrlichiosis is an infectious, noncontagious, seasonal (late fall-to-spring) disease reflecting the activity of ticks in the areas where horses are ridden or worked. equine ehrlichiosis is seen chiefly in the US in northern California, but is also seen in several other states, including Connecticut, Maine, Illinois, Arkansas, Washington, Pennsylvania, Colorado, Minnesota, and Florida. The disease has also been confirmed in British Columbia, Sweden, Great Britain, and South America.

Equine erlichiosis has been seen with increasing frequency over the past few years. It affects horses of all ages, and the symptoms are usually appear suddenly. Unlike many diseases, this condition causes less severe symptoms in horses that are less than three years old.

Equine ehrlichiosis is transmitted by ticks of the Ixodes family known as deer ticks. This condition is related to several other better-known diseases, such as Lyme disease and rocky mountain spotted fever. The clinical symptoms are seen most frequently in the winter, spring, and fall.

The bacteria from the tick bite tends to be engulfed by white blood cells and this causes the destruction of these infection- fighting white blood cells. After being infected, horses will be normal for 10-20 days prior to developing clinical signs.

After clinical signs develop, the horse will deteriorate rapidly and often will show mild signs of colic and become jaundiced over the next 3-4 days. Most horses will survive after 10-14 days of severe illness, but some will develop subclinical infection.

Severity of symptoms varies with the age of the horse and duration of the illness and may be mild. Horses that are younger than a year old may have a fever only. Horses 1-3 years old may develop fever, depression, mild limb edema, and ataxia. The fever, which is highest during the first 1-3 days of infection at 103-104°F (39.5-40°C), persists at 102-104°F (39-40°C) for 6-12 days. Signs become more severe over several days.

Although horses and people appear to be infected with strains of the same disease, it is believed that human exposure occurs through tick bites and not by direct transmission from horses to people.

This ehrlichial infection should not be confused with Potomac horse fever, a diarrheal disease.


  • Fever
  • Depression
  • Reluctance to move
  • Jaundice
  • Refusal to eat
  • Heart arrhythmias
  • Colic-like symptoms
  • Anemia
  • Mild limb swelling
  • Exacerbated concurrent infection (e.g., a leg wound or respiratory infection)
  • Loss of limb coordination


At this time, prevention is solely achieved by reducing exposure to ticks. No vaccine is currently available.


Diagnosis is made through a combination of clinical signs, blood analysis, environmental conditions, and response to treatment. With early and aggressive treatment, most horses become more normal within 24-36 hours and complete recovery is usually noticed within seven days.

Unlike many diseases, this disease responds well to intravenous oxytetracycline given for several days as prescribed by a veterinarian. Some horses will require more aggressive therapy, including IV fluids, diuretics, wrapping, etc.

Horses with severe lack of coordination and swelling the the limbs may benefit from short-term corticosteroid treatment with dexamethasone. Recovered horses are solidly immune for up to 2 years and are not carriers.

Viral encephalitis, primary liver disease, equine infectious anemia, purpura hemorrhagica, and viral arteritis are other horse diseases with similar symptoms. As with all serious equine diseases, a veterinarian should be called for a complete diagnosis and appropriate treatment at the first signs of equine ehrlichiosis.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.