Equine Ehrlichiosis

Also Known As

Equine granulocytic ehrlichiosis, Anaplasmosis


Equine ehrlichiosis is an infectious, noncontagious, seasonal (late fall-to-spring) disease reflecting the activity of the tick vectors at that time. It is seen chiefly in the US in northern California, but is also recognized in several other states, including Connecticut, Maine, Illinois, Arkansas, Washington, Pennsylvania, Colorado, Minnesota, and Florida. It 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 noted suddenly. Unlike many diseases, this condition causes less severe symptoms in horses that are less than 3 years old.

According to the Merck Veterinary Manual, the disease is caused by what has previously been known as Ehrlichia equi, but based on DNA sequence relationships, the organism is now referred to as Anaplasma phagocytophila. The organism has a wide host range; naturally occurring infections have been seen in horses, burros, dogs, llamas, and rodents. A rickettsia closely resembling A. phagocytophila, the human granulocytic ehrlichiosis (HGE) agent, has recently been implicated in cases of human illness in the upper Midwestern and Northeastern states in the US.

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 tends to be engulfed by white blood cells and causes accelerated destruction of these infection- fighting cells. After initial infection, 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.

The zoonotic risk is unknown at this time. Although horses and people appear to be infected with strains of the same agent, 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 caused by Neorickettsia risticii.


  • Fever
  • Depression
  • Reluctance to move
  • Jaundice
  • Anorexia
  • Transient ventricular arrhythmias
  • Colic-like symptoms
  • Anemia
  • Mild limb edema
  • Exacerbated concurrent infection (e.g., a leg wound or respiratory infection)
  • Ataxia


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 ataxia and edema may benefit from short-term corticosteroid treatment with dexamethasone. Recovered horses are solidly immune for up to 2 years and are not carriers.

Differential diagnoses that may confuse horse owners and veterinarians include viral encephalitis, primary liver disease, equine infectious anemia, purpura hemorrhagica, and viral arteritis.

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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