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Anaplasmosis in Horses (Equine Granulocytic Ehrlichiosis)


John E. Madigan

, DVM, MS, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis

Reviewed/Revised May 2019 | Modified Oct 2022

Anaplasmosis (formerly called equine granulocytic ehrlichiosis) is an infectious, seasonal disease, seen chiefly in the United States. Most cases occur in northern California but cases have been seen in several other states, including Connecticut, Illinois, Arkansas, Washington, Pennsylvania, Colorado, Minnesota, and Florida. Cases have also been confirmed in British Columbia, Sweden, Great Britain, and South America. The disease is seasonal in California, occurring in the late fall, winter, and spring.

This disease is caused by the rickettsial agent Anaplasma phagocytophilum, which is found in the bloodstream after infection is transmitted by tick bite. The risk of transmission to people is unclear at this time. Although horses and people appear to be infected with strains of the same agent, it is believed that people also acquire the infection from tick bites, and not directly from infected horses.

The severity of signs varies with the age of the animal and duration of the illness. Signs may be mild. Horses less than 1 year old may have a fever only; horses 1 to 3 years old develop fever, depression, mild limb swelling, and lack of coordination. Adults exhibit the characteristic signs of fever, poor appetite, depression, reluctance to move, limb swelling, and jaundice. Fever is highest during the first 1 to 3 days of infection, but may last for 6 to 12 days. Signs become more severe over several days. Any existing infection (such as a leg wound or respiratory infection) can be made worse. The infectious agent can be found in white blood cells 3-5 days after infection. DNA and antibody tests can also detect the disease.

The disease is easily treated in the early stages using appropriate antibiotics. The severity of the disease is variable; many horses recover after 14 days without treatment. However, rare fatalities have occurred that are believed to be associated with secondary infections. Horses with severe signs and neurologic signs may benefit from injectable corticosteroids. Recovered horses develop immunity for at least 2 years and are not carriers. Tick control measures are mandatory for control of the disease. There is no vaccine.

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