Anaplasma phagocytophilum Infection in Horses

Horses can suddenly lose their appetite, develop a fever, become lethargic (tired), and accumulate fluid on their lower legs (limb edema). The following can help determine the reason for these signs and minimize these problems:

  • If your horse suddenly has a loss of appetite, develops a fever, becomes reluctant to move, and accumulates fluid on the lower legs, contact your veterinarian.
  • Usually, only one horse on a property develops these signs.
  • Prompt diagnosis and treatment of Anaplasma phagocytophilum (formerly called Ehrlichia equi) infection can speed recovery.
  • Tick prevention, such as mowing pastures and minimizing bushes and woody areas around horse turnout areas, is important.

The Basics

Infection with A. phagocytophilumis a tick-borne infectious disease spread by Ixodes species ticks. This infection often occurs from spring through fall due to increased tick activity during this time of year. Illness usually occurs shortly after the tick bite. Affected horses may suddenly show signs such as appetite loss, fever, lethargy (tiredness), reluctance to move, and fluid accumulation on the lower limbs, which indent when pressed with a finger. Affected horses often develop a fever of 103°F or higher (normal temperature range for adult horses: 99°F to 101.5°F). If your horse suddenly develops these signs, it is important to contact your veterinarian. Usually, only one horse on a property is infected with A. phagocytophilum; however, multiple horses on one property are occasionally affected.

Diagnosis and Treatment

Your veterinarian will perform a thorough physical examination on your horse. Your veterinarian may also submit additional tests, such as blood tests, to confirm the infection. Your veterinarian may recommend treatment with antibiotics, either orally (by mouth) or intravenously (injected into a vein). Symptomatic treatment such as pain medication (for example, nonsteroidal anti-inflammatory drugs [NSAIDs]) can improve your horse’s comfort and appetite. Horses often respond positively to a short course of antibiotic treatment and recover completely. Occasionally, horses develop this disease again after treatment is finished and may require treatment for a longer period of time or with a different antibiotic. Once the infection has resolved, horses are often immune to this disease for a few years. 


The ideal method for preventing A. phagocytophilum infection is diligent tick control on a property. Ticks avoid sunlit areas, preferring bushes and shady locations; therefore, you should minimize bushes and overhanging tree branches close to the turnout area and mow the pasture to keep grass short. Prompt removal of ticks on your horse is important. In addition, tick repellants that contain permethrin can be useful. Scheduling your veterinarian to perform regular physical examinations can help to detect problems when they first begin and may be more amenable to treatment. Your veterinarian is accustomed to working with many types of horse farms, so ask him or her for tips on how to prevent disease on your property.