Anaplasma, a tick born disease, is a cause of fever in horses that was previously uncommon in the state of Michigan. Recently, two horses brought to Michigan State University for diagnostics and treatment due to fevers have been diagnosed with anaplasma. In horses, anaplasma is caused by the bacterium Anaplasma phagocytophila, and affected horses will have fevers, lethargy, and a decreased appetite. Additionally, some horses also will have limb edema (swelling), icterus (yellow discoloration of their mucus membranes/gums), petechiation (red spots on their mucus membrane and skin), and ataxia.

Anaplasma is NOT contagious horse-to-horse; however, it is transmitted by the ixodes tick (also called the deer tick or western black tick). These ticks are very common in Michigan, so it is not surprising that we are seeing horses with anaplasma. Horses typically begin showing signs of disease (fever, etc.) 8–12 days after being bitten by the tick.

Anaplasma can be diagnosed by finding the organism within the horse’s white blood cells, a PCR test, or IFA titers. Although the disease can be serious, with prompt and appropriate treatment by a veterinarian, most of the infected horses have an excellent prognosis.

If your horse is inappetent, lethargic, has a fever, or any of the other signs listed, please contact your veterinarian or Michigan State University's Large Animal Clinic for further evaluation and treatment. We are available to see horses on emergency 24/7/365.

MSU Large Animal Clinic