Anaplasmosis is the latest name for the tick-borne disease caused by the bacterium Anaplasma phagocytophilum. This organism was previously known as Ehrlichia equi, and the disease for years was called Equine Ehrlichiosis.

Anaplasma is transmitted to horses from bites of several tick species (Deer Tick and Western Black Legged Tick). The disease is not transmitted horse to horse.

The disease has been reported mostly in the United States and Europe. It is more common in specific regions of the U.S, particularly the Pacific Northwest and Northern California.

Horses with anaplasmosis are typically depressed and off feed, they have moderate to high persistent fevers, and often have swelling of the legs, belly, and the sheath in male horses. They may have runny, swollen eyes (conjunctivitis) and grayness in the clear part of the eye (cornea). Stallions can have swollen testicles.

A classic sign of anaplasmosis is the presence of small red or purple spots on the whites of eyes, gums and especially on the nasal mucous membranes or vulva of mares. These spots result from vasculitis - inflammation and resulting leakiness of blood vessels, which allows tiny bruises to form. Rarely, affected horses can be weak and wobbly, they may lie down and not be able to rise.

The blood of affected horses shows particular abnormalities, most notably the microscopic presence of the organism within particular white blood cells. The organism may be missed on a slide, and today a more sensitive and specific test is the PCR blood test.

Treatment involves the use of a particular class of antibiotic that kills intracellular organisms, the tetracycline group. In many cases, the diagnosis is presumed even before lab diagnostic results are known.