Anaplasmosis is a tick-borne disease caused by the bacterium Anaplasma phagocytophilum (formerly known as Ehrlichia phagocytophila and Ehrlichia equi) that infects white blood cells. It’s commonly confused with Potomac horse fever (PHF), which causes similar signs.
Scientists have determined the Anaplasma organism typically infects horses from late fall to early spring and is spread via Ixodes ticks (deer tick and Western black-legged tick), which also spread Lyme disease. The disease has been detected worldwide, including in North America, Africa, Asia, and Europe. Twenty percent of Ixodes ticks in equine-inhabited areas of the southwestern United States are infected with the organism (Roellig DM, et al. 2012). Horses are frequently exposed to the disease, with researchers reporting 41% exposure in Pennsylvania, 66% in Texas, and up to 100% in New York (Russel et al., 2021; Thompson et al., 2022).
Clinical signs can appear in an infected horse two weeks after exposure to ticks carrying A. phagocytophilum, making the disease difficult to recognize in early stages. Fever is the principal clinical sign and might last up to 10 days. Other common signs can last up to two weeks and include depression, inappetence, weakness, ataxia (incoordination) ventral edema (swelling of the limbs and beneath the chest/abdomen), and petechiation (small purple spots on mucous membranes).
A veterinarian should suspect anaplasmosis in a horse based on season, clinical signs, and bloodwork abnormalities such as decreased white blood cell count, red blood cell count, and platelet count. He or she can make a definitive diagnosis via blood buffy coat (white blood cell) smears showing at least three inclusion bodies called morulae in the white blood cells. However, since these are only visible about three days after onset of fever and can be missed on the smear, a negative blood smear does not necessarily mean the horse is uninfected. A more sensitive tool for diagnosis is a molecular test called a polymerase chain reaction (PCR) test. This can detect disease as early as three days prior to clinical signs appearing and as late as nine days after signs resolve (Franzen P, et al. 2005).
Since it can be difficult to obtain an immediate definitive diagnosis, and because treatments for PHF and anaplasmosis are similar, some veterinarians pursue treatment prior to confirming infection.
Intravenous antimicrobial oxytetracycline treatment usually helps resolve fever after 12-24 hours, and veterinarians typically continue with a course of doxycycline or continued oxytetracycline.
Owners can prevent Anaplasma infection by inspecting horses daily for ticks and applying tick repellent, such as permethrin products or cattle ear tags. No vaccine is currently in development, but horses that spontaneously recover might maintain immunity for up to 20 months.
Differentiating from PHF
The more widely recognized PHF can cause similar clinical signs as anaplasmosis of fever, depression, anorexia, edema, and decreased white blood cell count/red blood cell count/platelet count, during the late summer/early fall. The key difference between the two diseases is PHF more frequently causes laminitis (15-25% of cases), colic (5-10%), and severe diarrhea (10-30%) and results in a case fatality rate of up to 30% (Reed SM, et al. 2004). PHF is caused by a different agent, Neorickettsia risticii, a bacterium that infects the horse’s macrophages and intestinal cells and is usually spread by ingesting infected aquatic insects (mayflies and caddisflies) or freshwater snails near a water source such as a river.