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by Liz Howard, DVM

Loomis Basin Equine Medical Center



Tick fever is a blood-borne bacterial infection caused by Anaplasma Phagocytophilum. Previously known as Ehrlichia equi, this disease is found in Northern California during the late fall, winter, and spring months. Tick fever was first discovered in the foothills of Northern California in the 1960’s and has since been reported in multiple states, countries, and continents.

As the name indicates, tick fever is carried by Ixodes tick species, specifically the Western Black-legged tick (Ixodes pacificus) and the Deer tick (Ixodes scapularis). The pathogenesis of tick fever begins with reservoir hosts such as mice, rats, squirrels, and deer who carry the bacteria, Anaplasma phagocytophilum. When Ixodes tick larvae feed on these animals they too become infected with A. phagocytophilum. Ixodes larvae then grow into the nymph stage followed by the adult stage. Both the nymph stage and adult stage of these ticks are capable of transmitting the bacteria to horses, dogs, small ruminants, and humans. The incubation period after exposure is 10-14 days. Tick fever is not contagious from horse to horse.

The clinical presentation of tick fever consists of multiple symptoms that are commonly recognized by horse owners. Clinical signs include a high fever (102.9-106.5° F ), lethargy, decreased appetite, jaundice, broken blood vessels (petechiation) on mucous membranes, lower limb swelling, and neurologic signs.

Diagnosis of tick fever is made by evaluation of clinical signs, general blood work, and then confirmed with PCR testing. There are common changes observed on blood work including leukopenia (decrease white blood cell count), thrombocytopenia (decreased platelet count), and anemia (low red cell count). The most sensitive and specific test for Anaplasma phagocytophilum is PCR testing which recognizes the DNA of bacteria. PCR testing can be run in-house at LBEMC for a rapid diagnosis of tick fever.

Treatment of tick fever is generally very successful and horses improve significantly 24-48 hours after treatment begins. Treatment consists of intravenous antibiotics (oxytetracycline) followed by oral antibiotics (minocycline or doxycycline). Additionally, your horse will receive an NSAID such as banamine, for fever reduction. Owners can help by providing supportive care by offering fresh water and a variety of feed options.

Prevention can be difficult, especially for those who live in areas with heavy tick infestation. There is currently no vaccine for Anaplasma phagocytophilum. Tick control with topical products is the most effective way to protect your horse from infection.