Equine granulocytic ehrlichiosis (EGE) should be on the rule out list of any horse with persistent fever. Caused by the organism Anaplasma phagocytophilum (previously Ehrlichia equi and also called Anaplasma phagocytophilia) the prevalence of this organism appears to be much wider than seen in the past. This article discusses the organism, distribution, clinical signs, diagnosis, treatment, and prognosis of this disease.

Anaplasma phagocytophilum is a member of the tribe Ehrlichiae which are closely related to rickettsia. Some of the best-known members are the organisms that cause Potomac Horse Fever, Lymes Disease, and Rocky Mountain Spotted Fever. The disease has primarily been reported in CA but has also been seen in CO, IL, FL, WA, OR, CT, NJ, NY and should be suspected in almost every other state in continental United States. Other countries where it has been reported include Brazil, Canada, Germany, Switzerland, Sweden, Poland, and Israel. EGE is transmitted by ticks of the Ixodes genus, which includes the deer tick. Though this is a tick borne disease, cases are usually found in the fall and spring and where the weather is warm enough to support ticks year-round also in the winter time. One of the most important steps to preventing infection is controlling your horses exposure to ticks in areas where this disease is prevalent...more.

This organism seems to have the ability to infect most mammals but the natural host reservoir is uncertain. Rodents, deer, and even lizards and birds are purposed reservoirs. The horse does not seem to be a natural host to the infection as it does not create a persistent infection nor is the horse likely to be contagious under natural conditions.

The incubation time runs one to nine days in experimentally infected horses. Clinical signs are somewhat dependent on age. Most horses under 4 years of age that contract the disease have inapparent to mild symptoms. When clinical signs are evident, fever (102 to 106 degrees) and decreased appetite is the predominant sign and lasts one to nine days. Early on it may appear as a viral disease but some cases will worsen with the persistent fever becoming complicated with limb edema (swelling), petechiae (blood spots on the mucous membranes), icterus (yellowing of the whites of the eyes), and ataxia (stumbling) with a reluctance to move. It is at this point that further diagnostics are done and will usually lead to a diagnosis of EGE. Chronic cases have not been reported. Experimentally infected mares did not abort.

The disease can be diagnosed by looking for the organism in the granulocytic blood cells 3 to 10 days after symptoms develop. They appear as small dark staining dots or clusters in the cytoplasm. Anaplasma phagocytophilia is found in membrane-lined vacuoles within the neutrophilic and eosinophilic granulocytes. Large organisms approximately 0.2 um in diameter as well as large aggregates called morula 0.5 um in diameter. Organisms are visible under high, dry, or oil immersion objective with light microscopy. They stain blue with Wright-Leishman or Giemsa stains.

Several polymerase chain reaction (PCR) assays have been developed that are very sensitive and good for diagnosis before and after the appearance of the organisms appear in the blood. IFA (blood) test has been developed with infected horses becoming positive of the test around 21 days after infection and persists for months after infection.

Horses with no or mild symptoms recover uneventfully without treatment. When clinical signs are severe, treatment can hasten recovery. Like all ehrlichiae they are very sensitive to the tetracycline class of drugs. Rifampin has been used in growing animals to avoid teeth staining. Treatment consists of oxytetracycline at a dosage of 7 mg/kg slow IV once every 24 hours for three to seven days. Also doxycyline can be used orally at 10 mg / kg orally for the same length of time. Fever is gone within 48 hours of initiating treatment.

Prognosis is excellent for complete recovery.

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