Equine Granulocytic Ehrlichiosis (Anaplasmosis) in the Horse
by Michael A. Caruso III, VMD & Jacqueline Bartol, DVM, DACVIM
Anaplasma phagocytophilum is the etiologic, or causative, agent of equine granulocytic ehrlichiosis (EGE). What might be commonly known in the horse community as “Ehrlichia,” was recently renamed and should now be called “Anaplasma.” Anaplasma phagocytophilum is an intracellular, gram-negative bacterial organism that is found within granulocytes (white blood cells). On the east coast, the organism is spread by tick bites and is commonly localized within the tick Ixodes scapularis or deer tick, the same tick that spreads Lyme disease (Borellia burdorferi). As such, Lyme disease is often blamed for many tick-bite related abnormalities in the horse especially in New England, however Anaplasmosis should be considered as another differential diagnosis for horses stabled in tick-infested regions of the northeast.
In the horse, the severity of clinical signs is variable, based on a variety of factors including age of the horse, and may be evident anywhere from 1 to 12 days post-inoculation. After inoculation by tick-bite, horses over the age of 4 generally develop progressive signs of fever, depression, lethargy, partial anorexia, limb edema, petechiation (red, pinpoint lesions on mucous membranes) and icterus (yellow discoloration of mucous membranes) most commonly, in addition to ataxia, and reluctance to move in some cases. Both clinically and experimentally, horses younger than 4 years of age often show milder clinical signs including moderate fever, depression, moderate limb edema, and sometimes ataxia. In horses less than a year old, clinical signs of disease might not be evident. Immediately post-infection (during the first 1-2 days), fever is often high, ranging from 102.9-106.9 degrees Fahrenheit. Other than fever, other initial clinical signs can include reluctance to move, depression, ataxia, icterus, and petechiation of nasal septum mucosa. It is important to point out, that some horses only exhibit certain clinical signs and in the authors’ experience fever, depression, lethargy, and occasional body soreness/stiffness are manifested most frequently. Laboratory abnormalities may include leukopenia (decreased white blood cell count), thrombocytopenia (decreased platelet count), anemia (decreased red blood cell count), and inclusion bodies (intracellular bacterial capsules) within the white blood cells (neutrophils and eosinophils).
Diagnosis is often based on geographic location, presence of ticks on the horse (may or may not be present), typical clinical signs described above (especially fever, depression, lethargy), and abnormal laboratory findings described above (especially leukopenia and thrombocytopenia). There are some laboratory tests that are commercially available and include SNAP 4Dx (tests for presence of antibodies), indirect fluorescent antibody test, paired-titer testing, and PCR (polymerase chain reaction) test. While there are tests available, the authors’ experience is that clinical signs and response to treatment are more valuable than testing for EGE aside from identifying the organisms in the white blood cells on a blood smear which is diagnostic for the disease. When tests are performed and the horses' are treated, there is usually resolution of the clinical signs before the test results are finalized.
Treatment is often instituted primarily based on clinical signs, and response to treatment can be used as a subjective positive diagnosis. Intravenous oxytetracycline once daily for 3-5 days or one dose of IV oxytetracycline followed by one week of oral doxycycline is usually sufficient to resolve the disease. We also recommend treatment with Banamine to manage the fever and some horses need additional supportive therapies such as IV fluids and laminitis prevention due to the high fevers. However, if left untreated, the disease is often self-limiting.
The prognosis for recovery from Anaplasmosis is excellent, and horses that are promptly treated often have resolution of clinical signs within 12-24 hours. A full course of treatment is still recommended to be certain that the Anaplasma organism is eliminated. If you have any questions regarding Anaplasmosis, please contact you veterinarian or the veterinarians of New England Equine Medical & Surgical Center.