Century Equine

Equine Granulocytic Ehrlichiosis is caused by Anaplasma phagocytophilum, an intracellular rickettsial organism. Horses acquire this disease through tick bites in which the organism then infects the white blood cells, specifically neutrophils, of the animal. This is the reasoning for the term granulocytic, as neutrophils are a category of white blood cells called granulocytes. The incubation of the disease can be anywhere from 1-3 weeks meaning you may not actually find at tick on your horse when he is sick.

Clinical signs vary between horses. Younger horses may have a fever only while adults may have a fever, anemia, depression, limb edema, ataxia and anorexia. Some horses may also exhibit mild signs of icterus. The fever is usually high initially (>103F) and will persist without appropriate treatment. The underlying cause of this is a vasculitis, or inflammation of the blood vessels in your horse. This then leads to leaking of fluid from the vessels resulting in the limb edema and other clinical signs. Signs may progress however some horses will resolve the disease on their own. Morbidity ranges from mild to severe and mortality is rare.

Diagnosis can be done several ways. A blood smear may show cytoplasmic inclusion bodies in neutrophils. However, absence of inclusions bodies does not mean your horse isn’t infected as your smear may be too early in the course of the disease.  White blood cell counts often are low combined with a low red blood cell count and platelet count. Polymerase chain reaction (PCR) can be performed on blood to detect the presence of the organism or an indirect fluorescent antibody test can identify rising antibody levels. Other diseases, or differential diagnoses, could be any of the equine herpes virus, viral encephalitidies, viral arteritis, purpura hemorrhagica or equine infectious anemia.

Treatment is the use of antibiotics and anti-pyretics (fever). The most effective antibiotic is Oxytetracycline given IV, followed by oral Doxycyline if the horse is improving. Some horses require several doses of Oxytetracycline before they can be switched to Doxycycline. Be forewarned, other popular antibiotics like Penicillin and SMZs will not treat this disease and your horse will continue with a high fever. Banamine, or other non-steroidals are given for fever and inflammation. Most horses respond beautifully and are back to behaving normally after one dose of antibiotics. Recovered horses usually have a good immune response that will last upwards of 2 years and are not believed to be carriers.

There is no vaccine for this disease. Control is done through tick management. Keeping your fields mowed and checking your horses daily for ticks. There are also several spot-on tick treatment medications for horses that could be employed for those in heavily infested areas. Horses are not contagious so you do not have to quarantine horses however it is possible that the transfer of blood between horses (transfusions, using old needles) could also cause transmission of the disease. There is a rare possibility of zoonotic transmission to humans however you would need direct contact with the blood of your horse. The more likely scenario is you are also exposed to the same infected ticks as your horse.

This is a disease that I see quite a bit in this area. Continue to monitor your horses for ticks. They are often found around the ears/forelock, lower limbs, flank/sheath and tail-head. If your horse is anorexic, dull and has a high fever, please call your veterinarian before pre-treating your horse.