Tick-Borne Illness

Diseases transmitted by ticks are becoming more widespread. Recently, a case report was published detailing a common tick-borne illness in horses called equine granulocytic anaplasmosis (EGA), once called equine ehrlichiosis. An 11-year old Hanoverian-cross gelding had recently been imported from Canada in good health and appeared normal when ridden one morning. That afternoon, however, he became lethargic, anorexic, and somewhat ataxic. When his rectal temperature rose to 103°F, the veterinarian was called. By evening, he was becoming progressively more ataxic, couldn't turn his head to one side, and couldn't bear weight on his right forelimb. By early the next morning, he was unable to rise. The horse was referred to Tufts University School of Veterinary Medicine, where Rose Nolen-Walston, DVM, admitted him for complete evaluation.

Intravenous anesthesia was required to get the horse into a supportive sling as well as to collect a cerebrospinal fluid (CSF) sample. Blood was also drawn for further tests. Nolen-Walston was most concerned about viral encephalitis, equine protozoal myeloencephalitis, and to a lesser degree, trauma. Upon examination, the hind limbs were weak and moderately edematous (swollen), but capable of movement in the sling. The CSF was normal, but the gelding had mild anemia, a markedly low lymphocyte count, high serum creatine kinase, unconjugated bilirubin concentrations, and a very low number of platelets. Most notably, large numbers of neutrophils were found to contain Anaplasma phagocytophilum organisms, consistent with a diagnosis of EGA. The owner recalled a recent infestation of ticks on the gelding and other horses at the same barn. The gelding had many of the signs associated with A. phagocytophilum infection, which can include anemia, anorexia, ataxia, edema, jaundice, and orchitis (testis inflammation). The unusual sign in this case was recumbency; a neurological complication was suspected.

Nolen-Walston explains, "EGA usually responds to treatment with oxytetracycline very quickly. However, there may have been generalized vasculitis (inflammation of blood vessels) occurring in the brain, which would explain the recumbency, the edema, and why the response to treatment was slower." Luckily, the gelding responded well to oxytetracycline and neurological signs rapidly improved. The gelding had no neurological deficits after six days, and he was discharged after 12 days.

Horse owners are reminded that EGA occurs on both coasts in the United States, in South America, and in Europe. Thus, tick infestations should be treated promptly and horses monitored for signs of illness.

Nolen-Walston is now involved in a project studying inflammatory airway disease.

Nolen-Walston, R.; D'Oench, S.M.; Hanelt, L.M. Journal of the American Veterinary Medical Association, 224 (12): 1964-1966, 2004.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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