Being in the right place at the right time is sometimes the catalyst needed to catapult a bright, inquisitive mind into a world of discovery. This is what happened to John Madigan, DVM, MS, Dipl. ACVIM, ACAW, professor and researcher in medicine and epidemiology at the University of California, Davis (UC Davis), School of Veterinary Medicine, when he began veterinary practice in Northern California back in the 1970s. There, early in his career, he was faced with the challenging situation of treating a very sick horse with a puzzling illness, with clinical signs ranging from fever, lethargy, and ataxia (incoordination) to icterus (jaundice in the eyes), and mild petechiation (blood spots on the gums). Madigan wasn’t satisfied with simply treating the horse symptomatically, so he kept looking for answers.

He described this process of discovery during his delivery of the Frank J. Milne State of the Art Lecture at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah. The disease he ended up investigating for many years following his encounter with that sick horse is one that affects not just horses but also humans and other animals: tick fever, or granulocytic anaplasmosis, caused by Anaplasma phagocytophilum, previously known as Ehrlichia equi.


Madigan’s sleuthing began when he pulled blood from the horse and examined it under a microscope. Being a careful observer, he noticed strange-looking inclusion bodies within the neutrophils—white blood cells that are important for clearing infection in the body. Inclusion bodies are unusual structures found in the cytoplasm or nucleus of cells that may indicate certain types of infection. This clue led him to further investigation and, finally, answers. It turned out that he was dealing with a bacterial disease caused by Ehrlichia equi and transmitted by the very same ticks that infect humans an