Diagnosis of a tick-borne coinfection in a patient with persistent symptoms following treatment for Lyme disease.

A 67-year-old woman presented with 5 days of myalgias and fevers on completion of a 21-day course of amoxicillin for Lyme disease ( infection). She was found to have profound thrombocytopenia, as well as new anaemia and leucopenia. Workup revealed as the causative agent of her symptoms. The patient quickly improved after appropriate antimicrobial therapy directed against babesiosis was started. This case illustrates the importance of basic microbiology, including epidemiology and common vectors, when creating a differential diagnosis. Because the tick can harbour and transmit multiple parasites simultaneously, the possibility of coinfection should be considered in any patient not responding to appropriate initial medical therapy.