Severe Babesia microti Infection in an Immunocompetent Host in Pennsylvania.

Babesiosis, due to infection by a tick-borne protozoan (predominantly Babesia microti in North America), is an emerging health risk that is expanding into new areas and may be unfamiliar to clinicians in locations not previously considered endemic. Manifestations of infection can range from asymptomatic to life threatening, with severe disease more likely in those who have had a splenectomy, are immunocompromised, have chronic medical conditions, or are over 50 years of age. In this article, we describe an elderly but otherwise healthy man from an area not generally considered endemic for babesiosis who presented with severe hemolysis, acute renal failure, and high-level Babesia microti parasitemia; serological results suggestive of possible coinfection by Borrelia burgdorferi (the agent of Lyme disease, which is carried by the same tick as is Babesia microti) also was found. This report highlights that severe babesiosis can occur in an apparently normal host and underscores the continued geographic expansion of this pathogen and the need for early recognition and therapy.