Babesiosis is caused by microscopic parasites that infect red blood cells. Most human cases of Babesia infection in the United States are caused by the parasite Babesia microti. The primary method of transmission is through the bite of an infected black-legged tick (Ixodes scapularis); though, transmission through transfusion from an infected blood donor and congenital transmission (e.g. transplacental transmission) have been reported.
Babesia infections can range from subclinical to life-threatening. Clinical signs include hemolytic anemia and nonspecific influenza-like signs. Commonly associated symptoms include fever, chills, sweats, headache, myalgia, arthralgia, malaise, fatigue, and generalized weakness. Risk factors for severe babesiosis include asplenia (not having a spleen), advanced age, and impaired immune function.
Tick-borne transmission of babesiosis mostly occurs in the Northeast and upper Midwest, particularly, in New England (Connecticut, Massachusetts, and Rhode Island), New York, New Jersey, Wisconsin, and Minnesota. Nymph-stage ticks are mostly responsible for transmission of the disease, and most infections peak during the warm months. There have been no reported cases of babesiosis in West Virginia.