babesiosis bəbē˝bēō´sĭs [key], tick-borne disease caused by a protozoan of the genus Babesia. Babesiosis most commonly affects domestic and wild animals and can be a serious problem in cattle, but since the mid-20th cent. the disease has also been found in humans. In most cases the protozoal species is specific to a single host. The organisms enter the blood via a tick bite, then infect the red blood cells where they reproduce by cell division.

Human babesiosis, sometimes called Nantucket fever, was first diagnosed in healthy individuals after an outbreak on Nantucket Island, off the coast of Massachusetts, in the 1970s. The causative organisms are related to those that causes malaria, and are transmitted by the black-legged, or deer, ticks that also host the organisms that cause Lyme disease and human erhlichiosis. Babesiosis can also be transmitted through blood transfusion. The main symptoms are fever and chills, but especially in the elderly and in persons who have a compromised immune system or have had a splenectomy babesiosis can be more severe and sometimes fatal. Treatment is with a combination of antimalarial drugs.

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