Babesiosis is a blood infection caused by the parasite, Babesia. In the U.S., it is primarily spread to people by bites from infected blacklegged ticks, also called deer ticks (Ixodes scapularis). You may not notice the tick bite responsible for babesiosis because very small, young nymph ticks are the most likely to spread it. After infection, the parasite invades red blood cells, eventually causing them to burst. While rare, babesiosis has been transmitted by blood transfusions.
Babesiosis usually causes non-specific flu-like symptoms (e.g., fever, fatigue, chills, sweats, and headache), or none at all. However, it can cause life-threatening complications in some individuals. Because the parasite infects and destroys red blood cells, it can lead to a serious condition called hemolytic anemia. Those at highest risk include people with weakened immune systems, with underlying health conditions like kidney disease, the elderly, or individuals lacking a spleen. The spleen helps to clear infected red blood cells.
While there are more than 100 species of the Babesia parasite, only a handful of them infect people. Babesia microti causes most U.S. cases. In 2017, the Centers for Disease Control and Prevention (CDC) received notification of 1,994 confirmed cases in the U.S. Most cases occurred in the Northeast, Mid-Atlantic and Upper Midwest where the disease is well established (endemic).
There have been a handful of babesiosis cases on the West Coast. These are caused by a different parasite species, Babesia duncani. Different species of Babesia are found in Europe and Asia, and symptoms of infection may vary from the typical presentation. Babesiosis likely occurs worldwide, but little is known about the frequency of babesiosis in areas where malaria also exists. That's because both parasites infect red blood cells and the infections have similar, non-specific symptoms, making it very difficult to differentiate if the infection is caused by Plasmodium species (malaria) or Babesia parasites.
Borrelia burgdorferi (the bacteria that cause Lyme disease) and Babesia are transmitted by the same species of tick. About 20% of people with babesiosis also have Lyme disease. Those individuals have symptoms that are more severe and last longer than individuals with either infection alone.
Humans are not an essential host in the Babesia lifecycle and do not usually spread the infection to others. The parasite relies on rodents, primarily the white-footed mouse, and Ixodes ticks to complete its development. Human-to-human transmission of the infection is very uncommon.
Although rare, it is possible for babesiosis to be transmitted by blood transfusion because the vast majority of people with B. microti infections are asymptomatic and never diagnosed, so the donor is allowed to donate blood without knowing that the infection may be present. The first U.S. case of transfusion-transmitted babesiosis was reported in 1980. Since then, more than 200 cases of transfusion-associated infections have been documented.
Currently, the blood donor history questionnaire is used to identify and indefinitely defer individuals who have had a history of babesiosis. In 2018, the U.S. Food and Drug Administration approved the first donor screening test for B. microti and issued draft recommendations for a regional risk-based approach to donor screening in affected states.