Babesiosis is a disease triggered by the Babesia parasite, which is spread to humans through tick bites. In the United States, it is most commonly found in the Northeast and Midwest, particularly in southern New England, New York State, Wisconsin, and Minnesota. Typical symptoms are fever, chills, sweating, muscle aches, fatigue, and liver enlargement due to the breakdown of red blood cells. These symptoms usually appear 1 to 4 weeks after infection and can persist for several weeks. The illness is particularly dangerous for those with weakened immune systems, those without a spleen, and the elderly, and it can be fatal. Treatment generally involves antibiotics like clindamycin paired with quinine or atovaquone with azithromycin.Regarding the parasite, out of the over 100 reported Babesia species, only a few are known to infect humans. Babesia microti and Babesia divergens are the most commonly identified in human cases, though new variants, possibly different species, have recently been discovered. Babesia species can be mistaken for malaria, especially in regions where malaria is prevalent.The life cycle of Babesia microti involves two hosts: primarily the white-footed mouse (Peromyscus leucopus) and the deer tick (Ixodes dammini or I. scapularis). During a blood meal, an infected tick introduces sporozoites into the mouse host. These sporozoites invade red blood cells and reproduce asexually. Some parasites develop into male and female gametes, though indistinguishable under a light microscope. When a tick ingests these gametes, they unite and undergo a sporogonic cycle, producing sporozoites. While transovarial transmission is noted in larger Babesia species, it is not documented in smaller ones like B. microti. Humans become part of the cycle through tick bites, leading to asexual replication of the parasite in red blood cells, causing the disease symptoms. Humans are considered dead-end hosts, as ticks feeding on infected humans rarely transmit the parasite further. However, human-to-human transmission can occur through blood transfusions.Deer serve as hosts for adult ticks, indirectly influencing the Babesia cycle by affecting tick population sizes. As deer numbers grow, so does the tick population, increasing the risk of transmission.Diagnosis is achieved through microscopic examination of Giemsa-stained blood smears, with multiple smears sometimes necessary for confirmation. Antibody detection via an indirect fluorescent antibody test can support diagnosis. Inoculating hamsters or gerbils with a patient's blood can also aid diagnosis, as these animals typically develop parasitemia within 1 to 4 weeks. Treatment options include medication and exchange transfusions.
