Definition of Babesiosis

Reviewed on 3/29/2021

Babesiosis: An illness caused by the parasite Babesia which is transmitted from animals to humans by ticks. In the US, it is typically contracted in the Northeast or Midwest -- in southern New England or New York State and in Wisconsin or Minnesota. The signs and symptoms include fever, chills, sweating, myalgias (muscle aches), fatigue, hepatosplenomegaly (enlargement of the liver and spleen) and hemolytic anemia (anemia due to break-up of red cells). Symptoms typically occur after an incubation period of 1 to 4 weeks and can last several weeks. The disease is more severe in patients who are immunosuppressed, splenectomized (lack their spleen), or elderly. It can cause death. Treatment involves antibiotics, usually clindamycin and quinine or atovaquone and azithromycin.

The parasite: While more than 100 species of Babesia have been reported, only a few have been identified as causing human infections. Babesia microti and Babesia divergens have been identified in most human cases, but variants (considered different species) have been recently identified. Little is known about the occurrence of Babesia species in malarial areas where Babesia can easily be misdiagnosed as Plasmodium (the agent of malaria).

The life cycle of the parasite: (This contains some technical information.) The B. microti life cycle involves two hosts, which includes a rodent, primarily the white-footed mouse, Peromyscus leucopus. During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host. Sporozoites enter erythrocytes and undergo asexual reproduction (budding). In the blood, some parasites differentiate into male and female gametes although these cannot be distinguished at the light microscope level. The definitive host is a tick, in this case the deer tick, Ixodes dammini (I. scapularis). Once ingested by an appropriate tick, gametes unite and undergo a sporogonic cycle resulting in sporozoites. Transovarial transmission (also known as vertical, or hereditary, transmission) has been documented for "large" Babesia spp. but not for the "small" babesia, such as B. microti.

Humans enter the cycle when bitten by infected ticks. During a blood meal, a Babesia-infected tick introduces sporozoites into the human host. Sporozoites enter erythrocytes and undergo asexual replication (budding). Multiplication of the blood stage parasites is responsible for the clinical manifestations of the disease. Humans are, for all practical purposes, dead-end hosts and there is probably little, if any, subsequent transmission that occurs from ticks feeding on infected persons. However, human to human transmission can occur through blood transfusions.

Deer are the hosts upon which the adult ticks feed and are indirectly part of the Babesia cycle as they influence the tick population. When deer populations increase, the tick population also increases, thus heightening the potential for transmission.

The diagnosis: Diagnosis can be made by microscopic examination of thick and thin blood smears stained with Giemsa. Repeated blood smears may need to be examined to make the diagnosis. Antibody detection by indirect fluorescent antibody (IFA) test is a complementary diagnostic test. Isolation of Babesia by inoculation of the patient's blood into hamsters or gerbils may also assist in diagnosis. Animals inoculated with infective blood typically develop parasitemia (parasites circulating in their bloodstream) within 1 to 4 weeks.

Treatments in include medication and exchange transfusions.


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