How is babesiosis transmitted from ticks to humans?

Updated: Apr 01, 2021
  • Author: Rachel E Strength, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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I. scapularis has 3 developmental stages—larva, nymph, and adult—each of which requires a blood meal for development into the next stage. As a larva and nymph, the tick primarily feeds on rodents (eg, the white-footed mouse, Peromyscus leucopus); however, as an adult, the tick prefers to feed on the white-tailed deer, the primary host in the United States. Female ticks are impregnated while obtaining their blood meal on the deer, with the formation of up to 20,000 eggs. In contrast, cattle constitute the primary animal reservoir in Europe.

The clinical signs and symptoms of babesiosis are related to the parasitism of RBCs by Babesia. The ticks ingest Babesia from the host during feeding; they then multiply the protozoa in their gut wall and concentrate them in their salivary glands. When they feed again on a new host, they inoculate the new host with Babesia.

Entering the host’s bloodstream during the tick bite, the parasite infects RBCs, producing differentiated and undifferentiated trophozoites. Upon infection of the host erythrocyte, mature B. microti trophozoites undergo asynchronous asexual budding and divide into 2 or 4 merozoites. As parasites leave the erythrocyte, the membrane is damaged. The precise mechanism of hemolysis is unknown.

Babesia species in the host erythrocyte range from 1 to 5 µm in length. B. microti measures 2 × 1.5 µm, B. divergens measures 4 × 1.5 µm, and B. bovis measures 2.4 × 1.5 µm. As noted, the organisms are pear-shaped, oval, or round. Their ring form and peripheral location in the erythrocyte frequently lead to their being mistaken for Plasmodium falciparum. However, they differ from P. falciparum in that the schizogony is asynchronous, and massive hemolysis does not occur.

Alterations in RBC membranes cause decreased conformability and increased RBC adherence, which can lead to development of noncardiogenic pulmonary edema and acute respiratory distress syndrome (ARDS) among those severely affected. [1, 11]

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