babesiosis

Babesia microti is a malaria-like protozoan transmitted by the same deer ticks that carry Lyme disease and granulocytic ehrlichiosis. The white-footed mouse is the reservoir for B. microti, and in some areas, nearly all mice have a persistent low-level parasitemia.

B. microti survives well in refrigerated blood, and several cases of transfusion-acquired Babesiosis have been reported. Babesiae parasitize red blood cells and cause fever and hemolytic anemia. The symptoms are mild except in debilitated or splenectomized individuals, who develop severe and fatal parasitemias.

Morphology. In blood smears, Babesia resemble P. falciparum ring stages, although they lack hemozoin pigment and are more pleomorphic.

They form characteristic tetrads (Maltese cross), which are diagnostic if found. The level of B. microti parasitemia is a good indication of the severity of infection: 1% in mild cases and up to 30% in splenectomized persons, who also show marked erythrophagocytosis associated with the red blood cell destruction.

In fatal cases, the anatomic findings are related to shock and hypoxia and include jaundice, hepatic necrosis, acute renal tubular necrosis, adult respiratory distress syndrome, hemolysis, and visceral hemorrhages.