Human (Granulocytic) Anaplasmosis (HGA ... - Allina Health

BABESIOSIS
Babesiosis (originally called “Nantucket fever” due to its initially recognized occurrence in the
late 1960s on Nantucket Island) is a malaria-like illness caused by an apicomplexan protozoan.
These organisms are found worldwide. Although there are many species, only a few are identified
as causing human disease, including: B. microti, B. divergens, B. duncani (WA1 species),
and a strain designated MO-1. Like malaria, Babesia species infect erythrocytes, but unlike
malaria, babesiosis is transmitted by ticks, and the organism is found in a variety of animal
species that serve as reservoirs. Our previously mentioned “friend” Ixodes scapularis (deer
tick, blacklegged tick) is the usual tick vector.
Human infections in the United States of America occur mainly in the upper Midwest and
northeastern states due to infection by the rodent parasite Babesia microti (which infects the
white-footed mouse and other small mammals and which is the main species that infects people
in the United States). Tick larvae hatch in early summer from eggs laid in the spring by
adult female ticks, and become infected when they take a blood meal from infected whitefooted
mice. They molt into infected nymphs the following spring, and when they feed on
mice or humans in late spring or early summer, these hosts then also become infected. The
parasite is usually spread to humans by the young nymph stage of the tick (at which time the
tick is about the size of a poppy seed). In the fall, the nymphs molt into adults that feed on
white-tailed deer (that, according to some sources, do not become infected but nevertheless
support/amplify the tick population by providing the blood meals); if these infected adult ticks
feed instead on humans, they can infect humans at this time, as well. Thus, the main mode of
transmission to humans is through the bite of an infected tick (usually nymph, sometimes
adult), when humans engage in outdoor activities where Babesiosis is found.
In human infection, the sporozoites which are introduced into the bloodstream by the biting
tick attach to and enter the red blood cells (erythrocytes), where they mature into trophozoites
that eventually bud to form four merozoites (these stages are visible via light microscopy,
the four-merozoite stage sometimes characterized by the so-called “Maltese cross” formation
in infected red blood cells).
Babesia microti infection, Giemsa-stained thin smear
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