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Babesiosis, an infection by protozoans of the genus
Babesia, is transmitted by hard-bodied ticks.
Babesia
microti and Babesia divergens have been identified in most human
cases.
Although common in
animals, babesiosis is rare in humans.
The causative organisms,
parasites resembling those of malaria, invade and destroy
erythrocytes, but differs from malaria parasites in several important
ways. They are transmitted by ticks, make no pigment, produce no
sexual forms, and have no exerythrocytic stage.
Babesia infect a
variety of mammals including cattle, horses, and dogs.
The parasites are ingested by ticks when they feed on infected
mammals, multiply in the intestinal epithelium of the ticks, and
spread through the insect bodies.
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The infective organisms are then
transmitted in the saliva when the tick feeds again.
In Europe, most
reported cases are due to B. divergens and occur in splenectomized
patients. In the United States, B. microti is the agent most
frequently identified (Northeast and Midwest), and can occur in non-splenectomized
individuals. Two variants, have been reported in the U.S.
states of Washington and California (WA1- type and related parasites)
and Missouri (MO1).
After inoculation by the tick, Babesia invade
erythrocytes, where they appear as ameboid, round, rod-shaped or
irregularly shaped organisms.
They are 1 to 5 micrometer in diameter, and
with the Giemsa stain have a blue cytoplasm and a mass of red
chromatin.
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Splenectomy and diabetes are predisposing factors.
The incubation period varies from 2 to
6 weeks and is followed by sudden onset of chills and fever, sometimes
with muscle aches and pains, prostration, jaundice, dark urine,
diarrhea, and vomiting.
The
progressive invasion and destruction of red blood cells causes hemoglobinemia, hemoglobinuria, jaundice, and renal failure.
The
disease is usually self-limited, but uncontrolled infections can be
fatal.
Autopsies have found parasites in erythrocytes concentrated in
congested capillaries of many organs, and especially in the hepatic
sinusoids.
The diagnosis is made by identifying Babesia in thin blood
films.
Parasitized erythrocytes may also be identified in tissue
specimens.
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