Babesia microti

Babesia microti causes the disease babesiosis (also called piroplasmosis or red water fever). Babesia infections were longed thought of as important pathogens of cattle and other domestic mammals. Dogs and cats as well can infected. Babesia is generally considered a parasite of rodents with ticks (Ixodes species) as the vector, specifically the nymph stage of the ticks.

Disease / Pathology

Babesia microti infections produce malarialike symptoms in the host. Of the Babesia species able to infect humans (B. microti, B. divergent, and B. duncani) B. microti is the most common cause of babesiosis in humans.

Babesia microti infections are normally self limiting / asymptomatic. However infections can be serious especially in individuals that are immunocompromised, elderly, or asplenic.

Babesia divergens infections, which is most commonly found in Europe, tend to be reported most in splenetomized individuals and disease progression is generally more aggressive and symptomatic.

The parasites are similar to the plasmodium in that they will infect the red blood cells. In fact on the peripheral smear they are often mistaken for Plasmodium falciparum.

Babesia infections will produce symptoms associated with the destruction of red blood cells and mimic those of malaria of which it is often misdiagnosed as. These symptoms include fevers, hepatosplenomegaly, anemia due to red blood cell hemolysis, chills, malaise, sweating.

There is an incubation period of 1 to 4 weeks before symptoms appear. Symptoms can persist for several weeks. However in those individuals most prone to infections the symptoms can be more lengthy and severe if not treated.

Infections with B. divergens are significantly more severe and can prove to be fatal if not treated expeditiously.

Location in the Host

Babesia microti is located in the red blood cells (reticuloendothelial system) of the host.

Geographic Distribution

Babesia microti is endemic in the Northeastern parts of the United States and the Midwest. Human infections are especially prevalent along the southern coast of New England including Nantucket, Block Island, Martha’s Vineyard, and Long Island. Though it is not uncommon to find incidences of the disease farther inland.

Babesia divergens is most commonly isolated in Europe. Most cases reported in Europe are in splenectomized individuals.

Babesia duncani has been isolated in Washington and California.

Life Cycle

Image courtesy of CDC

Morphology & Diagnosis

Babesia microti trophozoites can be seen in the peripheral smear of infected individuals. A Wright’s stain smear is an excellent stain to visualize the parasites though the Geimsa stain can also be used as well.

Multiple red blood cells can be seen infected with the trophozoites, and multiple trophozoites can be seen inside the cells as pairs or tetrads. When in tetrads there can be seen the characteristic formation of a “Maltese cross” in the red blood cell. The trophozoites eventually rupture the red blood cell to go on and infect other red blood cells.

Frequently organisms can be seen extracellularly giving the smear what is often described as a dirty appearance.

The high parasitemia and multiply infected cells will frequently lead to a misdiagnosis of Plasmodium falciparum by the inexperienced parasitologist / hematologist.


Babesia in a blood smear – photo by W. Vientos
Photo by W. Vientos