Babesia spp.
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p635-636
2025-10-15
96
The genus Babesia includes approximately 100 species transmitted by ticks of the genus Ixodes. In addition to humans, these blood parasites infect a variety of wild and domestic animals. Cases of babesiosis have been documented worldwide, and several outbreaks in humans have occurred in the northeastern United States, particularly in Long Island, Cape Cod, and the islands off the East Coast (Homer). Although there are many species of Babesia, Babesia microti is the cause of most human infections in the United States, whereas B. divergens tends to be more common in Europe, is often found in splenectomized patients, and causes a more serious form of the disease.
GENERAL CHARACTERISTICS
Organism
Although the life cycle of Babesia spp. is similar to that of Plasmodium spp., no exoerythrocytic stage has been described; also, sporozoites injected by the bite of an infected tick invade erythrocytes directly. Once inside the erythrocytes, the trophozoites reproduce by binary fission rather than schizogony. Once the tick begins to take a blood meal; the sporozoites are injected into the host with the tick’s saliva.
The trophozoites of Babesia can mimic P. falciparum rings; however, there are differences that can help differentiate the two organisms (Figure 1). Babesia trophozoites vary in size from 1 to 5 µm; the smallest are smaller than P. falciparum rings. Also, ring forms outside of the RBCs and two to three rings per RBC are much more common in Babesia. The ring forms of Babesia tend to be very pleomorphic and range in size, even within a single RBC. The diagnostic tetrads, the Maltese Cross, though not seen in every specimen or species, may be present (see Figure 1).
Fig1. Babesia in red blood cells.
PATHOGENESIS AND SPECTRUM OF DISEASE
Babesiosis is clinically similar to malaria, and symptoms include high fever, myalgias, malaise, fatigue, hepatosplenomegaly, and anemia. Usually, B. microti infections in the United States occur in nonsplenectomized individuals and are relatively mild. Infections with some of the other Babesia spp. from the United States and with B. divergens in Europe occur in splenectomized or immunocompromised individuals and are clinically more serious. Mortality among symptomatic cases of B. microti infection in the United States is 5%, whereas that in B. divergens infection in Europe is around 40%. In both areas, risk factors for severe disease include increasing age, splenectomy, and a compromised immune system. Infections with Babesia species from California, Washington, and other western states tend to be more serious and can mimic the symptoms seen in B. divergens.
LABORATORY DIAGNOSIS
Routine Methods
The diagnosis of babesiosis should be considered for a patient with typical symptoms and a travel history including endemic areas, exposure to ticks, or recent blood transfusion. Examination of thick and thin stained blood films is the most direct approach to diagnosis. It is important to remember that the parasitemia may be low and these organisms tend to be routinely missed using auto mated hematology instruments.
Molecular Diagnostics
Although rare, molecular methods such as PCR are available in some laboratories.
THERAPY
Mild cases caused by B. microti usually resolve spontaneously, and in more serious cases, treatment with clindamycin and quinine or atovaquone and azithromycin is used. In very severe cases of B. microti infection and in B. divergens splenectomized or immunosuppressed patients, exchange transfusion can also be used in addition to antimicrobials.
PREVENTION
Personal protective measures, such as long pants, long sleeved shirts, and insect repellant, may reduce the risk of infection when outdoors in endemic areas for the tick vectors.
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