Babesiosis is a tick-borne infectious disease caused by parasites of the genus Babesia. These protozoans are obligate parasites of red blood cells (RBCs). Wild and domestic animals are the natural reservoirs of Babesia. Transmission to humans is incidental and was recognized only half a century ago. The vast majority of cases occur in the United States, where babesiosis has the status of an emerging infectious disease. Sporadic cases are reported in Europe and the rest of the world (see “Global Considerations,” below).
Babesia microti, a parasite of small rodents, is the etiologic agent of babesiosis in the northeastern United States. Highly endemic areas include Nantucket Island, Martha's Vineyard, Block Island, Shelter Island, eastern Long Island, and Fire Island. On the mainland, babesiosis is endemic in southeastern Massachusetts, coastal Rhode Island and Connecticut, central New Jersey, Wisconsin, and Minnesota. On the West Coast, the etiologic agent is Babesia duncani, a species closely related to those found in wildlife. The index case of infection with this species was reported from Washington State. Several cases were identified in northern California, and one case may have been contracted in central Oregon. Three cases of babesiosis caused by Babesia divergens–like organisms—from Washington State, Missouri, and Kentucky, respectively—have been reported.
The number of cases of B. microti illness has increased steadily over the last decade. In 2009, more than 700 cases were reported to the public health departments in endemic states. The prevalence of babesiosis caused by B. microti is underestimated because young healthy individuals typically experience a mild and self-limiting illness and may not seek medical attention. Accordingly, seroprevalence is much higher than the prevalence of clinical babesiosis.
The nymphal stage of the deer tick Ixodes scapularis is the primary vector for transmission of B. microti. Transmission occurs from May through September. The incubation period is 1–6 weeks long, with three-fourths of cases presenting in June and July. The vectors for transmission of B. duncani and B. divergens–like organisms remain unknown.
Babesiosis is occasionally acquired by transfusion of blood products, primarily packed RBCs; more than 70 such cases have been caused by B. microti, and two cases caused by B. duncani have been transmitted by this route. The incubation period lasts 1–9 weeks. Most cases occur in endemic areas during fall and winter. Some cases are diagnosed in nonendemic areas to which blood products have been imported from endemic areas. Of the 11 transfusion-related babesiosis deaths reported to the U.S. Food and Drug Administration since 1998, 10 have occurred since 2005.
Three cases of congenital babesiosis have been attributed to B. microti. Other cases of neonatal babesiosis have been acquired by transfusion or tick bite.
Patients experience a gradual onset of malaise, fatigue, and weakness. Fever exceeds 38°C, can reach 40.6°C, and is accompanied by one or several of the following: chills, sweats, headache, myalgia, anorexia, dry cough, arthralgia, and nausea. Less common symptoms include neck stiffness, sore throat, shortness of breath, abdominal pain, and weight loss. On physical examination, fever is the salient feature. Development of erythema chronicum migrans is suggestive of intercurrent Lyme disease. Ecchymoses and petechiae have been reported. Mild splenomegaly and hepatomegaly are occasionally noted. Lymphadenopathy is absent. Jaundice, slight pharyngeal erythema, retinal infarcts, and retinopathy with splinter hemorrhages are rare.
Severe B. Microti Illness
Severe babesiosis is associated with parasitemia levels of >4% and requires hospitalization. Risk factors include an age of>50 years, male gender, asplenia, HIV/AIDS, malignancy, and immunosuppression. Compared with patients hospitalized for other febrile illnesses, patients with severe babesiosis are more likely to report malaise, myalgia or arthralgia, and shortness of breath. Complications develop in ∼40% of hospitalized patients. Risk factors for complications are severe anemia (hemoglobin level ≤10 g/dL) and high-level parasitemia (>10%). Acute respiratory distress syndrome is the most common complication. Other complications include disseminated intravascular coagulation, congestive heart failure, and renal failure. Splenic infarcts and rupture have been reported. Strong predictors of poor outcome—defined as hospitalization for >2 weeks, stay in an intensive care unit for >2 days, or death—are male gender, alkaline phosphatase levels of >125 U/L, and white blood cell (WBC) counts of >5 × 109/L. The fatality rate is 5% among all hospitalized patients but is much higher (20%) among immunocompromised patients.
Other Babesial Infections
Cases of B. duncani infection range in severity from asymptomatic to fatal. Clinical manifestations are those reported for B. microti. All three reported patients infected with B. divergens–like organisms required hospitalization; one died.
A diagnosis of babesiosis should be considered for any patient who (1) presents with flu-like symptoms and has recently resided in or traveled to an endemic area or received a blood transfusion or (2) presents with symptoms of or has been diagnosed with Lyme disease or human granulocytotropic anaplasmosis.
Babesiosis is diagnosed by microscopic examination of Giemsa-stained thin blood smears, on which Babesia species appear as round or pear-shaped organisms. The ring form is most common and lacks the central brownish deposit (hemozoin) typical of Plasmodium falciparum trophozoites (Chap. e27). Other distinguishing features are the absence of schizonts and gametocytes and the occasional presence of tetrads (“Maltese crosses”), which are pathognomonic of infection with B. microti or B. duncani but are also noted in human RBCs infected with B. divergens–like organisms. When parasitized RBCs are rare (particularly at the onset of symptoms), identification of the parasite may require multiple blood smears over several days. If babesiosis is suspected but the ...