Babesiosis is a blood-borne infection transmitted by the bite of the deer tick (Ixodes scapularis). Most human cases of babesiosis are caused by the species B. microti. It is endemic in the Northeast and Midwest, and most cases occur in the summer months. The clinical manifestations vary from asymptomatic to severe or even fatal. Symptoms are non-specific, and this infection is often suspected when patients from an endemic area present with fevers, anemia, and other flu-like symptoms.
There are several laboratory tests to assist in the diagnosis. One traditional test is the direct demonstration of the organism intracellularly (in RBC’s) on stained smears of whole blood. The advantage of this test, in the hands of an experienced pathologist or technician, is that the result is available immediately. One potential disadvantage is that the organisms may not be visible below a certain level of parasitemia.
A more recently developed methodology for the direct demonstration of the organism in a clinical specimen (whole blood) is PCR. This methodology is capable of detecting minute quantities of the organism’s specific DNA, as few as several copies of DNA, making it a much more sensitive test than direct visualization on a blood smear. 104th to 105th fewer organisms are detectable by PCR compared to the visualization on a blood smear. For example, the limit of detection for this PCR methodology is approximately 10-20 Babesia organisms per ml of blood (corresponding to 1 out of 40,000,000 RBC’s infected). In contrast, the limit of detection of traditional blood smears is about 250,000 Babesia organisms per ml of blood (corresponding to 1 out of 4,000 RBC’s infected).
The detection of B. microti infection can also be established serologically. The most common methodology is the indirect fluorescent antibody test, or IgG IFA. Antibodies to B. microti may also be detected by IgM and IgG Western Blotting (WB). Serologic detection of infection is typically established by demonstrating seroconversion or a four-fold rise in titer on paired acute and convalescent specimens. The presence of a single high titer in a patient with a compatible history and symptomatology supports the presumptive diagnosis.
Early in the course of the infection the patient may not yet be symptomatic or is just beginning to become symptomatic. PCR testing at this stage is frequently positive, prior to a detectable serologic response. By the time the patient becomes symptomatic with Babesia infection, detectable antibodies are usually present and the PCR will often remain positive. In fact, B. microti DNA may be detected by PCR for many months in untreated patients and for shorter periods of time in treated patients. It is possible to monitor patients after treatment to document eventual PCR negativity (i.e. the clearance of B. microti DNA). Sometimes patients (especially if untreated) experience clinical relapse. It is appropriate to consider follow-up testing to determine possible active parasitemia.
Since Babesia parasitemia may occur in asymptomatic persons who may be blood donors, transfusion transmitted babesiosis is a recently recognized concern in blood banking.
The staff at the laboratory is highly trained in performing and interpreting these assays, has decades of experience in analyzing these tests in the context of various clinical situations, and is available to assist healthcare providers in interpreting tests and answering questions about the detection of tick-borne infections.