Babesia

Author:MBLOGSTU

Introduction

Infection by the malaria‐like protozoan, Babesia, has been recognized for over 100 years. The first written reference to babesiosis was possibly made in the Bible.

Also known as piroplasmosis, babesiosis was once considered primarily a disease of animals—affecting mostly livestock and other domesticated species. However, most human cases have now been reported in the United States, particularly in the northeastern region.

In the USA, B. microti is responsible for almost all reported cases, though recent studies document infections by newly identified Babesia species in California, Missouri, and Washington State. The emergence of human babesiosis appears to be linked to an increased deer population.

Moreover, several serological surveys suggest that infection with Babesia is more widespread than indicated by case reports, implying that most human babesiosis due to this parasite is subclinical.

Morphology

Babesia are intracellular parasites that are frequently described as malaria-like. They are piriform, round, or oval in shape; their appearance may vary with the species.

• B. microti: Normally found in rodents, it measures approximately 2.0 × 1.5 µm.
• B. bovis: A species found in cattle that can infect humans, measuring about 2.4 × 1.5 µm.
• B. divergens: Also a cattle parasite that may infect humans; it measures approximately 1.5 × 0.4 µm.

Babesia are often mistaken for Plasmodium falciparum (one of the malaria parasites) due to their intracellular ring forms and peripheral location within the erythrocyte. However, in contrast to Plasmodium, Babesia do not contain any hemoglobin-derived pigment. A key diagnostic feature of babesiosis is the presence of a distinctive tetrad (a “Maltese cross”) form of B. microti, which arises from division by budding rather than schizogeny.

Pathogenesis and Clinical Manifestations

Human cases of babesiosis vary between European and North American scenarios:

• In Europe, most cases (approximately 84%) have been reported and in some of these, bone marrow examinations have demonstrated hemophagocytosis.
• In the United States, severe infections with newly identified Babesia organisms have also occurred. For example, in Washington State the WA1 strain was identified in a patient with an intact spleen who suffered moderately severe disease.

Certain factors may predispose patients to more severe disease:

• Concurrent Lyme disease has been associated with prolonged and more severe symptomatology.
• Advanced age and underlying medical conditions (including splenectomy and HIV infection) are also risk factors for severe babesiosis, particularly with B. microti, which can cause prolonged and chronic infections.

Laboratory Diagnosis

The primary method for diagnosing babesiosis is through microscopic examination of Giemsa- or Wright-stained thick and thin blood smears. Most diagnostic experience has been with patients infected with B. microti.

Serologic Testing: For B. divergens, serologic testing is less useful in the diagnosis of acute infection because antibodies may not be detectable until approximately one week after symptom onset. However, serology may help in species identification.

For B. microti, serological testing using an indirect immunofluorescence test is available.

Molecular Detection: Polymerase chain reaction (PCR) holds promise as a sensitive and specific method for detecting B. microti.