Babesias

Morphology and differences from plasmodia

Protozoa of the genus Babesia are obligate intracellular parasites that localize exclusively inside erythrocytes. Their shape can be round or pear-like. The parasite is small, measuring 2 to 3 µm. Within the affected cell, they are most often located at the periphery of the erythrocyte.

On microscopic examination, babesiae are very difficult to visually distinguish from malaria agents. Externally, they closely resemble young ring forms of Plasmodium. Sometimes babesiae even take on a ring shape with two nuclei, making them virtually indistinguishable from Plasmodium falciparum.

However, babesiae have specific reproduction features:

• They are characterized by asynchronous budding.
• Pair forms are frequently found in the blood when the protozoa reproduce in pairs.

Life cycle

The pathogen's circulation cycle includes an intermediate host (human) and a vector (ixodid and argasid ticks). Furthermore, transovarian transmission of the pathogen from the female to her offspring occurs within the tick population.

Human infection occurs through the bite of an infected ixodid tick (genus Ixodes). At this moment, the merozoite enters the bloodstream.

Erythrocytic cycle in the human blood:

1. Invasion — the merozoite actively penetrates the erythrocyte.
2. Growth and development — the parasite exists and feeds inside the cell.
3. Reproduction — binary asexual division of the parasite takes place.
4. Tetrad formation — characteristic structures of four individuals form inside a single erythrocyte.
5. Release — the erythrocyte membrane ruptures (lysis occurs), and new merozoites enter the blood plasma.
6. Cycle repetition — the released merozoites invade new, healthy erythrocytes.

Vector infection occurs during blood feeding when the tick ingests merozoites from the blood of an infected person.

Epidemiology and Clinical Presentation

Human babesiosis is rare (the first case was described in Yugoslavia in 1957). The disease has geographical etiological features:

• USA: the main causative agent is B. microti (a rodent parasite).
• Europe: B. divergens (a cattle parasite) predominates.

The natural reservoirs of infection are field mice, other rodents, dogs, cats, and cows. In addition to tick bites, human infection can occur via blood transfusion.

The incubation period lasts from 2 to 4 weeks. In most cases, the infection is asymptomatic. If the disease manifests, it presents as a malaria-like illness. The key symptom triad includes:

• Chills;
• Fever;
• Hemolytic anemia.

Patients also report headache and muscle pain.

Severe course: The infection poses the greatest danger to individuals with splenic dysfunction or asplenic patients. Up to 10–15% of all erythrocytes can be affected in these patients, hemoglobinuria develops rapidly, and there is a high risk of a fatal outcome.

Laboratory diagnostics and treatment

Microscopic method (primary) Blood smears serve as the material for examination. The specimen is stained using the Romanowsky-Giemsa method: the parasite's cytoplasm stains blue, and the nucleus stains red. A pathognomonic (strictly specific) sign of babesiosis is the detection of tetrads in erythrocytes — groups consisting strictly of four trophozoites.

Serological method Indirect immunofluorescence assay (IFA) and enzyme-linked immunosorbent assay (ELISA) are used as additional methods. A diagnostic titer of specific antibodies is usually detected in the patient's blood 3–8 weeks after the onset of the disease.

Therapy and prophylaxis A combination of the antiprotozoal drug quinine and the antibiotic clindamycin is used to treat clinically overt forms. Prevention is exclusively non-specific: protection against tick bites (use of repellents, protective clothing) and vector population control.

Class and exam questions

What else to study on this topic

• Mechanism of hemoglobinuria development in severe forms of babesiosis
• Features of transovarian transmission of Babesia in tick populations
• Epidemiological specifics: differences between B. microti and B. divergens
• Risks and prevention of babesiosis transmission via blood transfusion

Other topics in the section "Apicomplexa (Malaria, Babesia, Toxoplasma, Sarcocystis, Cryptosporidium, etc.)"

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