Babesia is a microscopic parasite that infects red blood cells, and examining a blood sample under a microscope is a primary method for its detection. Direct visualization of the organism allows for a definitive diagnosis. Identifying the parasite is based on observing its distinct forms and characteristics within the blood.
The Babesia Parasite and Babesiosis Disease
Babesia is a single-celled protozoan parasite that causes the disease babesiosis. This organism is transmitted to humans through the bite of an infected tick, most commonly the blacklegged or deer tick. Transmission can also occur through contaminated blood transfusions or from an infected mother to her baby.
Once inside the body, the parasites invade and multiply within red blood cells. This replication process leads to the rupture of red blood cells, releasing more parasites to infect other cells. The destruction of red blood cells causes the symptoms of babesiosis, which can range from mild to severe. The most common species causing human infections in the United States is Babesia microti.
Microscope Slide Preparation for Babesia
A blood sample must be correctly prepared for microscopic examination. The process begins with collecting a blood sample, often from a finger prick or blood draw, from which two types of blood smears are made on glass slides: a thin smear and a thick smear.
The thin smear spreads a single drop of blood to create a layer one cell thick. This preparation is fixed with methanol to preserve the shape of the red blood cells and the parasites. The thick smear uses a larger drop of blood, which is not fixed, allowing red blood cells to break down and concentrate the parasites for easier detection.
Both smears are stained with a Giemsa stain, which colors the parasite’s nucleus red or purple and its cytoplasm blue. This makes them stand out against the red blood cells. After staining, the slides are examined under a microscope at high magnification.
Key Microscopic Features of Babesia
Babesia parasites are found inside red blood cells and are pleomorphic, meaning they appear in various shapes and sizes. A key distinguishing feature is that Babesia does not produce hemozoin, a dark pigment created from hemoglobin digestion. Multiple parasites can infect a single red blood cell, and they are sometimes seen outside of cells in the blood plasma after the host cell ruptures.
Common forms include:
- Small, ring-like structures with a dot of chromatin, which is the parasite’s genetic material.
- Pear-shaped (piroplasmic) organisms, often seen in pairs.
- Amoeboid forms with irregular shapes.
- The “Maltese cross” or tetrad formation, a characteristic structure where four daughter cells are arranged in a cross-like pattern.
Distinguishing Babesia from Similar Organisms
Differentiating Babesia from Plasmodium species that cause malaria is a diagnostic challenge because their ring forms can look similar. However, Babesia parasites are more pleomorphic compared to the uniform appearance of Plasmodium rings. The “Maltese cross” formation is also a definitive sign of Babesia, as this arrangement is not seen in malaria.
Unlike Plasmodium, Babesia does not produce hemozoin, a brownish pigment that Plasmodium leaves behind from digesting hemoglobin. While multiple parasites can infect a single red blood cell in both infections, the presence of four or more parasites in one cell is more indicative of Babesia. It is also possible to confuse Babesia with non-parasitic elements in a blood smear, such as platelets on top of a red blood cell. Careful focusing of the microscope can resolve this, as platelets will be on a different focal plane.