Chapter 136 : Plasmodium and Babesia*

The Plasmodium parasite is transmitted to humans when an infected female Anopheles mosquito takes a blood meal and inoculates sporozoites into the bloodstream ( 1 ). Sporozoites infect hepatocytes ( 2 ) and undergo asexual reproduction to form schizonts ( 3 ) in a process called exoerythrocytic schizogony (A). After 5 to 15 days, liver schizonts rupture to release hundreds of thousands of merozoites into the blood ( 4 ) which then infect erythrocytes ( 5 ). P. vivax and P. ovale have a dormant hypnozoite stage, which may remain in the liver and cause relapsing disease weeks to months later, while other human Plasmodium species do not have this stage. In erythrocytes, merozoites undergo asexual reproduction to form trophozoites and then schizonts in a process called erythrocytic schizogony (B). Schizonts rupture to release merozoites, which then infect other erythrocytes ( 6 ). This process is repeated every 1 to 3 days, resulting in the production of thousands to millions of infected erythrocytes in several days. Some merozoite-infected erythrocytes do not undergo asexual reproduction but instead develop into male microgametocytes and female macrogametocytes ( 7 ). When ingested by a female Anopheles mosquito during a blood meal ( 8 ), microgametocytes exflagellate to release microgametes, which penetrate macrogametes and form zygotes ( 9 ) via sexual reproduction in the sporogonic cycle (C). Zygotes become motile and elongated to form ookinetes ( 10 ) which invade the mosquito’s midgut wall and develop into oocysts ( 11 ). Over a period of 8 to 15 days, oocysts grow, rupture, and release thousands of sporozoites ( 12 ), which travel to the mosquito’s salivary glands to be inoculated into a new human host during the mosquito’s next blood meal ( 1 ). Life cycle courtesy of the CDC DPDx (http://www.cdc.gov/dpdx/). doi:10.1128/9781555817381.ch136.f1

Plasmodium morphology on thick blood films encompasses the entire spectrum of forms including trophozoites, schizonts, and gametocytes. Shown are P. falciparum early trophozoites in a moderate ( 1 ) and heavy ( 2 ) infection; P. vivax early trophozoites ( 3 ); P. malariae ( 4 ) and P. vivax ( 5 ) schizonts; and P. falciparum ( 6 ), P. malariae ( 7 ), and P. vivax ( 8 ) gametocytes. Neutrophils are shown for size comparison in panels 4 and 7. (Giemsa, ×1,000.) doi:10.1128/9781555817381.ch136.f2

The ideal location for microscopic examination of a thin film is the region of the feathered edge where the erythrocytes have minimal overlap and maintain their central pallor (middle). In this location, the erythrocyte infected with P. ovale is clearly enlarged and ovoid. When examining similar cells in regions of the film that are too thin (left) or thick (right), the morphology is distorted and may be misleading. doi:10.1128/9781555817381.ch136.f3

P. falciparum, successive developmental stages in Giemsa-stained thin blood films: early stage trophozoites/rings with a “headphone” form ( 1 ), rings with Maurer’s clefts ( 2 ), rings with appliqué forms and Maurer’s clefts ( 3 ), maturing trophozoites ( 4 ), early stage schizont ( 5 ), mature schizont ( 6 ) (courtesy of the CDC DPDx), macrogametocyte ( 7 ), and microgametocyte ( 8 ). Rings ( 1 , 2 , 3 ) and, less commonly, gametocytes ( 7 , 8 ) are found in peripheral blood, while other stages ( 4 – 6 ) are typically sequestered in the microvasculature. doi:10.1128/9781555817381.ch136.f4

P. malariae, successive developmental stages in Giemsa-stained thin blood films: early stage trophozoite/ring ( 1 ), “bird’s eye” ring form ( 2 ), mature trophozoites with “basket” form ( 3 ), mature trophozoite with “band” form ( 4 ), mature schizont ( 5 ), mature schizont showing rosette form with central pigment ( 6 ), macrogametocyte ( 7 ), and microgametocyte ( 8 ). doi:10.1128/9781555817381.ch136.f5

P. vivax, successive developmental stages in Giemsa-stained thin blood films: early stage trophozoite/ring, ( 1 ), maturing ring ( 2 ), mature amoeboid trophozoites ( 3 , 4 ), mature schizonts ( 5 , 6 ), macrogametocyte ( 7 ), microgametocyte ( 8 ). Note the Schüffner’s dots ( 2 , 4 , 6 , 8 ) and frequent molding of infected cells to neighboring erythrocytes. The infected erythrocytes are slightly larger than noninfected cells. doi:10.1128/9781555817381.ch136.f6

P. ovale, successive developmental stages in Giemsa-stained thin blood films: early stage trophozoite/ring ( 1 , 2 ), developing trophozoite ( 3 ), mature trophozoite with “comet cell” morphology ( 4 ); early stage schizont ( 5 ), mature schizont ( 6 ), macrogametocyte ( 7 ), microgametocyte ( 8 ). Note the Schüffner’s dots ( 3 , 4 , 8 ) and oval/elongated shape of the infected cells ( 4 , 6 , 8 ). The infected erythrocytes are slightly larger than noninfected cells. doi:10.1128/9781555817381.ch136.f7

P. knowlesi, successive developmental stages in Giemsa-stained thin blood films: early stage trophozoites/rings ( 1 to 4 ), developing trophozoite ( 5 to 7 ), mature trophozoite with “band” form ( 8 ), early stage schizont ( 9 and 10 ), mature schizont ( 11 and 12 ), immature gametocyte ( 13 ), mature microgametocyte ( 14 and 16 ), mature macrogametocyte ( 15 ). (Images reproduced from figures in reference 33 with permission.) doi:10.1128/9781555817381.ch136.f8

Cerebral malaria due to P. falciparum on tissue section (hematoxylin and eosin, ×400). The capillary in the center of the image contains multiple infected erythrocytes, seen primarily by their brown-black hemozoin pigment. doi:10.1128/9781555817381.ch136.f9

Babesia microti life cycle. The normal zoonotic life cycle involves a rodent (primarily Peromyscus leucopus, the white-footed mouse) and an Ixodes tick. The rodent becomes infected when an infected tick introduces B. microti sporozoites when taking a blood meal ( 1 ). Sporozoites enter the rodent erythrocytes and most undergo asexual reproduction ( 2 ), while some differentiate into female and male gametes ( 3 ). Ticks become infected when ingesting Babesia gametes in the blood of an infected rodent ( 4 ). Within the tick (A), the gametes undergo sexual reproduction to produce sporozoites ( 5 ). As with rodents, humans become infected through the bite of an infected Ixodes tick ( 6 ). Sporozoites enter human erythrocytes (B) and undergo asexual replication ( 7 ). Less commonly, humans are infected via blood transfusion ( 8 ). doi:10.1128/9781555817381.ch136.f10

Babesia morphology on thick blood film consists of ring forms resembling trophozoites of P. falciparum. Some pleomorphism is apparent. doi:10.1128/9781555817381.ch136.f11

Babesia morphology on thin blood film. Note the presence of small intraerythrocytic ring forms resembling early trophozoites of P. falciparum, as well as thicker and markedly pleomorphic ring forms. Characteristic extracellular forms (arrow) and a tetrad form (arrow head) are also seen. doi:10.1128/9781555817381.ch136.f12