Chapter 7 Answers to self-check questions

7.1 Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale, and Plasmodium knowlesi

7.2 An infected female Anopheles mosquito injects sporozoites into the bloodstream of the human host. The sporozoites migrate to the liver and multiply to form merozoites. The merozoites exit the hepatocytes encased in a merosome which travels to the lungs, lodges in the microcirculation and releases merozoites when the merosome membrane disintegrates. The merozoites invade host red blood cells, mature into the trophozoite stages and then enter shizogony. Rupture of the schizont releases merozoites which infect further red cells. Some trophozoites transform into gametocytes which are ingested by another mosquito when it feeds on the infected human host. The microgametocytes exflagellate and then fertilize the macrogametocytes to form the ookinete which embeds in the gut wall and develops into an oocyst. Multiple cell divisions occur in the oocyst to generate sporozoites which are released from the oocyst to travel to the mosquito’s salivary glands and are injected into another human host to perpetuate the life cycle.

7.3 Haemolytic anaemia due to merozoite release from schizonts; phagocytosis of infected red blood cells; splenic pitting; enhanced clearance of red cells with reduced deformability; increased reticuloendothelial and splenic function can lead to enhanced clearance of non-parasitized red cells.

7.4 Trophozoite appearances, merozoite numbers in schizonts, gametocyte morphology, size and shape of infected red cells, presence of multiply infected cells and the presence of Schüffner’s/James’s dots and Maurer’s clefts.

7.5 Experience of, and time available to, biomedical scientists; thick films are not suitable for species identification; peripheral blood can be ‘temporarily negative’ during episodes of parasite splenic sequestration; scanty infections are easily missed or can make species identification difficult; staining procedures; artefacts.

7.6 Rapid immunochromatography for malarial antigens that differentiate between falciparum and non-falciparum infection; quantitative buffy coat for parasite detection; PCR for parasite detection and species differentiation; assays for detection of anti-malaria antibodies.

7.7 Humans are the main hosts for malarial parasites but incidental hosts for babesiosis parasites. Both are transmitted by arthropod vectors, malaria by mosquitoes and babesiosis by ticks. Malarial parasites have a hepatic phase prior to red cell infection but babesiosis parasites infect red cells upon host entry. Both enter hosts as sporozoites and transform into trophozoites inside red cells. Unlike malaria, there is no parasitophorous vacuole formation when babesiosis parasites enter red cells. Both types of parasite cause haemolytic anaemia on red cell release but only malarial parasites show periodicity. Microscopical appearances common to P. falciparum malaria and Babesia species are normal red cell size, multiply-infected cells, small rings and absence of Schüffner’s and James’s dots. Babesia species are differentiated from P. falciparum by the presence of groups of Maltese Cross formations and lack of schizonts, gametocytes, Maurer’s clefts and pigment. Unlike malaria, babesiosis vectors are not re-infected when feeding on an infected human.

7.8 Wet preparation to detect presence of motile forms. Examination of Giemsa stained thick and thin films to detect parasites and identify species—T. cruzi can be confused with the non-pathogenic T. rangeli; T. brucei gambiense and T. brucei rhodesiense are morphologically indistinguishable. Quantitative buffy coat methodology, xenodiagnosis and PCR can also be used.

7.9 Light microscopy of a bone marrow aspirate, splenic aspirate, lymph nodes, liver biopsy or peripheral blood buffy coat. The parasites are contained within monocytes and macrophages. Rapid immunochromatography assays detect parasite antigens and PCR techniques detect and speciate the parasites. Serological testing has limited use in a diagnostic laboratory.

7.10 Peripheral blood, urine, hydrocele fluid or biopsies are stained with Giemsa or haematoxylin. Parasites can be concentrated in blood samples to increase chances of detection. Species identification is achieved by recognizing features of the sheath, cephalic space and the presence/arrangement of nuclei in the tail. Not all sheaths stain with Giemsa but can be visualized with haematoxylin. Rapid immunochromatography assays can detect W. bancrofti antigens and PCR techniques distinguish between W. bancrofti and B. malayi.

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