Protozoan Pathogens of Humans


Several protozoans are causes of important diseases that kill millions of animals and humans every year. In addition to known parasites, many so‐called opportunistic agents increase the infection risk, especially for immunocompromised people. Owing to the recent process in globalisation leading to a quick transportation of goods and people from one end of the world to the other, the chance for importation of agents of diseases has enormously increased. Thus new rush of epidemics or even pandemics will threaten overcrowded regions. This danger is furthermore supported by the phenomena of global warming, which lead to the further expansion of vectors of diseases. Therefore awareness is highly needed, if diseases should be recognised as early as possible.

Key Concepts:

  • Teaching in parasitology and epidemics must be strengthened at universities and governmental authorities.

  • Emergency plans have to be established to fight epidemics.

  • Research for new and better acting medicaments has to be strengthened.

Keywords: flagellates; sleeping sickness; amoebae; malaria; apicomplexa; ciliates; microsporidia; diarrhoea; opportunistic agents; cryptosporidia

Figure 1.

Scanning electron micrograph of a trophozoite of Trichomonas vaginalis, note the four anterior flagella, one lateral one and the terminal tip formed by the inner axostyle, which consists of a bundle of microtubules for stabilisation. ×4000.

Figure 2.

Giardia lamblia. Scanning electron micrograph of the ventral side of a trophozoite. Note the large ventral disc, which acts as holdfast system along the surface of the host intestinal cells. ×7500. Inset: light micrograph of a cyst. ×1000.

Figure 3.

Light micrographs of trypomastigote stages of Trypanosoma gambiense in human blood. ×1200.

Figure 4.

Light micrograph of a human bone marrow cell filled closely with amastigote stages of Leishmania donovani just before rupture. ×2000.

Figure 5.

Light micrographs of stages of Entamoeba histolytica: (a) trophozoite; (b) cyst (note inside the cyst the four nuclei, each with a central nucleolus), whereas the amoeba stage has only one nucleus. ×1500.

Figure 6.

Light micrograph of the trophozoite of the free‐living and accidentally infectious amoeba Acanthamoeba castellanii. This stage shows numerous fine filipodia, which are not present in the cyst stages. ×2000.

Figure 7.

Light micrograph of a Blastocystis trophozoite containing a large vacuole and inside the small surrounding cytoplasm several peripheral nuclei. ×1000.

Figure 8.

Light micrographs of Pneumocystis carinii: (a) amoeba‐like stage; (b) cyst‐like body including 8 separate ‘small trophozoites’. ×2500.

Figure 9.

Light micrographs of Toxoplasma gondii: (a) half‐moon shaped tachyzoites inside and outside of macrophages, ×1200; (b) tissue cyst in brain containing thousands of bradyzoites. ×800.

Figure 10.

Light micrographs of oocysts of Isospora belli from human faeces: (a) unsporulated stage, which is seen in fresh faeces; (b) sporulated stage with two sporocysts. The formation of the sporozoites inside the sporocysts occurs outside of the host. ×1500.

Figure 11.

Light micrographs of developmental stages of Cryptosporidium parvum: (a) oocysts from stool; (b) sections through developmental stages at the surface of intestinal cells. ×1000.

Figure 12.

Light micrographs of stages of Giemsa stained Plasmodium falciparum: (a) small signet‐ring stages; note the common double infection in two erythrocytes (=red blood cells); (b) banana‐shaped gamont. ×1000.

Figure 13.

Light micrograph of a large signet‐ring stage of Plasmodium vivax. Note that the host cell contains numerous Schüffner's dots. ×1200. Schüffner's dots (described by Schüffner, 1867–1949) are fine caveolae (=invaginations) at the surface of the Plasmodium vivax infected host reticulocytes when being filled with the Giemsa stain (which also colours the parasites inside the parasitophorous vacuole (PV)), whereas Maurer's clefts are enlargements of the PV.

Figure 14.

Light micrograph of a young schizont of Plasmodium ovale. Note that the shape of the host cell has changed and became somewhat ray‐like. This host cell also shows Schüffner's dots. ×1200.

Figure 15.

Light micrograph of two schizonts of Plasmodium malariae. Note that the parasitised red blood cell shows no alterations and no Schüffner's dots. ×1200.

Figure 16.

Light micrograph of a cyst of Balantidium coli. The included trophozoite contains a large macronucleus (MA). W, wall. ×1000.

Figure 17.

Light (inset) and transmission electron micrograph (TEM) of a spore of microsporidian parasites. In the TEM the coiled polar tube is seen cut longitudinally (upper half) and several times in cross‐section (periphery). Inset: ×1500; TEM ×11 000.



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Mehlhorn, Heinz(Sep 2011) Protozoan Pathogens of Humans. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001943.pub2]