Protozoan Pathogens: Identification


The identification of protozoan pathogens is based upon direct detection of the respective causative agent in clinical specimens and/or upon detection of specific immune reactions of the host. A considerable part of protozoan diagnostics still mainly relies on microscopy, however, in the past years molecular methods, particularly polymerase chain reaction (PCR)‐based techniques, have gained more and more importance. One of the big advantages of molecular techniques is that they usually allow identification below the genus level, which is often impossible by light microscopy. Serological tests are of great value in all tissue parasites and generally in most extra‐intestinal infections, whereas they only have limited importance in acute infections with short incubation times and in immunocompromised patients. Rapid card tests detecting parasite antigens and host antibodies are available for several important protozoan parasites and serve particularly well in the field setting.

Key Concepts:

  • Of the around 100 protozoan species that can infect humans, some are of prime importance for human health, including the causative agents of malaria, amebiasis, leishmaniasis and sleeping sickness.

  • Several protozoan infections show a more severe progression in the immunocompromised host, important examples are toxoplasmosis, pneumocystosis, cryptosporidiosis and visceral leishmaniasis.

  • The size of protozoan pathogens varies from ∼2 μm (amastigote Leishmania spp.) to ∼150 μm (Balantidium coli).

  • As parasite density in stool or body fluids may not be constant, the collection of repeated samples is often essential.

  • Proper collection, storage and transport of clinical specimens are of crucial importance for reliable laboratory diagnostics.

  • Accurate diagnosis includes both parasite detection and species (in some cases: genotype/serotype) identification.

  • Although numerous protocols for molecular detection of protozoan pathogens have been established in the past years, consent on the most reliable technique is missing for most protozoan taxa, thus demonstration of the causative agent in native material or stained smears is still the gold standard for detection.

  • Good microscopic expertise is essential: low parasite density and/or morphologic variability can result in false negative results, pseudo‐parasites and artefacts are common causes of false positive results.

  • In many protozoan taxa, morphology alone does not provide enough information for identification on or below the species level, this is today mainly based on molecular biological methods.

Keywords: protozoa; identification; detection; laboratory diagnosis

Figure 1.

Giardia duodenalis, cysts. Giemsa stain, ×1000.

Figure 10.

Acanthamoeba genotype T4, trophozoite. Phase contrast, ×400.

Figure 2.

Trichomonas vaginalis, trophozoite. Lugol's iodine stain, ×1000.

Figure 3.

Naegleria fowleri, trophozoite in brain. Giemsa stain, ×1000.

Figure 4.

Leishmania donovani, amastigotes in macrophage. Giemsa stain, ×1000.

Figure 5.

Plasmodium falciparum, young trophozoite (ring stage) in erythrocyte and gametocyte. Giemsa stain, ×1000.

Figure 6.

Babesia divergens, young trophozoite (budding form) in erythrocyte. Giemsa stain, ×1000.

Figure 7.

Balantidium coli, trophozoite. Native, ×1000.

Figure 8.

Blastocystis hominis, Lugol's iodine stain, ×1000.

Figure 9.

Entamoeba histolytica/dispar, cyst. Heidenhain's iron haematoxylin stain, ×1000. Note: Cysts of E. histolytica and E. dispar cannot be discriminated by morphology.



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Wiser MF (2010) Protozoa and Human Disease, 1st edn. New York and London: Garland Science, 250 pp.

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Walochnik, Julia, and Aspöck, Horst(Jan 2012) Protozoan Pathogens: Identification. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001949.pub3]