Parasitic Single-Celled Eukaryotes

Parasitic Amoebae

Entamoeba histolytica
Examine the cysts under oil immersion. The cysts should be 10-20 m in diameter, and will contain 4 nuclei when mature, and chromatoid bars with rounded ends (Fig. 1.1),. Trophs of the invasive form generally are 20-40 m in diameter and do not form cysts. The invasive form often will have red blood cells in the food vacuoles. Trophs of the benign form range from 7-19 m, and do form cysts. The cytoplasm of the trophs is fine-grained, and tends to have few or no vacuoles. The nucleus of E. histolytica has a thin membrane with delicate beading, and a central endosome (Fig. 1.1),. Entamoeba hartmanni is very similar in morphology to E. histolytica, but generally is smaller (troph 5-11 m; cyst 5-10 m).

Entamoeba coli
Examine the troph stage under oil immersion. Trophs are on the order of 15-50 m in diameter and do form cysts. In contrast to the trophs of E. histolytica, these trophs have a coarsely granular, vacuolated cytoplasm. The structure of the nucleus differs from that of E. histolytica in that the membrane is thick with coarse beading, and the endosome is eccentric (Fig. 1.2). The cysts are 10-33 m and, in contrast with E. histolytica, contain 8 nuclei when mature and have irregular, splintered chromatoid bars (Fig. 1.2).

Parasitic Flagellates

Trypanosoma cruzi
Examine trypomastigotes of Trypanosoma cruz under oil immersion, and compare them to the diagram in Fig. 3.4 of your textbook (p. 51), noting the nucleus, kinetoplast, and undulating membrane. Typically, the number of trypomastigotes in the blood is low, as the majority of individuals occur as amastigotes in tissue pseudocysts.

Giardia lamblia
Examine trophozoites of Giardia lamblia under oil immersion, and compare them to the diagram in Fig. 3.4 of your textbook (p. 51). Cysts of G. lamblia might have 2 nuclei when immature, but typically have 4.

Phylum Apicomplexa

Class Sporozoea

Subclass Coccidiasina

Order Eucoccidiorida

Family Plasmodiidae

Plasmodium vivax
Examine a blood smear under oil immersion. Most of the cells you will find will be troph stages in red blood cells. Use the pictures in Fig. 3.22 and 3.23 (pages 82 and 83) in your textbook to help you identify ring-stage trophs, ameboid trophs, and schizonts (=meronts), which are recognizable from the fragmented nuclei (which stain purple in Wrights stain). Gametocytes also might be present. Distinguishing characteristics of P. vivax include enlarged, decolorized host cells with Schffners dots (pink stippling of the red blood cell membrane), large parasites, schizonts forming 12-14 merozoites, and rounded gametocytes. Small amounts of hemozoin (black pigment) might be present. Use Fig. 1.3 to avoid confusion between infected red blood cells and the normal leukocytes found in human blood.

Plasmodium falciparum
Examine the blood smear under oil immersion. Use the pictures in Fig. 3.21 - 3.23 (pages 81- 83) in your textbook to help you identify the stages you encounter. Plasmodium falciparum is the only species of Plasmodium infecting humans that will multiply infect a single red blood cell. As the troph grows, it remains in the ring form, resembling the early troph stage of P. vivax. The mature schizont produces 8-24 merozoites (median=16). Infected host cells do not present with Schffners dots, and hemozoin granules are numerous and larger than those produced in P. vivax infections. Gametocytes are crescent shaped. Generally, only ring stages and gametocytes of P. falciparum are found in the peripheral blood.

Class Conoidasina

Subclass Gregarinasina

Gregarina sp.
Members of this genus are cephaline gregarines, occurring in the digestive tract of cockroaches (Blattodea) and orthopterans. Examine the slide containing whole mounts of Gregarina sp., noting the anterior protomerite and posterior deutomerite separated by a septum.

Phylum Ciliophora

Trichodina sp.
Examine specimens of Trichodina under oil immersion, and compare them to Fig. 3.28B on page 98 of your textbook.

Phylum Dicyemida

Dicyema sp.
Examine the preparation from a cephalopod kidney, and compare the rhombogen to Figs. 9.5 and 9.6 on pages 292 and 293 of your textbook. Note the axial cell enclosing the darkly stained axoblasts, the elongate jacket cells surrounding the axial cell, and the apical cluster of cells (the calotte) used for attachment.