Entamoeba histolytica
Examine the cysts under oil immersion. The cysts should be 10-20 µm 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 µm 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 µm, 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 µm; cyst 5-10 µm).
Entamoeba coli
Examine the troph stage under oil immersion.
Trophs are on the order of 15-50 µm 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 µm and, in contrast with E. histolytica, contain 8
nuclei when mature and have irregular, splintered chromatoid bars
(Fig. 1.2).
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.
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
Wright𠏋 stain). Gametocytes also might be present. Distinguishing characteristics of
P. vivax include enlarged, decolorized host cells with Schüffner𠏋 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 Schüffner𠏋 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.
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.
Trichodina sp.
Examine specimens of Trichodina under oil immersion,
and compare them to Fig. 3.28B on page 98 of your textbook.
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.
