T. cruzihas overcome two problems. Firstly it avoids the host’s response by ‘hiding’
within cells where it multiplies and then invades their cells. Secondly it avoids the prob-
lem of distribution and transmission by invading the bloodstream.
Many protozoa take advantage of the fact that cells in tissue and organs are embedded
in a semi-solid matrix — a ground substance. Most cells are adjacent to at least one other
cell but they remain in contact with the fluids in the matrix to receive their nutrients,
oxygen etc. and to dispose of their waste products. The majority of the infective stages
of the parasitic protozoa are smaller than the host cells and are able move through
matrix in search of an appropriate cell or enter the bloodstream through a fine capillary
ending. The parasites move either by means of a flagellum or cilia or amoeboid-type
movement.
The cellular architecture of most host organs is such that the individuals cells are in
contact with intercellular fluid-filled spaces. Liver cells are arranged in columns two
cells wide with an interstitial space between the columns. This provides ample room for
the movement of phagocytic cells as well as parasites.
n SUMMARY
An outline of the general characteristics of the protozoa is presented. The characteristics
and taxonomy of the parasitic protozoa that infect mainly humans and domestic animals is
described. Examples of the parasitic Sarcomastigophora, Sarcodina and the Apicomplexa
are presented. A brief description is given of aspects of the morphology, physiology and
reproduction of the most important examples of parasitic protozoa.End of chapter questions
Question 2.1 Give the names of the different types of outer coverings of protozoa.
Question 2.2 List the types of organelles found within the cytoplasm of protozoa.
Question 2.3 How do protozoa reproduce?
Question 2.4 How many different forms of asexual reproduction have been observed
among the protozoa?
Question 2.5 What is a cyst; how and why do they form?
Question 2.6 How do protozoa feed?
Question 2.7 In which of the protozoan phyla are located the main parasites of economic
importance?
Question 2.8 Name some examples of intercellular and extracellular parasitic protozoa.
Question 2.9 By what methods do parasitic protozoa move?
Question 2.10 What are spore-forming protozoa? Give some examples.
Question 2.11 Name some of the human diseases caused by parasitic protozoa.
Question 2.12 Name some flagellates that are the cause of diseases in both humans and
domestic animals.
Question 2.13 Describe the process of schizogony.
Question 2.14 How many different forms occur during the life-history of an intracellular
protozoan?
Question 2.15 Name and describe the different phases associated with the parasitic
flagellated protozoa.
Question 2.16 What is the reticulo-endothelial system?
PROTOZOA