n Lesions form in the host’s skin close to the site of the insect bite and most infected
cells remain in the vicinity of the lesion.L. mexicanaand L. brasiliensisare both capable of invading mucocartilage and mucous
membranes and disfiguring lesions can develop (cutaneous leishmaniasis).
L. donovanipromastigotes invade macrophages and transform into amastigotes which
can invade macrophages within the spleen, liver and bone marrow (visceral leishmaniasis).4.4.1.6Toxoplasma gondii
Toxoplasma gondiiis a protozoan coccidian parasite that has both an asexual and a sexual
stage in its life-history. The definitive host is the cat (Felis domestica) and the intermedi-
ate host is a rodent.n The infection in the rodent consists of a pseudocyst located in either muscle or brain.
Each pseudocyst contains numerous bradyzoites which are released into the lumen
of the cat’s stomach. The individual bradyzoites invade the epithelial cells of the small
intestine and undergo asexual multiplication to form merozoites.
n The released merozoites invade non-infected epithelial cells and the population of the
parasites increases within the cat. Some of the merozoites develop into male and
female gametes which fuse to form a zygote that transforms into an oocyte.
n The oocyte nucleus divides and numerous sporozoites from within the oocyst pass out
via the cat’s faeces.The oocysts are eaten by rodents and accidentally by man. The oocyst releases the sporo-
zoites into the gut of the rodent or man and these then invade intestinal macrophages.n They are able to resist being digested by the macrophage and transform into tachyzoites.
Binary fission occurs within the macrophage and daughter tachyzoites are released to
infect other cells within the host.
n Once the population has reached a certain density the division of tachyzoites stops and
they transform into pseudocysts within various tissues, especially the brain. This
change in development may be due to the host’s immune response.If the host is a pregnant female the tachyzoites can cross the placenta and infect the
foetus. In humans the tachyzoites invade the neural tissue, the myocardium, lungs and
liver. Inflammatory lesions develop as a result of the cell invasion and this can lead to
permanent damage in the foetus.4.4.1.7Cryptosporidiumspp
Cryptosporidiumis a gut protozoan parasite of man and domestic animals.n Sporulated oocysts each containing four sporozoites are swallowed.
n The sporozoites emerge in the small intestine and become attached to the surface of
the epithelial cells of the gut mucosa.
n The sporozoites are surrounded by elongated microvilli which fuse and envelop the sporo-
zoite; and the parasite becomes intracellular and forms into a trophozoite.
n A schizont develops from the trophozoite which divides to form eight merozoites (first
generation). The cell ruptures and releases the merozoites and these attach to new non-
infected epithelial cells and by schizogony produce four second generation merozoites.PARASITOLOGY