membrane. This stage remains extracellular in the blood and body fluids and mul-
tiplies asexually by binary fission.
n After numerous divisions the parasites can reach a concentration of 1,500/mm^3 and
at this stage they begin to invade cerebrospinal fluid and interstitial spaces.
n At a certain population density a number of the trypanosomes transform into stumpy
forms (an intermediate form). These tend to congregate in the peripheral blood
vessels where they are taken up by the fly while it feeds.
n The stumpy forms migrate into the fly’s midgut and transform into procyclic try-
panomastigotes. They divide asexually and the new generation migrates to the salivary
gland and proventriculus of the fly where they transform into the metacyclic form
(metatrypanosomes). Depending upon the environmental temperature and the
species of Glossina, the cycle within the fly takes about 25–50 days.4.4.1.4Trypanosma cruzi
T. cruziis found in Central and S. America and the main vector hosts are the blood-feeding
triatomid bugs Rhodniusspp and Panstrongylusspp. One of the main features of this para-
site is that it is mostly intracellular.
n The vector bites a mammal near its eyes or mouth and defaecates at the site of the bite,
which causes a local itch. The resultant scratching helps the infective stage of T. cruzi
to enter the host at the site of the bite.
n The infective stage, the trypanomastigote, actively penetrates a host cell and transforms
into an amastigote. Amastigotes undergo asexual multiplication forming a new genera-
tion of trypanomastigotes to be released into the bloodstream once the cell dies.
n The trypanomastigotes are able to invade a range of cells including cells of the reticulo-
endothelial system, heart muscle cells, glial cells and cells of the urinogenital tract.
n After several multiplication phases a new generation of amastigotes develops and
accumulates in a primary chagoma, a lesion that forms on the face. When a vector feeds,
it acquires from these sites amastigotes which transform into epimastigotes within the
hindgut of the insect. These undergo repeated divisions and can remain within the bug
for 1–2 years, ie the rest of the vector’s life-span.4.4.1.5Leishmaniaspp
Leishmaniaspp are spread by the biting sandflies of the genera Phlebotomaand Lutzomyia.
L. tropica, L. donovani, L. mexicana, L. brasiliensis, and L. peruvanaare all species which infect
humans. Leishmaniaspp are found throughout the warmer regions of the world and have
numerous reservoir hosts (a zoonosis). Leishmaniaspp can cause either a cutaneous,
mucocutaneous or visceral disease. The promastigotes are found within the gut of the
vector and the amastigotes within the macrophages of the mammalian host.
n A feeding sandfly takes up blood containing amastigotes inside a macrophage. The
insect digests the macrophages and releases the amastigotes into its stomach. These
are transformed into promastigotes and undergo binary fission. The new generation
of promastigotes migrates into the insect’s proboscis ready to be injected into the next
host during the insect’s next meal.
n The promastigotes injected into the host are phagocytosed by the host’s macrophages.
n The promastigotes are able to resist being lysed by the phagocytic cells and transform
into amastigotes which undergo asexual division. The host cell is eventually disrupted,
releasing the amastigotes into blood and body fluids.PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE