n The resting stage is an oval cyst with four nuclei that lacks certain subcellular
organelles (mainly hydrogenosomes) and has attenuated flagellae and blepharoblasts.
The cyst passes out of the host via the faeces and becomes the infective stage. The rigid
outer wall protects this stage from environmental changes.
n Cysts, which are the infective stage, are swallowed and once within the gut two fully
formed trophozoites grow within the cytoplasm. These become attached to the
epithelial cells of the gut and then undergo the process of binary fission producing numer-
ous trophozoites.n 4.4 INDIRECT LIFE-CYCLES
These are parasites that have more than one host in their life-cycle. This group includes
parasites that during the process of transfering from the definitive host to the inter-
mediate host may have free-living stages. Alternatively they may use a vector to trans-
mit the parasite from one host to another, thus avoiding the free-living distributive
phase.
n 4.4.1 PROTOZOA WITH MORE THAN ONE HOST
4.4.1.1Plasmodiumspp
These are the intracellular blood parasites (see Figs 2.4 and 2.5) that cause the disease
known as malaria. Four different species of Plasmodiuminfect humans and they are P.
falciparum, P. vivax, P. ovaleand P. malariae. The genus Plasmodiumtends to be host-
specific. The majority of vertebrates are infected either by species of Plasmodiumor by a
related intracellular protozoan parasite.
The transmission of Plasmodiumfrom one host to another is via a blood sucking arthro-
pod. In man it is the female mosquito of the Anophelinae or Culicinae that is responsi-
ble for the transmission of the parasites. Although there nearly 800 species of Anopheles
and Culexonly a limited number transmit malaria.
The mouthparts of the mosquito can pierce the skin of the host and reach into the
capillaries to obtain a blood meal. If sporozoites — spindle shaped motile forms of the
parasite — are present in the salivary glands of the mosquito, they are inoculated into
the host while the mosquito absorbs blood from the host.
The sporozoites circulate round the body via the blood and within 1–30 min penet-
rate the liver and then into the hepatocytes — the exoerythrocytic (non-red blood cell)
stage. Once established within an hepatocyte, the sporozoite undergoes a multiplicative
process known as exoerythrocytic schizogony. Numerous uninucleate merozoites are
formed as a result of schizogonony.
P. vivaxsporozoites take 6–8 days to mature and produce about 10,000 merozoites;
P. ovaletakes 9 days to produce about 15,000 merozoites; P. malariaetakes 12–16 days
to produce 2,000 merozoites and P. falciparum5–7 days to produce 40,000 merozoites,
all from a single sporozoite.
The hepatic merozoites exit from the liver tissue into the circulating blood. They
become attached to receptor sites located on red blood cells before invasion to form the
erythrocytic stage. Once inside an erythrocyte the merozoite develops into a feeding tropho-
zoite. A parasitophorous vacuole forms round the trophozoite and the trophozoite takes
on a ‘signet-ring’ shape.
The trophozoite develops into a schizont which undergoes schizogony to produce
numerous blood stage merozoites. The erythrocyte eventually bursts releasing the
merozoites into the circulation and these then invade non-infected red blood cells. The
result is a dramatic increase in parasitaemia and, when the number of parasites reaches
PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE