n The new merozoites are released and attach to a non-infected epithelial cell before becom-
ing internalised. Within the new cells the merozoites transform into either a male
microgametocyte or a female macrogametocyte. The male microgametocyte under-
goes several divisions to produce microgametes which then fuse with the macrogametocyte
to form a zygote.
n The zygote differentiates into an oocyst which is shed with the faeces. If the oocyst is
swallowed by a new host they then sporulate.n 4.5 PARASITIC HELMINTHS TRANSMITTED BY VECTORS
The majority of these are nematodes that do not have a free-living stage and rely on
arthropods that feed on vertebrate body fluids for transmission from one host to
another.
n 4.5.1WUCHERERIA BANCROFTI
W. bancroftiis a filarian nematode that is one of the causes of filariasis in humans. As far
as is known this parasite is specific to humans and has no reservoir hosts. There are three
‘geographical variants’ each with a different genus of mosquito as its vector. In the South
Pacific the vector is a species of Aedes, a mosquito that feeds nocturnally.
The adult W. bancroftiworms live within the lymphatic system of the lower limbs of
humans.
n The male is approximately 4 ×0.1 cm and the female 8 ×0.3 cm long. The females
are ovaviviparous and give birth to live microfilariae (L 1 larvae) which enter the lym-
phatic system and then into the circulating bloodstream.
n The microfilariae require a relatively high oxygen tension and tend to concentrate in
the capillaries of the lungs during the day. During the night while the host is sleeping
the oxygen tension of the peripheral blood increases and the microfilariae migrate out
of the lungs. This coincides with the nocturnal feeding habits of the mosquito. During
feeding the mosquito takes up microfilariae with the blood meal.
n The larvae first accumulate within the flight muscles of the mosquito and then
undergo three moults to form L 3 larvae. These larvae migrate to the mosquito’s
mouthparts and are deposited on the skin while feeding. The larvae ‘crawl’ through
the wound and migrate through the subcutaneous tissues into the lymphatic system and
take about a year to reach maturity. After copulation the females give birth to live
microfilariae, a process which can continue for up to 8 years.n 4.5.2ONCHOCERCA VOLVULUS
The adult parasite (a nematode) lives in the subcutaneous tissues of (mainly) humans
and the microfilariae migrate round the body. If a larva reaches the host’s eye and
remains there this condition can lead to permanent blindness, commonly known as
river blindness.
The vector host is the biting blackfly Simulium damnosumwhich completes its life-cycle
in the ‘white water’ of river rapids.
n From the subcutaneous tissues microfilariae enter the fly while it feeds. The
microfilariae move from the fly’s mouth into the muscle fibres of the fly’s thorax.
After two moults the larvae migrate into the proboscis to be in position for the next
occasion when the fly feeds. During feeding the larvae withdraw from the proboscis.
The larvae are deposited on the skin and enter the host through the wound.PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE