n The larvae that migrate into the striated muscles eventually become ‘encysted’. The
muscle cells that surround the parasite, the so-called nurse cells, provide the nutrients
and stimulus for larval development. After 20 days a cyst composed mainly of host mate-
rial with some input from the parasite has formed around each larva. The larvae coil
into a characteristic ‘spiral’ shape within the cyst. As the cyst ages the host deposits
calcium on the outer wall of the cyst. Calcified cysts can be detected by X-ray analysis.The movement of the larvae from the gut lumen into the lamina propia causes local
inflammation and infiltration of leukocytes. Those larvae that do not enter striated
muscle often end up in the heart and central nervous system where they can cause
disruption of the functioning of those particular organs.
The adults are eventually expelled from the gut whereas the larvae of Trichinella
migrate to the striated muscle and remain within the host. Striated muscle from the para-
site’s point of view provides an ideal environment. Once within the muscle tissue the
parasite is only exposed to limited aspects of the immune system but is in the ideal place
for transference to the next host. Muscle tissue is nearly always the preferred food of
carnivores.
n 4.1.5STRONGYLOIDESSPP
Strongyloides stercoralisare small nematodes (2 ×0.04 mm) that live in the small intestine
of humans.
n A mature worm first develops a male gonad and produces sperm. The male gonad then
regresses and a functioning female gonad develops followed by self-fertilisation.
Apparently parasitic males do not exist or have not yet been found.
n After fertilisation the embryonated eggs in the intestinal lumen hatch into first stage
larvae. The larvae moult in the colon into second stage larvae and these are deposited,
via the faeces, into the soil.
n Larvae that are delayed in the colon moult into third stage larvae, burrow into the gut
mucosa and enter the circulation — a process known as autoinfection.
n Larvae that are deposited into relatively warm and damp soil moult twice more before
become free-living adults.
n Adults of both sexes are found in the free-living phase and females produce embry-
onated eggs. The larvae hatching from these eggs moult four times before developing
into free-living adults.
n Third stage free-living filariform larvae are infective and when the situation arises are
able to penetrate the skin of a suitable host (see Fig. 4.4). The migration route of the
larvae through the body to the gut is similar to that of Ancylostoma.n 4.1.6TOXOCARA CANIS
Toxocara canisis a zoonotic nematode parasite of man and dogs. The life-history of this
parasite is to a certain extent controlled by the hormones of the adult female dog (see
Fig. 4.5).
n Adult worms, found only in puppies, are 5–8 cm long. Eggs are only passed by
puppies up to the age of five weeks. If the soil is oxygenated and warm the eggs take
about 2–6 weeks to embryonate and become infective.
n A lactating bitch can become infected by ingesting eggs while cleaning the puppies and
the subsequent larvae enter a dormant phase.PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE