Trichinosis disease in humans is normally caused by the ingestion of poorly-cooked pork containing infective stages of this parasitic roundworm (nematode). It is now becoming rare in the developed world following the success of meat inspection programs.
The life cycle of this parasite involves adults living in the cells lining the intestine (epithelial cells) of carnivores such as pigs, foxes and cats. After fertilisation female worms release about 1,500 juveniles which migrate via the blood to the liver, from there to the heart, lungs and then all round the body via the systemic circulation.
When they reach skeletal muscle the parasites form a type of cyst, called a parasite-nurse-cell complex, which is nourished by a blood supply from the host. Juvenile worms can remain viable for up to 25 years while encysted in this manner.
If meat containing cysts is eaten by humans, the juvenile worms are released from their nurse cells in the intestine and grow, through several moults, into adults.
In humans this can cause nausea, vomiting and diarrhoea.
Unfortunately for this worm humans are 'dead-end host'’ as we are rarely eaten by other carnivores, so that the juvenile worms are not passed onto other animals.
In Australia, a different species, Trichinella pseudospiralis, occurs in native carnivorous marsupials (e.g. spotted quolls and Tasmanian Devils) and this species also presents a potential problem for humans.
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