LAB 3.7 Tapeworms & Flukes
The digestive system of the sheep is essentially the same as
that of the beef animal, so the spare time will be used for a brief
outline of the main parasites detected by meat inspectors.
Three tapeworms are most important in Ontario. A cysticercus
(plural cysticerci) is the larval stage. Infection of cattle and pigs
in Ontario may originate from animal feed contaminated with human
faeces.
1. Taenia saginata - a large
tapeworm in the human gut, with infection from cysticerci in beef.
2. Taenia solium - a large
tapeworm in the human gut, with infection from cysticerci in pork.
3. Echinococcus granulosus -
a small tapeworm in the dog's gut, with infection from cysticerci in
sheep meat. The great danger to humans is accidental ingestion of
dog faeces - usually a child playing in an area contaimated by dog
faeces or a pet owner licked by a contaminated dog. Inside the human,
the parasite is in the wrong host but may continue development to form
very large hydatid cysts in a vital organ like the brain (the hydatid
cyst is something like a compound cysticercus).
Ontario sheep may have Fasciola
hepatica flukes in their livers. Infected livers are
condemned. The flukes greatly reduce the growth of sheep and
lambs and are important for producers. Sheep are infected by eating
grass on which are the infective larval stages (metacercaria) of the
fluke. The flukes migrate to the ducts in the sheep's liver - which
then are blocked, thus preventing the normal release of bile to aid in
the digestion of fats. There is an intermediate host - the infective
eggs in sheep faeces develop into a free-swimming miracidium which
invades a small fresh-water snail (usually Lymnaea bulimoides in
Ontario). In the snail it multiplies through two stages of development
(sporocyst and redia) to release free-swimming cercaria larvae. If the
pasture is properly drained there will be no water snails and no
infection. Cattle and deer may also contain liver flukes (Fascioloides
magna).
Trichinella spiralis is a small
nematode worm. It is rare in Ontario pork. The adult lives
in the gut of the pig (or rodent or bird) and the larvae encyst between
its muscle fibres. The worms are very small (microscopic) and cannot be
detected by visual examination of the meat without using a microscope
or a diagnostic test kit. If a human gets infected, large numbers
of worms may create massive damage as they migrate through the
body. Pigs get infected if they eat rodents or birds that wander
into their pens. Although it is rare, it is essential for
consumers to cook pork to at least 60 degrees Centigrade. If you
wish to eat your pork rare (pink) - freeze the pork first.
LINKS
Overhead showing life
cycles of Taenia saginata and T. solium
Overhead showing life
cycle of Fasciola hepatica
Overhead showing life
cycle of Trichinella spiralis