ANIMAL PARASITES


The common parasitic diseases of domestic animals are caused by the
following groups of worms: _Flukes_ or _trematoides_; _tapeworms_ or
_Cestoides_; _thorn-headed worms_ or _Acanthocephales_; and _round-worms_
or _Nematoids_. Flat worms, such as tapeworms and flukes, require secondary
hosts. The immature and mature forms of tapeworms are parasites of
vertebrate animals, but an invertebrate host is necessary for the
completion of the life cycle of the fluke. The hog is the only specie of
domestic animals that becomes a host for the thorn-headed worm. The
round-worm is a very common parasite. There are many species belonging to
this class.

[Illustration: FIG. 70.--Liver flukes.]

DISTOMA HEPATICUM (COMMON LIVER FLUKE).--Sheep are the most common hosts
for this parasite. It is present in the gall ducts and livers, and causes a
disease of the liver known as liver rot. The liver fluke is flat or
leaf-like and from thirteen to fifteen mm. long (Fig. 70). The head portion
is conical. It has an oval and ventral sucker, and the body is covered with
scaly spines. The eggs are oval and brownish in color.

_The life history_, in brief, is as follows: Each adult is capable of
producing an immense number of eggs which are carried down the bile ducts
with the bile to the intestine, and are passed off with the faeces. Under
favorable conditions for incubation, such as warm, moist surroundings, the
ova or eggs hatch and the _ciliated embryos_ become freed. The embryo next
penetrates into the body of certain snails and encysts. The _sporocyst_, as
it is now called, develops into a third generation known as _redia_ which