lives in the intestine of the ox, where it may attain the length of two
feet, while in the intestine of the lamb it reaches at most the length
of about nineteen inches. It is sometimes present in large numbers in
lambs, causing difficulty of digestion with resulting emaciation,
poverty of blood, and even death. Bladder-worm still unknown.
ORDER: =Trematoda= (FLUKES).
Flattened parasitic worms, tongue-shaped or leaf-shaped, living on or in
a host, and provided with one or two ventral suckers. The intestine
forks into two branches immediately behind the gullet, and these two
divisions in some species (_e.g._ the large liver-fluke) branch again
repeatedly in a tree-like fashion. Anus and blood-vessels absent. Almost
all flukes are hermaphrodite; they fertilize themselves. All the forms
living within a host (_i.e._ endoparasitic forms) lay an immense number
of small eggs, from which larvæ are hatched that become asexually
reproducing animals. These or their progeny produce once more the sexual
form. The development is therefore an example of metagenesis (p. 16),
but there is a change of host. Only the two liver-flukes need be dealt
with here.
The =Large Liver Fluke= (_Distoma hepaticum_),
(Fig. 141), is flat, but swollen in the middle, when it contains a large
number of eggs, under which circumstances the edges only are flat. The
last are of a dirty brown colour, while the middle of the body is
greyish yellow. There is a triangular projection at the front end of the
body with a sucker at the apex, and another at hinder end of the
triangle. The outer surface of the body is covered with minute
backwardly projecting spines, which present no hindrance to the forward
creeping of the body in the bile-ducts, but make backward creeping or
gliding impossible.
[Illustration:
FIG. 141.—The Liver Fluke (_Distoma hepaticum_), natural size.
]
Eggs ¹⁄₁₈₀ of an inch long, with a red shell possessing a greenish
sheen. They can only develop if they get into water, and in wet years
the conditions are very favourable on damp meadows, for under such
conditions the sheep-dung, with the contained fluke eggs, is liable to
fall into pools of water. But many of the eggs can develop in years that
are not so wet and on dryer fields, as they may be carried into a ditch
by rain or on the feet of the sheep. The larva hatched from the egg of
the liver-fluke is ¹⁄₁₂₅ of an inch long, elongated in shape, and swims
about freely in the water by means of a covering of cilia (Fig. 142,
_a_). It soon finds its way into the lung cavity of a small water snail
(_Lymnæus truncatulus_, Fig. 142, _i_), loses its covering of cilia, and
becomes broader, even almost spherical. In the free-swimming larva there
are already to be found a collection of germinal cells in the hinder
part of the body; later on, these cells repeatedly divide and form
little heaps, which ultimately become fresh individuals. The larva has,
therefore, passed into a second stage, and this is now known as a
_sporocyst_ (Fig. 142, _b_, _c_). The stage developed asexually from the
heap of germinal cells is distinguished from the sporocyst by a
differently shaped body and by the possession of a gut (with mouth,
pharynx, and intestine, but no anus), which is absent in the latter.
This stage is termed a “_redia_” (Fig. 142, _d_). From five to eight of
them are usually developed within one sporocyst. Germinal cells are also
present in the redia, and they develop into little heaps of cells from
which, while the weather is warm, new rediæ are developed, that live in
the lung cavity of the snail, like their parents. When the weather
becomes colder, “_cercariæ_” (Fig. 142, _f_) are developed from the
germinal cells, some fifteen to twenty of them within a single redia
(Fig. 142, _e_). The cercaria has a long tail, by means of which it can
propel itself in the water, after leaving the mother redia and the
snail. It possesses two suckers, and a forked intestine without anus.
After the cercariæ have moved about in the water for some time they fix
themselves, by means of their suckers, to various plants growing in the
water. The tail being now superfluous is lost, the cercaria becomes
spherical, and contracts somewhat within an investment which it secretes
(Fig. 142, _g_). When, later on, the pools in which the cercariæ live
dry up, the plants which were before submerged are now on dry land, and
may be devoured by sheep, together with the encysted cercariæ found upon
them. The wall of the cyst is dissolved in the intestine of the sheep,
and the contained cercaria gradually becomes a young liver-fluke, which
at first is oval and very small (Fig. 142, _h_), while, later on, its
anterior third becomes the triangular projection at the front end of the
adult fluke, and its posterior two-thirds grow very considerably.
Meanwhile the young liver-fluke has travelled from the intestine of the
sheep into its bile-ducts. It appears that the fluke is completely grown
within a few weeks after its introduction into the body of the sheep.
Infection almost always takes place in summer or autumn. The life
history of the liver-fluke exemplifies change of host and metagenesis
(p. 16). From the egg of a sexual worm a free-swimming larva is
developed, which becomes an asexual _sporocyst_ within the snail. This
sporocyst produces asexual _rediæ_, and from these, also asexually, are
developed _cercariæ_, which leave the snail, pass into water, attach
themselves to plants, and reach the intestine of the sheep later on. The
cercariæ become flukes by a process of metamorphosis, and are to be
regarded as fluke larvæ.
[Illustration:
FIG. 142.—Liver Fluke: _a_, free-swimming ciliated larva; _b_, young
_Sporocyst_, a few days after migration into the snail; _c_,
_sporocyst_, containing rediæ in various stages of development; _d_,
_redia_, containing balls of germinal cells; _e_, mature _redia_,
containing its brood of cercariæ; _f_, _mature cercaria_; _g_,
just-encysted _cercaria_; _h_, young fluke, soon after entering
sheep; _i_, the water snail, _Lymnæus truncatulus_.
_a_, _b_, _c_, _d_ × 200; _e_ × 150; _f_ × 300; _g_ × 150; _h_ × 25;
_i_, natural size and × 3½.
]
It is easily explained why the “fluke disease,” or “liver rot,” caused
by the fluke should appear much oftener in some regions than others.
Low-lying pastures which are now and then flooded are best suited for
the development of the disease, since the fluke eggs have a better
chance of developing upon them. This is because at certain times of the
year there are in such spots numerous submerged plants on which
_cercariæ_ encyst, and which, later on, when the pools dry up, are
nibbled by sheep. It seems that the _cercariæ_ mostly attach themselves
to certain kinds of plants, hence the local nature of the disease, which
of course also depends on the presence of _Lymnæus truncatulus_ (Fig.
142, _i_).
Fluke disease first makes itself apparent in the sheep one or two months
after infection. If the sheep does not die the parasites often remain in
its liver till the next spring. The liver of a single sheep may contain
from 200 to 250 individuals. They live in the various branches of the
bile-duct, and the irritation they set up first causes an unusual
quantity of blood to flow to the liver. Inflammation and internal
bleeding may result, and gall stones are deposited in the thickened
walls of the biliary passages. The flow of bile from the liver is made
difficult or impossible. Later on, the parasites draw so much blood from
the liver that this organ is very insufficiently nourished, so that no
more bile is secreted, and the liver substance shrivels up. In
consequence of these changes the digestion is incomplete, and the whole
body of the sheep is very insufficiently nourished. The diseased sheep
are therefore sluggish and enfeebled; they eat little, but drink a great
deal, and suffer from poverty of blood. Their wool gets dry, and does
not hold together, and they become emaciated. Dropsy and diarrhœa set
in, and jaundice often makes its appearance. If a sheep is infested by a
tolerably large number of flukes, death is almost always the final
result.
“The best _preventative_ is a trustworthy and careful shepherd,”[5] who
will not let the sheep feed in spots where previous experience shows the
disease is usually contracted during damp summers. Furthermore, sheep
badly infested should be slaughtered as soon as possible, so that the
flukes they contain may not get to the egg-laying stage. Sick sheep
should be fed with nourishing food (various kinds of hay, oats; with
addition of cooking salt), so that young sheep containing but few flukes
may get back their strength, while this treatment will make the badly
infested sheep die more quickly.
Footnote 5:
Spinola.
Oxen are also frequently affected by fluke disease in many regions,
other animals to a less extent.