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The fluke worm goes through a very complicated life cycle arriving at
maturity after numerous changes or stages of development, which although
well-known may be very briefly noticed here. The researches of Thomas and
Leuckhart have pretty well cleared up the life history of the fluke. The eggs
provided with an operculum are expelled with the faeces; within 4 to 6 weeks,
an embryo is hatched, and this has a ciliated skin which is provided with a
sting at one end. This embryo penetrates into the respiratory tract of
some molluscs, and chiefly into the Lymnæa peregra, and L. truncatula and
it appears to be probable into some other in this country as L. truncatula does
not appear to be very common, here. After a period varying from three to six
weeks the embryo is transformed into a "sporocyst" provided with germinative
cells which are developed in order to form Redia, these latter in undergoing
a new change give birth directly according to Leuckhart, indirectly according
to Thomas to the "cercaria" small microscopic beings provided with a tail,
which live at liberty in the water. A single distoma egg gives birth to nearly
a thousand cercaria.
Leuckhart states that the animals ingest the cercaria either by swallowing
the Lymnæa with the food, or by drinking water in which they are suspend-
ed. Thomas has demonstrated the fact very clearly that the cercaria leave
their host's body in order to swim in the water and fix themselves upon aquatic
plants or damp meadow grass. They lose the tail and become ball shaped and
surrounded by a clammy mass, so that they become cyst like having a diameter
of two or three millimetres and they are of snowy whiteness: They preserve
their vitality for several weeks. Animals ingest them by browsing the grass to
which they are adhering.
It is perhaps unnecessary to discuss the subject of how the flukes arrive
at the liver in this disease. Most of the worms when they have invaded
the liver remain in the biliary canals, but some penetrate the walls of these
canals and enter into the liver substance which they destroy and furrow, they
pass through Glissons capsule as well as the serious membrane which covers it
and may produce perihepatitis or a peritonitis, others penetrate into the
branches of the portal system, others finally enter the central hepatis veins and
thence pass to distant parts of the body. After passing through the right
side of the heart the parasites reach the lung, where they produce haemorrhagic
centres and also make caverns with bloody contents. This is especially the
case with cattle in which cysts and tubercules are found which are of the size
of a hen's egg at times, with thick walls which are calcified at times and contain
the parasite swimming in a brownish viscous fluid. Morot has found these para-
sites in the slaughter-houses of Troyes in 4 per cent. of the cattle slaughtered,
and they are very common in this country.
The main post-mortem changes are found in the liver, they dffer mainly ac-
cording to the period at which they are observed. In most cases the left lobe
is first affected. The liver is enlarged, its weight may be doubled, its edges are
rounded and it is changed in color, upon a yellow, gray, or dirty brownish ground
we observe dark red or dark reddish brown spots which are round, linear or bran-
ched. The surface of the liver presents a coarse appearance, is irregular, and
there is sometimes perihepatitis. We frequently observe pin head or millet
sized openings in the capsule through which a bloody dirty material oozes.
When an incision is made on the surface young flukes may be found immediately
under the capsule. On section we see changes in the biliary canals and liver
substance; there are many lacunar chambered spaces of a very irregular form
and full of bloody reddish brown material and fluke worms in an imperfectly
developed state. The biliary canals have ordinarily trebled or quadrupled in
size, in cattle they reach the diameter of one's thumb which gives the liver a
cystic appearance. In the dilated biliary canals we find fluke worms and their
eggs in great quantities. The mucous membrane of the canals owing to the
irritation produced by the rough skins of the worms becomes the seat of a
purulent catarrh, is considerably thickened and sometimes incrustated with
phosphate of lime.
In the branches of the portal system we may find thrombosis produced
by fluke worms floating free within the blood vessels. The gall bladder