Page:Popular Science Monthly Volume 23.djvu/766

746 the snails after nursing the young flukes through three generations distribute the tadpole cercariæ, which convey the infection back to the sheep, and it there inhabits the liver and causes the disease.

—Now that the life-history of the fluke is known, it is not difficult to comprehend the conditions necessary for its existence. There must be: (1) Fluke-eggs on the ground; (2) wet ground or water during warm weather—(3) the snail Limnæus truncatulus (?); and (4) sheep allowed to feed upon the infested ground.

Under the first condition it may be said that wherever fluked sheep are kept we shall have fluke-eggs. In some districts flukes are always to be found, and where the conditions are the most favorable a sudden outbreak may be expected. The disease sometimes appears in quarters where it was previously unknown, and may have been introduced in manure containing fluke-eggs, or adhering to the feet of cattle, dogs, or men. The eggs and young flukes (embryos) may be conveyed by running streams, floods, etc. Other animals than sheep are infested with the parasite, and rabbits and hares may be the means of introducing the plague. The production of the fluke-eggs may be prevented by killing the sheep so soon as they are found suffering from the "rot." If there is a suspicion that a flock is attacked, one of the members exhibiting the strongest signs may be killed and its liver examined. If rotted sheep are kept, they should be on dry ground, where the fluke-eggs can not fall on wet land, or be swept into brooks by the descending rains. The manure of infested sheep should not be placed on wet land, and the livers of rotted sheep ought to be destroyed or deeply buried.

The remedy for the second condition is a simple one, but not always easy of application. Wet land should be thoroughly drained, and, besides preventing the rot, it will greatly improve the pasturage. When draining can not be done, lime or salt may be scattered over the surface. These substances will destroy the embryos, the more developed encysted form of the fluke, and the snails which serve as hosts. The salt or lime should be applied in early summer, when the young flukes are present in the greatest numbers.

There seems to be only one snail in England which is host to the young flukes, and the accounts of Professor Thomas in exposing other species of Limnæus to the embryos of the fluke are most interesting. The Limnæus truncatulus (Fig. 5) is said not to exist in the United States, though several other kinds of snails belonging to the same large genus are found here. It is probable that with us some other species than Limnæus truncatulus serves as the host of the intermediate forms of our liver-flukes. Draining the wet land will reduce the number of snails; and dressings of lime or salt, as above mentioned, also destroy them. The lime should be scattered especially on or near marshy places and along ditches. If a pasture has been flooded,