6. What is the damage from the sheep-tick? Give treatment.
7. Describe the injury from scabies and mange.
8. Give treatments for these diseases.
9. Mention the several poultry mites and tell how to treat them.
CHAPTER XXV
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 escape from the cyst. The daughter redia or cercaria, as they are now termed, leave the body of the snail and finally become encysted on the stems of grass, cresses and weeds. When taken into the digestive tract of the animal grazing over infested ground, the immature flukes are freed by the digestive juices. They then pass from the intestine into the bile ducts. The period of development varies from ten to twenty weeks; each sporocyst may give rise to from five to eight redia and each redia to from twelve to twenty cercaria.