Fig. 19.—The common liver fluke (Fasciola hepatica).
Two species of flukes occurring in the liver and lungs are known to affect cattle in the United States. These parasites are flat, leaf-like worms; one of them, the common liver fluke (Fasciola hepatica, fig. 19), is less than an inch in length, while the other, the large American fluke (Fasciola magna, fig. 20), is considerably larger when full grown. In their life history these flukes depend on snails as intermediate hosts. At a certain stage of development the young flukes leave the snails, become encysted on stalks of grass (fig. 21), or fall into drinking water, and finally may be swallowed by cattle. Stiles writes as follows:
Fig. 20.—The large American fluke (Fasciola magna).
Flukes may produce a serious, often fatal, disease, more especially in younger animals. The symptoms are somewhat similar to those produced by worms in the stomach. The first symptoms are generally overlooked, the disease not attracting attention until the appetite is diminished; rumination becomes irregular, the animals become hidebound, and the coat dull and staring. The staring coat is due to the contraction of the muscles of the hair follicles. The visible mucous membranes become pale, eyes become dull, there is running at the eyes, and the animal gradually becomes emaciated. As the disease advances the milk supply is lessened, fever appears, there is generally great thirst, but the appetite almost ceases; edematous swellings appear on the belly, breast, etc.; diarrhea at first alternates with constipation, but finally becomes continuous. The disease lasts from two to five months, when the most extreme cases succumb.
Most of the German cattle are said to be infested with liver flukes, but even when a large number are present the nourishment of the cattle is not disturbed. Thickening of the gall ducts, so that a so-called "Medusa's head" forms on the surface of the liver toward the stomach, appears in even well-nourished animals; even in cases of a cirrhosis of the liver it is seldom that any effect upon the cattle's health can be noticed, and so long as a portion of the liver tissue about twice the size of the fist remains intact, the nourishment of the animal may be comparatively good. It is rare that one sees a generalized edema in slaughtered cattle as a result of fluke invasion, and even in the heaviest infections of young cattle only emaciation is noticed.
Treatment.—Medicinal treatment is unsatisfactory. The disease may be prevented to a considerable extent by giving animals plenty of salt, and by introducing carp, frogs, and toads into infected districts; these animals destroy the young stages of the parasite and feed upon the snails which serve as intermediate hosts.