Swine become infected with this parasite by eating scraps of infected meat, or the offal of their own kind, or by eating infected rats. The rat, through its cannibalistic propensities, becomes infected frequently, and is one of the chief factors in the wide dissemination of the disease. Human infection is practically accidental and self-limited; biologically speaking, man as a host does not enter into the calculations of the parasite.
Treatment of established trichiniasis infection is palliative, not truly remedial. The parasites, once inside the body, cannot be materially affected by the administration of any drug. While cure of trichiniasis is thus difficult, if not impossible, prevention is very simple. The thorough cooking of all food is sufficient to preclude infection. This relatively simple means of destroying the larvae is a more certain as well as less expensive method of preventing infection than is the laborious microscopic examination of the tissues of every slaughtered hog. In Germany between 1881 and 1898 over 32 per cent of 6,329 cases of trichinosis that were investigated were traced to meat that had been microscopically examined and passed as free from trichinae.[92] On the other hand, thorough cooking removes all possibility of danger.
TENIASIS
Various tapeworm or cestode infections are contracted by eating meat containing the parasite. Particular species of tapeworm usually infest the flesh of specific hosts, as Tenia saginata in the beef and Tenia solium in the hog. The dwarf tapeworm, Hymenolepis nana, develops in rats, and the human infections with this parasite occasionally observed are probably caused by contamination of food by these animals.
Fig. 8.—Cysticercus cellulosae in pig's tongue. (After Neumann and Mayer.)
Sometimes the existence of the tapeworm in man is restricted to the alimentary tract and the symptoms vary from trivial to severe, but sometimes (Tenia solium) the larval stage of the tapeworm invades the tissues and becomes encysted in various organs (brain, eye, etc.), where, as in the case of cerebral infection, it may result fatally. The encysted larva of Tenia solium was at one time regarded as an independent animal species and named Cysticercus cellulosae. The condition known as "measly pork" is produced by the occurrence of this encysted parasite.
So-called hydatid disease is due to the cystic growth produced by the larva of a species of tapeworm (Echinococcus) inhabiting the intestine of the dog. Human infection may be caused by contaminated food as well as more directly by hands soiled with petting infected dogs. Several varieties of tapeworms infesting fish, especially certain fresh-water species, may be introduced into the human body in raw or partly cooked fish.
Methods for the prevention of tapeworm infection include the destruction of the larvae by heat—that is, the thorough cooking of all meat and fish—and the minimization of close contact with those animals, such as the dog and cat, that are likely to harbor parasites. Cleanliness in the preparation and serving of food, and attention to hand-washing before meals, and especially after touching pet animals, are necessary corollaries.