[83] _Hygiea_, LXXV (1913), 1. [84] _Progrès méd._, 3d series, XXVI (1910), 25. [85] Ledingham and Arkwright, _The Carrier Problem in Infectious Diseases_, pp. 152-53. [86] _Jour. Hyg._, XI (1911), 24. [87] _Münch. med. Wchnschr._, LIV (1907), 979. [88] See, for example, H. Langer and Thomann, _Deutsche med. Wchnschr._, XL (1914), 493. [89] _Ztschr. f. Infektionsk. ... d. Haustiere_, VIII (1910), 237. CHAPTER VII ANIMAL PARASITES Not only pathogenic bacteria but certain kinds of animal parasites sometimes enter the human body in or upon articles of food. One of the most important of these is the parasite causing trichiniasis. TRICHINIASIS Trichiniasis or trichinosis is a disease characterized by fever, muscular pains, an enormous increase in the eosinophil blood corpuscles, and other more or less well-defined symptoms; at the onset it is sometimes mistaken by physicians for typhoid fever. The responsible parasite is a roundworm (_Trichinella spiralis_, formerly known as _Trichina_) which is swallowed while in its encysted larval stage in raw or imperfectly cooked pork.[90] The cysts or envelopes in which the parasites live are dissolved by the digestive fluids and the young larvae which are liberated develop in the small intestine to the adult worm, usually within two days. The young embryos, which are produced in great numbers by the mature worms, gain entrance to the lymph channels and blood stream, and after about ten days begin to invade the muscles--a procedure which gives rise to many of the most characteristic symptoms of the infection. It is estimated that in severe cases as many as fifty million embryos may enter the circulation. The parasites finally quiet down and become encysted in the muscle tissue and the symptoms, as a rule, gradually subside. Ingestion of a large number of parasites at one time often results fatally, the mortality from trichiniasis being on the average somewhat over 5 per cent and rising in some outbreaks to a much higher figure (30 per cent). On the other hand, many infections are so light as to pass unnoticed. Williams[91] found _Trichinella_ embryos present in 5.4 per cent of the bodies of persons dying from other causes. Such findings are considered to indicate that occasional slight _Trichinella_ infections even in the United States are quite common. This might indeed be expected from the frequent occurrence of infection in swine, about 6 per cent of these animals being found to harbor the parasite. [Illustration: FIG. 7.--Trichinae encysted in intercostal muscle of pig. (About 35×1.) (After Neumann and Mayer.)]