Whatever be the precise degree of danger from food infection by healthy paratyphoid-carriers (man or domestic animals), it is obvious that general measures of care and cleanliness will be more or less of a safeguard. As with typhoid fever so all outbreaks of paratyphoid should be thoroughly investigated in order that the sources of infection may be found and eliminated. The possible connection of rats and mice with these outbreaks should furnish an additional incentive to lessen the number of such vermin as well as to adopt measures of protecting food against their visits.
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.
Fig. 7.—Trichinae encysted in intercostal muscle of pig. (About 35×1.) (After Neumann and Mayer.)
The specific symptoms (such as the muscular pain) of trichiniasis may be due in part to mechanical damage of the muscle tissue, but it is also probable that they are partly due to toxic products exuded by the worms and partly to the introduction of alien protein material—the protein of the worm—into the tissues. Secondary bacterial infection is also a possibility, but there is little evidence to prove that this is an important factor in most cases of trichiniasis. The various stages observed in the progress of the disease are plainly connected with the different phases of the worm's development—the initial localization in the intestines, the invasion of the muscles, and the final encystment.