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.

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.

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.

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.

UNCINARIASIS

Hookworm infection (uncinariasis, ankylostomiasis) is commonly caused by infection through the skin of the feet, but the possibility of mouth infection cannot be disregarded, and in regions where hookworm disease exists methods of guarding against food contamination should be practiced, as well as other precautions. Billings and Hickey[93] believe that hookworm disease is contracted by unconscious coprophagy (from raw vegetables) much more frequently than is generally supposed.

OTHER PARASITES

A number of other parasitic worms (e.g., _Strongyloides_, _Ascaris_ or eelworm, and _Oxyuria_ or pinworm) may conceivably enter the human body in contaminated food, and while, as in hookworm disease, other modes of infection are probably more important, the liability to occasional infection by uncooked food must not be overlooked.

Various forms of dysentery or diarrhea have been attributed to infection with _Giardia (Lamblia) intestinalis_. Observations made by Fantham and Porter[94] upon cases contracted in Gallipoli and Flanders have given support to this view. Strains of this parasite of human origin have been shown to be pathogenic for mice and kittens. It is considered possible that these animals may act as reservoirs of infection and spread the disease by contamination of human food.

FOOTNOTES:

[90] The consumption of raw sausage made with pig meat is particularly likely to give rise to trichiniasis.

[91] _Jour. Med. Research_, VI (1901), 64.

[92] Edelmann, Mohler, and Eichhorn, _Meat Hygiene_, 1916, p. 182.

[93] _Jour. Amer. Med. Assoc._, LXVII (1916), 1908.

[94] _Brit. Med. Jour._, II (1916), 139.