INTESTINAL WORMS.
BY JOSEPH LEIDY, M.D.
All animals, except in general the simple cell-forms constituting the sub-kingdom of protozoa, under ordinary circumstances are more or less liable to be infested with others, called parasites, which commonly live at the expense of their hosts, frequently with little or no inconvenience, but often causing discomfort and suffering even unto death. Parasites are distinguished as external and internal, the two being mostly of a widely different character. The former chiefly pertain to the division of arthropoda, or animals with jointed limbs, as exemplified by lice, fleas, and flies of the class of insects, mites of the class of arachnides, and epizoans and isopods of the class of crustaceans.
Internal parasites, from their usual habitation named entozoa, are commonly observed in the intestines of animals, and hence their distinction as intestinal worms. The name has proved to be appropriate, for investigations have shown that most entozoa, observed from time to time in other parts of the bodies of animals, pass part of their life in the intestinal canal of the same or of some other animal.
By far the greater number of entozoa are peculiar animals, constituting the chief part of the scolecides, an extensive group of the sub-kingdom of vermes or worms. Of this group they comprise the orders of CESTODES, or tape-worms; the ACANTHOCEPHALI, or thorn-headed worms; the TREMATODES, or fluke-worms; and the greater portion of the NEMATODES, or thread-worms. Many entozoa also belong to the protozoa, but these, so far as relates to man in a medical point of view, appear unimportant, and will therefore not here enter into consideration.
In the course of their life entozoa undergo changes of form and condition, and pass these in different organs of the same or of different animals, and it may be for a brief period externally or in a non-parasitic state. In many instances, as in the tape-worms and the fluke-worms, the transformations accompanying the changes are of so extraordinary a character that until their life-history was investigated the successive metamorphoses were viewed as distinct animals. Mostly, the entozoa pass one stage of existence within the intestine of some animal, and another stage in different organs of other animals. Many, perhaps most species, in each stage are peculiar to one or a few nearly-related animals, but others of the same kind infest a number of different animals. The animals infested by the same parasite may be remotely as well as nearly related. Thus the Tænia saginata, or beef tape-worm, in the mature state lives in the small intestine of man only, but in its juvenile or larval condition in the flesh meat of the ox. The Tænia elliptica, the common tape-worm of the intestine of the dog, in the larval condition lives in the louse of this animal. The liver-fluke, Distomum hepaticum, occasionally found in the liver of man, but of common occurrence in the sheep, to which it proves so destructive in the affection known as rot, in the juvenile condition lives in a little fresh-water snail of the genus Lymneus. The guinea-worm, Filaria medinensis, which in the mature state is found beneath the skin of man, in the larval condition inhabits the minute crustacean cyclops of stagnant waters.
As would be reasonably supposed, entozoa commonly gain access to their hosts through the food and drink, though in the case of aquatic animals they also obtain entrance directly through the integument from the surrounding medium. So long as they remain in the intestinal canal they may occasion little trouble or inconvenience. When they are numerous in this position or proportionately large, according to their peculiar nature they may produce more or less suffering and even the most serious consequences. Generally, however, it is when they occupy other positions, to which they have migrated from the intestine, that they induce aggravated symptoms proportioned to their numbers and the nature of the organs they infest.
Many species of entozoa have been discovered in man, and most of them are peculiar in kind. Many are common, and, while some are widely extended, others are more or less restricted to certain localities. They are variable in their frequency, largely proportioned to the prevalence of habits which are favorable to their transmission, and which, though under control, are more or less disregarded. Some species are so rare in their occurrence that they seem to be accidental, and therefore of comparatively little interest to the physician.