Part 155

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.

In general, the frequency of occurrence of intestinal worms is proportioned to the extent of use of uncooked or insufficiently cooked meats, the drinking of unfiltered standing waters, uncleanly habits, and the intimacy of association with domestic animals. It therefore follows that important prophylactics against infection by parasites are properly-cooked food, the use of spring or freely-running water or filtered standing water, cleanly habits, and the avoidance of intimacy with domestic animals.

The Cestodes, or Tape-worms.

Tape-worms in the mature condition inhabit the intestines of vertebrate animals, and are usually conspicuous for their long, tape-like appearance and jointed character. In the juvenile or larval state they infest the various organs, except the interior of the intestinal canal, of both vertebrates and invertebrates, and in this condition are so diminutive and inconspicuous that until a comparatively recent period they for the most part remained unnoticed, and when known their relationship with the mature forms was not recognized.

The mature tape-worm, as ordinarily observed, is a long, soft, flat, white worm, which from its resemblance has received its familiar name. {932} It has a small head, succeeded by a short, more slender neck gradually widening into the body, which is divided transversely into segments. These, which are usually called joints or links, and also named proglottides, are so many individuals, and finally become separated to hold an independent existence. The tape-worm clings to the mucous membrane of the intestine by its head, which is provided for the purpose with suckers, and in many cases also with circlets of hooks. The segments of the body are incessantly produced by gradual growth and successive division of the neck, and as they enlarge they become more distinct and develop within a bisexual generative apparatus for each. The worm has neither mouth nor intestine, but is nourished by imbibition from the surrounding liquid in which it lies constantly bathed. A pair of longitudinal vessels commences in the head and extends throughout the body, one on each side, and in some genera is joined by a transverse vessel at the fore and back part of every segment. The mature segments have no body-cavity, but are occupied with a complex bisexual generative apparatus, which is self-impregnating. Finally the uterus, usually much ramified, becomes especially conspicuous through distension with eggs, and the rest of the organs for the most part become atrophied. The ripe segments successively detach themselves from those in advance, often singly and not infrequently several linked together. In this condition, often in lively movement, they are discharged with the feces, and thus commonly render themselves obvious to their host. Subsequently they may continue to live a brief period externally in a non-parasitic condition. Ordinarily, in moist excrement, or in water or similar materials, they will remain alive for several days.

After the discharge of the tape-worm segments, together with the eggs which had been previously laid by the latter and those still contained within them, any or all may be swallowed by animals feeding in places where the infected excrement has been deposited. When the proglottides and eggs are taken into the stomach they are digested and the embryos or proscolices are liberated.

The embryo or proscolex of the tape-worm is a microscopic spherical or oval body, provided at one pole with three pairs of divergent spicules, by which it is enabled to penetrate the walls of the stomach or intestine of its host. From these positions the embryo migrates either directly or through the blood-vessels to some other organ, most frequently the liver or the muscles. Having reached its destination, it becomes fixed in position, and for a time remains comparatively quiescent, but undergoes further development. The embryo loses its spicules and is transformed into the larval form or scolex. In most species of tape-worms the scolex is simple or individual in character, and consists of a head like that of the parent or mature worm, with a neck ending in a capacious cyst, within which the head and neck are inverted. In this form the scolex is contained in a sac of connective tissue induced by the presence of the parasite. Such sacs, frequently observed imbedded in the flesh, liver, lungs, and other organs of animals, are familiarly known as measles. In this condition the scolices of certain tape-worms have long been known, but as their relationship was not recognized, they were viewed as distinct species of parasites and described as cysticerci. In other species of tape-worms the scolex is of compound character; that is to say, the embryo {933} in its further development gives rise to the production of one or more groups of individuals in conjunction. The compound scolex thus forms a sac or a group of sacs, the basis of hydatid tumors. These occur of various sizes, even up to that of a child's head, and may occupy any organ of the body. They consist of a spherical sac or group of sacs, simple in character or containing others, ranging in size from that of a mustard-seed to that of a marble, or larger to that of a walnut, enclosed in an envelope of connective tissue induced by the presence of the parasite. The sacs are filled with liquid, and have, attached within or free and floating, or less frequently attached without, variable numbers of little white grains, which on examination with the microscope exhibit the same constitution as the simple scolex above described. As in the case of the cysticerci of measles, the scolices of hydatids have long been known, but as their relationship with the mature forms was unrecognized until lately, they were regarded as distinct parasites and described as echinococci and coenuri. Sometimes the compound scolex fails in development further than the production of the sacs, which then constitute the so-called acephalocysts.

Measles with their occupants, when retained in the muscles or other organs, ordinarily undergo no further development, but ultimately, after some months to a year or two, undergo degradation. The larva or scolex dies and atrophies; the measle degenerates, and often becomes the focus of calcareous deposit, shrinks to a little cicatrix, and may finally disappear. Of a more serious nature is the tape-worm embryo which produces the hydatid tumor. With the increase of this, proportioned to the production of sacs and scolices, it may become so large as greatly to interfere with the function of the organ it occupies, and according to the nature of this organ will be the gravity of the affection.

When, however, the flesh or other parts of animals affected with measles or hydatids containing active scolices are used as food in a raw or insufficiently cooked state, the meats are digested in the stomach and the scolices liberated to pursue their further development. Passing into the small intestine, the active scolex everts its head from its caudal sac, which atrophies and disappears, and the parasite attaches itself to the mucous membrane, and rapidly develops and grows into the conspicuous and familiar form of the adult tape-worm. The duration of life of the latter while maintaining its position in the intestine is uncertain, but under favorable circumstances it commonly continues for years, and thus, with the incessant production of ripe segments charged with eggs, it becomes a constant focus of infection.

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Three species of tape-worm in the mature condition are common parasites of man, living in the small intestine. They are the Tænia saginata, Tænia solium, and Bothriocephalus latus.

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TÆNIA SAGINATA.--SYNONYMS: Tænia mediocanellata; Beef tape-worm; Unarmed tape-worm; Fat tape-worm.

Larval condition: Cysticercus saginata; Beef measle-worm.

This, which is now regarded as the most common tape-worm of man, is named the beef tape-worm because it is derived from the beef used as food. In the mature condition it lives only in the small intestine of {934} man, and in the juvenile condition it lives in the ox. Its frequency is proportioned to the prevalence of the custom of eating beef in a raw or insufficiently cooked state, conjoined with the careless habit of leaving human excrement in pastures where it is accessible to cattle.

The mature beef tape-worm is commonly observed as a soft, yellowish-white, thickish, band-like worm, ranging from six to twenty feet or more in length. The head, about the size of a yellow mustard-seed, is rounded quadrate and provided with four equidistant hemispherical suckers. Succeeding the head is a short, slightly narrower, flattened neck, which merges into the gradually widening and segmented body. The segments, at first narrow fore and aft and several times wider than the length, become successively larger, proportionately longer, more distinct, and quadrate in outline; and finally the length may exceed the breadth two or three times. A full-grown tape-worm may possess twelve hundred segments and more, and specimens are recorded as reaching a length of thirty feet. The larger segments measure from a quarter of an inch to an inch long and from three to four lines wide. The larger or riper segments exhibit on one border, irregularly alternating on the two sides, at or near the middle, a papilla in which is the external aperture of the genital apparatus. In the fully-ripe segments the uterus, distended with eggs, may be obscurely seen through the wall of the body, but is rendered more visible by drying the segments, moderately compressed, between two pieces of glass. It appears as a long, narrow, white or brownish median line or tube, giving off laterally numerous short, transverse, more or less branching tubes.

The worm in its usual position lies along the course of the intestine in loose coils, and exhibits lively movements, alternately shortening and elongating, expanding and contracting the head, and protruding and retracting the suckers. The ripe segments spontaneously detach themselves, and may be found scattered along the large intestine ready to be discharged with the excrement, or, as is sometimes the case, they may spontaneously creep from the anus. Rarely more than a single worm infests a person at the time. The species is of rapid growth. According to Perroncito, quoted by Cobbold, a mature worm was reared from a beef measle, swallowed by a student, in fifty-four days.

It is estimated that the number of eggs in the mature segments of the beef tape-worm amounts to about 35,000. As the full-grown worm may consist of 1200 segments, and there is reason to believe these are renewed several times annually, we learn that the whole number of eggs produced by a single individual is enormous. The ripe segments, attached to the parent or becoming spontaneously detached, lay their eggs in the intestine to be discharged with the feces. When more or less emptied they shrink and appear reduced in size, and in this condition are expelled or spontaneously creep from the anus. If the ripe segments are forcibly expelled and are alive, they will lay their eggs in the feces externally. The ripe eggs are brown, oval, about 0.03 mm. long, and have a thick shell, with an outer vertically striated envelope.

As previously intimated, the common source of the beef tape-worm in man is the use of raw or insufficiently cooked beef affected with measles. The ox becomes infested by swallowing the eggs, or, it may be, even the entire segment, of a tape-worm deposited with feces in the {935} pastures of cattle. The measles usually occur in the muscles, including the heart, though they have also been noticed in the liver and lungs. They appear, in beef, as oval, whitish bodies from the size of a mustard-seed to that of a pea. They consist of a sac of connective tissue containing the larval tape-worm or cysticercus. Measles under ordinary circumstances are seldom noticed in beef, and when they occur are commonly few in number.

According to the latest authorities--Leuckart, Cobbold, Stein, and others--the beef tape-worm is the most common of the cestodes which infest man. Until within about thirty years it was generally not distinguished from the pork tape-worm, and this was accordingly regarded as the most common human species. Since the writer distinctly recognized the beef tape-worm within the last twenty years, all the specimens of Tæniæ, from people of Philadelphia and its vicinity, that have been submitted to him for examination--perhaps in all about fifty--have appeared to belong solely to Tænia saginata. The prevalence of this species with us is no doubt due to the common custom of eating underdone or too rare beef, while the pork tape-worm is comparatively rare, as with us pork is only used in a well-cooked condition.

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TÆNIA SOLIUM.--SYNONYMS: The Pork tape-worm; Solitary tape-worm; Armed tape-worm.

Larval condition: Cysticercus cellulosæ; Pork measle-worm.

Until a recent period this species was generally regarded as the most common tape-worm of man--a view which in great measure was due to the circumstance that the beef tape-worm was not distinguished from it. It was called the solitary tape-worm, still expressed by the specific name, from the impression that it rarely occurred otherwise than single at a time in a person. This has also proved to be incorrect, likewise due to the two kinds of tape-worms having been confounded together; for while the beef tape-worm most commonly occurs solitary, the pork tape-worm not unfrequently occurs with several together.

The species is now appropriately named the pork tape-worm, as indicating its common source--pork used as food. The frequency of the parasite is proportioned to the prevalence of the custom of using pork in a raw or imperfectly cooked state, conjoined with that of depositing excrement where it may be accessible to hogs. In the mature condition the pork tape-worm is peculiar to man and lives in the small intestine, but in the larval condition, though especially infesting the hog, it also occasionally infests man, and lives in any organs of the body, but mostly the muscles, liver, and lungs.

The mature pork tape-worm, as commonly seen, is a soft white, thin, band-like worm, from five to ten feet long and about four lines where widest. The head is spheroid, about the size of that of an ordinary pin, and smaller than that of the beef tape-worm. It is furnished with four hemispherical cup-like suckers, and the summit forms a blunt papilla armed with a double circle of twenty-five or twenty-six hooks. The neck is narrow, thread-like, about an inch long, and merges into the segmented body, which gradually widens to the extent mentioned. The segments, at first much wider than long, as they successively enlarge also become more distinct and proportionately longer, so that the more {936} posterior ripe ones are as long as they are wide, and often longer, though not to the same degree as in the beef tape-worm. The genital papilla, with its external aperture, is marginal as in the latter. The fully-developed uterus is quite distinctive in character from that of the beef tape-worm. The median tube is coarser, and the lateral branches are likewise coarser, much fewer--half the number or fewer--less branched, and less crowded. The ripe and often spontaneously detached segments are commonly longer than broad, more or less elliptical in outline, with truncated ends, and usually measure about half an inch in length by about a third in breadth. The ripe eggs resemble those of the beef tape-worm, but are usually spheroid in shape.

The common source of the pork tape-worm is pork affected with measles eaten in the raw or insufficiently cooked state. The hog becomes affected with measles when it has access to human excrement containing eggs and ripe segments of the tape-worm, which it eats with avidity. The eggs, with their already developed embryos, when swallowed, undergo the same series of transformations and course as those indicated in the account of the beef tape-worm. Pork affected with measles is much more common than beef affected in the same way, and is frequently a subject of ordinary observation. From the difference in habit of the hog and ox this is what might have been suspected; and the fact that the beef tape-worm is more common than the pork tape-worm is to be explained from the circumstance that fresh beef is in more general use than pork, and is usually employed less thoroughly cooked.