A Manual of Clinical Diagnosis

by James Campbell Todd

CHAPTER VI

ANIMAL PARASITES

Animal parasites are common in all countries, but are especially abundant in the tropics, where almost every native is host for one or more varieties. Because of our growing intercourse with these regions, the subject is assuming increasing importance in this country. Many parasites, hitherto comparatively unknown here, will probably become common.

Some parasites produce no symptoms, even when present in large numbers. Others cause very serious symptoms. Only those which have clinical interest will be considered here. The illustrations will give a better idea of their appearance than any description. They belong to three classes: I. Protozoa. II. Vermes. III. Arthropoda.

I. PROTOZOA

1. Amoeba Coli Dysenteriæ.—This organism is found, often in large numbers, in the stools of tropical dysentery and in the pus and walls of hepatic abscesses associated with dysentery, and is generally regarded as the cause of the disease. It is a colorless, granular cell, 20 to 40 µ in diameter (Fig. 91). It contains one or more distinct vacuoles; a round nucleus, which ordinarily is obscured by the granules; and frequently red blood-corpuscles and bacteria. When at rest, its shape is spheric; but upon a warm slide it exhibits the characteristic ameboid motion, constantly changing its shape or moving slowly about. This motion is its most distinctive feature. Other amebæ, resembling the pathogenic variety but smaller (10 to 15 µ in diameter), are sometimes found in normal feces.

FIG. 91.—Amoeba coli in intestinal mucus, with blood-corpuscles and bacteria (Lösch).

When the presence of amebæ is suspected, the stool should be passed into a warm vessel and kept warm until and during the examination. A warm stage can be improvised from a plate of copper with a hole cut in the center. This is placed upon the stage of the microscope, and one of the projecting ends is heated with a small flame. Amebæ are most likely to be found in grayish or blood-streaked particles of mucus. Favorable material for examination can be obtained at one's convenience by inserting into the rectum a large catheter with roughly cut lateral openings. A sufficient amount of mucus or fecal matter will usually be brought away by it.

2. Trichomonas Vaginalis.—The acid discharge of catarrhal vaginitis sometimes contains this parasite in abundance. It is oval or pear-shaped, one to three times the diameter of a red blood-corpuscle in length, and has a cluster of flagella at one end (Fig. 92). It is not unlike a pus-corpuscle in size and general appearance, but is actively motile. When in motion the flagella are not easily seen. No pathogenic significance is ascribed to it. Other varieties of the genus have been found in the feces, the urine, and the sputum.

FIG. 92.—Trichomonas vaginalis (after Kölliker and Scanzoni).

A similar but somewhat smaller organism, Cercomonas hominis (Fig. 93), has been found in the feces in a variety of diarrheal conditions and in from 10 to 25 per cent. of healthy persons in tropical regions.

FIG. 93.—Cercomonas hominis: A, Larger variety; B, smaller variety (Davaine).

In urine or vaginal discharges these organisms might be mistaken for spermatozoa by one who is entirely unfamiliar with the appearance of either.

3. Paramoecium Coli (Balantidium Coli).—This parasite is an occasional inhabitant of the colon of man, and sometimes produces diarrhea. It is an oval organism, about 0.1 mm. long, is covered with cilia, and contains a bean-shaped nucleus, two contractile vacuoles, and variously sized granules (Fig. 94).

FIG. 94.—Balantidium (Paramoecium) coli (Eichhorst).

4. Hemosporidia.—This is a large group of parasites with two life-cycles: one in the blood-corpuscles or plasma of a vertebrate host—man, mammals, birds, reptiles; the other in the body of some insect. The malarial parasite, already described; the organism (Pirosoma bigeminum) producing Texas fever in cattle; and the questionable parasite (Piroplasma hominis), which has been described as the cause of "tick fever" of Montana, belong to the group.

5. Trypanosomes have been mentioned ([p. 195]).

II. VERMES

1. Cestoda.—Tape-worms are very common parasites of both man and the animals. The most important are Tænia saginata, Tænia solium, Bothriocephalus latus, and Tænia echinococcus. They all pass a larval stage in the body of an intermediate host. In the adult stage they consist of a linear series of flat, rectangular segments (proglottides), at one end of which is a smaller segment, the scolex or head, especially adapted for attachment to the host. The series represents a colony, of which the scolex is ancestor. The proglottides are sexually complete individuals, derived from the scolex by budding. With exception of the immature segments near the scolex, each contains a uterus filled with ova. The three tape-worms first mentioned are distinguished from one another mainly by the structure of the scolex and of the uterus ([Fig. 100]). The scolex should be studied with a low-power objective. The uterus is best seen by pressing the segment out between two plates of glass.

FIG. 95.—Tænia saginata (Eichhorst).

(1) Tænia Saginata or Mediocanellata (Fig. 95).—This, the beef tape-worm, is the common tape-worm of the United States. Its length sometimes exceeds twenty-five feet. The middle segments measure about one-fourth by one-half inch. The scolex is about the size of a pin-head, and is surrounded by four sucking discs, but has no hooklets (Fig. 96). The uterus extends along the middle line of the segment and gives off about twenty branches upon each side ([Fig. 100]). The larval stage is passed in the muscles of various animals, especially cattle, where it lies encysted (cysticercus stage).

The larva is ingested with the meat, its capsule is dissolved by the digestive juices, and it attaches itself to the intestinal wall by means of its suckers. It then develops into the mature worm.

The ova are present in the stools of infected persons, often in great numbers. They are spheric or ovoid, yellow in color, and have a thick, radially striated shell ([Fig. 101]). Their greatest diameter is 30 to 40 µ (about four or five times the diameter of a red blood-corpuscle). Vegetable cells, which are generally present in the feces, are often mistaken for them.

FIG. 96.—Head of Tænia saginata (Mosler and Peiper).	FIG. 97.—Head of Tænia solium (Mosler and Peiper).

(2) Tænia solium, the pork tape-worm is very rare in this country. It is usually much shorter than Tænia saginata. The scolex is surrounded by four sucking discs, and has a projection, or rostellum, with a double row of horny hooklets (Fig. 97). The uterus has only seven to ten branches ([Fig. 100]).

The ova closely resemble those of Tænia saginata, but are a little smaller ([Fig. 101]).

(3) Bothriocephalus latus, the fish tape-worm, is the largest parasite of man, sometimes reaching fifty feet in length, although generally not more than half so long. It is common in some countries of Europe and in Japan, but is very rare in this country. The head is not unlike the bowl of a spoon in shape. It is unprovided with either suckers or hooklets, but has two longitudinal grooves which serve the same purpose (Fig. 98). The uterus, which is situated in the center of the segment, is roset-shaped ([Fig. 100]).

FIG. 98.—Head of Bothriocephalus latus: a, a, Bothridies; b, neck (Blanchard).

The larval stage is found in fish, especially the pike.

FIG. 99.—Egg of Bothriocephalus latus, showing lid and yolk granules (photograph by F. C. Wood).

The ova are characteristic. They measure about 45 by 70 µ, are brown in color, and are filled with small spherules. The shell is thin, and has a small hinged lid at one end (Fig. 99).

Bothriocephalus latus is interesting clinically because it often causes a very severe grade of anemia.

FIG. 100.—Segments of—(1) Tænia saginata; (2) Bothriocephalus latus; (3) Tænia solium, showing arrangement of uterus.

FIG. 101.—Comparative size of eggs of intestinal parasites: a, Tænia solium; b, Tænia saginata; c, Ascaris lumbricoides; d, Trichocephalus dispar; e, Oxyuris vermicularis (after Strümpell).

FIG. 102.—Tænia echinococcus; enlarged (Mosler and Peiper).

(4) Tænia Echinococcus.—The mature form of this tape-worm inhabits the intestine of the dog and wolf. The larvæ develop in cattle and sheep ordinarily, but are sometimes found in man, where they give rise to echinococcus or "hydatid" disease. The condition is unusual in America, but is common in Iceland and Australia.

The adult parasite is 2.5 to 5 mm. long, and consists of only four segments (Fig. 102). It contains many ova. When the ova reach the digestive tract of man, the embryos are set free and find their way to the liver, lung, or other organ, where they develop into cysts, thus losing their identity. Other cysts, called "daughter cysts," are formed within these. The cyst-wall is made up of two layers, from the inner of which develop larvæ which are identical with the head, or scolex, of the mature parasite. These are ovoid structures about 0.3 mm. long. Each has four lateral suckers and a rostellum surmounted by a double circular row of horny hooklets. The rostellum with its hooklets is frequently invaginated into the body.

Diagnosis of echinococcus disease depends upon detection of scolices, free hooklets, or particles of cyst-wall, which is characteristically laminated and usually has curled edges. These can be found in fluid withdrawn from the cysts, or, less frequently, in the sputum or the urine when the disease involves the lung or kidney (Figs. [55] and 103).

FIG. 103.—Contents of echinococcus cyst, showing hooklets, scolices, and cholesterin crystals (Wood).

The cyst-fluid is clear, between 1.002 and 1.010 in specific gravity, and contains a notable amount of sodium chlorid, but no albumin.

2. Nematoda.—(1) Ascaris Lumbricoides.—The female is 20 to 40 cm. long and about 6 mm. thick; the male, a little more than half as large. Their color is reddish or brown. They are the common "round-worms" so frequently found in children. Their habitat is the small intestine. Large numbers are sometimes present.

FIG. 104.—Ascaris lumbricoides (female) (Mosler and Peiper).

The diagnosis is made by detection of the worms or ova in the feces. The latter are generally numerous. They are elliptic, measuring about 50 by 70 µ, and have an unsegmented protoplasm (Fig. 105). The shell is thick, and is surrounded by an uneven gelatinous envelop which is often stained with bile.

FIG. 105.—Eggs of Ascaris lumbricoides (Mosler and Peiper).

(2) Oxyuris Vermicularis.—This is the "thread-worm" or "pin-worm" which inhabits the colon and rectum, especially of young children. Its presence should be suspected in all unexplained cases of pruritus ani. The female is about 1 cm. long; the male, about 0.6 cm. (Fig. 106).

FIG. 106.—Oxyuris vermicularis and egg: a, Natural size; b, egg (after Heller).

The worms are not infrequently found in the feces; the ova, rarely. The latter are best found by scraping the skin at the margins of the anus, where they are deposited by the female. They are asymmetrically oval, about 50 µ in length, and often contain a partially developed embryo.

(3) Filaria Sanguinis Hominis.—A description of this worm will be found in the chapter upon the Blood, [p. 194]. The embryos are sometimes found in urine and chylous fluids from the serous cavities. Their motion is then usually less active than when in blood. That shown in Fig. 107 was alive sixty hours after removal of the fluid. Embryos were present in the blood of the same patient.

FIG. 107.—Filaria sanguinis hominis (embryo) in chylous hydrocele fluid; length, 300 µ; width, 8 µ. A number of red blood-corpuscles also appear (studied through courtesy of Dr. S. D. Van Meter).

FIG. 108.—Trichinella spiralis (larvæ) from head of right gastrocnemius muscle; seventh week of disease (two-thirds objective; eye-piece 4) (Boston).

(4) Trichina (Trichinella) Spiralis.—This is a very small worm, not exceeding 3 mm. in length when fully developed. Infection in man occurs from ingestion of insufficiently cooked pork, which contains encysted embryos. These reach maturity in the small intestine. The female produces great numbers of young, which migrate to the voluntary muscles, chiefly near the tendinous extremities, and there become encysted.

Trichiniasis is generally accompanied by a marked eosinophilia. The diagnosis is made by teasing out upon a slide a bit of muscle, obtained preferably from the outer head of the gastrocnemius, the insertion of the deltoid, or the lower portion of the biceps. The coiled embryos can easily be seen with a two-thirds objective (Fig. 108).

FIG. 109.—Uncinaria duodenalis: a, Male (natural size); b, female (natural size); c, male (enlarged); d, female (enlarged); e, head; f, f, f, eggs (after v. Jaksch).

(5) Uncinaria.—The two varieties of this worm, Uncinaria duodenalis and Uncinaria Americana, are among the more harmful of the animal parasites. They inhabit the small intestine, usually in great numbers, and commonly produce a severe and often fatal anemia. Infection is common in subtropical regions, notably in Egypt, in some European countries, and, especially, in Porto Rico and the West Indies, where about 90 per cent. of the rural population is infected. It is much more common in the United States than was formerly supposed.

The adult worms are seldom found in the feces, but may appear after a dose of thymol followed by a brisk purge. They resemble Oxyuris vermicularis to the naked eye. Ova are usually present in enormous numbers. Those of Uncinaria duodenalis measure about 30 by 50 µ; of Uncinaria Americana, somewhat more. They have a thin, smooth, transparent shell, and their protoplasm is divided into 2, 4, 8, or more rounded segments (Fig. 110).

FIG. 110.—Four eggs of the New World hook-worm (Uncinaria Americana), in the one-, two-, and four-cell stages. The egg showing three cells is a lateral view of a four-cell stage. Greatly enlarged (after Stiles).

(6) Strongyloides Intestinalis.—Infection with this worm is by no means so rare in this country as the few clinical reports would indicate. It is very common in subtropical countries, notably in Italy and in southern China. It seems probable that the parasite is the cause of "Cochin China diarrhea," although some authorities regard it as harmless.

FIG. 111.—Strongyloides intestinalis: A, Mature female; B, rhabditiform larva; C, filariform larva (after Braun).

The adult worm, which reproduces by parthenogenesis, is about 2 mm. long. It inhabits the upper portion of the small intestine, but neither it nor the ova appear in the stool unless an active diarrhea exists. Ordinarily the eggs hatch in the intestine, and when infection is severe, embryos can be found in the feces in large numbers. These are the "rhabditiform embryos," which measure about 0.40 by 0.02 mm. They are actively motile, and are best found by making a small depression in the fecal mass, filling it with water, and standing in a warm place (preferably an incubator) for twelve to twenty-four hours. The embryos will collect in the water, and can be easily found with a two-thirds objective.

Outside the body the rhabditiform embryos develop into a free-living, sexually differentiated generation. The young of this generation are the more slender "filariform embryos" (Fig. 111). Infection can occur either through these embryos of the free-living generation, or by direct transformation of rhabditiform into filariform embryos and these into the parthenogenic parasitic adult.

(7) Trichocephalus Dispar (T. Trichiurus).—This, the "whip-worm," is 4 or 5 cm. long. Its anterior portion is slender and thread-like, while the posterior portion is thicker (Fig. 112). It is widely distributed geographically, and is one of the most common of intestinal parasites in this country. It lives in the large intestine, especially the cecum, with its slender extremity embedded in the mucous membrane. Whip-worms do not, as a rule, produce any symptoms, although gastro-intestinal disturbances, nervous symptoms, and anemia have been ascribed to them. They, as well as many other intestinal parasites, are probably an important factor in etiology of appendicitis, typhoid fever, and other intestinal infections. The damage which they do to the mucous membrane favors bacterial invasion.

FIG. 112.—Trichocephalus dispar; a, Female; b, male (natural size) (Heller).

The number present is usually small. The worms themselves are rarely found in the feces. The ova, which are not often abundant, are easily recognized. They are brown, ovoid in shape, about 50 µ long, and have a button-like projection at each end ([Fig. 101]).

III. ARTHROPODA

Most of these are external parasites, and the reader is referred to the standard works upon diseases of the skin for descriptions. The itch-mite (Acarus scabiei) and the louse (Pediculus capitis, corporis, vel pubis) are the more common members of the group.

A number of flies may deposit their ova in wounds or in such of the body cavities as they can reach, and the resulting maggots may cause intense irritation. Ova may be swallowed with the food and the maggots appear in the feces. Probably most important is the "screw worm," the larva of Compsomyia macellaria, infection with which is not rare in some parts of the United States. The ova are most commonly deposited in the nasal passages, and the larvæ, which may be present in great numbers, burrow through the soft parts, cartilage, and even bone, always with serious and often with fatal results.