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). |