BLADDER, DISEASES OF ; FACE, DISEASES OF. For paralysis of the muscles of the eye, see SQUINTING.
PARASITES.—Parasites are living plants or animals which live either temporarily or continuously on or in the bodies of other plants or animals, and draw their nourishment from their host. It is noteworthy that there are hardly any plants or animals known which do not support some one or more varieties of parasites, and in the higher animals as well as in the more complex plants the number of these parasites may run into the hundreds. These parasites are responsible for the causation of many diseases in both the plant and the animal kingdoms, and when it is realised that practically all plant diseases, for instance, are due to some form of microscopic parasite, one can appreciate the great importance that this subject has for the farmer. This is not the place, however, to discuss the subject of plant or animal parasites in their effects on plants, for only the discussion of parasites on man can he entered into. The parasites of the lower animals (dogs, cats, sheep, rats, cows, pigs, fish, turtles, snakes, frogs, spiders, etc.) occupy a very important place in the zoological system, but this again is outside the realm of the present work.
The parasites which are most inimical to man may be divided into two classes : plant parasites and animal parasites. Of the former, the bacteria and moulds are most important ; whereas among the latter certain minute protozoa, certain forms of insects, and certain worms are most commonly met with. In the chapter on BACTERIA full consideration has been given to the subject of plant parasites, and in the consideration of bacterial diseases such as typhoid fever, tuberculosis, meningitis, etc., what is known regarding the bacteria that cause them is fully discussed. As for the animal parasites, the group of minute, one-celled organisms called frolozoa arc of extraordinary interest, since it has been shown only within comparatively recent times that they are responsible for certain diseases. This is probably true for malaria, possibly for yellow fever, undoubtedly for the disease known as the sleeping sickness. These diseases are all due to the presence and growth of minute animals within the human body, these animals belonging to the lowest stages in the animal scale. It is well known, furthermore, that certain forms of
dysentery and certain conditions such as abscess of the liver are due to minute protozoa, and it is not at all unlikely that certain forms of tious disease beri-beri, certain skin diseases, and even possibly syphilis may be demonstrated as due to minute animal parasites.
More characteristic parasites, however, are certain forms of insects, which include mites as well as the true insects. The diseases that are clue to insect parasites are not very numerous. Of the sixteen or nineteen systematic orders belonging to these forms, science recognises about four as contributing to parasitic diseases of man : the flies, the fleas, the bugs, the lice, and the order to which the body-lice belong. Stinging insects are not here included. Two-winged insects are of interest in that occasionally they deposit their eggs in certain parts of the body. They are very rare ; but the eggs of flies, etc., are known to occur as parasitic on the head and parts of the skin of people who are extremely uncleanly.
Flies, however, are of considerable interest when one bears in mind that they are often responsible for the spread of many diseases, particularly of infectious ophthalmia, where the flies, lighting on the eyes of an infected patient, transmit the infection to the eyes of a sleeping healthy person ; and for the continuation of typhoid in camps where this disease is common.
The screw-worm, or Lucili macellaria, is a worm that occurs in the warmer portions of the United States, and is known to deposit its eggs in the nostrils of individuals when they are asleep, causing a very chronic catarrh of the nose. The jigger, or jigger-flea, is a native of the tropics. It is known to burrow into the skin, especially beneath the toenails, and there its eggs are laid, at times producing ulcers. It is one of the dreaded insects of the south. The best way to get rid of this parasite is by the introduction of a sharp knife-point, the object being to get the insect out without bursting the skin if possible, as this accident is apt not only to spread the eggs, but also to disseminate an acrid serum which forms foul ulcers.