TUBERCULOSIS, an infectious, com municable disease caused by the bacillus of tuberculosis. The bacillus induces the formation of little nodules called tubercles. These tubercles may grow in size through the continued action of the organisms; they may soften, break down and be expelled, leaving behind an ulcer or a cavity; they may become hard by a process of sclerosis; or they may calcify. In addition to the local mani festations the disease produces general symp toms like elevation of the body temperature, increased pulse-rate and loss of weight. It is popularly known under a variety of other names as consumption, phthisis, decline, debility, hectic fever and when localized in a special part or tissue, as Pott's disease or hunchback, scrofula, hip-joint disease, white swelling (tuberculosis of elbow) and lupus (tuberculosis of skin). Contrary to prevalent lay opinion, when prop erly treated, it is a very curable affection.
Distribution of the Disease, Geographical — Tuberculosis is the most universal of all dis eases. It is found in every part of the world, and has been known from the beginning of his tory. It was accurately described by Hippoc rates (460 s.c.), and by Galen (200 A.D.). It is most prevalent, however, in large cities and especially in overcrowded districts.
Distribution According to No race is exempt, but some races appear less resistant than others. Indians, when brought into civili zation, prove especially susceptible. Among the other races in this country the negroes seem to he the most susceptible with next in order the Irish, while Jews stand at the end of the list. In the general mortality about one-tenth and in the mortality between the ages of 15 and 60 about one-fourth of all deaths are due to it. The number of clinical cases in a community is about 10 times the number of deaths in a year (Rosenau). Eighty per cent of these manifest it in the lungs.
Distribution Among Most ani mals are more or less susceptible. Among domestic animals it is found most frequently in cattle and swine, though sheep and horses are not exempt. Dogs and cats manifest it rarely. It is also found in birds (fowl) and fish. Wild animals in their native haunts seem less susceptible, yet in domestication it is the most common cause of death. Rabbits, guinea-pigs rats and mice may acquire it. Guinea-pigs are
especially susceptible to experimental inocula tion and are, therefore, commonly used for this purpose. Though frequent in adult cattle, it is infrequent in calves (Nocard), showing that direct heredity plays no part.
Etiology The actual cause of the disease is the tubercle-bacillus described by Robert Koch in 1882. This is a minute vegetable non-motile organism in the shape of a rod or lead-pencil, measuring about three microns of an inch) in length, and about four to six times longer than broad. It is visible only under the higher powers of the microscope, a oil-immersion lens being usually used to study it. Its principal characteristic is its be havior toward aniline dyes. It requires the strongest dyes to stain it, but when stained it holds the dye so tenaciously that exposure even to strong mineral acids for a reasonable time fails to decolorize it. This characteristic fur nishes the most ready means for its recognition. It is quite parasitic in nature, growing on but few artificial media, namely, blood-serum, glycerine-agar, bouillon or potato, best on the first. It grows only at the body temperature C.). It is slow in growth, and becomes apparent only from 5 to 14 days after inocula tion of the medium. Exposure moist to a tem perature of 60° C. for 15 minutes, or boiling, kills it, though freezing has no effect on it. It is killed by direct sunlight within a variable period of time (from 15 minutes on), depend ing on the season and the character of the medium containing the organism; by diffuse sunlight near a window in a week or two. In growing (either parasitically or without the body) the organism elaborates a chemical product highly poisonous to most animals. It is this poison circulating in the blood which produces the general symptoms of the disease, such as fever, increased heartbeat, emaciation, etc. Tubercle-bacilli found in different animals differ in their characteristics. The human, I ovine, avian and fish varieties have been dif ferentiated. One of the principal points to be noted is the slow propagation of the organisms with the consequent slow development of the disease. The actual infection usually precedes the manifestation of the disease by scarcely ever less than two and frequently as many as 20 years.