. Animal parasites and human disease. Medical parasitology; Insects as carriers of disease. 40 SPIROCHETES the granule stage and may thus be transmitted in dust or on the bodies of flies. Spirochceta bronchialis, causing a form of bron- chitis, is probably transmitted in this way. There is a wonderful variation in the size and form of spi- rochetes and also in their mode of life. A few species are free- living and of very large size, in fact almost visible to the naked eye ($• mm. in length), and there are many large species which live as harmless commensals with various mollusks. The disease-
. Animal parasites and human disease. Medical parasitology; Insects as carriers of disease. 40 SPIROCHETES the granule stage and may thus be transmitted in dust or on the bodies of flies. Spirochceta bronchialis, causing a form of bron- chitis, is probably transmitted in this way. There is a wonderful variation in the size and form of spi- rochetes and also in their mode of life. A few species are free- living and of very large size, in fact almost visible to the naked eye ($• mm. in length), and there are many large species which live as harmless commensals with various mollusks. The disease-causing species (some examples of which are shown in. B Fig. 5. Types of parasitic spirochetes. A, Sp. duttoni; B, Sp. novyi; C, Sp. pallida; D, Sp. refringens; E, Sp. balanitidis; F, Sp. vincenti; G, Sp. icterohemor- rhagioe. X about 1500. (After various authors.) Fig. 5) are very much smaller, often being so delicate and slender as to be hardly visible under the highest powers of the micro- scope. Not all the small spirochetes of vertebrates are patho- genic however; two species occur almost invariably in the human mouth, living on the tartar of the teeth and about the roots of the teeth, and yet, normally at least, cause no ill effects. One of these inhabitants of our mouths, Sp. buccalis, is a relatively short blunt species, but the other, Sp. dentium, is excessively slender, and practically indistinguishable when living from the spirochete of syphilis. Other harmless spirochetes occur in various stagnating secretions or excretions of the body, about the tonsils, and in the intestinal mucus. Spirochetes and Disease. — There is some question about how many distinct human diseases are caused by spirochetes. The mere presence of spirochetes in sores or diseased tissue is not sufficient reason for believing that they are the direct cause of the diseased condition, for, like many bacteria, they are often found in exposed sores which are known to be due to other causes. Spiro
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