Yaws is neither hereditary nor congenital. A pregnant mother suffering from yaws does not give birth to a child suffering from the same disease, nor one which will subsequently develop yaws unless the virus be first introduced directly through a breach of surface after birth. It is not conveyed by the milk; nor does a suckling suffering from yaws necessarily infect its nurse.
Age, sex, occupation, race.— Although two-thirds of the cases in the West Indies occur before puberty, no age is exempt. Three males are infected to every female attacked. Occupation has no manifest influence. In the West Indies, Europeans, Chinese, and Indians catch the disease if exposed to the contagium.
The virus.— Both Pierez and Nicholls found a micrococcus in yaws tissue and in the exudation. Cultures of this micro-organism introduced into certain animals did not give rise to the disease
In 1905, by overstaining with Leishman's and Giemsa's stains, Castellani demonstrated in scrapings of yaws tissues the presence of an extremely delicate treponema, Treponema pertenue (Spirochœta pertenuis, S. pallidula), very like the spirocbæte of syphilis. Castellani's observations have been amply confirmed.
To demonstrate the treponema, slides should be prepared from scrapings of an incised yaw papule before it has ruptured. The dried films are then overstained by Giemsa's or Leishman's method. The Burri or indian-ink method (see p. 231) provides a rapid and convenient means of demonstrating the spirochsetes as thin wavy lines on a dark background. They can only be seen in a living state by the dark-ground illumination method. A fully developed yaw is unsuitable, because, in consequence of its having been exposed to external sources of contamination, a variety of organisms will be present and may confuse the observer. Opinions differ with regard to the dimensions, presence or absence of flagella and undulating membrane, and other minute details; suffice it to say that T. pertenue to the ordinary observer is morphologically indistinguishable from the corresponding germ of syphilis. Ranken, using
