Salvarsan is more specific for yaws than it is for syphilis and the percentage of positive Wassermann tests is as great in yaws as in syphilis.
Notwithstanding the above points, which would indicate a close relationship, all authorities are now agreed that clinical and pathological evidence show the two diseases to be separate entities.
Epidemiology.—Charlouis inoculated 32 Chinese prisoners with scrapings from yaws lesions. The disease developed in 28 of them, first showing itself at the site of inoculation.
Paulet inoculated 14 negroes with yaws material and after a period of incubation of from twelve to twenty days a primary lesion appeared, to be followed by the generalized eruption. In naturally acquired yaws the period of incubation is from three to six weeks. These experiments are in line with the known fact that any skin abrasion which comes in contact with a yaws lesion becomes infected, as when the mother nurses an infant with lesions on its face and develops a yaws lesion at the site of some fissure about the nipple.
Yaws shows a striking limitation to the tropics and in a disease so communicable by direct contact it seems remarkable that it does not spread from the occasional case introduced into temperate regions. In the tropical world it seems limited to low level areas. Another feature of yaws epidemiology is the vastly greater susceptibility of colored races, even those of mixed white blood showing a certain degree of immunity.
All evidence is against a congenital form of yaws.
In particular are flies important factors in the transmission of the disease, transferring the secretions from yaws lesions to abrasions or ulcers on the skin of healthy persons.
The greater the attention to personal hygiene the less probable is the spread of yaws, so that Europeans are rarely infected while the disease may be prevalent in the native population.
In countries where it is prevalent it is chiefly a disease of children, the adults possessing immunity as the result of attacks in childhood.