Yaws may be evolving less conspicuous lesions.Yaws is most common among children, who spread it by playing together. It is not thought to be transmitted from mother to child in the womb. Yaws is not a venereal disease.T. pallidum pertenue has been identified in nonhuman primates (baboons, chimpanzees, and gorillas) and experimental inoculation of human beings with a simian isolate causes yaws-like disease. However, no evidence exists of cross-transmission between human beings and primates, but more research is needed to discount the possibility of a yaws animal reservoir in nonhuman primates. Diagnosis
Most often the diagnosis is made clinically. Dark field microscopy of samples taken from early lesions (particularly ulcerative lesions) may show the responsible bacteria; the spirochaetes are only 0.3 µm wide by 6–20 µm long, so light-field microscopy does not suffice.A microscopic examination of a biopsy of a yaw may show skin with clear epidermal hyperplasia (a type of skin thickening) and papillomatosis (a type of surface irregularity), often with focal spongiosis (an accumulation of fluid in specific part of the epidermis). Immune system cells, neutrophils and plasma cells, accumulate in the skin, in densities that may cause microabscesses.Warthin–Starry or Levaditi silver stains selectively stain T. pallidum, and direct and indirect immunofluorescence and immunoperoxidase tests can detect polyclonal antibodies to T. pallidums. Histology often shows some spatial features which distinguish yaws from syphilis (syphilis is more likely to be found in the dermis, not the epidermis, and shows more endothelial cell proliferation and vascular obliteration).Blood-serum (serological) tests are increasingly done at the point of care.
They include a growing range of treponemal and nontreponemal assays. Treponemal tests are more specific, and are positive for any one who has ever been infected with yaws; they include the Treponema pallidum particle agglutination assay. Nontreponemal assays can be used to indicate the progress of an infection and a cure, and positive results weaken and may become negative after recovery, especially after a case treated early. They include the venereal disease research laboratory (VDRL; requires microscopy) and rapid plasma reagin (RPR; naked-eye result) tests, both of which flocculate patient-derived antibodies with antigens.Serological tests cannot distinguish yaws from the closely related syphilis; no test distinguishing yaws from syphilis is widely available. The two genomes differ by about 0.2%. PCR and DNA sequencing can distinguish the two. There are also no common blood tests which distinguish among the four treponematoses: syphilis (Treponema pallidum pallidum), yaws (Treponema pallidum pertenue), bejel (Treponema pallidum endemicum), and pinta (Treponema carateum).Haemophilus ducreyi infections can cause skin conditions that mimic primary yaws. People infected with Haemophilus ducreyi lesions may or may not also have latent yaws, and thus may or may not test positive on serological tests. This was discovered in the mid-2010s. It seems that a recently diverged strain of Haemophilus ducreyi has evolved from being a sexually transmitted infection to being a skin ulcer pathogen that looks like yaws.Yaws has been reported in nonendemic countries.