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Syphilis is caused by Treponema pallidum, a slender, tightly coiled bacterium in the family Spirochaetaceae. Too slender to be seen on direct microscopy, T. pallidum can be detected with darkfield microscopy and has a characteristic rotary movement with central flexion, which is diagnostic. The organism cannot be cultured in vitro.



In the vast majority of cases, syphilis is acquired by sexual intercourse with an infectious individual, during which T. pallidum gains access to the subcutaneous tissues through intact mucous membranes or microperforations in the skin. Disease transmission is most likely during early (primary and secondary) syphilis, especially when active mucocutaneous lesions are present. An infectious syphilitic individual has about a 50% chance of transmitting the disease during a sexual encounter. In general, patients can no longer transmit the disease by sexual contact 4 years after infection.

Other means of transmission include close contact with an active lesion, passage through the placenta or during delivery (congenital syphilis), blood transfusion, and accidental direct inoculation, as in a needle stick injury. In developed countries, transfusion syphilis is extremely rare, owing to routine screening of donated blood for syphilis since the 1930s.

Syphilis is most common among sexually active persons with multiple partners, making it is necessary to rule out other sexually transmitted diseases, especially HIV, when a diagnosis of syphilis is made and vice versa. Because of the high transmission rate of syphilis, empiric treatment of incubating syphilis in sexual contacts is warranted whenever possible.


Men's Health