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Overview of the Etiologic Agent

Peak rates of syphilis in the United States occurred during the 1940s. Discovery of and subsequent treatment with penicillin therapy, along with public health programs, account for the dramatic decrease of syphilis among the U.S. population seen today. Still, syphilis is a major sexually transmitted disease (STD) and outbreaks of the disease have occurred in recent years. There were 52,995 cases of syphilis reported in 1996.

The incidence of disease is greatest in rural and urban areas of the South in the United States. The disease is also disproportionately distributed among the poor, minorities, and individuals who have multiple partners.

Syphilis is caused by the organism Treponema pallidum, a member of the family Spirochaetaceae. The organism is a tightly coiled, corkscrew-shaped bacterium. It cannot be cultured in vitro, and can only be viewed under darkfield or electron microscopy. The organism is too thin to be seen under light microscopy.


Transmission of the organism occurs by sexual contact. Thirty to forty percent of persons exposed to an infected person will acquire the bacterium. T. pallidum can also be transmitted via kissing, blood transfusion, or from a mother to her fetus through the placenta. The bacterium thus enters the body through abrasions in the skin or mucous membranes. The incubation period of syphilis is between 9 and 90 days, with an average of 21 days.

Signs, Symptoms and Treatment

There are four stages of an infection with T. pallidum.

The first stage of the disease, called primary syphilis, occurs when the microorganisms infect their entry site, activate the immune system, and cause a lesion. This lesion, called a chancre, begins looking like a pimple but later evolves into an indurated ulcer that has smooth and firm borders. A significant feature of the chancre is that it is painless. If the chancre occurs in a spot that is difficult to see, like the vagina, cervix or rectum, the infected person may not notice it. The vast majority of the infected will experience one single chancre, but multiple chancres can occur. The lesion goes away in 1 to 6 weeks without any medical treatment, and does not produce a scar. During primary syphilis a person may also have swollen lymph nodes.