Syphilis - Primary

Primary syphilis is the initial stage of infection caused by the spirochete *Treponema pallidum*, characterized by the appearance of a painless ulcer (chancre) at the site of inoculation, typically occurring about 3 weeks after exposure.

Definition

Primary syphilis is the initial stage of infection caused by the spirochete *Treponema pallidum*, characterized by the appearance of a painless ulcer (chancre) at the site of inoculation, typically occurring about 3 weeks after exposure.

Epidemiology

Occurs worldwide; more common in sexually active adults aged 20–40 years
Higher prevalence among men who have sex with men (MSM)
Increased risk in individuals with HIV infection
Transmission occurs primarily via sexual contact (genital, anal, or oral)
Incubation period ranges from 10 to 90 days (average 21 days)

Etiology

Caused by *Treponema pallidum*, a motile spirochete bacterium
Transmitted through direct contact with infectious lesions during sexual activity
Can also be transmitted via blood transfusion or vertically from mother to fetus
Bacterium penetrates intact mucous membranes or microscopic abrasions in skin

Pathophysiology

*T. pallidum* enters through mucosal or skin breaks and multiplies locally
Local inflammatory response leads to formation of a painless chancre at the inoculation site
Organisms spread via lymphatic and hematogenous routes to distant tissues
Regional lymphadenopathy develops due to immune response
If untreated, progresses to secondary syphilis within 4–10 weeks