Chlamydial infection is a sexually transmitted disease (STD) present in epidemic proportions, representing the largest number of cases reported to the Centers for Disease Control and Prevention (CDC) for any condition. Nearly 1.5 million cases are reported annually to the CDC, increasing at a rate of 5% to 8% per year. The prevalence of Chlamydia infection varies by clinical setting. Among women seen in primary care practice, the prevalence is 3% to 5%. In family planning clinics, the prevalence increases to 9%; in STD clinics, the rate rises to 17% to 28%. Prevalence is very high among adolescents (18%), and one study of college campus women found 50% to be infected. Among female military recruits, the prevalence of chlamydial infection was 9%; the rate for the youngest recruits, 17 years of age, was 12%. In the past 20 years, the reported rates of chlamydial infection have increased by over 250%. Although much of this rise is accounted for by increased screening (such screening is one of the parameters used to measure and reward quality of care), chlamydial infection remains very prevalent in the United States.

Women are at greater risk for contracting C. trachomatis infection than are men, and they also suffer more serious consequences. The risk in a single act of unprotected intercourse with an infected partner is 40% for women and 20% for men. The increased risk for women reflects the fact that they receive an ejaculate of infected secretions from their partner’s genital tract. Men do not receive such a large inoculum unless they have an intact foreskin, which can serve as a reservoir for the woman’s infected cervical secretions.

Among women with documented urogenital infection, the cervix is infected in 75% and the urethra in 50%. Endometritis can be demonstrated in about 33%; it is often clinically silent, but infection may spread to the fallopian tubes. In studies of salpingitis, Chlamydia is recovered from the tubes in up to 50% of subjects. Vaginal infection is rare.

Chlamydiae may also colonize the pharynx and rectum in women engaging in oral and anal sex, respectively. Among those attending an STD clinic, the rate for oral recovery of organisms was 3.2%; the rectal colonization rate was 5.2%. Rectal involvement has been noted even in the absence of rectal intercourse.

In men, the urethra is the predominant site of infection, with more than 82% having a symptomatic or visible urethral
discharge. In 1% to 2% of infected men, the infection ascends to the epididymis to produce acute scrotal pain and discomfort.

Men who have sex with men (MSM) have higher rates of oral and anal recovery of organisms. Lymphogranuloma strains may be present.

Women who have sex with women (WSW) are not free from risk of chlamydial infection, although rate of transmission is not well-established. The infection is likely to have originated from contact with a male partner. Prevalence appears higher than previously thought.

Age is a powerful predictor of risk for infection. Women younger than 21 years are at the greatest risk. Other important risk factors include having had a new partner in the last 2 months, more than one partner in the last 6 months, or a partner known to have other partners. Nonetheless, even among women who are monogamous or who have been sexually inactive in the last 2 months, prevalence can be as high as 7% to 10%.

Other significant predictors of chlamydial infection identified by multivariate analysis include African American race, low level of education, unprotected intercourse, mucopurulent cervical discharge, and induced mucosal bleeding on swabbing of the cervix. Among patients with gonococcal infection, 30% to 50% have concurrent chlamydial disease. In a large surveillance study of urban residents younger than the age of 45 years, the rate for either infection was 15% in black women, 6.4% in black men, 2.8% in white men, and 1.3% in white women. The risk of C. trachomatis infection was highest in the 18- to 20-year-old group (8.0%), but Neisseria gonorrhoeae infection was unexpectedly high in the 31- to 35-year-old group (10.2%).