C. trachomatis infection is the most commonly reported notifiable disease in the United States. It is among the most prevalent of all STDs, and since 1994, has comprised the largest proportion of all STDs reported to CDC (Table 1). Studies also demonstrate the high prevalence of chlamydial infections in the general U.S. population. Among young adults (aged 18–26 years) who participated in the nationally representative National Longitudinal Study of Adolescent Health (Add Health) during 2001–2002, chlamydia prevalence was 4.2%.1
Chlamydial infections in women, which are usually asymptomatic, can result in PID, which is a major cause of infertility, ectopic pregnancy, and chronic pelvic pain. Data from a randomized controlled trial of chlamydia screening in a managed care setting suggested that screening programs can lead to as much as a 60% reduction in the incidence of PID.2 As with other inflammatory STDs, chlamydial infection can facilitate the transmission of human immunodeficiency virus (HIV) infection.3 In addition, pregnant women infected with chlamydia can pass the infection to their infants during delivery, potentially resulting in neonatal ophthalmia and pneumonia. Because of the large burden of disease and risks associated with infection, CDC recommends that all sexually active women younger than age 26 years receive an annual chlamydia screening.4
The increase in reported chlamydial infections during the last 20 years reflects the expansion of chlamydia screening activities, the use of increasingly sensitive diagnostic tests, an increased emphasis on case reporting from providers and laboratories, and improvements in the information systems used for reporting. However, many women who are at risk are still not being tested—reflecting, in part, the lack of awareness among some health care providers and the limited resources available to support these screenings.
Chlamydia screening and reporting are likely to continue to expand in response to the Healthcare Effectiveness Data and Information Set (HEDIS) annual measure, which assesses chlamydia screening coverage of sexually active young women who receive medical care through commercial or Medicaid managed care organizations.5 The annual chlamydia screening rate increased from 25.3% in 2000 to 41.6% in 2007 among sexually active females aged 16–25 years (or aged 16–26 years during 2000–2002) who were enrolled in commercial or Medicaid health plans in the United States during 2000–2007.6 In 2008, women aged 16–20 years in commercial plans had a chlamydia screening coverage rate of 40.1%, while those in Medicaid had a rate of 52.7%.7
To better monitor trends in disease burden in defined populations during the expansion of chlamydia screening activities, data on chlamydia positivity and prevalence among people screened in a variety of settings are used. In most instances, test positivity serves as a reasonable approximation of prevalence.8
In 2009, a total of 1,244,180 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 409.2 cases per 100,000 population, which is an increase of 2.8% compared with the rate of 398.1 in 2008. During 1990–2009, the rate of reported chlamydial infection increased from 160.2 to 409.2 cases per 100,000 population (Figure 1, Table 1).
Chlamydia by Region
During 2000–2009, chlamydia rates increased in all regions (Figure 2). In 2009, rates were highest in the South (452.4 per 100,000 population), followed by the Midwest (401.9) and the West (383.0) (Table 3). Rates have consistently remained lowest in the Northeast (363.9).
Chlamydia by State
Chlamydia by Metropolitan Statistical Area
In 2009, the chlamydia rate per 100,000 population in the 50 most populous metropolitan statistical areas (MSAs) increased (Table 6). Among women, the 2009 rate of 629.3 cases was a 3.3% increase over the 2008 rate of 609.0 cases (Table 7). The 2009 rate among men (248.3) increased 6.1% from the 2008 rate (234.1) (Table 8). In 2009, 58.4% of chlamydia cases were reported by these MSAs.
Chlamydia by County
Counties in the United States with the highest chlamydia case rates per 100,000 population were located primarily in the Southeast and West, including Alaska (Figure 4). In 2009, a total of 710 (22.6%) of 3,141 counties had rates higher than 400.0 cases per 100,000 population. Fifty-four counties and independent cities reported 40% of all chlamydia cases in 2009 (Table 9).
Chlamydia by Sex
In 2009, the overall rate of reported chlamydial infection among women in all 50 states and the District of Columbia (592.2 cases per 100,000 females) was almost three times higher than the rate among men (219.3 cases per 100,000 males), likely reflecting a larger number of women screened for this infection (Figure 1, Tables 4 and 5). The lower rates among men also suggest that many of the sex partners of women with chlamydia are not receiving a diagnosis of chlamydia or being reported as having chlamydia.
However, with the advent of highly sensitive nucleic acid amplification tests (NAATs) that can be performed on urine, chlamydial infection is increasingly being diagnosed in symptomatic and asymptomatic men. During 2005–2009, the chlamydial infection rate among men increased 37.6% (from 159.4 to 219.3 cases per 100,000 males) compared with a 20.3% increase among women during the same period (from 492.2 to 592.2 cases per 100,000 females).
Chlamydia by Age
Among women, the highest age-specific rates of reported chlamydia in 2009 were among those aged 15–19 years (3,329.3 cases per 100,000 females) and 20–24 years (3,273.9 cases per 100,000 females) (Figure 5, Table 10). Age-specific rates among men, although substantially lower than the rates among women, were highest in those aged 20–24 years (1,120.6 cases per 100,000 males) (Figure 5, Table 10).
Chlamydia by Race/Ethnicity
In 2009, chlamydia rates were highest among black men and women (Figure 6, Table 11B). The rate of chlamydia in blacks was more than eight times higher than that in whites (1,559.1 and 178.8 cases per 100,000 population, respectively). During 2005–2009, rates among blacks increased 26.3% (from 1,234.2 to 1,559.1). Among whites, rates increased 17.9% (from 151.7 to 178.8). The rates among American Indians/Alaska Natives (776.5) and Hispanics (504.2) were 4.3 and 2.8 times higher, respectively, than that of whites.
Chlamydia by Reporting Source
Most chlamydia cases reported in 2009 were from venues outside of STD clinics (Table A2). Over time, the proportion of cases reported from non-STD clinic sites has continued to increase (Figure 7). In 2009, among women, only 11.5% of chlamydia cases were reported through an STD clinic (Figure 8). Most cases among women were reported from private physicians/health maintenance organizations (HMOs) (39.7%). In contrast, among men, 33.1% of chlamydia cases were reported from an STD clinic in 2009 and 26.9% were reported from private physicians/HMOs.
Chlamydia Prevalence in the Population
The National Health and Nutrition Examination Survey (NHANES) is a nationally representative survey of the U.S. civilian, noninstitutionalized population aged 14–39 years that provides an important measure of chlamydia disease burden. During 1999–2002, the overall prevalence of chlamydial infection was 2.2% and was similar in males and females (2.0% and 2.5%, respectively).9 Prevalence was higher among non-Hispanic blacks than non-Hispanic whites in all age groups (Figure 9).
Chlamydia screening and prevalence monitoring activities were initiated in Region X of the U.S. Department of Health and Human Services (HHS) in 1988 as a CDC-supported demonstration project. In 1993, chlamydia screening services for women were expanded to three additional HHS regions (III, VII, and VIII) and in 1995, to the remaining HHS regions (I, II, IV, V, VI, and IX). In some regions, federally funded chlamydia screening supplements local- and state-funded screening programs. Screening criteria and practices vary by region and state. See Definitions of HHS Regions in the Appendix for details.
In 2009, the median state-specific chlamydia test positivity among women aged 15–24 years who were tested during visits to selected family planning clinics in all 50 states, the District of Columbia, Puerto Rico, and the Virgin Islands was 7.5% (range: 3.5% to 15.5%) (Figures 10 and 11). Since 1997, median chlamydia positivity has increased steadily. This increase is likely because of increasing use of more sensitive test technology. See Chlamydia and Gonorrhea Prevalence Monitoring in the Appendix for details.
Chlamydia test positivity among women aged 15–24 years screened in family planning clinics fluctuated in all 10 HHS regions during 2005–2009 (Figure 12). Positivity has remained fairly stable in six regions (I, II, III, IV, V, and X). In the remaining four regions (VI, VII, VIII, and IX), positivity increased slightly during 2005–2009.
The positivity trend data in Figure 12 are not adjusted for changes in laboratory test methods and associated increases in test sensitivity. Using more sensitive tests has been shown to affect positivity.10 Use of NAATs in family planning clinics to screen women aged 15–24 years for chlamydia is increasingly widespread (Figure 13). In four HHS regions (I, V, VII, and VIII), NAATs were used nearly exclusively during 2005–2009. In four other regions (III, IV, VI, and IX), NAATs usage was 97% or higher in 2009. The remaining two regions used NAATs more than 80% of the time in 2008.
Chlamydia Among Special Populations
More information on chlamydia screening programs for women of reproductive age and chlamydia among adolescents, minority populations, and people in corrections facilities is presented in the Special Focus Profiles.
Both prevalence and the number of reported cases of C. trachomatis infections remain high among most age groups, racial/ethnic groups, geographic areas, and both sexes. The reported number of chlamydia cases is higher among women, especially those of younger age (15–19 and 20–24 years), but this finding could be a reflection of screening recommendations. Racial differences also persist; rates among blacks continue to be substantially higher than rates among other racial/ethnic groups.
3 Fleming DT, Wasserheit JN. From epidemiological synergy to public health policy and practice: the contribution of other sexually transmitted diseases to sexual transmission of HIV infection. Sex Transm Infect. 1999;75:3-17.
4 Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2006. MMWR Recomm Rep. 2006;55(No.RR-11):1-94.
6 Centers for Disease Control and Prevention. Chlamydia screening among sexually active young female enrollees of health plans — United States, 2000–2007. MMWR Morb Mortal Wkly Rep. 2009;58(14):362-5.
9 Datta SD, Sternberg M, Johnson RE, Berman S, Papp JR, McQuillan G, et al. Gonorrhea and chlamydia in the United States among persons 14 to 39 years of age, 1999 to 2002. Ann Intern Med. 2007;147(2):89-96.