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. Based on estimates from national surveys conducted from 1999–2008, chlamydia prevalence is 6.8% among sexually active females aged 14–19 years.1
Chlamydial infections in women are usually asymptomatic. However, these can result in pelvic inflammatory disease (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 might 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 annual chlamydia screening.4
The Healthcare Effectiveness Data and Information Set (HEDIS) contains a measure which assesses chlamydia screening coverage of sexually active young women who receive medical care through commercial or Medicaid managed care organizations.5 Among sexually-active women aged 16–24 years in commercial plans, chlamydia screening increased from 23.1% in 2001 to 45.0% in 2011. During the same time period, the screening rate among sexually-active women aged 16–24 years covered by Medicaid increased from 40.4% to 58.0%.6 Although chlamydia screening is expanding, 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.
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. To supplement case report data, chlamydia positivity and prevalence among people screened in a variety of settings are monitored.
In 2012, a total of 1,422,976 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 456.7 cases per 100,000 population, only a 0.7% increase compared with the rate of 453.4 in 2011. During 1992–2012, the rate of reported chlamydial infection increased from 182.3 to 456.7 cases per 100,000 population (Figure 1, Table 1).
Chlamydia by Region
During 2003–2012, chlamydia rates increased in all regions (Figure 2). In 2012, rates were highest in the South (496.9 per 100,000 population), followed by the Midwest (452.1), the West (426.5), and the Northeast (417.8) (Table 3).
Chlamydia by State
In 2012, chlamydia rates by state ranged from 233.0 cases per 100,000 population in New Hampshire to 774.0 cases in Mississippi (Figure 3, Table 2); the rate in the District of Columbia was 1,101.6 cases per 100,000 (Table 3).
Chlamydia by Metropolitan Statistical Area
In 2012, the chlamydia rate per 100,000 population in the 50 most populous metropolitan statistical areas (MSAs) was similar to the rate in 2011 (481.1 and 480.9 cases, respectively) (Table 6). In 2012, 56.8% of chlamydia cases were reported by these MSAs. Among women in these MSAs, the 2012 rate of 661.8 cases per 100,000 females was similar to the 2011 rate of 667.6 cases per 100,000 females (Table 7). Among men, the 2012 rate (291.3 per 100,000 males) increased 2.6% from the 2011 rate (284.0 cases per 100,000 males) (Table 8).
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 2012, 927 (29.5%) of 3,142 counties had rates higher than 400.0 cases per 100,000 population. Seventy counties and independent cities reported 44% of all chlamydia cases in 2012 (Table 9).
Chlamydia by Sex
During 1995–2011, chlamydia rates among females increased each year (Figure 1). In 2012, the overall rate of reported chlamydial infection among women in all 50 states and the District of Columbia (643.3 cases per 100,000 females) was similiar to the reported case rate in 2011 (643.4 cases per 100,000 females). This is the first time since nationwide reporting began in 1995 that chlamydia case rates among females did not increase.
The overall case rate among males increased 3.2% during 2011–2012 (254.4 to 262.6 cases per 100,000 males). As in previous years, the reported case rate among females was about two times the case rate among men in 2012, 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 2008–2012, the reported chlamydial infection rate among men increased 25.5% (from 209.3 to 262.6 cases per 100,000 males) compared with a 11.0% increase among women during the same period (from 579.4 to 643.3 cases per 100,000 females).
Chlamydia by Age
Chlamydia rates are highest among adolescents and young adults aged 15–24 years (Table 10). Among those aged 15–19 years, rates increased 8.9% during 2008–2011 (1,947.7 to 2,120.8 cases per 100,000 population) and then decreased 5.6% during 2011–2012 (2,120.8 to 2,001.7 cases per 100,000 population). Among those aged 20–24 years, rates increased 18.1% during 2008–2011 (2,075.9 to 2,450.8 cases per 100,000) and then increased slightly (2.1%) during 2011–2012 (2,450.8 to 2,501.5 cases per 100,000).
Among women, the highest age-specific rates of reported chlamydia in 2012 were among those aged 15–19 years (3,291.5 cases per 100,000 females) and 20–24 years (3,695.5 cases per 100,000 females) (Figure 5, Table 10). Within these age ranges, reported rates were highest among women aged 18 years (4,666.3 cases per 100,000 females), aged 19 years (4,921.1 cases per 100,000 females), and aged 20 years (4,647.5 cases per 100,000 females) (Table 12). After increasing steadily from 2000 to 2011, during 2011–2012, rates among women aged 15–19 years decreased 5.6% (3,485.2 to 3,291.5 cases per 100,000 females). Rates increased slightly (1.8%) among women aged 20–24 years (3,630.0 to 3,695.5 cases per 100,000 females) during 2011–2012.
Age-specific rates among men, although substantially lower than the rates among women, were highest in those aged 20–24 years (1,350.4 cases per 100,000 males) (Figure 5, Table 10). Similar to trends in women, after increasing for the last decade, during 2011–2012 reported case rates among men aged 15–19 years decreased 5.1% (816.3 to 774.8 cases per 100,000 males). During 2011–2012, reported cases among men aged 20–24 years increased slightly (1,307.8 to 1350.4 cases per 100,000 males).
Chlamydia by Race/Ethnicity
Among the 48 jurisdictions (47 states and the District of Columbia) that submitted data in the new race and ethnicity categories in 2012 according to the revised OMB standards, chlamydia rates were highest among black men and women (Figure L, Table 11B). The rate of chlamydia among blacks was almost seven times the rate among whites (1,229.4 and 179.6 cases per 100,000 population, respectively). The rate among American Indians/Alaska Natives (728.2 cases per 100,000) was 4.1 times the rate among whites. The rate among Hispanics (380.3 cases per 100,000) was 2.1 times the rate among whites. The rate among Native Hawaiians/Other Pacific Islanders (590.4 cases per 100,000) was 3.3 times the rate among whites. The rate among Asians was lower than the rate among whites (112.9 cases and 179.6 cases per 100,000, respectively).
Among the 39 jurisdictions (38 states and the District of Columbia) that submitted data in the new race and ethnicity categories from 2008–2012 according to the OMB standards, rates among blacks increased 3.7% (from 1,186.5 to 1,230.6 cases per 100,000). Among whites, rates increased 38.5% (from 134.4 to 186.2 cases per 100,000) (Figure 6).
Chlamydia by Reporting Source
Most chlamydia cases reported in 2012 were from venues outside of STD clinics (Figure 8 and Table A2). Over time, the proportion of cases reported from non-STD clinic sites has continued to increase (Figure 7). In 2012, among women, only 6.9% of chlamydia cases were reported through an STD clinic (Figure 8 ). Most cases among women were reported from private physicians/health maintenance organizations (HMOs) (38.5%). Among men, 21.4% of chlamydia cases were reported from an STD clinic in 2012 and 27.7% 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, non-institutionalized population aged 14–39 years that provides an important measure of chlamydia disease burden. From 1999–2000 to 2007–08, there was an estimated 40% reduction (95% Confidence Interval [CI]: 8%, 61%) in prevalence among persons aged 14–39 years.7 During 2005–2008, the overall prevalence of chlamydia among persons aged 14–39 years was 1.5% (95% CI: 1.2%, 1.9%). Prevalence was highest among non-Hispanic blacks (5.9%, 95% CI: 4.5%, 7.7%) (Figure 10).
Chlamydia Positivity in Selected Populations
In 2005, the STD Surveillance Network (SSuN) was established to improve the capacity of national, state, and local STD programs to detect, monitor, and respond to trends in STDs. In 2012, a total of 42 STD clinics at 12 sites collected enhanced behavioral information on patients who presented for care to these clinics. More detailed information about SSuN methodology can be found in the STD Surveillance Network section of the Appendix, Interpreting STD Surveillance Data.
In 2012, the proportion of STD clinic patients testing positive for chlamydia varied by age, sex, and sexual behavior. Adolescent men who have sex with women (MSW) had the highest prevalence (26.4%). Among MSW and women, prevalence among those tested decreased with age. The variation in prevalence by age was not as pronounced for men who have sex with men (MSM) (Figure 9).
During the mid-1990s to 2011, chlamydia positivity among women screened in family planning and prenatal care clinics participating in infertility prevention activities were sent to CDC to monitor chlamydia prevalence. As the national infertility prevention program expanded, these data became difficult to interpret as trends were influenced by changes in screening coverage, screening criteria, and test technologies, as well as demographic changes in patients attending clinics reporting data to CDC. These issues could not be addressed with the limited variables that were collected at the national level. Chlamydia positivity data continue to be useful locally to inform clinic-based screening recommendations and to identify at-risk populations in need of prevention interventions, but are no longer collected to monitor national trends in chlamydia.
Chlamydia Among Special Populations
More information on chlamydia among women of reproductive age, adolescents and young adults, men who have sex with men, and minority populations is presented in the Special Focus Profiles.
Chlamydia continues to be the most commonly reported nationally notifiable disease with 1,422,976 cases reported in 2012. For the first time since 1995, chlamydia case rates among females did not increase. For the first time since 2000, chlamydia case rates decreased among both males and females aged 15–19 years. However, both test positivity and the number of reported cases of C. trachomatis infections remain high among most age groups, racial/ethnic groups, geographic areas, and both sexes. Racial differences also persist; reported case rates and prevalence estimates among blacks continue to be substantially higher than among other racial/ethnic groups.
1 Centers for Disease Control and Prevention. CDC Grand Rounds: Chlamydia prevention: challenges and strategies for reducing disease burden and sequelae. MMWR Morb Mortal Wkly Rep. 2011;60(12):370-3.
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, 2010; No.59(RR-12):1-110. Erratum in: MMWR Recomm Rep. 2011;60(1):18.
7 Datta SD, Torrone E, Kruszon-Moran D, Berman S, Johnson R, Satterwhite CL, Papp J, Weinstock H. Chlamydia trachomatis trends in the United States among persons 14 to 39 years of age, 1999-2008. Sex Transm Dis. 2012 Feb;39(2):92-6.