Chlamydia, caused by infection with Chlamydia trachomatis, is the most common 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 US population, particularly among young women.1
Chlamydial infections in women are usually asymptomatic.2 Untreated infection can result in pelvic inflammatory disease (PID), which is a major cause of infertility, ectopic pregnancy, and chronic pelvic pain. Data from randomized controlled trials of chlamydia screening suggested that screening programs can lead to a reduction in the incidence of PID.3,4 As with other inflammatory STDs, chlamydial infection could facilitate the transmission of HIV infection.5 In addition, pregnant women infected with chlamydia can pass the infection to their infants during delivery, potentially resulting in ophthalmia neonatorum, which can lead to blindness, and pneumonia.6 Because of the large burden of disease and risks associated with infection, CDC recommends annual chlamydia screening for all sexually active women younger than age 25 years and women ≥25 years at increased risk for infection (e.g., women with new or multiple sex partners).7
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. Among sexually active women aged 16–24 years in commercial health maintenance organization (HMO) plans, chlamydia screening increased from 23.1% in 2001 to 48.3% in 2016. Among sexually-active women aged 16–24 years covered by Medicaid, screening rates increased from 40.4% in 2001 to 57.3% in 2016.8 Although chlamydia screening has expanded over the past two decades, 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.
Interpreting Rates of Reported Cases of Chlamydia
Trends in rates of reported cases of chlamydia are influenced by changes in incidence of infection, as well as changes in diagnostic, screening, and reporting practices. As chlamydial infections are usually asymptomatic, the number of infections identified and reported can increase as more people are screened even when incidence is flat or decreasing. During 2000–2011, the expanded use of more sensitive diagnostic tests (e.g., nucleic acid amplification tests [NAATs]) likely increased the number of infections identified and reported independently of increases in incidence. Also, although chlamydia has been a nationally notifiable condition since 1995, it was not until 2000 that all 50 states and the District of Columbia required reporting of chlamydia cases. National case rates prior to 2000 reflect incomplete reporting. The increased use of electronic laboratory reporting over the last decade or so also likely increased the proportion of diagnosed cases reported. Consequently, an increasing chlamydia case rate over time may reflect increases in incidence of infection, screening coverage, and use of more sensitive tests, as well as more complete reporting. Likewise, decreases in chlamydia case rates may suggest decreases in incidence of infection or screening coverage.
Chlamydia — United States
In 2017, a total of 1,708,569 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 528.8 cases per 100,000 population. During 2000–2011, the rate of reported chlamydial infection increased from 251.4 to 453.4 cases per 100,000 population (Figure 1, Table 1). During 2011–2013, the rate of reported cases decreased to 443.5 cases per 100,000 population, followed by an increase in the rate of reported cases over each of the next four years. During 2016–2017, the rate increased 6.9%, from 494.7 to 528.8 cases per 100,000 population (Figure 1, Table 1).
Chlamydia by Region
In 2017, rates of reported cases of chlamydia were highest in the South (560.4 cases per 100,000 population, 6.8% increase from 2016), followed by the West (524.3, 8.8% increase from 2016), Midwest (514.4, 5.6% increase from 2016), and Northeast (483.3, 6.2% increase from 2016) (Table 3). During 2008–2012, rates of reported cases of chlamydia increased in all regions (Figure 2). During 2012–2013, rates decreased in the Northeast, Midwest, and South and remained stable in the West. During 2013–2017, rates increased in all regions, with the largest increase occurring in the West (421.1 to 524.3 cases per 100,000 population, 24.5% increase) (Figure 2, Table 3).
Chlamydia by State
In 2017, rates of reported cases of chlamydia by state ranged from 226.1 cases per 100,000 population in West Virginia to 799.8 cases per 100,000 population in Alaska (Figure 3, Table 2); the rate in the District of Columbia was 1,337.0 cases per 100,000 population (Table 3). During 2016–2017, rates of reported chlamydia cases increased in 46 states and the District of Columbia. The rate of reported chlamydia cases in 2017 was above the US total in 19 states (Table 2).
Chlamydia by Metropolitan Statistical Area
The rate of reported cases of chlamydia in the 50 most populous metropolitan statistical areas (MSAs) increased 8.3% during 2016–2017 (518.8 to 561.7 cases per 100,000 population, respectively) (Table 6). In 2017, 58.5% of chlamydia cases were reported by these MSAs. During 2016–2017, the rate of reported cases of chlamydia in these MSAs increased 6.2% among women (663.0 to 704.1 cases per 100,000 females) and 12.1% among men (367.0 to 411.3 cases per 100,000 males) (Tables 7 and 8).
Chlamydia by County
In 2017, 630 (20.1%) of 3,140 counties had rates of reported chlamydia higher than 542 cases per 100,000 population (Figure 4). Seventy counties and independent cities reported 43% of all chlamydia cases in 2017 (Table 9). Of the 70 counties and independent cities reporting the highest number of chlamydia cases, 46 (65.7%) were located in the South and West (Table 9).
Chlamydia by Sex
In 2017, 1,127,651 cases of chlamydia were reported among females for a rate of 687.4 cases per 100,000 females (Table 4). After increasing each year during 2000–2011, the rate of reported chlamydia cases among females decreased during 2011–2013, followed by an increase in the rate of reported cases over each of the next four years (Figure 1). The total rate increase during 2013–2017 among females was 11.1%.
Among males, 577,644 cases of chlamydia were reported in 2017 for a rate of 363.1 cases per 100,000 males (Table 5). The rate of reported cases among males increased each year during 2000–2017, with the exception of 2012–2013, when rates remained stable (Figure 1). During 2016–2017 alone, the rate among men increased 10.5%; however, during 2013–2017, rates of reported cases among men increased 39.3% (compared with an 11.1% increase among women) (Tables 4 and 5). This pronounced increase among men could be attributed to either increased transmission or improved case identification (e.g., through intensified extra-genital screening efforts) among gay, bisexual, and other men who have sex with men (collectively referred to as MSM). This cannot be assessed, however, as most jurisdictions do not routinely report sex of sex partner or anatomic site of infection.
Despite this considerable increase in men, the rate of reported chlamydia cases among females was still about two times the rate among males in 2017, likely reflecting a larger number of women screened for this infection (Figure 1, Tables 4 and 5). The lower rate among men also suggests that many of the sex partners of women with chlamydia are not receiving a diagnosis of chlamydia or being reported as having chlamydia.
Chlamydia by Age
The rates of reported cases of chlamydia were highest among adolescents and young adults aged 15–24 years during 2013–2017 (Table 10). In 2017, the age-specific rate of reported cases of chlamydia among 15–19 year olds was 2072.4 cases per 100,000 population and the rate among 20–24 year olds was 2,820.3 cases per 100,000 population (Table 10).
In 2017, 97.4% of all reported chlamydia cases in women were among those aged 15–44 years. The highest age-specific rates of reported cases of chlamydia in 2017 were among those aged 15–19 years (3,265.7 cases per 100,000 females) and 20–24 years (3,985.8 cases per 100,000 females) (Figure 5, Table 10). Within these age groups, rates were highest among women aged 19 years (5,398.6 cases per 100,000 females) and 20 years (5,141.4 cases per 100,000 females) (Table 12).
Increases have been observed in recent years in rates of reported cases of chlamydia among all age groups in females aged 15–44 years (Figure 6). Specifically, the rate of reported cases among women aged 15–19 and 20–24 years increased over the last three and four years, respectively. The rate among 15–19 year olds increased 6.5% during 2016–2017, with a total increase of 10.7% during 2014–2017 (2,949.3 to 3,265.7 cases per 100,000 females) (Table 10). The rate among 20–24 year olds increased 3.7% during 2016–2017, with a total increase of 10.9% during 2013–2017 (3,594.2 to 3,985.8 cases per 100,000 females) (Table 10).
In 2017, 94.2% of all reported chlamydia cases in men were among those aged 15–44 years. The age specific rates of reported cases of chlamydia among men, although substantially lower than rates among women, were highest in those aged 20–24 years (1,705.4 cases per 100,000 males) (Figure 5, Table 10). Similar to trends in women, increases have been observed in rates of reported cases of chlamydia among all age groups in males aged 15–44 years (Figure 7). Specifically, the rate of reported cases among men aged 15–19 and 20–24 years increased over the last three and four years, respectively. The rate among 15–19 year olds increased 11.1% during 2016–2017, with a total increase of 28.0% during 2014–2017 (722.9 to 924.5 cases per 100,000 males). The rate among 20–24 year olds increased 7.8% during 2016–2017, with a total increase of 30.1% during 2013–2017 (1,310.9 to 1,705.4 cases per 100,000 males) (Table 10).
Chlamydia by Race/Hispanic Ethnicity
Rates of reported cases of chlamydia were highest among Black, American Indian/Alaska Native (AI/AN), and Native Hawaiian/Other Pacific Islander (NHOPI) women (Figure S, Table 11B). Overall, the rate of reported cases of chlamydia among Blacks was 5.6 times the rate among Whites (1,175.8 and 211.3 cases per 100,000 population, respectively). The rate among AI/ANs (781.2 cases per 100,000 population) was 3.7 times the rate among Whites. The rate among NHOPIs (715.4 cases per 100,000 population) was 3.4 times the rate among Whites. The rate among Hispanics (404.1 cases per 100,000 population) was 1.9 times the rate among Whites. The rate among Asians (129.6 cases per 100,000 population) was 0.6 times the rate among Whites.
During 2013–2017, rates of reported chlamydia cases increased among all racial and Hispanic ethnicity groups, with AI/ANs increasing 3.7%, Asians 29.6%, Blacks 6.1%, NHOPIs 19.4%, Whites 20.2%, Multirace 59.9%, and Hispanics 10.5% (Figure 8). During 2016–2017, rates increased among all racial and Hispanic ethnicity groups (AI/ANs: 5.0%, Asians: 10.7%, Blacks: 5.2%, NHOPIs: 11.0%, Whites: 5.8%, Multirace: 11.4%, and Hispanics: 9.5%) (Figure 8).
More information on chlamydia rates among race/Hispanic ethnicity groups can be found in the Special Focus Profiles.
Chlamydia by Reporting Source
In 2017, 5.8% of chlamydia cases were reported from STD clinics, 78.6% were reported from venues outside of STD clinics, and 15.6% had an unknown reporting source (Table A2). Over time, the proportion of male cases reported from STD clinic sites has decreased substantially, from 30.5% in 2008 to 9.7% in 2017 (Figure 9). In 2017, among women, only 3.8% of chlamydia cases were reported through an STD clinic (Table A2). A large proportion of cases among women (31.6%) were reported from private physicians/HMOs in 2017 (Figure 10). Among men, 24.8% of cases were reported from private physicians/HMOs (Figure 9).
Chlamydia Prevalence in the Population
The National Health and Nutrition Examination Survey (NHANES; see Section A2.4 in the Appendix) is a nationally representative survey of the US civilian, non-institutionalized population that provides an important measure of chlamydia disease burden in respondents aged 14–39 years. During 2013–2016, the overall prevalence of chlamydia among persons aged 14–39 years was 1.7% (95% Confidence Interval [CI]: 1.3–2.1) (Figure 11). Among sexually active females aged 14–24 years, the population targeted for screening, prevalence was 4.3% (95% CI: 2.7–5.8), with the highest prevalence among Mexican American females (10.0%, 95% CI: 4.0–15.9) (Figure 12).
Chlamydia Positivity in Selected Populations
The STD Surveillance Network (SSuN) is an ongoing collaboration of 10 state, county, and city health departments collecting enhanced clinical and behavioral information among patients attending 30 STD clinics in the SSuN jurisdictions (See Section A2.2 of the Appendix).
In 2017, the proportion of STD clinic patients testing positive for chlamydia varied by age, sex, and sexual behavior. Adolescent men who have sex with women only (MSW) had the highest positivity (39.2%), either reflecting disproportionate testing of men with urethritis or targeted testing of partners of women diagnosed with chlamydia. Positivity among all those tested decreased with age, though the variation in positivity by age was not as pronounced for MSM (Figure 13). The overall positivity, represented by the average of the mean value by the 10 SSuN jurisdictions, was 17.5% for MSM, 15.0% for MSW, and 11.5% for women.
Chlamydia Among Special Populations
More information on chlamydia among women of reproductive age, adolescents and young adults, MSM, and minority populations is presented in the Special Focus Profiles.
Chlamydia continues to be the most commonly reported nationally notifiable disease, with 1,708,569 cases reported in 2017 and increasing rates of reported cases over each of the last four years. Rates of reported chlamydia cases increased 6.9% during 2016–2017. The Southern region of the US reported the highest rate of chlamydial infection in 2017; the West reported the largest rate increase during 2016–2017. In 2017, the rate of reported cases of chlamydia in women was nearly two times the rate in men. However, during 2013–2017, the rate in men increased 39.3%, whereas the rate in women increased only 11.1%. Potential reasons for this considerable increase in male cases could be due to a true increase in infections or to improved screening coverage in men, especially increased extra-genital screening in MSM.
The facilities reporting chlamydial infections have changed over the last 10 years or so, with most (78.6%) chlamydia cases in 2017 reported from venues outside of STD clinics. The proportion of men being diagnosed with chlamydia in STD clinics decreased 68.2% from 30.5% in 2008 to 9.7% in 2017; in 2017 alone, approximately one-third of chlamydia cases among women were reported from private physicians/HMOs. Racial differences also persist; reported case rates among Blacks continue to be substantially higher than among all other racial/Hispanic ethnicity groups. Rates of reported cases of chlamydia were next highest among AI/AN and NHOPI women. Ultimately, both test positivity and the number of reported cases of C. trachomatis infections remain high among most age groups, racial/Hispanic ethnicity groups, geographic areas, and both sexes.
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4. Oakeshott P, Kerry S, Aghaizu A, et al. Randomised controlled trial of screening for Chlamydia trachomatis to prevent pelvic inflammatory disease: The POPI (prevention of pelvic infection) trial. BMJ 2010; 340: c1642.
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8. National Committee for Quality Assurance. The State of Healthcare Quality 2017. Available at: http://www.ncqa.org/report-cards/health-plans/state-of-health-care-quality/2017-table-of-contents/chlamydia-screeningExternal. Accessed June 29, 2018.