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 U.S. 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. 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).6
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 47.0% in 2014. Among sexually-active women aged 16–24 years covered by Medicaid, screening rates increased from 40.4% in 2001 to 58.0% in 2011, then decreased to 54.6% in 2014.7 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. Expanded use of more sensitive diagnostic tests (e.g., nucleic acid amplification tests [NAATs]) can also increase the number of infections identified and reported independently of increases in incidence. Although chlamydia has been a nationally notifiable condition since 1994, 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. Additionally, increasing use of electronic laboratory reporting has likely increased the proportion of diagnosed cases that are reported. Consequently, an increasing chlamydia case rate 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 2015, a total of 1,526,658 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 478.8 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. The rate of reported cases then increased in 2014 and again in 2015. During 2014–2015, the rate increased 5.9%, from 452.2 to 478.8 cases per 100,000 population (Figure 1, Table 1).
Chlamydia by Region
In 2015, rates of reported cases of chlamydia were highest in the South (520.5 cases per 100,000 population, 7.0% increase from 2014), followed by the Midwest (464.8, 3.8% increase from 2014), West (464.4, 6.4% increase from 2014), and Northeast (425.9, 5.1% increase from 2014) (Table 3). During 2006–2012, 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 2013–2015, rates increased in all regions, with the largest increase occurring in the West (421.1 to 464.4 cases per 100,000 population, 10.3% increase) (Table 3).
Chlamydia by State
In 2015, rates of reported cases of chlamydia by state ranged from 233.3 cases per 100,000 population in New Hampshire to 768.3 cases in Alaska (Figure 3, Table 2); the rate in the District of Columbia was 1,198.1 cases per 100,000 population (Table 3). During 2014–2015, rates of reported chlamydia increased in 37 states and the District of Columbia. The rate of reported chlamydia cases in 2015 was above the U.S. total in 18 states.
Chlamydia by Metropolitan Statistical Area
The rate of reported cases of chlamydia per 100,000 population in the 50 most populous metropolitan statistical areas (MSAs) increased 5.5% during 2014–2015 (469.1 to 494.8 cases per 100,000 population, respectively) (Table 6). In 2015, 56.7% of chlamydia cases were reported by these MSAs. During 2014–2015, the rate of reported cases of chlamydia in these MSAs increased 3.1% among women (626.7 to 646.4 cases per 100,000 females) and 10.5% among men (303.4 to 335.4 cases per 100,000 males) (Tables 7 and 8).
Chlamydia by County
In 2015, 784 (25.0%) of 3,141 counties had rates of reported chlamydia higher than 444 cases per 100,000 population (Figure 4). Seventy counties and independent cities reported 43.0% of all chlamydia cases in 2015 (Table 9). Of the 70 counties and independent cities reporting the highest number of chlamydia cases, 49 (70.0%) were located in the South and West (Table 9).
Chlamydia by Sex
In 2015, 1,045,143 cases of chlamydia were reported among females for a rate of 645.5 cases per 100,000 females (Table 4). After increasing each year during 2000–2011, the rate among females decreased during 2011–2013 from 643.4 to 619.0 cases per 100,000 females (Figure 1). The rate among females increased 0.4% during 2013–2014 and increased 3.8% during 2014–2015, for a total increase of 4.3% since 2013.
After remaining stable during 2012–2013, the rate of reported cases of chlamydia among males increased each year during 2013–2015 (for a total increase of 17.1%). During 2014–2015, the rate among men increased 10.5%, from 276.1 to 305.2 per 100,000 males. As in previous years, the rate of reported chlamydia cases among females was about two times the rate among males in 2015, 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.
However, with the advent of highly sensitive NAATs that can be performed on urine, chlamydial infection is increasingly being diagnosed in symptomatic and asymptomatic men. During 2011–2015, rates of reported cases among men increased 20.0% (from 254.4 to 305.2 cases per 100,000 males) compared with a 0.3% increase among women during the same time period (from 643.4 to 645.5 cases per 100,000 females) (Tables 4 and 5).
Chlamydia by Age
The rate of reported cases of chlamydia are highest among adolescents and young adults aged 15–24 years (Table 10). In 2015, the rate of reported cases of chlamydia among 15–19 year olds was 1,857.8 cases per 100,000 population and the rate among 20–24 year olds was 2,574.9 cases per 100,000 population (Table 10).
Among females, the highest age-specific rates of reported cases of chlamydia in 2015 were among those aged 15–19 years (2,994.4 cases per 100,000 females) and 20–24 years (3,730.3 cases per 100,000 females) (Figure 5, Table 10). Within these age groups, rates were highest among women aged 19 years (4,790.9 cases per 100,000 females) and 20 years (4,646.2 cases per 100,000 females) (Table 12). After increasing steadily during 2000–2011, the rate among women aged 15–19 years decreased each year during 2011–2014 (for a total decrease of 15.4%), but increased 1.5% during 2014–2015. The rate increased 2.7% among women aged 20–24 years during 2014–2015 (3,632.7 to 3730.3 per 100,000 females) (Table 10).
In 2015, 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 (1,467.8 cases per 100,000 males) (Figure 5, Table 10). Similar to trends in women, after increasing for the last decade, rates among men aged 15–19 years decreased each year during 2011–2014 (for a total decrease of 11.5%), but increased 6.3% during 2014–2015 (722.4 to 767.6 per 100,000 males). Among men aged 20–24 years, the rate increased 7.8% during 2014–2015 (1,361.3 to 1,467.8 cases per 100,000 males).
Chlamydia by Race/Ethnicity
Among the 50 states that submitted race and ethnicity data in 2015 according to Office of Management and Budget (OMB) standards (see Appendix A Section A1.5), rates of reported cases of chlamydia were highest among Black men and women (Figure P, Table 11B). The rate of reported cases of chlamydia among Blacks was 5.9 times the rate among Whites (1,097.6 and 187.2 cases per 100,000 population, respectively). The rate among American Indians/Alaska Natives (709.1 cases per 100,000 population) was 3.8 times the rate among Whites. The rate among Hispanics (372.7 cases per 100,000 population) was 2.0 times the rate among Whites. The rate among Native Hawaiians/Other Pacific Islanders (622.1 cases per 100,000 population) was 3.3 times the rate among Whites. The rate among Asians was lower than the rate among Whites (114.1 cases per 100,000 population).
During 2011–2015, 45 states submitted race and ethnicity data according to the OMB standards (see Appendix A Section A1.5). During 2011–2015, rates of reported chlamydia cases increased among Asians (7.8%), Native Hawaiians/Other Pacific Islanders (8.9%), Whites (14.6%), and Multirace (43.1%), and decreased in Blacks (11.2%) (Figure 6). Rates were stable among American Indians/Alaska Natives and Hispanics during 2011–2015. During 2014–2015, rates increased among Whites (2.6%), Asians (7.2%), Multirace (5.2%), and Hispanics (2.0%), and decreased among American Indians/Alaska Natives (3.5%). Rates were stable among Blacks and Native Hawaiians/Other Pacific Islanders during 2014–2015 (Figure 6).
More information on chlamydia rates among race/ethnicity groups can be found in the Special Focus Profiles.
Chlamydia by Reporting Source
In 2015, 7.2% of chlamydia cases were reported from STD clinics, 78.4% were reported from venues outside of STD clinics, and 14.4% had an unknown source of report (Table A2). Over time, the proportion of male cases reported from STD clinic sites has decreased substantially, from 32.7% in 2006 to 12.9% in 2015 (Figure 7). In 2015, among women, only 4.5% of chlamydia cases were reported through an STD clinic (Table A2). A large proportion of cases among women (32.8%) were reported from private physicians/ HMOs (Figure 8). Among men, 12.9% of chlamydia cases were reported from an STD clinic in 2015 and 24.4% were reported from private physicians/HMOs (Table A2, Figure 7).
Chlamydia Prevalence in the Population
The National Health and Nutrition Examination Survey (NHANES; see Appendix A Section A2.4) is a nationally representative survey of the U.S. civilian, noninstitutionalized population that provides an important measure of chlamydia disease burden in respondents aged 14–39 years. During 2007–2012, the overall prevalence of chlamydia among persons aged 14–39 years was 1.7% (95% Confidence Interval [CI]: 1.4–2.0) (Figure 10). Among sexually active females aged 14–24 years, the population targeted for screening, prevalence was 4.7% (95% CI: 3.2–6.1), with the highest prevalence among non-Hispanic Black females (13.5%, 95% CI: 9.2–17.7) (Figure 11).1
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 Appendix A Section A2.2).
In 2015, 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 prevalence (31.2%), either reflecting disproportionate testing of men with urethritis or targeted testing of partners of women diagnosed with chlamydia. Prevalence among all those tested decreased with age, though the variation in prevalence by age was not as pronounced for gay, bisexual, and other men who have sex with men (collectively referred to as MSM) (Figure 9).
Chlamydia Among Special Populations
Chlamydia continues to be the most commonly reported nationally notifiable disease, with 1,526,658 cases reported in 2015 and increasing rates of reported cases over each of the last two years. Rates of reported chlamydia cases increased 5.9% during 2014–2015. The Southern region of the U.S. reported the highest rate of chlamydial infection in 2015, as well as the largest rate increase during 2014–2015, at 7.0%. However, the Western region experienced the largest rate increase in reported chlamydia cases during 2011–2015, at 10.7%. In 2015, the rate of reported cases of chlamydia in women was 2.1 times the rate in men; however, the rate in men increased 20.0% during 2011–2015, whereas, the rate in women increased only 0.3% during the same time frame. After decreasing during 2011–2014, the rate among women aged 15–19 years increased 1.5% during 2014–2015.
The facilities reporting chlamydial infections have changed over the last 10 years, with most (78.4%) chlamydia cases in 2015 reported from venues outside of STD clinics. The proportion of men being diagnosed with chlamydia in STD clinics decreased 60.6% from 32.7% in 2006 to 12.9% in 2015, and approximately one-third of chlamydia cases among women were reported from private physicians/HMOs. Racial differences also persist; reported case rates and prevalence estimates among Blacks continue to be substantially higher than among all other racial/ethnic groups. 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.
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.
5 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(1): 3-17.
6 Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, 2015. MMWR Recomm Rep. 2015; 64(RR-3): 1-137.
Erratum in: MMWR 2015; 64(33): 924.
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- Page last updated: October 18, 2016
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