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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.1,2
Chlamydial infections in women are usually asymptomatic. However, 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 might facilitate the transmission of human immunodeficiency virus (HIV) infection.5 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 25 years receive annual chlamydia screening.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 plans, chlamydia screening increased from 23.1% in 2001 to 46.2% in 2013. 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 54.9% in 2013.7 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 diagnostics tests (e.g., nucleic acid amplification tests) can also increase the number of infections identified and reported independent 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 2014, a total of 1,441,789 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 456.1 cases per 100,000 population. During 1993–2011, the rate of reported chlamydial infection increased from 178.0 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. During 2013–2014, the national rate of reported cases increased 2.8% to 456.1 cases per 100,000.
Chlamydia by Region
In 2014, rates of reported chlamydia were highest in the South (492.3 per 100,000 population), followed by the Midwest (448.9), the West (441.8), and the Northeast (406.9) (Table 3). Between 2005–2012, 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 2013–2014, rates increased in all regions with the largest increase in the West (421.1 to 441.8 cases per 100,000) (Table 3).
Chlamydia by State
In 2014, rates of reported cases of chlamydia by state ranged from 254.5 cases per 100,000 population in West Virginia to 787.5 cases in Alaska (Figure 3, Table 2); the rate in the District of Columbia was 818.8 cases per 100,000 (Table 3). During 2013–2014, rates of reported chlamydia increased in 40 states.
Chlamydia by Metropolitan Statistical Area
In 2014, the rate of reported cases of chlamydia per 100,000 population in the 50 most populous metropolitan statistical areas (MSAs) increased 3.6% from the rate in 2013 (458.3 and 474.6 cases per 100,000, respectively) (Table 6). In 2014, 56.9% of chlamydia cases were reported by these MSAs. In these MSAs, the rate among women increased 1.7% during 2013–2014 (623.8 to 634.5 cases per 100,000) (Table 7) and the rate among men increased 8.1% (283.8 to 306.8 cases per 100,000) (Table 8).
Chlamydia by County
Counties in the United States with the highest rates of reported cases of chlamydia were located primarily in the South and West, including Alaska (Figure 4). In 2014, 785 (25.0%) of 3,142 counties had rates higher than 439 cases per 100,000 population. Seventy counties and independent cities reported 43% of all chlamydia cases in 2014 (Table 9).
Chlamydia by Sex
In 2014, 1,006,441 cases of chlamydia were reported among females for a case rate of 627.2 per 100,000 females. During 1995–2011, the rate among females increased each year (Figure 1). During 2011–2013, the rate decreased from 643.4 to 619.0 cases per 100,000 females and during 2013–2014, the rate increased 1.3% to 627.2 cases per 100,000 (Table 4).
After remaining stable during 2012–2013, the overall case rate among males increased (6.8%) during 2013–2014 (260.6 to 278.4 cases per 100,000 males). As in previous years, the reported case rate among females was about two times the case rate among males in 2014, 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 nucleic acid amplification tests (NAATs) that can be performed on urine, chlamydial infection is increasingly being diagnosed in symptomatic and asymptomatic men. During 2010–2014, the rate of reported cases among men increased 22.4% (from 233.2 to 278.4 cases per 100,000 males) compared with a 6.0% increase among women during the same period (from 605.1 to 627.2 cases per 100,000 females).
Chlamydia by Age
Rates of reported cases of chlamydia are highest among adolescents and young adults aged 15–24 years (Table 10). In 2014, the rate among 15–19 year olds was 1,804.0 cases per 100,000 and the rate among 20–24 year olds was 2,484.6 cases per 100,000.
Among women, the highest age-specific rates of reported chlamydia in 2014 were among those aged 15–19 years (2,941.0 cases per 100,000 females) and 20–24 years (3,651.1 cases per 100,000 females) (Figure 5, Table 10). Within these age ranges, reported rates were highest among women aged 19 years (4,640.4 cases per 100,000 females) and aged 20 years (4,567.5 cases per 100,000 females) (Table 12). After increasing steadily during 2000–2011, the rate among women aged 15–19 years decreased 5.6% during 2011–2012, decreased 8.7% during 2012–2013, and decreased 4.2% during 2013–2014. The rate increased among women aged 20–24 years during 2011–2014 (3,630.0 to 3,651.1 per 100,000 females) (Table 10).
Age-specific rates among men, although substantially lower than the rates among women, were highest in those aged 20–24 years (1,368.3 cases per 100,000 males) (Figure 5, Table 10). Similar to trends in women, after increasing for the last decade, reported case rate among men aged 15–19 years decreased 5.1% during 2011–2012 and decreased 9.0% during 2012–2013. Rates among men aged 15–19 years decreased slightly during 2013–2014 (722.9 to 718. 3 per 100,000 males). Among men aged 20–24 years, the reported case rate increased 4.4% during 2013–2014 (1,310.9 to 1,368.3 cases per 100,000 males).
Chlamydia by Race/Ethnicity
Among the 48 states that submitted data in the race and ethnicity categories in 2014 according to Office of Management and Budget (OMB) standards (see Section A1.5 in the Appendix), rates of reported cases of chlamydia were highest among non-Hispanic black men and women (Table 11B). The rate of chlamydia among non-Hispanic blacks was 6.2 times the rate among non-Hispanic whites (1,117.9 and 180.6 cases per 100,000 population, respectively). The rate among American Indians/Alaska Natives (668.8 cases per 100,000) was 3.7 times the rate among whites. The rate among Hispanics (380.6 cases per 100,000) was 2.1 times the rate among whites. The rate among Native Hawaiians/Other Pacific Islanders (625.1 cases per 100,000) was 3.5 times the rate among whites. The rate among Asians was lower than the rate among whites (112.0 cases and 180.6 cases per 100,000, respectively).
During 2010–2014, 43 states submitted chlamydia case report data in the race and ethnicity categories according to the OMB standards (see Section A1.5 in the Appendix). Between 2010–2014, rates increased among all races and ethnicities, except blacks (Figure 6). During 2013–2014, rates increased among whites (2.1%) and Asians (5.2%), and decreased 2.4% among American Indians/Alaska Natives. Rates were stable among blacks, Hispanics, Native Hawaiians/Other Pacific Islanders during 2013–2014 (Figure 6).
More information on chlamydia rates among race/ethnicity groups can be found in the Special Focus Profiles.
Chlamydia by Reporting Source
Most chlamydia cases reported in 2014 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 2014, among women, only 4.9% of chlamydia cases were reported through an STD clinic (Figure 8). Most cases among women were reported from private physicians/health maintenance organizations (HMOs) (33.6%). Among men, 14.9% of chlamydia cases were reported from an STD clinic in 2014 and 25.0% were reported from private physicians/HMOs.
Chlamydia Prevalence in the Population
The National Health and Nutrition Examination Survey (NHANES; see Section A2.4 in the Appendix for more information) 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. 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 highest prevalence among 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 states and independently funded cities collecting enhanced clinical and behavioral information among patients attending STD clinics in the SSuN jurisdictions. Due to a transition from SSuN Cycle 2 to Cycle 3, data for this report include information from patients attending STD clinics during 2014 in the 6 jurisdictions that overlap cycles. See Section A2.2 of the Appendix for more information about SSuN methodology.
In 2014, 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 (28.4%), either reflecting disproportionate testing of men with urethritis or targeted testing of partners of females 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 (MSM) (Figure 9).
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 1,441,789 cases reported in 2014. During 2013–2014, rates of reported chlamydia increased 2.8% overall, but decreased 4.2% among 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 Torrone E, Papp J, Weinstock H; Centers for Disease Control and Prevention (CDC). Prevalence of Chlamydia trachomatis genital infection among persons aged 14-39 years--United States, 2007-2012. MMWR Morb Mortal Wkly Rep. 2014 Sep 26;63(38):834-8.
4 Oakeshott P, Kerry S, Aghaizu A, Atherton H, Hay S, Taylor- Robinson D, 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. doi: 10.1136/bmj.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;75:3-17.
6 Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines, MMWR Morb Mortal Wkly Rep.2010; No.59(RR-12):1-110. Erratum in: MMWR Recomm Rep. 2011;60(1):18
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