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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, 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 26 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 45.1% in 2012. During the same time period, the screening rate among sexually-active women aged 16–24 years covered by Medicaid increased from 40.4% to 57.1%.7 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.
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.
In 2013, a total of 1,401,906 chlamydial infections were reported to CDC in 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 446.6 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 2011–2012, the national rate of reported cases remained stable (453.4 to 453.3 cases per 100,000). During 2012–2013, the rate decreased 1.5% to 446.6 cases per 100,000. This is the first time since national reporting began that the rate of reported cases of chlamydia has decreased.
Chlamydia by Region
In 2013, rates of reported chlamydia were highest in the South (485.1 per 100,000 population), followed by the Midwest (439.0), the West (424.9), and the Northeast (403.3) (Table 3). Between 2004–2012, rates of reported cases of chlamydia increased in all regions (Figure 2). During 2012–2013, the rate decreased in the Northeast by 3.1%, in the Midwest by 2.7%, and in the South by 1.4% and increased in the West by 0.6% (Table 3).
Chlamydia by State
In 2013, rates of reported cases of chlamydia by state ranged from 236.2 cases per 100,000 population in New Hampshire to 789.4 cases in Alaska (Figure 3, Table 2); the rate in the District of Columbia was 1,104.4 cases per 100,000 (Table 3). During 2012–2013, rates of reported chlamydia decreased in 26 states and in the District of Columbia.
Chlamydia by Metropolitan Statistical Area
In 2013, the rate of reported cases of chlamydia per 100,000 population in the 50 most populous metropolitan statistical areas (MSAs) decreased 2.4% from the rate in 2012 (462.7 and 474.2 cases per 100,000, respectively) (Table 6). In 2013, 56.5% of chlamydia cases were reported by these MSAs. Among women in these MSAs, the rate decreased 3.6% during 2012–2013 (653.1 to 629.6 cases per 100,000) (Table 7). Among men, the 2013 rate (286.7 per 100,000 males) was similar to the 2012 rate (286.5 cases per 100,000 males) (Table 8).
Chlamydia by County
Counties in the United States with the highest rates of reported cases of chlamydia were located primarily in the Southeast and West, including Alaska (Figure 4). In 2013, 866 (27.6%) of 3,142 counties had rates higher than 400.0 cases per 100,000 population. Seventy counties and independent cities reported 43% of all chlamydia cases in 2013 (Table 9).
Chlamydia by Sex
In 2013, 993,348 cases of chlamydia were reported among females for a case rate of 623.1 per 100,000 females. During 1995–2011, the rate among females increased each year (Figure 1). During 2011–2012 the rate decreased slightly (0.7%) from 643.4 to 638.7 cases per 100,000 and during 2012–2013 decreased 2.4% from 638.7 to 623.1 cases per 100,000 (Table 4).
The overall case rate among males increased slightly (0.8%) during 2012–2013 (260.6 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 males in 2013, 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 2009–2013, the rate of reported cases among men increased 21.0% (from 217.1 to 262.6 cases per 100,000 males) compared with a 6.2% increase among women during the same period (from 586.7 to 623.1 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 2013, the rate among 15–19 year olds was 1,852.1 cases per 100,000 and the rate among 20–24 year olds was 2,451.6 cases per 100,000.
Among women, the highest age-specific rates of reported chlamydia in 2013 were among those aged 15–19 years (3,043.3 cases per 100,000 females) and 20–24 years (3,621.1 cases per 100,000 females) (Figure 5, Table 10). Within these age ranges, reported rates were highest among women aged 19 years (4,767.2 cases per 100,000 females) and aged 20 years (4,507.3 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 and decreased 8.7% during 2012–2013. The rate increased slightly (1.8%) among women aged 20–24 years during 2011–2012 and was stable during 2012–2013.
Age-specific rates among men, although substantially lower than the rates among women, were highest in those aged 20–24 years (1,325.6 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. Among men aged 20–24 years, the reported case rate was similar in 2012 and 2013 (1,322.8 and 1,325.6 cases per 100,000 males).
Chlamydia by Race/Ethnicity
Among the 47 jurisdictions (46 states and the District of Columbia) that submitted data in the race and ethnicity categories in 2013 according to Office of Management and Budget (OMB) standards, rates of reported cases of chlamydia were highest among black men and women (Figure L, Table 11B). The rate of chlamydia among blacks was 6.4 times the rate among whites (1,147.2 and 180.3 cases per 100,000 population, respectively). The rate among American Indians/Alaska Natives (697.9 cases per 100,000) was 3.9 times the rate among whites. The rate among Hispanics (377.0 cases per 100,000) was 2.1 times the rate among whites. The rate among Native Hawaiians/Other Pacific Islanders (633.3 cases per 100,000) was 3.5 times the rate among whites. The rate among Asians was lower than the rate among whites (111.5 cases and 180.3 cases per 100,000, respectively).
During 2009–2013, 40 jurisdictions (39 states and the District of Columbia) submitted chlamydia case report data in the race and ethnicity categories according to the OMB standards. Between 2009–2012, rates increased among all races and ethnicities (Figure 6). During 2012–2013, rates decreased among American Indians/Alaska Natives (5.0%), among blacks (6.8%), and among whites (0.8%), were stable among Hispanics, and increased 10.0% among Native Hawaiians/Other Pacific Islanders (Figure 6).
Chlamydia by Reporting Source
Most chlamydia cases reported in 2013 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 2013, among women, only 5.5% of chlamydia cases were reported through an STD clinic (Figure 8). Most cases among women were reported from private physicians/health maintenance organizations (HMOs) (33.2%). Among men, 17.2% of chlamydia cases were reported from an STD clinic in 2013 and 24.3% 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.1 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 2013, 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 2013, 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 (31.7%), likely reflecting targeted testing of partners of females diagnosed with chlamydia. Among MSW and women, prevalence among those tested decreased with age. 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 1,401,906 cases reported in 2013. For the first time since national reporting began, the rate of reported cases of chlamydia decreased. Decreases in the overall rate were driven by decreases among women; in particular, decreases among women 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 Datta SD, Torrone E, Kruszon-Moran D, Berman S, Johnson R, Satterwhite CL, et al. Chlamydia trachomatis trends in the United States among persons 14 to 39 years of age, 1999-2008. Sex Transm Dis. 2012;39(2):92-6. doi: 10.1097/OLQ.0b013e31823e2ff7.
2 Miller WC, Ford CA, Morris M, Handcock MS, Schmitz JL, Hobbs MM, et al. Prevalence of chlamydial and gonococcal infections among young adults in the United States. JAMA. 2004 12;291(18):2229-36.
3 Scholes D, Stergachis A, Heidrich FE, Andrilla H, Holmes KK, Stamm WE. Prevention of pelvic inflammatory disease by screening for cervical chlamydial infection. N Engl J Med. 1996;34(21):1362-6.
4 Oakeshott P, Kerry S, Aghaizu A, Atherton H, Hay S, Taylor-Robinson D, et al. Randomized 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, 2010 MMWR Morb Mortal Wkly Rep. 2010; No.59(RR-12):1-110. Erratum in: MMWR Recomm Rep. 2011;60(1):18.
7 National Committee for Quality Assurance. The state of healthcare quality 2013. Washington (DC): National Committee for Quality Assurance; 2013. p. 79-81.
- Page last reviewed: December 16, 2014 (archived document)
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