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Chlamydia trachomatis infections are the most commonly-reported notifiable disease in the United States. They are among the most prevalent of all STDs and, since 1994, have comprised the largest proportion of all STDs reported to CDC (Table 1). Recent studies also demonstrate the high prevalence of chlamydial infections in the general U.S. population. From 1999 to 2002, chlamydia prevalence among participants (aged 14-39 years) in the National Health and Nutrition Examination Survey was 2.2%.1 Among young adults (18-26 years of age) participating in the National Longitudinal Study of Adolescent Health from 2001 to 2002, chlamydia prevalence was 4.2%.2

In women, chlamydial infections, which are usually asymptomatic, may 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 a reduction in the incidence of PID by as much as 60%.3 As with other inflammatory STDs, chlamydial infection can facilitate the transmission of HIV infection. In addition, pregnant women infected with chlamydia can pass the infection to their infants during delivery, potentially resulting in neonatal ophthalmia and pneumonia. Due to the large burden of disease and risks associated with infection, CDC recommends screening all sexually active women younger than 26 years of age for chlamydia annually.4

The increase in reported chlamydial infections during the last 10 years reflects the expansion of chlamydia screening activities, use of increasingly sensitive diagnostic tests, an increased emphasis on case reporting from providers and laboratories, improvements in the information systems for reporting, and, possibly, true increases in disease. However, many women who are at risk are still not being tested, reflecting, in part, lack of awareness among some health care providers and limited resources available to support screening. Chlamydia screening and reporting are likely to continue to expand further in response to the Healthcare Effectiveness Data and Information Set (HEDIS) measure for chlamydia screening of sexually active women 15 through 25 years of age who receive medical care through commercial or Medicaid managed care organizations.5

To better monitor trends in disease burden in defined populations during the expansion of chlamydia screening activities, data on chlamydia positivity among persons screened in a variety of settings are used. In most instances, test positivity serves as a reasonable approximation of prevalence.6

Chlamydia - United States

In 2000, for the first time, all 50 states and the District of Columbia had regulations requiring the reporting of chlamydia cases.

In 2006, 1,030,911 chlamydial infections were reported to CDC from 50 states and the District of Columbia (Table 1). This is the first time reported cases of chlamydia have exceeded 1 million. This case count corresponds to a rate of 347.8 cases per 100,000 population, an increase of 5.6% compared with the rate of 329.4 in 2005. The reported number of chlamydial infections was almost three times the number of reported cases of gonorrhea (358,366 gonorrhea cases were reported in 2006) (Table 1).

From 1987 through 2006, the rate of reported chlamydial infection increased from 50.8 to 347.8 cases per 100,000 population (Figure 1, Table 1).

Chlamydia by Region

For the years 1997-2001, chlamydia rates in the southern region of the United States were slightly higher than in any other region of the country (Figure 2, Table 3). For the years 2002-2006, overall rates were comparable in the Midwest, West, and South. Rates have consistently remained lowest in the Northeast. In 2006, rates increased in the South, West, and Northeast (363.3, 357.9, 299.0 cases per 100,000 population, respectively) and remained the same in the Midwest (352.4 cases).

Chlamydia by State

In 2006, chlamydia rates per 100,000 population by state ranged from 152.4 cases in New Hampshire to 681.8 cases in Alaska (Figure 3, Table 2). Thirty states, the District of Columbia, and Guam had chlamydia case rates higher than 300 cases per 100,000 population.

Chlamydia by Metropolitan Statistical Area (MSA)

In 2006, the chlamydia case rate per 100,000 population in the 50 most populous MSAs increased overall, among both women and men (Table 7). Among women, the 2006 case rate of 533.8 is a 4.4% increase over the 2005 case rate of 511.3 (Table 8). The 2006 case rate among men (191.5 per 100,000 population) increased 7.5% from the 2005 case rate (178.2) (Table 9). In 2006, 56.7% of chlamydia cases were reported by these MSAs.

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 2006, 842 (26.8%) of 3,140 counties had rates greater than 300.0 cases per 100,000 population. Rates per 100,000 population were 150.0 or less in 1,296 counties (41.3%) and between 150.1 and 300.0 in 1,002 counties (31.9%). Eighty-nine counties and independent cities reported 50% of all chlamydia cases in 2006. Fifty-four with the greatest number of cases are shown in Table 6, with case rates ranging from 213.3 (Miami-Dade County, Florida) to 1,330.3 (St. Louis (City), Missouri) per 100,000 population.

Chlamydia by Reporting Source

The majority of chlamydia cases reported in 2006 were reported from venues outside of STD clinics (Figure 5, Table A2). Among women, only 12.0% of chlamydia cases were reported through an STD clinic (93,169 of 775,788 total cases). In contrast, among men, 32.7% of chlamydia cases were reported through an STD clinic in 2006 (82,638 of 252,630 total cases).

Chlamydia by Race/Ethnicity

In 2006, chlamydia rates increased for all racial and ethnic groups except Asian/Pacific Islanders (Figure 6, Table 11B). The rate of chlamydia among African Americans was over eight times higher than that of whites (1,275.0 and 153.1 cases per 100,000, respectively). The rates among American Indian/Alaska Natives (797.3) and Hispanics (477.0) were also higher than that of whites (5.2 and 3.1 times higher, respectively). In 2006, the chlamydia case rate per 100,000 population among Asian/Pacific Islanders was 132.1, a decrease of 11.0% from the 2005 rate (148.4).

Chlamydia by Sex

In 2006, the overall rate of reported chlamydial infection among women in the United States (515.8 cases per 100,000 females) was almost three times as high as the rate among men (173.0 cases per 100,000 males), likely reflecting a greater 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 being diagnosed or reported as having chlamydia. However, with the advent of highly sensitive nucleic acid amplification tests that can be performed on urine, symptomatic and asymptomatic men are increasingly being diagnosed with chlamydial infection. From 2002 through 2006, the chlamydial infection rate in men increased by 36.4% (from 126.8 to 173.0 cases per 100,000 males) compared with a 15.9% increase in women during the same period (from 445.0 to 515.8 cases per 100,000 females).

Chlamydia by Age and Sex

Among women, the highest age-specific rates of reported chlamydia in 2006 were among those 15 to 19 years of age (2,862.7 cases per 100,000 females) and 20 to 24 years of age (2,797.0 cases per 100,000 females) (Figure 7, Table 10). These increased rates in women may, in part, reflect increased screening in this group. Age-specific rates among men, while substantially lower than the rates among women, were highest in the 20- to 24-year-old age group (856.9 cases per 100,000 males) (Figure 7, Table 10).

Chlamydia Screening and Prevalence Monitoring Project

Chlamydia screening and prevalence monitoring activities were initiated in Health and Human Services (HHS) Region X (Alaska, Idaho, Oregon, Washington) in 1988 as a CDC-supported demonstration project. In 1993, chlamydia screening services for women were expanded to three additional HHS regions (III, VII, and VIII) and, in 1995, to the remaining HHS regions (I, II, IV, V, VI, and IX). In some regions, federally-funded chlamydia screening supplements local-and state-funded screening programs. Screening criteria and practices vary by region and state.

In 2006, the median state-specific chlamydia test positivity among 15- to 24-year-old women who were screened during visits to selected family planning clinics in all states and outlying areas was 6.7% (range 2.8% to 16.9%) (Figures 8 and 9). See Appendix (Chlamydia, Gonorrhea, and Syphilis Prevalence Monitoring) for details.

To examine trends in regional chlamydia positivity, rates are adjusted to account for changes in laboratory test methods and associated increases in test sensitivity (Figure 10, see Appendix).7 Even after adjustment, chlamydia test positivity has remained fairly stable in all 10 HHS regions between 2002 and 2006. Positivity slightly decreased in three regions from 2005 to 2006, increased in five regions, and remained the same in two regions.

Chlamydia Among Special Populations

Additional information on chlamydia screening programs for women of reproductive age and chlamydia among adolescents and minority populations can be found in the Special Focus Profiles.

Chlamydia Summary

Both prevalence and reported cases of genital Chlamydia trachomatis infections remain high across age groups, race/ethnicity groups, geographic locales, and both sexes. The burden of chlamydia appears higher among women, especially those of younger age (15 to 19 and 20 to 24 years of age), but this may be a reflection of which persons are screened. Racial differences also persist; case rates among African Americans continue to be substantially higher than rates among other race/ethnicity groups.

1 Datta SD, Sternberg M, Johnson RE, Berman S, Papp JR, McQuillan G, Weinstock H. Gonorrhea and chlamydia in the United States among persons 14 to 39 years of age, 1999 to 2002. Ann Intern Med 2007;147(2):89-96.

2 Miller WC, Ford CA, Morris M, Handcock MD, Schmitz JL, Hobbs MM, Cohen MS, Mullan Harris K, Udry JR. Prevalence of chlamydial and gonococcal infections among young adults in the United States. JAMA 2004;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):1362-66.

4 Centers for Disease Control and Prevention. Sexually Transmitted Diseases Treatment Guidelines, 2006. MMWR, 2006;55(No.RR-11):38.

5 National Committee for Quality Assurance (NCQA). HEDIS 2000: Technical Specifications, Washington, DC, 1999, pp. 68-70, 285-286.

6 Dicker LW, Mosure DJ, Levine WC. Chlamydia positivity versus prevalence: what’s the difference? Sexually Transmitted Diseases 1998;25:251-3.

7 Dicker LW, Mosure DJ, Levine WC, et al. Impact of switching laboratory tests on reported trends in Chlamydia trachomatis infections. Am J Epidemiol 2000;51:430-5

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