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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). In women, chlamydia 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%.1 As with other inflammatory STDs, chlamydia 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 aged less than 26 years for chlamydia.2

The increase in reported chlamydia 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, and improvements in the information systems for reporting. 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 Health Plan Employer Data and Information Set (HEDIS) measure for chlamydia screening of sexually active women 15 through 25 years of age who receive medical care through managed care organizations or Medicaid.3 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.4

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 2005, 976,445 chlamydia infections were reported to CDC from 50 states and the District of Columbia (Table 1). This case count corresponds to a rate of 332.5 cases per 100,000 population, an increase of 5.1% compared with the rate of 316.5 in 2004. The reported number of chlamydia infections was almost three times the number of reported cases of gonorrhea (339,593 gonorrhea cases were reported in 2005) (Table 1).

From 1986 through 2005, the rates of reported chlamydia infection increased from 35.2 to 332.5 cases per 100,000 population (Figure 1, Table 1). The continuing increase in reported cases likely represents the further expansion of screening for this infection, the development and use of more sensitive screening tests, and more complete national reporting.

Chlamydia by Sex

In 2005, the overall rate of reported chlamydia infection among women in the United States (496.5 cases per 100,000 females) was over three times higher than the rate among men (161.1 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 diagnosed or reported. 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 chlamydia infection. From 2001 through 2005, the chlamydia infection rate in men increased by 43.5% (from 112.3 to 161.1 cases per 100,000 males) compared with a 15.6% increase in women over the same period (from 429.6 to 496.5 cases per 100,000 females).

Chlamydia by Region

For the years 1996–2001, the chlamydia rates in the Southern region of the United States were slightly higher than the rates in any other region of the country (Figure 2, Table 3). For the years 2002–2005, overall rates were comparable in the Midwest, West, and South (353.7, 343.6, 338.1 cases per 100,000 population, respectively). Although slight increases occurred in all regions, rates have remained lowest in the Northeast since 1996. In 2005, the case rate per 100,000 population in the Northeast was 282.5.

Chlamydia by State

In 2005, chlamydia rates per 100,000 population by state ranged from 141.7 cases in New Hampshire to 732.6 cases in Mississippi (Figure 3, Table 2). Thirty-two states and one outlying area had chlamydia case rates higher than 300.0 cases per 100,000 population.

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 2005, 799 (25.4%) 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 1,391 counties (44.3%) and between 150.1 and 300.0 in 950 counties (30.3%).

Chlamydia by Reporting Source

The majority of chlamydia cases reported in 2005 were reported through non-STD clinics (Figure 5, Table A2). Among women, only 12.5% of chlamydia cases were reported through an STD clinic (92,229 of 740,371 total cases). Women are more frequently asymptomatic and less likely than men to seek care at an STD clinic. In contrast, among men, 33.8% of chlamydia cases were reported through an STD clinic (78,677 of 232,781 total cases).

Chlamydia by Race

In 2005, chlamydia rates increased for all race/ethnic groups (Figure 6, Table 10B). The rate of chlamydia among blacks was over eight times higher than that of whites (1,247.0 and 152.1 cases per 100,000, respectively). The rates among American Indian/Alaska Natives (748.7) and Hispanics (459.0) were also higher than that of whites (4.9 and 3.0 times higher, respectively).

Chlamydia by Age and Sex

Among women, the highest age-specific rates of reported chlamydia in 2005 were among 15- to 19-year-olds (2,796.6 cases per 100,000 females) and 20- to 24-year- olds (2,691.1 cases per 100,000 females). These increased rates in women may be, in part, due to increased screening in this group. Age-specific rates among men, while substantially lower than the rates in women, were highest in the 20- to 24-year-olds (804.7 cases per 100,000 males) (Figure 7, Table 9).

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 2005, the median state-specific chlamydia test positivity among 15- to 24-year-old women who were tested during visits to selected family planning clinics in all states and outlying areas was 6.3% (range 3.0% to 20.3%) (Figures 8 and 9). See Appendix (Chlamydia, Gonorrhea, and Syphilis Prevalence Monitoring) for details.

To examine trends in regional chlamdyia positivity, rates were adjusted to account for changes in laboratory test methods and associated increases in test sensitivity (Figure 10, see Appendix).5 Even after adjustment, chlamydia test positivity has remained fairly stable within regions from 2001–2005. Positivity slightly decreased in six of ten HHS regions from 2004 through 2005, increased in three regions, and remained the same in one region.

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.

1 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.

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

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

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

5 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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