Sunday, February 5, 2012


Author: Dr H. Hunter Handsfield Clinical Professor of Medicine University of Washington


CHLAMYDIA : The Most Common Bacterial STD

Chlamydia trachomatis inclusion bodies. Source: Public Health Image Library (CDC)

Genital infection with chlamydia—scientific name Chlamydia trachomatis—is the most common bacterial sexually transmitted disease (STD) in industrialized countries and probably worldwide.  Three to 4 million cases are estimated to occur in the United States each year and the World Health Organization estimates at least 90 million annual cases worldwide. The primary anatomic sites of infection are the urethra in men and the uterine cervix and urethra in women.  Infections of the rectum, acquired through receptive anal intercourse, also are common, especially among men who have sex with men (MSM).  As for most STDs, chlamydia presents the greatest health threat in women, primarily through infection that ascends from the cervix to involve the uterus and Fallopian tubes, resulting in pelvic inflammatory disease (PID).  Chlamydial PID in turn often results in scarring or blockage of the tubes, making chlamydia one of the most common causes of female infertility and ectopic (tubal) pregnancy.  Chlamydia also can lead to serious health consequences in infants born to infected mothers.  Most infections initially are asymptomatic or cause outwardly trivial symptoms, which paradoxically increases the frequency and severity of long-term consequences by delaying diagnosis and treatment.  As for all STDs associated with genital inflammation, chlamydia enhances susceptibility of infected persons to acquiring human immunodeficiency virus (HIV), contributing to the worldwide AIDS pandemic.

Lymphogranuloma venereum (LGV) is a rare STD caused by particular strains of chlamydia bacteria.  LGV has made a recent comeback among MSM in industrialized countries, but remains rare overall.  Some strains of C. trachomatis cause the blinding eye disease trachoma; as reflected in the scientific name of the organism, trachoma was actually the first chlamydial disease identified.  The strains that cause trachoma are not sexually transmitted, but are passed from person to person by flies, contaminated clothing, or bedclothes, or by nonsexual person-to-person contact.  Historically, trachoma was one of the most common causes of blindness worldwide and it remains an important public health problem in some developing countries.  Other related organisms, not transmitted sexually, are C. pneumoniae, an occasional cause of respiratory infection, and C. psittaci, an infection of birds that sometimes causes pneumonia in humans.
This Knol addresses sexually transmitted chlamydia in adults.  The information on frequency, risk factors, and populations at risk emphasize the United States and is largely applicable to Western Europe, Australia and New Zealand.  However, routine diagnostic testing and other prevention strategies have not been widely implemented in most other countries, so that reliable epidemiologic data are scarce.  Thus, in most of the world the incidence, prevalence, and frequency of complications is likely to be substantially higher than in the United States, with the likely exception of some predominantly Islamic countries.  The information on symptoms, complications, treatment, and prevention principles applies to all geographic areas. 


In the United States and many industrialized countries, chlamydia cases are routinely reported to health authorities, such as state or local health departments.  The US Centers for Disease Control and Prevention (CDC) summarizes the annual data.  Reported data, however, are incomplete; many cases are not diagnosed at all and others are not reported or counted. Therefore extrapolation and often creative interpretation, as well as special research efforts, are needed to estimate the actual infection rates.  The incidence of chlamydia (or any other disease) is the number of cases occurring over a particular time, such as infections per year).  The prevalence of a condition is the number of cases that exist at any particular moment, such as the percent of the population carrying the infection.  Measures of both incidence and prevalence are used to estimate the frequency of chlamydia and other STDs. 
This graph shows the incidence of reported chlamydia in the United States from 1987 to 2006.  Slightly over 1 million cases were reported in 2006, corresponding to 348 per 100,000 members of the population.  After adjusting for infections that go undiagnosed and for infections not reported to health authorities, about 4 million new infections are estimated to occur annually in the United States.  The higher rate in females compared with males in Figure 1 is more apparent than real, because the data show reported infections, not actual case rates.  More women than men have regular chlamydia testing, so that more cases are diagnosed; the actual incidence is about the same in both sexes.  The upward sloping curves in the figure are related more to increased testing over time, and the rising frequency with which cases are reported, than to actually rising rates of infection.
The bar graph illustrates the dramatic association of chlamydia with younger age, especially in women and girls.  For every 100,000 15- to 19-year-old girls in the United States, 2,863 (almost 3%) became infected with chlamydia in 2006.  Since only about 60% of teen girls are sexually active, the adjusted rate approximates 4,500 per 100,000.  That is, almost 5% of sexually active teen girls nationwide acquire chlamydia every year.  Even this dramatic figure underestimates the rate in some population groups.  The rates vary widely between geographic areas and by socioeconomic status and race/ethnicity; among minority populations in some parts of the country, up to 30% of teens acquire chlamydia each year.  The rates undoubtedly are substantially higher in most countries of Eastern Europe, Africa, and Asia.
The overall frequency of chlamydia in the US has risen modestly in the past decade, compared to apparent declines in the 1990s; a similar trend has been observed in Sweden.  There is controversy about the reasons, but most experts attribute the changes to increased testing, use of increasingly accurate diagnostic tests, and a true rise in incidence.  One theory suggests that improved case detection is having an unintended side effect:  because of earlier diagnosis and treatment of many infections, people may develop less complete immunity to reinfection, so that they remain more susceptible to repeat infection, possibly resulting in a vicious circle of increased case rates as a paradoxical consequence of improved prevention.  Nationwide, in 1999-2002, at any point in time 2% of all men and women were carrying chlamydia, including 4.6% of teen girls and 3.2% of 20-29 year old men.  Among patients at especially high risk, such as those obtaining health care at health department STD clinics, typically 5-20% of patients are infected.  Among people with gonorrhea, 10% to 30% also have chlamydia.  However, the reverse is not true:  among persons with chlamydia, fewer than 1% are also infected with gonorrhea.

Except for the strains that cause trachoma, chlamydia is transmitted almost exclusively by penile-vaginal or penile-anal sex.  Despite common beliefs to the contrary, chlamydia uncommonly infects the mouth or throat, and unlike gonorrhea, transmission from an oral partner to the genitals through fellatio or cunnilingus is rare, if it occurs at all.  Chlamydia is common among MSM, in whom it is transmitted predominantly by penile-anal intercourse.  Rectum to rectum transmission, through fingers, sex toys, and shared douching equipment, probably accounts for most LGV infections in MSM.


The hallmarks of chlamydial infection are symptoms that result directly from infection of the urethra in men, the cervix and sometimes the urethra in women, and the rectum of either men or women.

Urethral Infection in Men
Up to half of men who acquire infection of the urethra remain asymptomatic.  When symptoms occur, they usually start 7-21 days after exposure, although sometimes the infection shows up several weeks after acquisition.  Among men with symptoms, the most common clinical diagnosis is nongonococcal urethritis (NGU).  The main manifestations of NGU are abnormal discharge from the penis, typically cloudy or white in color.  Sometimes there dysuria, i.e., discomfort in the penis when urinating, which may range in severity from a minor sensation of itchiness in the urethra to, rarely, severe pain.
The name itself distinguishes NGU from gonorrhea, or gonococcal urethritis.  (The term nonspecific urethritis, or NSU, is used in some countries for the same condition.)  However, chlamydia accounts for only 20% to 40% of NGU cases.  Some of the remainder are caused by the sexually transmitted bacteria Mycoplasma genitalium and others might result from specific strains of Ureaplasma urealyticum.  Four percent to 5% of cases are due to herpes simplex virus, adenovirus, or Trichomonas vaginalis.  No known cause can be found for at least 30% to 40% of NGU cases, although epidemiologic data support the notion that almost all cases are sexually acquired.  Most likely, as yet undiscovered bacteria often are responsible.  A few cases may be due to immunological causes rather than infection per se.  One theory is that prior infection with chlamydia, gonorrhea or other organisms sometimes might stimulate an aberrant immune response that causes symptoms in the absence of continuing infection per se.

Genital Infection in Women

Up to 80% of infected women remain asymptomatic, and when symptoms occur they are mild and nonspecific; that is, infected women may notice nothing different than normal day-to-day variations in the amount of vaginal discharge and other minor genital discomfort.  Women who notice chlamydia symptoms typically experience increased vaginal discharge and sometimes urinary urgency or discomfort on urination, sometimes mimicking the symptoms of non-sexually transmitted urinary tract infection.  Spotty vaginal bleeding between menstrual periods, sometimes triggered by sex, also is common, and some infected women experience heavier than normal bleeding when they menstruate.  Pain in the lower abdomen indicates the development of PID, the main complication.  
Examination by an experienced clinician may reveal cervicitis, i.e., inflammatory signs of the uterine cervix, such as increased redness or abnormal discharge of mucus or pus from the cervical opening, sometimes characterized as mucopurulent cervicitis (MPC), illustrated in the first photo to the right.  Gentle swabbing of the cervical opening may induce bleeding (second photo).  As with NGU in men, many infections other than chlamydia cause MPC in women.  Most cases of MPC are not associated with known sexually transmitted organisms and, as for NGU, half the time no infection at all can be identified as the cause.

Other Manifestations of Uncomplicated Chlamydial Infection
Most infections of the rectum are asymptomatic, but some infected persons notice discharge of mucus or pus from the anus, pain, itching of the anus, and sometimes rectal bleeding.  A few people with genital chlamydia also have conjunctivitis, resulting in irritation, itching, and other signs similar to “pink-eye”.  Such cases probably result primarily from auto-inoculation, i.e., self-infection after contamination of the fingers with genital secretions.

Natural Course of Untreated Infection
Without treatment, chlamydial infections can persist in the genital tract for several months, sometimes a year or more, especially in women.  The longest proved case persisted 4 years, but rare infections might persist even longer.  Therefore, detection of chlamydia in a person who has not been an apparent risk for STD—for example, a monogamous person or someone who has not recently been sexually active—does not necessarily mean a partner has been unfaithful or that the patient has not been truthful about recent sexual activity.  Nevertheless, the large majority of infections apparently are cleared by the immune system within several months, so that new infection usually indicates recent sexual exposure to an infected partner.


Pelvic Inflammatory Disease.  The main complication of chlamydia is PID, the result of lower genital tract chlamydia that migrates to the uterus and fallopian tubes.  PID typically causes lower abdominal pain, sometimes with fever, and often abnormal vaginal bleeding, including both bleeding between periods and abnormally heavy menstruation.  Severe cases can include abscesses of the Fallopian tubes or elsewhere in the lower abdomen, sometimes requiring surgery, including hysterectomy, for cure.  Symptomatic PID is estimated to occur in 10% to 15% of women with chlamydia.  PID is especially common with repeat chlamydial infection.  Asymptomatic PID also is common, and chlamydia can be detected in the uterus of most infected women, usually without obvious illness, and many women who are infertile or who have an ectopic (tubal) pregnancy because chlamydia scarred their Fallopian tubes have no recollection of abdominal pain or other PID symptoms.  Chlamydia is responsible for a large proportion, perhaps a majority, of tubal infertility and ectopic pregnancy cases in industrialized countries.
Epididymitis.  The most common chlamydia complication in men is acute epididymitis, infection of the sperm collecting ducts and testicle; it is the male counterpart of PID.  Epididymitis results in painful enlargement of the testicle, usually on one side only, often accompanied by fever and signs of NGU, described above.  Marked swelling and tenderness are the rule, and epididymitis rarely explains mild forms of testicular pain or discomfort.  The photograph illustrates a typical case; the enlargement of the right testicle is apparent.

Reactive Arthritis.
  Reactive arthritis is a relatively uncommon complication characterized by pain and swelling of certain joints, eye inflammation, a unique skin rash, and NGU in men or cervicitis in women.  The NGU or cervicitis can be confusing to both patient and physician, since they can occur either from the initial chlamydial infection or as an immunologic reaction, part of the reactive arthritis syndrome itself. Reactive arthritis results from an abnormal immune response in susceptible individuals, triggered by any of several infections of the genital or gastrointestinal tract.  Among sexually active young persons, chlamydia is the most common inciting infection.

Infection of the Newborn.
  The most common manifestation of transmission to the newborn from an infected mother is conjunctivitis (eye infection), often called ophthalmia neonatorum.  Exposed infants also can develop chlamydial pneumonia or an asthma-like chronic bronchitis, with chronic cough persisting through several weeks or months of age.  The occurrence of neonatal chlamydia represents a failure of prevention, since all pregnant women should be tested and, if infected, treated before delivery.

Lymphogranuloma Venereum. LGV has two forms.  The “classical” version, occurring in both men and women, causes swelling and inflammation of the lymph nodes in the groin, sometimes progressing to rupture of the nodes and drainage of pus.  Today, the most common version is infection of the rectum (proctitis) in MSM, often with severe pain, bleeding, and constipation.  Modern LGV in MSM appears not to be transmitted by penile anal intercourse, and even with the modern resurgence of LGV, few MSM have been diagnosed with genital infection or with inflamed lymph nodes in the groin.  These facts are consistent with rectum to rectum transmission, discussed above.  Even with the recent resurgence in MSM, LGV remains rare.


Chlamydia can be suspected by the trained clinician, based on symptoms and physical examination.  However, the findings are identical to those caused by other genital, rectal, and abdominal infections.  Further, the physical examination often is entirely normal in patients with or without symptoms.  Therefore, laboratory testing is necessary for definitive diagnosis.

Identification of Chlamydia trachomatis
The main diagnostic method is to identify chlamydia at the site of infection, usually after collecting a specimen on a swab or by testing voided urine.  Historically, cell culture was the preferred method, i.e., to inoculate a specimen into cells that support the growth of chlamydia bacteria, then identify chlamydia growing in the infected cells.  However, cell culture is cumbersome, time consuming, and expensive; while still useful as a research tool, it is not a practical method for widespread use.  In the 1980s a brief period passed in which tests were developed that used chemical methods to identify components of the organism, or its DNA, in swab specimens.  These tests were more efficient than culture, but they were plagued by both false positive and false negative test results.
The past 15 years has seen the development of the current gold standard assays, the nucleic acid amplification tests (NAAT).  With NAAT, minute quantities of DNA can be identified in clinical specimens.  By designing the test to identify a particular organism’s DNA, NAATs are both highly sensitive, i.e., capable of detecting almost all chlamydial infections, and highly specific, i.e., avoiding false positive results.  The most common and best known NAAT methodology is the polymerase chain reaction (PCR), which is now used in a wide range of diagnostic tests for many infectious diseases.  Other NAAT methods include the ligase chain reaction (LCR), the DNA strand displacement assay (SDA), and transcription mediated analysis (TMA).  One or more commercial tests to detect chlamydia has been developed with each of these technologies.  In the United States, the predominant, currently available tests are based on TMA (Aptima®, GenProbe), PCR (Amplicor®, Roche), or SDA (Probe-Tec®, Becton Dickinson).
One of the main advantages of NAAT compared with culture and other diagnostic tests for chlamydia is that NAATs are accurate when used to test voided urine or self-collected vaginal swabs.  Therefore, widespread screening of sexually active persons by NAAT can be carried out without cumbersome pelvic examinations in women or painful passage of swabs into men’s urethras.  These characteristics make testing more practical in clinics and physicians’ offices, and also allow performance of screening tests in non-medical settings, such as schools, places of employment, and even through the mail.  In the future, NAAT kits may be available for self-testing at home, as is now routine for pregnancy testing.   Recent research shows that the NAATs also give reliable results when used to test rectal specimens, making the tests useful in diagnosing chlamydia in MSM.  Their performance for testing pharyngeal (throat) specimens remains in question, but this is not an important limitation; throat infection is uncommon and testing for it is rarely if ever necessary.  Today NAATs account for almost all testing for chlamydia, as well as most testing for gonorrhea, in the United States and other industrialized countries.

Blood Tests
Serology, i.e., testing of blood for antibody produced by the immune system in response to infection, has an important role in the diagnosis of many infectious diseases.  However, blood tests have little role in diagnosing most chlamydial infections.  Serological tests have been used in research, and sometimes they are useful in diagnosing the underlying cause of female infertility, by identifying women with Fallopian tube obstruction due to chlamydia.  Serology also is used to diagnose LGV when NAAT testing of genital or rectal specimens does not reveal the LGV strains of C. trachomatis, particularly in patients with chronic, longstanding infection.


Authorities such as CDC and WHO recommend routine treatment when chlamydia is suspected, without awaiting diagnostic confirmation.  For example, treatment against chlamydia is routine for the sex partners of infected persons and for patients with MPC, NGU, proctitis, PID, or epididymitis, before NAAT results are known.  Similarly, because 10% to 30% of persons with gonorrhea are co-infected, treatment of gonorrhea routinely is accompanied by drugs active against chlamydia.  Table 1 lists the regimens recommended by CDC.
       Table 1.  Treatment of Uncomplicated Chlamydial Infections in Nonpregnant Adults

                        Recommended Regimens

                                Azithromycin* 1.0 gram in a single oral dose
                                Doxycycline+ 100 milligrams orally twice daily for 7 days

                        Alternative Regimens

                                Levofloxacin# 500 milligrams orally once daily for 7 days
                                Ofloxacin@ 300 milligrams orally twice daily for 7 days
                                Erythromycin+ 500 mg orally 4 times daily for 7 days
                        *  Zithromax® (Pfizer)
                        +  Many brands and generics
                        #  Levaquin® (Ortho-McNeil)
                        @ Floxin (Ortho-McNeil)

Routine Treatment.  The two primary drug classes used for treatment of chlamydia are the macrolide antibiotics and the tetracyclines.  The former group includes azithromycin (Zithromax®), clarithromycin (Biaxin®), erythromycin (many generic brands), and others.  The tetracyclines include doxycycline (Vibramycin® and generics), minocycline (Minocin® and generics), and tetracycline itself.  In most industrialized countries, the primary treatments recommended by CDC, WHO, and various health departments are either a single oral dose of azithromycin, 1.0 gram, or doxycycline, usually in a dose of 100 milligrams (0.1 gram) by mouth, twice daily for 7 days.  Either regimen is at least 95% effective (often 99% effective) in curing uncomplicated chlamydia.
Alternative Drugs.  Levofloxacin (Levaquin®) or other drugs of the fluoroquinolone class sometimes are used when azithromycin or doxycycline cannot be given, or when treatment must be started and the exact cause of the patient’s condition is not known, as when chlamydia and urinary tract infection both are possible explanations for the patient’s infection.  However, not all fluoroquinolones are useful against chlamydia.  For example, ciprofloxacin (Cipro®) is not effective.  The tetracyclines should not be used in pregnant women; erythromycin or amoxicillin are recommended as alternatives to treat pregnant women if azithromycin is not available or not tolerated.  PID and epididymitis are treated with combinations of drugs, designed to eradicate gonorrhea and various other bacteria that can cause or contribute to the infection. 

Treatment of Sex Partners
The Importance of Partner Treatment.  The sex partners of persons with chlamydia require treatment to protect their own health and that of other sex partners who might be at risk.  All STDs are selectively transmitted by infected persons without symptoms.  Many persons with genital discharge or other symptoms recognize the health implications, stop having sex, and seek care.  By contrast, those without symptoms have no awareness of the risk and continue sexual activity.  Therefore, the person responsible for a particular patient’s chlamydia is not likely to have symptoms that lead him or her to seek health care.  This principle underlies a central tenet of STD prevention:  infected patients and their health care providers must take active steps to inform partners and assure treatment.  With few exceptions, local or state health departments in the United States, as well as their counterparts in other countries, lack the resources to assist in notifying the partners of persons with chlamydia or gonorrhea.  Therefore, assuring treatment of partners is the responsibility of the infected person and the health care provider.
Ideally, the partners should be professionally examined and counseled before being treated.  However, many patients are unable or unwilling to contact the partners who they believe infected them.  Other partners cannot be located or contacted, and still others do not seek care despite being informed, often because they do not believe they are infected, especially if they have no STD symptoms.  Accordingly, it is likely that through these routine approaches, only about half of all partners of persons with chlamydia or gonorrhea receive timely treatment.  The remainder help to sustain the chlamydia epidemic, and they often re-infect the original patient.
Expedited Partner Treatment.  The concept of expedited partner treatment (EPT) has evolved in recent years to address the difficulty of assuring that partners receive treatment.  Under EPT—also known as patient-delivered partner therapy, or PDPT—infected patients are given extra drug or a prescription to be taken to their sex partners, without requiring partners to visit a provider’s office or clinic.  Other mechanisms also can be employed, such as direct retrieval of medication by the partner from the provider’s office (without examination) or from a pharmacy.  Three randomized, controlled clinical trials among persons with chlamydia or gonorrhea have demonstrated that EPT results in substantially more partners receiving treatment in comparison with historically routine partner management, i.e., notifying partners to attend for personal evaluation.  Most important, all three trials showed that reinfection of the initially infected person was less frequent with EPT than with routine management.  
Even before the research trials were conducted, EPT had been used frequently by health care providers in the United States and elsewhere, as a pragmatic accommodation to assure partner treatment and to protect their patients from reinfection.  CDC and other authorities now recommend EPT as a routine option to assure treatment of the partners of heterosexual men and women with chlamydia or gonorrhea. Health authorities have hesitated to recommend routine use of EPT among MSM, who tend to have higher rates of other STDs, especially undiagnosed HIV infection, along with their gonorrhea or chlamydial infections.  However, EPT remains an option when other methods to assure partner treatment are not likely to be successful.

Follow-up Testing
Test of Cure.  The recommended treatments are sufficiently reliable that routine retesting to assure cure generally is not necessary.  However, test of cure is recommended in pregnant women, both because the risk of treatment failure is elevated and because the consequences of inadequately treated infection may be severe.  Tests of cure also are recommended for infected persons who might be unable to comply with the complete course of treatment or if atypical antibiotic regimens are used.  Tests of cure should be done 3 weeks or more after completion of treatment.  Earlier testing risks false positive results, because NAATs can detect chlamydial DNA up to 2-3 weeks after the organism has been killed, resulting in false positive test results. 
Rescreening.  Although routine test of cure is generally not recommended for persons treated for chlamydia or gonorrhea, CDC and other health agencies now recommend “rescreening” in its place.  Rescreening means delayed retesting of infected persons, i.e. a few months after treatment.  Among men and women with either chlamydia or gonorrhea, 10% to 20% are found to be infected again when tested 3-6 months later.  Most cases result from reinfection, either from an untreated partner or by sex with other partners.  However, some cases represent persistent infection, despite normally effective antibiotic treatment; in these cases, rescreening serves the same role as earlier test of cure.  Rescreening 3 months after treatment is advised for all patients with chlamydia or gonorrhea, regardless of the patients’ certainty that their partners were treated, whether or not condoms were used for sex following treatment, and whether or not their current sex partners are believed to be at risk.


Prevention and control of STD can be viewed from the perspective of the individual at risk, who prefers to avoid infection (or, if infected, to prevent complications); and from the perspective of health care providers and prevention agencies, such as health departments.  These strategies have considerable overlap.  For example, health care providers are advised to routinely test patients at risk for chlamydia, but sexually active persons can request testing when the provider does not take the initiative.  Similarly, providers should promote condom use, but only persons at risk can choose to use them.

Public Health Prevention Strategies
The mainstay of chlamydia prevention is laboratory-based screening, i.e., routine testing of persons at risk.  Because most infections are asymptomatic, case finding cannot rely on attendance by persons in health care providers’ offices or clinics.  Owing to the high frequency of chlamydia in all sexually active populations, screening recommendations are not even based on indicators of STD risk, such as number of sex partners or new sexual partnerships.  Rather, CDC and other health agencies recommend routine testing by NAAT for all women age 25 and under who are sexually active (including monogamous women) whenever they seek health care for any reason.  Laboratory screening of men theoretically would contribute to control, but widespread male screening is difficult because outwardly healthy young men do not frequently visit health care providers.  However, screening tests for men are useful when those at risk can be readily accessed, such as men who are incarcerated, military recruits, and sometimes in schools.
In addition to diagnostic screening tests, important public health chlamydia prevention strategies include promulgation and use of recommended treatment regimens, assuring treatment of infected persons’ sex partners, counseling and education programs, professional education of health care providers in clinical management and prevention, and surveillance through required reporting of infections to local and state health departments, which in turn permits the efficient targeting of prevention resources where needed.

Personal Prevention
Consistent use of condoms for vaginal or anal intercourse outside committed, mutually monogamous relationships is a core prevention strategy for all STDs and HIV infection.  Chlamydia is no exception.  No sexually transmitted pathogen passes through an intact condom, and properly used condoms—i.e., placement prior to genital penetration, with prompt removal after sex, before erection is lost—are almost 100% effective.  (Apparent failures in preventing chlamydia are due mostly to improper use or condom rupture, not to actual failures of intact condoms.)  Specific sexual practices also influence the risk of STD.  Unprotected vaginal and anal sex risks transmission from either partner to the other. Chlamydia is rarely if ever transmitted in either direction by cunnilingus or fellatio, even though these practices confer modest risk for gonorrhea, herpes, and syphilis.  In general, however, unprotected oral-genital sex carries substantially lower risk for transmission of STDs, including HIV, than unprotected vaginal or anal intercourse.
Selecting sex partners at low risk is important as well in preventing chlamydia and other STDs.  Potential partners at especially high risk are commercial sex workers, persons believed to have multiple partners combined with inconsistent condom use, and users of illicit drugs.  Intuitively, having fewer sex partners enhances sexual safety while increased partner numbers increases the risk of chlamydia and all STDs.  However, partner number per se is less a risk factor than how and where partners are selected and whether or not condoms are consistently used.  Sexually active persons outside mutually monogamous relationships, especially teens and young adults, periodically should seek routine screening tests for chlamydia and other common STDs, especially gonorrhea, syphilis, HIV, and sometimes genital herpes.  The frequency of recommended screening depends on risk.  Annual testing makes sense for most sexually active young persons, but some—such as MSM with multiple partners, sexually active teens, and commercial sex workers—should be tested as often as 2-4 times per year.  Finally, all sexually active persons should be aware of the common symptoms of STD and promptly seek care when such health problems appear, either in themselves or their sex partners.  


1.    Stamm WE.  Chlamydia trachomatis Infections of the Adult.  Chapter 32 in Holmes KK, et al (ed).  Sexually Transmitted Diseases, 4th edition. New York, McGraw-Hill, 2008:575-93. The main chapter in the premier STD textbook, by one of the world's top experts in chlamydial infections.
2.    Handsfield HH. Color Atlas and Synopsis of Sexually Transmitted Diseases, 2nd edition. Chapter 2, Chlamydial Infections.  New York, McGraw-Hill, 2001:12-19. A succinct overview with extensive clinical photographs illustrating symptoms, intended for health professionals but easily understood by educated readers.
3.    Datta SD, et al.  Gonorrhea and chlamydia in the United States among Persons 14 to 39 Years of Age, 1999 to 2002.  Annals of Internal Medicine 2007;147:89-96. The report of the most comprehensive national survey of these STDs in the US.
4.    CDC.  Sexually Transmitted Disease Surveillance, 2006.  Atlanta, GA, November 2007. Annually updated statistics on reported STD in the US.
5.    CDC.  Sexually Transmitted Diseases Treatment Guidelines, 2006.  Morbidity and Mortality Weekly Report 2006;55:RR-11.  CDC's treatment recommendations, supplemented by extensive summaries of key information about all STDs.
6.    CDC.  Expedited Partner Therapy in the Management of Sexually Transmitted Diseases, 2006.  CDC's summary of outcomes of research studies and recommendations use of streamlined approaches to assure that partners of persons with gonorrhea or chlamydia receive treatment.
7.    Brunham RC, et al.  The unexpected impact of a Chlamydia trachomatis control program on susceptibility to reinfection.  Journal of Infectious Diseases 2005;192:1836-44. A thoughtful analysis of why chlamydia rates might be rising despite intensive prevention, suggesting that earlier treatment reduces immunity, leaving persons susceptible to reinfection.
8.    Golden MR, et al.  Impact of expedited sex partner treatment on recurrent or persistent gonorrhea or chlamydial infection:  a randomized controlled trial.  New England Journal of Medicine 2005;352:676-85. The most comprehensive of three research studies documenting the effectiveness of streamlined treatment of sex partners.
9.    Handsfield HH.  Nongonococcal urethritis:  a few answers but mostly questions (editorial).  Journal of Infectious Diseases 2006;193:333-5. A succinct summary of the causes of NGU and the remaining uncertainties about the syndrome.
10.    Scholes D, et al.  Prevention of pelvic inflammatory disease by screening for cervical chlamydial infection. New England Journal of Medicine 1996;334:1362–1366. The primary research study documenting that routine testing prevents the most important complication of chlamydia.
11.    Marrazzo JM, et al.  Impact of patient characteristics on performance of nucleic acid amplification tests and DNA probe for detection of Chlamydia trachomatis in women with genital infections. Journal of Clinical Microbiology 2005;4:577-84. The science behind the modern tests for chlamydia.