What is chlamydia?

Chlamydia is a sexually transmitted infection caused by bacteria. There are three main species of Chlamydia causing human infection: Chlamydia trachomatis, Chlamydia pneumoniae (also known as Chlamydophila pneumoniae), and Chlamydia psittacci.
C. pneumoniae and C. psittacci cause respiratory infections.
C. trachomatis causes infections that can affect the eyes, urogenital area, or cause pneumonia in infants born to infected mothers, depending on how the infection is transmitted. Trachoma is a nasty infection of the eyes that forms small granules along the conjunctiva (thin membrane covering the surface of the eye and inner surface of the eyelid). This can lead to ulceration, scarring and blindness. C. trachomatis infection is one of the most common sexually transmitted infections (STIs) in the United States, UK and Australia. C. trachomatis is often included in the category of non-gonnococcal urethritis. A chlamydia symptom is urethritis (inflammation of the urethra, the tube that carries urine from the bladder) that is not caused by Neisseria gonorrhoea.

Statistics on chlamydia

Chlamydia organisms have a worldwide distribution.
C. trachomatis is now extremely common and the most common sexually transmitted infection in Australia. In particular, men and women between 15 and 25 years of age are commonly affected. It is commonly asymptomatic, so is easily passed on between sexual partners. There has been a steady rise in the incidence of C. trachomatis, with a 76% increase between 1995 and 1999. Trachoma is common in the Aboriginal population of Australia, being an important causative factor for corneal scarring and blindness.
C. pneumoniae and C. psittacci are seen worldwide, being common causes of atypical pneumonia.


Chlamydia is currently the most common STD in men. Young, sexually active men are at greatest risk. 20% of men with gonorrhoea will also have chlamydia. Men suffer an anterior urethritis but approximately one quarter of men will have mild or no symptoms at all. 40% of non-gonococcal urethritis is due to chlamydia.


In women, the infection typically occurs in the endocervix and is very common with up to 5% of women affected. Women are much more commonly asymptomatic. Approximately 40% of women with gonorrhoea will have coexisting chlamydial infection.


As C. trachomatis is sexually transmitted, it does not normally occur in children. However, infants can acquire infections whilst in the womb of an infected mother. Infants get infections of the eyes, nose, throat or lungs. Neonatal conjunctivitis causes swelling, discharge and erythema, and if not treated may lead to pneumonia. Chlamydial conjunctivitis occurs in approximately 25% of infants who have an infected mother. Infant pneumonia usually develops a few weeks after birth and occurs in 10-20% of infants exposed to the bug at birth.

Risk factors for chlamydia

C. trachomatis:

  • For genito-urinary infection, unprotected intercourse is the main cause of chlamydia infection. Although patients with any sexually transmitted infection (STI) are at increased risk of co-infection with another STI, co-infection of chlamydia and gonorrhea is most common. Unprotected sex (without condoms) with a new partner (within 3 months) or sexual intercourse with many partners increases the risk of chlamydial infection. Oral or anal sex can cause pharyngitis and proctitis respectively.
  • C. trachomatis can be transmitted via the birth canal of an infected mother, and neonates exposed to chlamydia at birth may develop conjunctivitis 5-13 days later.
  • Ocular infection (trachoma or adult inclusion conjunctivitis) is transmitted through contaminated hands, droplets, clothing, and flies.

C. pneumoniae:

  • Respiratory infection with C. pneumoniae is transmitted from person to person through infectious aerosols.

C. psittacci

  • C. psittacci is carried by many species of birds (especially parrots) and can be transmitted to people in contact with the infected birds.

Progression of chlamydia

  • C. trachomatis can cause urethritis (inflammation of the urethra) in men. In women it causes cervicitis (inflammation of the cervix – the opening of the womb), urethritis or salpingitis (inflammation of the fallopian tubes which carry eggs from the ovaries). Reactive arthritis is a systemic syndrome affecting the eyes, joints and urethra and is frequently preceded by genital chlamydia infection.
  • LGV is a type of genital infection in which there is a genital lesion, followed by inflammation and swelling of lymph nodes in the inguinal region and systemic signs, with chronic ulceration possibly as a sequelae.
  • Trachoma can lead to progressive inflammation of the conjunctiva on the eyelid, with scarring of the eyelid and inturning of the eyelashes – which can lead to corneal abrasions an potentially blindness.
  • Infant pneumonia and neonatal conjunctivitis normally occur 1 to 3 weeks after birth and are usually uncomplicated.
  • C. psittacci and C. pneumoniae are usually uncomplicated. However, C. psittacci can cause a severe pneumonia with disease affecting several organ systems in epidemic cases.

How is chlamydia diagnosed?

  • Laboratory test may reveal general signs of chlamydia infection – raised white cell count, elevated CRP (a non-specific marker of inflammation).
  • CXR may reveal an atypical pneumonia picture in C. pneumoniae and C. psittacci.

Prognosis of chlamydia

Early antibiotic treatment is extremely successful and may prevent the development of long-term complications.
If these infections are not treated, they can lead to serious diseases and may be passed onto sexual partners. Untreated genital infection in men can lead to prostatitis, epididymitis and infertility. In women, the infection can spread to uterus (womb) and fallopian tubes leading to a condition known as pelvic inflammatory disease (PID). This causes abdominal pain, vaginal discharge, pain on sexual intercourse and can lead to infertility and abnormal pregnancies.
Ocular trachoma can lead to blindness due to chronic scarring of the cornea.

Treatment of chlamydia

Genital chlamydia treatment with a course of antibiotics after samples for laboratory tests are taken. Regimes for uncomplicated infection include 100mg 12-hourly doxycycline for 7 days, or 1g single dose oral azithromycin.
It is important that you tell anyone you have had sex with that you have chlamydia infection, so they can be treated to prevent nasty complications. This is particularly important as chlamydia may be asymptomatic. All sexual partners must be checked. Patients are guaranteed confidentiality. In addition, you should not have sexual intercourse until the full course of antibiotics for treatment of chlamydia is completed. It is also recommended to avoid alcohol whilst receiving treatment for chlamydia as it can inhibit the action of antibiotics and alcohol may lead to sexual activity.
Newborn baby chlamydia infections should be treated with erythromycin for 10-14 days.
For respiratory infection with C. pneumoniae and C. psittacci – tetracycline or erythromycin for 10 to 14 days. Chlamydia re-treatment is often needed, and infected birds with C. psittacci should also be treated with chlortetracycline hydrochloride.

More information

Sexually transmitted infections (STIs)
For more information on different types of sexually transmitted infections, prevention of STIs, treatments and effects on fertility, see Sexually transmitted infections (STIs).


Chlamydia prevention

Prevention of chlamydia infections include:

  • Always use condoms: These are the best contraceptive method to prevent transmission of sexually transmitted diseases such as chlamydia;
  • Have long-term relationships where both partners are not infected and neither person has additional partners;
  • Limit your sexual partners: The less people you have sex with, the less likelihood you will become infected with the pathogenic Chlamydia micro-organism;
  • Talk to your partners about sexually transmitted diseases;
  • Get early treatment of chlamydia if you suspect you show symptoms of chlamydia.


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