Chlamydia are obligately intracellular bacteria. They lack several metabolic and biosynthetic pathways and depend on host cells for intermediates including ATP. They are sometimes referred to as ATP parasites.
They exist in two unique life stages: (1) extracellular infectious particles called elementary bodies and (2) intracellular replicative forms called reticulate bodies. Reticulate bodies form inclusions in the cytoplasm of infected cells and give rise to the term inclusion conjunctivitis, one of the disease entities caused by the principal human pathogenic species Chlamydia trachomatis.
Trachoma and inclusion conjunctivitis are distinct entities caused by different strains (often referred to as serovars) of Chlamydia trachomatis.
Trachoma is a major cause of blindness in the developing world as a result of chronic and repeat infections resulting in corneal scarring and opacification. It has been eradicated in North America.
Further reading CDC – Trachoma
Inclusion conjunctivitis is caused by “genital strains” of Chlamydia trachomatis commonly found in North America. Though it does occur in adults in conjunction with genital disease, the most common manifestation is ophthalmia neonatorum – conjunctivitis of newborns as a result of passage through an infected birth canal. This is the main reason that all newborns have erythromycin ointment or another topical antibiotic applied to their eyes shortly after birth.
Chlamydia trachomatis is a very common cause of sexually transmitted disease in North America. Although asymptomatic infection is possible, disease in males is most often characterized by urethral discharge and pain indistinguishable from the disease caused by the other principal pathogen, Neisseria gonorrheae.
Women mainly suffer from cervicitis and pelvic inflammatory disease which is likely the most common preventable cause of infertility. Urethritis (sometimes referred to as the “acute urethral syndrome”) also occurs and should be thought of when women complain of dysuria but do not have signs of bacterial cystitis. However many infections in females are completely asymptomatic making women the main “reservoir” of the organism.
Further reading CDC – STD Chlamydia
Lymphogranuloma venereum is chronic sexually transmitted disease, uncommon in North America but highly prevalent in Africa, Asia and South America, that is caused by specific serotypes of Chlamydia trachomatis (L1, L2 and L3) that are distinct from the serotypes that are prevalent in North America and cause cervicitis and urethritis.
Newborn Respiratory Infection
Exposure to genital strains of C. trachomatis at birth can result in interstitial pneumonitis approximately 2 – 3 weeks after birth. The illness is usually mild with a distinctive staccato cough. Therapy with macrolide antibiotics is curative.
Because chlamydia are obligately intracellular organisms culture can only be achieved by infecting receptive cell cultures. This is the most specific way of identifying C. trachomatis. However, it is not a sensitive way of finding chlamydia and is no longer a routine method of diagnosing chlamydia disease.
Many antibody based antigen detection methodologies are available for the detection of chlamydia organisms.
Nucleic Acid Methodologies
Several amplication methods (e.g. PCR) are commercially available for detecting chlamydia. This is currently the method used most commonly to identify Chlamydia trachomatis from clinical samples in Canada.
Chlamydia pneumonia is a relatively recently described species and is a fairly common cause of pneumonia and bronchitis in adults. Disease is similar to that caused by Mycoplasma pneumoniae and viruses and the distinction can be very difficult.
Specific diagnosis is made serologically, however few laboratories are equipped for the necessary specialized testing.
Further reading CDC – Chlamydia pneumoniae
Psittacosis is caused by infection with C. psittaci an organism endemic in many birds. Infection occurs by inhalation of dried bird excrement and disease can be severe. It is manifest by pulmonary and systemic symptoms. As expected bird handlers are at greatest risk. Diagnosis is confirmed serologically.
Further reading CDC – Chlamydia psittaci