Harrisons Manual of Medicine, 18th Ed.

CHAPTER 107. Chlamydial Infections


• Chlamydiae are obligate intracellular bacteria, possess both DNA and RNA (a characteristic that distinguishes them from viruses), and have a cell wall similar to that of gram-negative bacteria.

• These organisms have a complex reproductive cycle and exist in two forms.

– The elementary body (the infective form) is adapted for extracellular survival, while the reticulate body is adapted for intracellular survival and multiplication.

– Within 18–24 h after infection of the cell, reticulate bodies have replicated and begin to condense into elementary bodies that are released to infect other cells or people.

• Three chlamydial species infect humans: Chlamydia trachomatis, C. psittaci, and C. pneumoniae.

– CF tests and enzyme immunoassays that detect lipopolysaccharide identify chlamydiae only to the genus level.

– The microimmunofluorescence (MIF) test can differentiate among the three species.



See Chap. 92.



• Trachoma is a chronic conjunctivitis caused by C. trachomatis serovars A, B, Ba, and C. Transmission occurs through contact with ocular discharge from infected pts, which can also be transferred by flies.

• AIC is an acute eye infection in adults exposed to infected genital secretions and in their newborns. This infection is caused by sexually transmitted C. trachomatis strains, usually serovars D through K.


Trachoma is a leading cause of preventable infectious blindness, with ~6 million pts having been affected. In the hyperendemic regions of northern and sub–Saharan Africa, the Middle East, and parts of Asia, the prevalence of trachoma is ~100% by the third year of life. Reinfection and persistent infection are common.

Clinical Manifestations

Both trachoma and AIC present clinically as conjunctivitis characterized by small lymphoid follicles in the conjunctiva, although trachoma usually starts insidiously before 2 years of age.

• With progression, there is inflammatory leukocytic infiltration and superficial vascularization (pannus formation) of the cornea.

– Scarring eventually distorts the eyelids, turning lashes inward and abrading the eyeball (trichiasis and entropion).

– The corneal epithelium eventually ulcerates, with subsequent scarring and blindness.

– Destruction of goblet cells, lacrimal ducts, and glands causes dry-eye syndrome (xerosis), with resultant corneal opacity and secondary bacterial corneal ulcers.

• AIC is an acute unilateral follicular conjunctivitis with preauricular lymphadenopathy and presents similarly to acute conjunctivitis due to adenovirus or HSV.

– Corneal inflammation is evidenced by discrete opacities, punctate epithelial erosions, and superficial corneal vascularization.

– Left untreated, the disease may persist for 6 weeks to 2 years.


Clinical diagnosis is based on the presence of two of the following signs: lymphoid follicles on the upper tarsal conjunctiva, typical conjunctival scarring, vascular pannus, or limbal follicles.

• Intracytoplasmic chlamydial inclusions are found in 10–60% of Giemsa-stained conjunctival smears from children with severe inflammation.

• However, chlamydial nucleic acid amplification tests are more sensitive in detecting infection.


• AIC responds to azithromycin (a single 1-g oral dose) or doxycycline (100 mg PO bid for 7 days). Treatment of sexual partners is needed to prevent ocular reinfection and chlamydial genital disease.


Etiology and Epidemiology

Most avian species can harbor C. psittaci, but psittacine birds (e.g., parrots, parakeets) are most often infected; human infections are uncommon and occur only as a zoonosis.

• Exposure is greatest in poultry workers and in owners of pet birds.

• Present in nasal secretions, excreta, tissues, and feathers of infected birds, C. psittaci is transmitted to humans by direct contact with infected birds or by inhalation of aerosols. Transmission from person to person has never been documented.

• As a result of quarantine of imported birds and improved veterinary-hygienic measures, outbreaks and sporadic cases of psittacosis are now rare, with fewer than 50 confirmed cases reported in the U.S. each year.

Clinical Manifestations

Psittacosis in humans can range in severity from asymptomatic or mild infections to acute primary atypical pneumonia (which can be fatal in 10% of untreated cases) to severe chronic pneumonia.

• After an incubation period of >5–19 days, pts present with fever, chills, muscular aches and pains, severe headaches, hepatomegaly and/or splenomegaly, and gastrointestinal symptoms.

• Cardiac complications may include endocarditis and myocarditis.


This diagnosis is confirmed by serologic studies.

• The gold standard is the MIF test.

• Any antibody titer >1:16 or a 4-fold rise between paired acute- and convalescent-phase serum samples, in combination with a clinically compatible syndrome, can be used to diagnose psittacosis.

TREATMENT C. psittaci Infections

• Tetracycline (250 mg PO qid for 3 weeks) is the antibiotic of choice.

• Erythromycin (500 mg PO qid) is an alternative agent.



C. pneumoniae is a common cause of human respiratory diseases, primarily in young adults.

• Seroprevalence rates of 40–70% demonstrate that C. pneumoniae is widespread worldwide. Seropositivity is first detected at school age and then increases by ~10% per decade.

• The role of C. pneumoniae in atherosclerotic disease has long been discussed, but large-scale treatment studies have cast doubts on the etiologic role of this organism in this disease.

Clinical Manifestations

The clinical spectrum of C. pneumoniae infection includes acute pharyngitis, sinusitis, bronchitis, and pneumonia.

• Pneumonia due to C. pneumoniae resembles that due to Mycoplasma pneumoniae. Pts have antecedent upper respiratory tract symptoms, fever, nonproductive cough, minimal findings on auscultation, small segmental infiltrates on chest x-ray, and no leukocytosis.

– Primary infection is more severe than reinfection.

– Elderly pts can have severe disease.


Serology is the most clinically useful means for diagnosing C. pneumoniae infection.

• The diagnosis of acute C. pneumoniae infection requires demonstration of a 4-fold rise in titer between acute- and convalescent-phase serum samples.

• Culture of the organism is difficult and is not routinely attempted. PCR assays for C. pneumoniae are currently available only for research purposes.

TREATMENT C. pneumoniae Infections

• Erythromycin or tetracycline (2 g/d for 10–14 days) is recommended.

• Other macrolides (e.g., azithromycin) or quinolones (e.g., levofloxacin) are alternative agents.


For a more detailed discussion, see Gaydos CA, Quinn TC: Chlamydial Infections, Chap. 176, p. 1421, in HPIM-18.