Chlamydia pneumonia is an acute pulmonary inflammation caused by Chlamydia pneumoniae (CP). Most cases are mild, with an insidious onset, and there is no gender difference in incidence. It can occur throughout the year and commonly affects both the upper and lower respiratory tracts, causing pharyngitis, laryngitis, tonsillitis, sinusitis, bronchitis, and pneumonia. Chlamydia pneumonia is more common in school-aged children, but it is rare in children under 3 years old. Outbreaks can occur in semi-enclosed environments such as households, schools, military barracks, and other densely populated workplaces, accounting for 10%-20% of community-acquired pneumonia (CAP) cases.
Etiology and Pathogenesis
CP is an obligate intracellular bacterium-like parasite belonging to the family Chlamydiaceae. Another species, Chlamydia psittaci, can also cause pneumonia in humans. CP has a biphasic life cycle consisting of an elementary body (EB) and a reticulate body (RB). The EB is a dense, spherical structure approximately 0.2-0.4 μm in diameter and is infectious, while the RB, also known as the initial body, is approximately 0.5-1 μm in diameter and is the replicative, non-infectious form of CP. CP is a human pathogen that is transmitted from human to human, primarily through respiratory droplets, though transmission via contaminated objects is also possible. Older individuals, those with poor nutrition, chronic obstructive pulmonary disease (COPD), or weakened immune function are more susceptible to infection.
Clinical Manifestations
The onset is often insidious, with early symptoms resembling those of Mycoplasma pneumonia. Symptoms are usually mild and include fever, rigors, myalgia, dry cough, non-pleuritic chest pain, headache, malaise, and fatigue, with hemoptysis being rare. Patients with pharyngitis may experience sore throat and hoarseness. Some individuals exhibit a biphasic disease course: initially presenting with pharyngitis that improves with symptomatic treatment, followed by pneumonia or bronchitis in 1-3 weeks, with worsening cough. A small proportion of patients may be asymptomatic. CP infection may also involve extrapulmonary manifestations, such as otitis media, arthritis, thyroiditis, encephalitis, and Guillain-Barré syndrome. Physical examination of the lungs typically reveals no abnormalities, though moist rales may occasionally be heard.
Laboratory and Other Examinations
White blood cell counts are usually normal or slightly elevated, and erythrocyte sedimentation rate (ESR) is often increased. Direct isolation of CP from sputum, throat swabs, pharyngeal secretions, or bronchoalveolar lavage fluid is considered the gold standard for diagnosis. However, CP cannot be cultured outside of host cells and requires inoculation into respiratory-derived cell lines (e.g., HEp-2 and HL cell lines), which is labor-intensive and generally limited to research settings, making it difficult to implement in most hospitals. Currently, the diagnosis of Chlamydia pneumonia primarily relies on serological tests. In primary infections, an acute-phase serum IgM titer ≥ 1:32 or a fourfold increase in paired acute and convalescent-phase serum IgM or IgG titers confirms the diagnosis. In reinfections, an IgG titer ≥ 1:512 or a fourfold increase in IgG titer, or a fourfold increase in convalescent-phase IgM, is diagnostic. PCR testing for DNA amplification in respiratory specimens is also available and is mostly used in clinical epidemiological studies.
Chest X-rays in the early stages typically show unilateral alveolar infiltrates in the lower lobes. In later stages, bilateral lesions may develop, characterized by a mix of interstitial and alveolar infiltrates. Lesions may persist for several weeks. Primary infections are predominantly associated with alveolar infiltrates, while reinfections involve a mix of alveolar and interstitial changes.
Diagnosis and Differential Diagnosis
Diagnosis requires a comprehensive analysis of respiratory and systemic symptoms, chest X-ray findings, microbiological and serological results. CP infection should be suspected in pneumonia patients with persistent dry cough who do not respond to β-lactam antibiotics. Since Chlamydia pneumonia lacks specific clinical features, definitive diagnosis relies on specialized tests such as pathogen isolation and serological detection. Differential diagnosis includes Mycoplasma pneumonia and viral pneumonia.
Treatment
Macrolide antibiotics are the first-line treatment, including erythromycin, roxithromycin, azithromycin, and clarithromycin. Fluoroquinolones (e.g., levofloxacin, moxifloxacin) and tetracyclines (e.g., doxycycline) are also effective. The treatment course typically lasts 14-21 days. Symptomatic treatment can be provided for fever, dry cough, headache, and other symptoms.