Chlamydial pneumonia is an acute lung inflammation caused by Chlamydia pneumoniae (CP)， mostly mild, with insidious onset, without gender difference, and can occur in all seasons. The upper and lower respiratory tract can be involved, and pharyngitis, laryngitis, tonsillitis, sinusitis, bronchitis, and pneumonia may occur. Chlamydial pneumonia is more common in school-age children, but less common in children under age 3. Outbreaks may occur in semi-enclosed environments such as homes, schools, military camps, and other work areas with concentrated populations. The disease accounts for 10% - 20% of community-acquired pneumonia.
Etiology and pathogenesis
CP is an obligate intracellular bacterioid parasite, belonging to the family Chlamydiaceae. Chlamydia psittaci can also cause pneumonia in humans. CP has biphasic life cycle consisting of elementary body and initial body. The elementary body is a dense sphere with a diameter of 0.2 - 0.4 μm and is infectious, while the initial body, also known as reticulate body, with a diameter of approximately 0.51 μm, is proliferative but not infectious. CP is a pathogen with human-to-human transmission, and is transmitted through respiratory droplets or pollutants. The older, frail, malnourished, or immunocompromised individuals or patients with chronic obstructive pulmonary disease are susceptible to the disease.
Clinical manifestations
The onset is often insidious, and the early manifestations are upper respiratory tract infection symptoms, resembling those of mycoplasmal pneumonia. Symptoms are fever, rigors, myalgia, dry cough, non-pleuritic thoracodynia, headache, malaise, fatigue, and rarely hemoptysis. Patients with pharyngitis present with pharyngeal pain and hoarseness. Some patients may present with a two-stage disease course. Pharyngitis improves after symptomatic treatment in the early stage, and pneumonia or bronchitis occurs in 1 - 3 weeks, with aggravated cough. Few patients may be asymptomatic. CP infection may also be accompanied by extrapulmonary manifestations, such as otitis media, arthritis, thyroiditis, encephalitis, and Guillain-Barre syndrome. Physical examination reveals no abnormalities in the lungs, but occasionally moist crackles can be heard
Laboratory and auxiliary examinations
The white blood cell count is normal or slightly elevated, and the erythrocyte sedimentation is increased. Direct isolation of CP from sputum, throat swabs, throat secretions, and bronchoalveolar lavage fluid is the gold standard for diagnosis. However, CP cannot be cultured in vitro. It needs to be inoculated and cultured in cell lines derived from the respiratory tract, such as Hep-2 and HL cell lines. The operation is cumbersome, and is generally only used for scientific research. Currently, the diagnosis of chlamydial pneumonia mainly relies on serology. In patients with primary infection, a diagnosis can be established if the IgM titer of serum specimens in the acute phase is ≥ 1:32 or if there is a 4-fold or greater increase in serum IgM or IgG in the acute and convalescent phases. In reinfected patients, the IgG titer ≥ 1:512 or with a fourfold or greater increase in IgM in the convalescent phase are present. PCR can also be used to amplify DNA from respiratory specimens, and is mostly used in clinical epidemiological investigations.
X-ray shows unilateral, lower lobar alveolar exudation in the early stage, followed by bilateral lesions, manifested by a mixture of interstitial and alveolar exudation, and the lesions can last for several weeks. Patients with primary infection mostly have alveolar exudation, while patients with reinfection have a mixture of alveolar exudation and interstitial lesions.
Diagnosis and differential diagnosis
Comprehensive analysis should be conducted by combining respiratory and systemic symptoms, x-ray, etiology, and serology. In patients with pneumonia who are ineffective in treatment with β-lactam antibiotics and have persistent dry cough, CP infection should be alerted. The disease has no specific clinical manifestations, and the diagnosis is mainly based on specific examinations, such as pathogen isolation and serological testing. The disease should be differentiated from mycoplasmal pneumonia and viral pneumonia.
Treatment
Macrolides, such as erythromycin, roxithromycin, azithromycin, and clarithromycin, are preferred. Quinolones such as levofloxacin and moxifloxacin and tetracyclines such as doxycycline also have good curative effects. The duration of treatment is 14 - 21 days. Fever, dry cough, and headache may require symptomatic treatment.