Chlamydia pneumonia is an acute inflammatory disease of the lungs caused by Chlamydia pneumoniae, often involving the upper and lower respiratory tracts and causing pharyngitis, laryngitis, tonsillitis, sinusitis, bronchitis, and pneumonia. It is often prevalent in populations in congregate settings, such as the military, schools, and families, and usually infects all family members, but is less common in children under 3 years of age. Etiology and pathogenesis Chlamydia pneumoniae is a specialized intracellular bacterial-like parasite that belongs to the Chlamydia family. Also causing pneumonia in humans is Chlamydia psittaci. The morphology of Chlamydia pneumoniae varies, with the protozoa being dense and spherical, with a diameter of about 0.2-0.4?m. The reticulum is about 0.51?m in diameter and is the proliferative form of Chlamydia, which has no infectivity. Chlamydia pneumoniae is a human pathogen that is transmitted from one person to another, probably mainly through respiratory droplets, but also through contaminants. Older, weaker, malnourished, COPD, and immunocompromised individuals are susceptible to infection. After infection, immunity is weak and easily recurrent. Clinical manifestations The onset of the disease is insidious, and early manifestations are symptoms of upper respiratory tract infection. Clinically it is quite similar to mycoplasma pneumonia. The symptoms are usually mild, with fever, chills, myalgia, dry cough, non-pleuritic chest pain, headache, malaise and malaise. Rarely, hemoptysis is present. Those who develop pharyngitis present with sore throat and hoarseness. Some patients may present with a two-stage course: pharyngitis at the beginning, which improves with symptomatic treatment, followed by pneumonia or bronchitis 1-3 weeks later, with worsening cough. A small number of patients may be asymptomatic. Chlamydia pneumoniae infection may also be accompanied by extrapulmonary manifestations such as otitis media, arthritis, thyroiditis, encephalitis, Guillain-Barré syndrome, etc. Laboratory and other tests Blood leukocytes are normal or slightly elevated, and blood sedimentation is accelerated. Chlamydia pneumoniae can be isolated directly from sputum, pharyngeal swabs, pharyngeal secretions, and bronchoalveolar lavage fluid. DNA amplification of respiratory specimens can also be performed by PCR. In primary infections, serum IgM can be detected early, and serum specimens in the acute phase such as IgM antibody titers of more than 1:16 or double serum IgM or IgG antibodies in the acute and recovery phases have more than 4-fold elevation. (IgG titers in reinfected individuals) 1:512 or 4-fold higher, or greater elevation of IgM in the recovery phase. Pharyngeal swab isolation of Chlamydia pneumoniae is the gold standard for diagnosis. Radiographic chest radiographs show predominantly unilateral, lower lobe alveolar exudate. There may be a small to moderate amount of pleural effusion, most often present early in the course of the disease. Chlamydia pneumoniae pneumonia can often develop bilaterally, showing a mixture of interstitial and alveolar exudate, and the lesions can persist for several weeks. Patients with primary infection tend to show alveolar exudate on chest radiographs, while those with reinfection have a mixture of alveolar exudate and interstitial lesions. Diagnosis and differential diagnosis Chlamydia pneumoniae infection lacks specific clinical manifestations, and confirmation of diagnosis is based mainly on specific laboratory tests regarding etiology, such as pathogen isolation and serological testing. A comprehensive analysis should be made in combination with respiratory and systemic symptoms, X-ray examinations, pathogenic and serologic tests. As the application of β-endophthalamide antibacterial drugs is ineffective in patients with pneumonia, patients should be alerted to Chlamydia pneumoniae infection if they continue to have a dry cough. Treatment Erythromycin is preferred for Chlamydia pneumonia, but doxycycline or clarithromycin may also be used, both for 14- 21 days. Azithromycin 0.5g/d for 5 days. Fluoroquinolones can also be used. Symptomatic treatment is available for fever, dry cough, headache, etc.