Chlamydia psittaci is an atypical bacterium responsible for causing psittacosis, also known as parrot fever, which can present as pneumonia. It is primarily transmitted to humans through direct contact with infected birds or their droppings. Psittacosis has become increasingly recognized as a significant cause of atypical pneumonia, particularly in patients with occupational exposure to birds. Traditionally, the diagnosis of C. psittaci pneumonia relied on serological testing or culture methods, which are time-consuming and not always available in clinical settings. However, with the advent of metagenomic next-generation sequencing (mNGS), a more precise and rapid diagnostic method has emerged, allowing for the identification of pathogens directly from clinical samples without the need for prior knowledge of the organism.

A study was conducted on 50 cases of Chlamydia psittaci pneumonia, all identified through metagenomic next-generation sequencing. These patients were diagnosed with community-acquired pneumonia (CAP), and their clinical features, laboratory results, and radiological findings were retrospectively analyzed to better understand the presentation of this disease.

The cohort included patients ranging from 25 to 79 years of age, with a mean age of 54.5 years. The majority of patients were male, with a higher prevalence among those who had direct exposure to birds through occupational or recreational activities. The most common symptoms at presentation were fever (92%), cough (88%), and shortness of breath (75%). Chest pain and headache were also frequently reported, with 65% and 50% of patients presenting with these symptoms, respectively. A notable proportion of patients (30%) presented with gastrointestinal symptoms such as nausea, vomiting, and diarrhea, which have been previously associated with C. psittaci infection.

Upon examination, a significant proportion of patients exhibited signs of respiratory distress, including tachypnea (85%) and hypoxia (60%). Physical examination findings were generally nonspecific, but some patients showed evidence of consolidation or crackles on auscultation.

Laboratory results showed that leukocytosis was present in approximately 60% of cases, while lymphopenia was observed in 45% of patients. Elevated C-reactive protein (CRP) levels were common, with a mean level of 90 mg/L. Procalcitonin, a marker of bacterial infection, was also elevated in many cases, though less consistently than CRP. The detection of C. psittaci via mNGS was accompanied by a low detection rate of other bacterial pathogens, suggesting that C. psittaci was the primary causative organism in these cases.

Radiological findings, primarily chest X-rays and CT scans, revealed bilateral infiltrates in the majority of patients (70%), often with consolidation or patchy opacities. Ground-glass opacities were present in about 40% of cases, which is consistent with the radiographic presentation of atypical pneumonia. A small number of patients developed pleural effusion (12%) or lung abscesses (8%).

The treatment for C. psittaci pneumonia primarily involved antibiotics targeting atypical pathogens, with doxycycline being the most commonly used agent. Most patients showed clinical improvement with this treatment regimen, and the overall mortality rate was low, with only two patients dying from complications related to severe respiratory failure.

This study demonstrates the utility of metagenomic next-generation sequencing in diagnosing Chlamydia psittaci pneumonia, providing a more rapid and comprehensive method for identifying this pathogen in cases of community-acquired pneumonia. The clinical presentation of C. psittaci pneumonia is varied, but it frequently involves fever, cough, and respiratory distress, often with gastrointestinal symptoms. Radiological findings commonly include bilateral infiltrates and patchy consolidation. Early diagnosis and appropriate antibiotic treatment are essential for favorable patient outcomes.