Human infection of psittacosis is mainly caused by close contact with infected birds or poultry and inhalation of bird secretions or contaminated aerosols; human-to-human transmission is rare [8, 11]. Most reported psittaci pneumonia are sporadic cases. In this study, 4 cases of psittaci pneumonia were a clustered occurrence; the 4 cases were medical staff in the same ward. However, clinical epidemiological investigation revealed no similar infections among their close contacts. Therefore, the fresh air conditioning system was suspected to be the possible source of infection; after the air conditioning system was turned off and cleaned, no new cases occurred. Although its original source is still under investigation, the COVID-19 pneumonia is at least confirmed to spread between humans through droplets and close contacts. Therefore, COVID-19 pneumonia can often be traced back to close contacts of the infection or virus carriers.
Previous studies have shown that the amount and percentage of peripheral lymphocytes are usually normal in psittaci pneumonia, whereas peripheral lymphocytes are usually decreased in COVID-19 pneumonia [8, 11–14]. In this study, no significant differences in amount and percentage of peripheral lymphocytes were observed between the two groups of patients. However, erythrocyte sedimentation rate and CPR had increased in almost all patients with the psittaci pneumonia, yet they were normal in patients with COVID-19 pneumonia.
In this study, 9 of the 10 patients with psittaci pneumonia were diagnosed with unilateral lung involvement, and lesions were all located in the lower lung in the first HRCT imaging diagnosis. The lesions were mainly solid with or without pleural effusion. Our observation is consistent with a previous report that the lung lesions of psittaci pneumonia usually occurred in a single lower lung . Among the 13 patients with COVID-19 pneumonia, GGO and GGO with consolidation were the main lesions , and the lesions were mainly emerged in the peripheral lung zone. In addition, unlike psittaci pneumonia, pleural effusion is rarely observed in COVID-19 pneumonia . Therefore, these two diseases could be easily distinguished based on the distributions of the lesions, the main imaging manifestations, and accompanying signs on the first HRCT. Our dynamic observations demonstrated that the lesions in psittaci pneumonia typically changed over the course of a few days, whereas the lesions in COVID-19 pneumonia changed more rapidly, often in a few hours (Data not shown). Therefore, multiple re-examinations of lung HRCTs and dynamic observations of the lung lesion changes could benefit the differentiation of the two diseases.
After humans are infected with Chlamydia psittaci, the pathogen enters the body, and it first enters the macrophages of the liver and spleen to proliferate; then, it enters the lungs and other organs through the bloodstream . A previous study shows that an inflammatory reaction in psittaci pneumonia is observed around blood vessels and spread to the surroundings, and therefore causes lobular and interstitial pneumonia . In contrast, COVID-19 viruses directly invade the bronchial epithelium and mainly cause bronchiolitis and peripheral inflammation. On HRCT imaging, the consolidation is caused by perivascular inflammation, while GGO is caused by inflammation spreading to the surrounding area. The early lesions of psittaci pneumonia may form a typical "anti-halo sign" due to the central inflammatory exudation and the repair of the marginal zone. In the early stage of COVID-19 pneumonia, it is mainly inflammatory exudation in the interstitium of the lung, and GGO is the main manifestation on CT. The thickened interlobular septum and intralobular septum line shadow superimposed on the background of GGO can form a typical "paving stone-like change". In the middle and late stages of the disease, bacterial fibrous mucus exudation appeared in the alveolar cavity, and the density of the lesion increased, which forms the paving stone-like changes with consolidation on CT .
The images of COVID-19 pneumonia and psittaci pneumonia should also be distinguished from other atypical pneumonia, such as: SARS, avian influenza virus pneumonia, and legionella pneumonia. Psittaci pneumonia is often a single lesion localizing in the right or left lower lobe. COVID-19 pneumonia is often multiple lung lesions. Patients without underlying lung diseases generally have no mediastinal and hilar lymphadenopathy, and pleural thickening or effusion. The imaging characteristics of SARS include that the lesions change rapidly, even in hours, and the lesions are single or multiple, and more common in the middle and lower lung lobes. Some SARS patients may have inconsistence between symptoms and image performance; that is, the symptoms are mild, and the lung shadows are obvious . Avian influenza virus pneumonia is characterized by lung consolidation and ground-glass shadows as the main imaging features of the lungs with multiple lobes and multiple segments involved, and the lower and posterior lesions of the lungs are more severe than the upper and anterior lesions; this indicates that the lesion has characteristics of distribution along gravity . Legionella pneumonia is a widespread multilobe distribution of fibrinous purulent bronchopneumonia. It is often manifested as multilobe and multi-segmental invasion on images, and lesions may appear in the inner segment of the lung field. In severe cases, lung cavities or lung abscesses may appear .
We acknowledged that this study has several limitations. First, the sample size is small for both the psittaci pneumonia and COVID-19 pneumonia patients. Thus, they were not stratified for disease severity, complications, the time from first symptom occurrence to the HRCT, sex, and age. Secondly, since our hospital is not a designated hospital for COVID-19 pneumonia, there is a lack of analysis on dynamic image changes and a follow-up study.