Comparative Study of HRCT Imaging Characteristics of Psittaci Pneumonia and COVID-19 Pneumonia

Backgroud: Both Chlamydia psittaci and COVID-19 virus can cause lung inammation, which manifests extremely similarly in clinical symptoms and imaging. Especially during the epidemic of COVID-19, psittacosis pneumonia is easily misdiagnosed as COVID-19 pneumonia. The identication of the chest imaging between the two diseases is of special signicance when the epidemiological contact history is unclear, and the etiology and nucleic acid test results are not available. This study conducts to compare the imaging characteristics on chest high-resolution CTs (HRCT) between patients with psittaci pneumonia and COVID-19 pneumonia. Methods: A retrospective analysis of the imaging characteristics on chest HRCTs of 10 psittaci pneumonia patients and 13 COVID-19 pneumonia patients. The similarities and differences in HRCT images of patients with psittaci pneumonia and COVID-19 pneumonia were analyzed. Results: HRCT showed that among the 10 psittaci pneumonia patients, 8 cases (80.00%) had single lobe involvement, and 2 cases (20.00%) had multiple lobe involvement. Among the 13 COVID-19 pneumonia patients, 2 cases had single lobe involvement (15.38%), and 11 cases had multiple lobe involvement (84.62%). The types of lesions in 10 psittaci pneumonia patients included simple consolidation in 5 cases (50.00%), and ground-glass opacity (GGO) with consolidation in 5 cases (50.00%). The types of lesions in 13 COVID-19 pneumonia patients included simple GGO in 6 cases (46.15%), GGO with consolidation in 4 cases (30.77%), GGO with paving stone sign in 2 cases (15.38%), and simple consolidation in 1 case (7.69%). Lymphadenopathy was observed in 1 psittaci pneumonia patient (10.00%) and 1 COVID-19 pneumonia patient (7.69%). Among the 10 psittaci pneumonia patients, 8 cases (80.00%) had bronchial ination, and 6 patients (60.00%) had pleural effusion. Among the 13 COVID-19 pneumonia patients, 5 patients (38.46%)

New coronavirus pneumonia is a severe acute respiratory infection (SARI) caused by the COVID-19 virus.
The clinical symptoms of COVID-19 pneumonia are atypical, and the diagnosis requires viral nucleic acid testing. Both Chlamydia psittaci and COVID-19 virus can cause lung in ammation, which manifests extremely similarly in clinical symptoms and imaging. Especially during the epidemic of COVID-19, psittacosis pneumonia is easily misdiagnosed as COVID-19 pneumonia [9]. The identi cation of the chest imaging between the two diseases is of special signi cance when the epidemiological contact history is unclear, and the etiology and nucleic acid test results are not available.
This study retrospectively analyzed the high-resolution CT (HRCT) images of 10 psittaci pneumonia patients and 13 COVID-19 pneumonia patients during the COVID pandemic; this was conducted in order to provide early differential diagnoses of psittaci and COVID-19 pneumonia.

General information
Clinical data from 10 cases of psittaci pneumonia and 13 cases of COVID -19 pneumonia were collected from January 2020 to November 2020. All 10 psittaci pneumonia patients were diagnosed with Chlamydia psittaci through second-generation gene sequencing. COVID-19 patients were diagnosed according to the National Health and Construction Commission's "New Coronavirus Infection Pneumonia Diagnosis and Treatment Program" [10].

Instruments and scanning methods
Patients were scanned using chest non-contrast HRCT, and data were collected using Siemens emotion (Siemens Medical Solutions). Scanning parameters were as followed: tube voltage 120 kV, tube current 100-200mA, slice thickness 0.625-5mm, pitch 0.75-1.5, and calibration 0.625-5mm. The lung window and the mediastinal window were reconstructed with a high-spatial-resolution algorithm. The lung window level was set to -700 HU, and the width was set to 1700 HU; the mediastinal window level was set to 40 HU, and the width level was set to 350 HU. The scanning range was from the entrance of the thorax to the level of the posterior costophrenic angle, and an end-inspiratory scan was taken. The MRP method was adopted for image post-processing to obtain multi-directional two-dimensional images of human tissues and organs from the original horizontal axis images.

Image analysis
Two physicians, both with over 10 years of experience in diagnostic imaging, performed image diagnoses to determine the location and imaging characteristics of the lesion. If the diagnosis results are inconsistent between these two physicians, a consensus was reached after discussion. Consolidation was de ned as an increased density of lung parenchyma with obscuration of the underlying vessels.
Ground-glass opacity (GGO) was de ned as an increase in lung parenchymal opaci cation without obscuration of the underlying vessels (opposed to consolidation). The "paving stone" sign was de ned as thickened interlobular septa and intralobular lines superimposed on a background of GGO, similar to irregular paving stones. The bronchial sign refers to the air-lled, low-density bronchial shadow in the high-density lung tissue without air.

Results
General information, clinical manifestations, and laboratory tests Ten patients (7 males and 3 females) with psittaci pneumonia were between the ages of 26-68 years old, with an average age of 44.6 years. All 10 patients visited the doctor with fever as the main symptom, which was accompanied with cough in 2 cases, shortness of breath in 2 cases, and abdominal pain in one case. Thirteen COVID-19 patients (8 males and 5 females) were between the ages of 27-79 years old, with an average age of 51.4 years. Similarly, these ten patients also visited the doctor with fever as the main symptom, but 2 patients also had cough and 1 patient had fatigue. Three cases were asymptomatic patients. The laboratory results of the two groups of patients are presented in Table 1. Other signs: Among the 10 cases with psittaci pneumonia, one case (10.00%) had lymphadenopathy, eight cases (80.00%) showed signs of bronchial in ation, and six cases (60.00%) had pleural effusion. Among the 13 cases with COVID-19 pneumonia, one case (7.69%) had lymphadenopathy, and ve cases (38.46%) showed signs of bronchial in ation without sign of pleural effusion in all cases. The main imaging manifestations and accompanying signs of psittaci and COVID-19 pneumonia are shown in Table 2.

Discussion
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 con rmed 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][12][13][14]. In this study, no signi cant 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 rst 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 [15]. Among the 13 patients with COVID-19 pneumonia, GGO and GGO with consolidation were the main lesions [16], 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 [17]. Therefore, these two diseases could be easily distinguished based on the distributions of the lesions, the main imaging manifestations, and accompanying signs on the rst 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 reexaminations of lung HRCTs and dynamic observations of the lung lesion changes could bene t the differentiation of the two diseases.
After humans are infected with Chlamydia psittaci, the pathogen enters the body, and it rst enters the macrophages of the liver and spleen to proliferate; then, it enters the lungs and other organs through the bloodstream [18]. A previous study shows that an in ammatory reaction in psittaci pneumonia is observed around blood vessels and spread to the surroundings, and therefore causes lobular and interstitial pneumonia [19]. In contrast, COVID-19 viruses directly invade the bronchial epithelium and mainly cause bronchiolitis and peripheral in ammation. On HRCT imaging, the consolidation is caused by perivascular in ammation, while GGO is caused by in ammation spreading to the surrounding area. The early lesions of psittaci pneumonia may form a typical "anti-halo sign" due to the central in ammatory exudation and the repair of the marginal zone. In the early stage of COVID-19 pneumonia, it is mainly in ammatory 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 brous 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 [21].
The images of COVID-19 pneumonia and psittaci pneumonia should also be distinguished from other atypical pneumonia, such as: SARS, avian in uenza 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 [21]. Avian in uenza 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 [22]. Legionella pneumonia is a widespread multilobe distribution of brinous 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 eld. In severe cases, lung cavities or lung abscesses may appear [23].
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 strati ed for disease severity, complications, the time from rst 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.

Conclusion
In this study, results suggests that chest HRCT can objectively re ect the large changes in the lung lesions of psittaci pneumonia and COVID-19 pneumonia, and it can provide help for their early clinical diagnosis, early isolation, and early treatment.

Declarations
Ethics approval and Consent to participate: This study was approved by the Ethics Committee of the Second Xiangya Hospital of Central South University. All methods were carried out in accordance with relevant guidelines and regulations. Written informed consent was obtained from all participants.
Consent for publication: Consent for publication was obtained from every individual.
Availability of data and materials: All data generated or analysed during this study are included in this published article Competing interests: The authors declare that they have no competing interests