Computed tomography findings and severity scores in Chlamydia psittaci pneumonia: a retrospective study of 69 cases with clinical correlation

BACKGROUND: Chlamydia psittaci pneumonia (CPP) is a rare but potentially severe zoonotic disease.This study aimed to characterize the computed tomography(CT)findings of CPP and evaluate the utility of CT severity scores for predicting intensive care unit(ICU) admission. METHODS: We retrospectively analyzed patients diagnosed with CPP between January 2022 and September 2025.Patients were divided into ICU and non-ICU groups based on disease severity and ICU admission.Demographic, clinical, laboratory, and radiological data were collected.Two radiologists independently reviewed CT images to record imaging features and calculate two scores: the chest CT score(CTS, range 0-25 based on lobar involvement)and the chest CT severity score(CTSS, range 0-40 based on 20 lung segments).Multivariable logistic regression and receiver operating characteristic(ROC)curve analyses were performed to identify predictors of ICU admission.We further compared the predictive performance of CTS with that of the CURB - 65 (confusion, urea, respiratory rate, blood pressure, age ≥ 65 years) score, a widely - utilized clinical severity assessment tool for community - acquired pneumonia. The CURB - 65 score was calculated for each patient upon admission. RESULTS: A total of 69 patients were included(45 male,24 female; mean age 61.6 ± 12.5years).A history of poultry or bird exposure was reported in 65 patients(94.2%).The ICU group comprised 22 patients(31.9%).Among survivors(n = 65),follow-up showed complete clinical recovery, and repeat CT imaging demonstrated complete resolution or marked improvement of interstitial abnormalities in those with available follow-up scans, supporting the reversible acute nature of these changes.In the ICU group, both the CTS (9.9 ± 6.4 vs. 5.9 ± 3.4, P = 0.011) and CTSS (13.5 ± 9.0 vs. 9.0 ± 4.8, P = 0.035) were significantly elevated. ROC analysis indicated that the area under the curve was 0.710 for CTS (cut - off 7.5; sensitivity 63.6%; specificity 76.6%) and 0.657 for CTSS. The CURB - 65 score yielded an area under the curve (AUC) of 0.633 (95% confidence interval [CI]: 0.476-0.790, P = 0.076). The DeLong test showed no statistically significant difference between the AUCs of the CTS and CURB - 65 (P = 0.452). Notably, only the CTS reached statistical significance in predicting intensive care unit (ICU) admission, implying that computed tomography (CT) - based severity assessment might offer prognostic information that cannot be captured solely by clinical score. CONCLUSION: This study provides a detailed imaging characterization of CPP.CT severity scores, particularly the CTS, are independently associated with ICU admission and may serve as adjunctive tools for early risk stratification.The reversible nature of interstitial changes on follow-up supports their acute inflammatory origin.Compared with CURB-65,the CTS offered a numerically higher AUC and provided significant prognostic information where clinical scoring alone did not.