Clinical and Radiological Features of an Adenovirus Type 7 Outbreak in Split-Dalmatia County, Croatia, 2022-2023

Human adenoviruses (HAdVs) are known to be highly contagious pathogens. They are commonly associated with mild respiratory infections in young children but can also cause severe life-threatening infections. Human adenovirus types 4 and 7 have frequently been reported to cause pneumonia in immunocompetent youths and adults. In this retrospective study, we analyzed the clinical, laboratory, radiological, and microbiological features, as well as the treatment and outcomes of an adenovirus outbreak in 185 patients who were admitted to the Emergency Unit of the Departments of Infectious Diseases and Pediatrics, University Hospital of Split, Croatia, between October 2022 and April 2023. An unusual increase in the frequency of adenovirus pneumonia was observed, especially in adults, followed by respiratory failure and complications such as pulmonary embolism. The most common chest X-ray findings were unilateral patchy opacity and unilateral reticulations (11.6%), followed by unilateral lobar pneumonia (7.1%). The predominant CT presentation was unilateral lobar pneumonia with multiple patchy ground glass opacities (23.5%) or lobar pneumonia with mixed opacities (17.6%). We found a low correlation between Brixia score and C-reactive protein in adults and no correlation in children. Adenovirus type 7 was almost exclusively isolated from patients with pneumonia. Most of our patients with severe or critical adenovirus pneumonia were immunocompetent adults without any medical history. So far, only a few studies have presented the radiological features of HAdV pneumonia, which generally did not reveal lobar pneumonia in a substantial percentage. Our research also demonstrated an unusual presentation of adenovirus infection complicated with pulmonary embolism, which has rarely been reported in previous studies. The aforementioned HAdV outbreak indicates the necessity for further research, especially in the context of effective antiviral therapy and infection prevention.