COVID-19-like symptoms observed in Chinese tree shrews infected with SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic continues to pose a global threat to the human population. Identifying animal species susceptible to infection with the SARS-CoV-2/ HCoV-19 pathogen is essential for controlling the outbreak and for testing valid prophylactics or therapeutics based on animal model studies. Here, different aged Chinese tree shrews (adult group, 1 year old; old group, 5–6 years old), which are close relatives to primates, were infected with SARS-CoV-2. X-ray, viral shedding, laboratory, and histological analyses were performed on different days post-inoculation (dpi). Results showed that Chinese tree shrews could be infected by SARS-CoV-2. Lung infiltrates were visible in X-ray radiographs in most infected animals. Viral RNA was consistently detected in lung tissues from infected animals at 3, 5, and 7 dpi, along with alterations in related parameters from routine blood tests and serum biochemistry, including increased levels of aspartate aminotransferase (AST) and blood urea nitrogen (BUN). Histological analysis of lung tissues from animals at 3 dpi (adult group) and 7 dpi (old group) showed thickened alveolar septa and interstitial hemorrhage. Several differences were found between the two different aged groups in regard to viral shedding peak. Our results indicate that Chinese tree shrews have the potential to be used as animal models for SARS-CoV-2 infection. Tree shrews were divided into two groups, i.e., adult group ( n =13, black circles) and old group ( n =7, red circles), and inoculated with SARS-CoV-2 (black triangle) via intranasal, ocular, and oral routes. At 0, 3, 5, 7, 11 (old group), or 14 (adult group) days post-inoculation (dpi), clinical examinations were performed to show tree shrew status. Animals from each group were euthanized and necropsied at indicated dpi for virological and pathological assays. Circled areas on radiographs are regions showing lung infiltrates. Viral loads in serum and throat/rectal swabs (A), lung lobes (B), and other tissues (C) from infected tree shrews measured using quantitative real-time PCR. Each graphic represents one individual, with color referring to indicated dpi. For instance, three infected animals sacrificed at 3 dpi were defined by square, circle, and diamond, respectively. Marks in blue refer to time point 3 dpi. A: Lesions in lungs. Representative view of ventral lungs of infected tree shrew obtained via necropsy at 3 dpi in adult group or at 7dpi in old group compared to healthy control. White box indicates lungs showing focal areas of hyperemia. B, C: Representative hematoxylin and eosin staining (B) and immunofluorescence analysis of viral antigen NP (C) in lung tissues from tree shrews at 3 dpi in adult group or at 7 dpi in old group. Blue arrows in A refer to lesions. Yellow arrows in B indicate pulmonary lymphocytes or neutrophils. White arrows in C indicate SARS-CoV-2 NP staining (green) in pneumocytes, with DAPI staining in blue. PBS and IgG indicate tissue sample from healthy animal in control group treated with PBS but no SARS-CoV-2. Scale bars: 100 µm. Values are means± SD . P -values were calculated using paired Student’s t test. Each circle represents one individual, with “Before” and “After” referring to indicated parameter in same animal before SARS-CoV-2 infection and at time of euthanasia, respectively. We grouped all infected animals in each group together, regardless of different infection times of different animals.