Age-Dependent Progression of SARS-CoV-2 Infection in Syrian Hamsters

Abstract

In late 2019, an outbreak of a severe respiratory disease caused by an emerging coronavirus, SARS-CoV-2, resulted in high morbidity and mortality in infected humans. Complete understanding of COVID-19, the multi-faceted disease caused by SARS-CoV-2, requires suitable small animal models, as does the development and evaluation of vaccines and antivirals. Since age-dependent differences of COVID-19 were identified in humans, we compared the course of SARS-CoV-2 infection in young and aged Syrian hamsters. We show that virus replication in the upper and lower respiratory tract was independent of the age of the animals. However, older hamsters exhibited more pronounced and consistent weight loss. In situ hybridization in the lungs identified viral RNA in bronchial epithelium, alveolar epithelial cells type I and II, and macrophages. Histopathology revealed clear age-dependent differences, with young hamsters launching earlier and stronger immune cell influx than aged hamsters. The latter developed conspicuous alveolar and perivascular edema, indicating vascular leakage. In contrast, we observed rapid lung recovery at day 14 after infection only in young hamsters. We propose that comparative assessment in young versus aged hamsters of SARS-CoV-2 vaccines and treatments may yield valuable information, as this small-animal model appears to mirror age-dependent differences in human patients.

Keywords: COVID-19; Mesocricetus auratus; age-related disease; animal model; cellular tropism; coronavirus; histopathology; in situ hybridization; pneumonia; serology.

Figure 1

Body weight changes, body temperatures, and viral loads of young versus aged Syrian hamsters infected intranasally with severe acute respiratory syndrome (SARS)-CoV-2. Individual relative body weights of (A) mock-infected, (B) young, and (C) aged hamsters over the course of 14 days after infection are given. Kruskal–Wallis tests with Dunn’s multiple comparison post hoc tests for each time point revealed significant differences at 2 dpi (mock vs. young; p < 0.01) and 2–5 dpi (mock vs. old; p < 0.01). (D) Temperature changes (as means with SD). Viral loads were determined from homogenized right cranial lung lobes. (E) Virus titers of 25 mg of lung tissue determined by plaque assay in Vero E6 cells, and (F) corresponding virus genome copy numbers as determined by RT-qPCR. Viral loads were also determined by RT-qPCR in (G) bucco-laryngeal swabs, (H) nasal washes and (I) 25 µL of whole blood samples. The color codes represent mock-infected (green), infected young (6-week-old, blue) and aged hamsters (33- to 35-week-old, red).

Figure 2

Lung histopathology (A–E) and detection of SARS-CoV-2 RNA (F) at different time points after infection. (A) Time-dependent course of pneumonia in young and adult hamsters representative of each group (n = 3 for 2, 3, and 5 dpi, n = 1 for 7 dpi and n = 2 for 14 dpi; Bar = 0.5 cm) (B) Affected areas of inflammation were identified histologically for each lung and compared between the groups. (C) Lung inflammation score taking into account (i) severity of pulmonary inflammation; (ii) bronchitis (iii) bronchial and alveolar necrosis; iv) hyperplasia of alveolar epithelial cells type II. (D) Immune cell influx score taking into account the infiltration of lung tissue with (i) neutrophils; (ii) macrophages; (iii) lymphocytes; (iv) perivascular lymphocytic cuffing; and (E) edema score including (i) alveolar and (ii) perivascular edema. Scores and parameters in C to E were graded as absent (0), minimal (1), mild (2), moderate (3), or severe (4) as described [35]. (F) Time-dependent distribution of SARS-CoV-2 RNA signals in young and adult hamsters representative of each group as detected by in situ hybridization (group sizes as in A; for digital image analysis of the differences, see Figure S3; bar = 0.5 cm).