Dose-dependent response to infection with SARS-CoV-2 in the ferret model: evidence of protection to re-challenge
Authors: Kathryn A. Ryan et al.
Journal/ Pre-Print: BioRxiv
Tags: Immunology/Immunity, Animal Model
1. Ferrets exposed to 5x106 and 5x104 pfu SARS-CoV-2 shed viral RNA in the nose for up to 20 days post-infection (dpi). Only 1 out of 6 ferrets exposed to 5x102 pfu showed RNA shedding in nasal washes, indicating a limited natural dissemination of SARS-CoV-2 in the nasal mucosa. Peak of viral RNA shedding in nose was at day 3 pi, with a consistent high detection in the lungs of one euthanised animal per group.
2. At day 3 and 5 pathological signs of mild multifocal bronchopneumonia was observed in 5-15% of the lung of high dose, 5% of the lung in medium dosed, and occasionally in low SARS-CoV-2 dosed ferrets.
3. Primary infection, even at low doses, elicits a protective immunity to reinfections established at day 28 post-infection. Protection is observed in animals infected with the lowest dose and developing low titers of neutralising antibodies. Regardless of the primary dose, all animals showed T cell reactivity to spike peptides indicative of the development of a protective adaptive humoral and cellular immune response.
This works summarises pilot experiments modelling COVID19 and SARS-Cov-2 infection in ferrets using three different doses (5 x 106, 5 x 104 and 5 x 102 pfu). Viral RNA shedding and persistence in the nasal airways, as well as lung pathology showed similar kinetics in animals receiving 5 x 104 or more . However, systemic involvement was limited as evidenced by the lack of body weight loss and fever in most infected animals. Overall, the peak of nasal RNA shedding was at day 3, decaying from day 11-13 and persisting up to day 20 pi. Only high- and mid-dose animals developed mild multifocal bronchopneumonia alongside liver inflammation. No involvement of the gut is reported. Ferrets of the high and medium infection doses developed significant titers of neutralising antibodies and were protected from re-challenged at day 28 pi. Protection from lung pathology associated with a significant T cell response to spike peptides ex vivo. Remarkably, protection was also observed in ferrets receiving the lower infection dose, which developed lower titers of neutralising antibodies. These observations suggest that ferrets are of limited use to model severe COVID19.
Impact for SARS-CoV2/COVID19 research efforts
Characterise a ferret model for COVID19 and SARS-Cov-2 infection.
· In vivo study (ferrets)
Strengths and limitations of the paper
Novelty: It has been shown previously that ferrets parallel certain aspects of COVID19 pathology in humans(Y.Kim et al. Cell & Host Microbes 2020) but this study shows that disease is greatly influenced by the dose of virus used in the primary infection. Also, this provides another piece of evidence that primary infections induce short-term immunity to re-infections in animals.
Standing in the field: Kinetics of virus replication in vivo are seldomly reproduced across different strains of the same virus. The observations might hold true only for the strain used (Victoria/1/202026 SARS-CoV-2), and further studies might require performing similar kinetics studies before estimating ID50 for a particular disease parameter.
The magnitude of systemic involvement in ferrets suggests the development of a milder disease, with no fever or significant weight loss. Even at high doses, lung involvement is no bigger than 15%. Altogether, these observations suggest that ferrets are of limited use to model severe COVID19.
Appropriate statistics: No. Low animal numbers and no independent experimentation. Lack of statistical testing throughout the manuscript.
Viral model used:
1. Victoria/1/202026 SARS-CoV-2 strain.
Main limitations: The authors should consider the use of at the very least another strain of the SARS-Cov-2.