Animal Models of COVID-19. I. Comparative Virology and Disease Pathogenesis.

The Coronavirus Disease 2019 (COVID-19) pandemic has fueled unprecedented development of animal models to understand disease pathogenesis, test therapeutics, and support vaccine development. Models previously developed to study severe acute respiratory syndrome coronavirus (SARS-CoV) have been rapidly deployed to study SARS-CoV-2. However, it has become clear that despite the common use of ACE2 as a receptor for both viruses, the host range of the 2 viruses does not entirely overlap. Distinct ACE2-interacting residues within the receptor binding domain of SARS-CoV and SARS-CoV-2, as well as species differences in additional proteases needed for activation and internalization of the virus, are likely sources of host differences between the 2 viruses. Spontaneous models include rhesus and cynomolgus macaques, African Green monkeys, hamsters, and ferrets. Viral shedding and transmission studies are more frequently reported in spontaneous models. Mice can be infected with SARS-CoV; however, mouse and rat ACE2 does not support SARS-CoV-2 infection. Murine models for COVID-19 are induced through genetic adaptation of SARS-CoV-2, creation of chimeric SARS-CoV and SARS-CoV-2 viruses, use of human ACE2 knock-in and transgenic mice, and viral transfection of wild-type mice with human ACE2. Core aspects of COVID-19 are faithfully reproduced across species and model. These include the acute nature and predominantly respiratory source of viral shedding, acute transient and nonfatal disease with a largely pulmonary phenotype, similar short-term immune responses, and age-enhanced disease. Severity of disease and tissue involvement (particularly brain) in transgenic mice varies by promoter. To date, these models have provided a remarkably consistent template on which to test therapeutics, understand immune responses, and test vaccine approaches. The role of comorbidity in disease severity and the range of severe organ-specific pathology in humans remains to be accurately modeled.