Introduction

First detected in late 2019, the COVID-19 pandemic caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has since been declared a public health emergency of international concern, approaching eleven million human infections and causing over half a million human deaths by early July 2020 (Dong, Du, & Gardner, 2020). The zoonotic virus was first identified in Wuhan, China and the initial cluster of cases was initially believed to be linked to a live ‘wet’ seafood and animal market, although results of back tracing activities have now called this into question (Letzter, 2020; Woodward, 2020). Phylogenetically related to SARS-CoV-1, which caused the disease SARS that emerged from China in 2003, SARS-CoV-2 is also believed to have emerged from a betacoronavirus circulating in rhinolophid (horseshoe) bats (Lau et al., 2020; Zhou et al., 2020), which then made the species jump to humans, potentially by way of an intermediate animal host. Various animal species including snakes, turtles and pangolins have been proposed as the intermediate host (Andersen, Rambaut, Lipkin, Holmes, & Garry, 2020; Lam et al., 2020a; Lam et al., 2020b; Liu et al., 2020; Xiao et al., 2020; S.-L. Zhai et al., 2020; Zhang, Wu, & Zhang, 2020), with no consensus yet being reached.
From the initial zoonotic spill-over event in China, the virus quickly demonstrated efficient human-to-human transmission, with virus in respiratory droplets expelled by COVID-19 patients during coughing, sneezing and talking reportedly remaining viable for multiple hours in aerosols and up to days on some surfaces (Meselson, 2020; van Doremalen et al., 2020). With billions of people around the world harbouring the virus, and the known animal-origin and high recombination rates of SARS-coronaviruses (Stavrinides & Guttman, 2004), reverse spill-over of SARS-CoV-2 from humans to animals (anthroponotic transmission) was surely inevitable.
Incalculable numbers of human-animal interactions occur each day in domestic, agricultural, research, recreational, educational, therapeutic, entertainment and wild settings. Over 470 million dogs and 370 million cats were kept as pets worldwide in 2018 (Bedford, 2020), with the USA, China and Russia the top three dog and cat owning nations (Walden, n.d.). Over half of 27,000 surveyed households owned at least one pet in 2016, and increases in pet ownership are reportedly highest in China, India and Latin America (GfK, 2016). Worldwide, an estimated 70 billion terrestrial animals were raised and slaughtered for human consumption in 2018, with farm animal production the fastest growing agricultural sub-sector and increases particularly expected to occur in developing countries (Ilea, 2009). Flourishing legal and illegal wildlife trades exist worldwide, with the global trafficking in illegal wildlife commodities including pangolin scales, elephant ivory, tiger bone and bear bile estimated at $7-23 billion USD in 2016 (TRAFFIC, 2020). Encroachment of humans into previously uninhabited areas for logging, cropping and urbanisation can create diverse wildlife-livestock-human interfaces that represent critical points for cross-species transmission and emergence of pathogens (Hassell, Begon, Ward, & Fèvre, 2017). Over 60% of recently identified emerging disease events have been zoonotic – the majority of which have had a wildlife origin – and such events have been increasing significantly in recent times (Jones et al., 2008; Wang & Crameri, 2014).
As recent outbreaks of zoonotic diseases with pandemic and panzootic potential – including highly pathogenic avian influenza, SARS and Ebola virus – have demonstrated, an understanding of the human-animal interface and its relevance to disease transmission remains a critical control point. For the current pandemic, determination of the susceptibility of various animal species to infection with SARS-CoV-2 and the role of animals in the epidemiology of the disease are critical to informing appropriate human and veterinary public health responses.
Many simulation models have been developed to assess the likelihood of SARS-CoV-2 susceptibility in animal hosts. These studies aim to classify animal susceptibility or elucidate possible intermediate or reservoir host species based on the implied affinity of the species’ angiotensin-converting enzyme 2 (ACE2) receptor binding domain sites for the SARS-CoV-2 spike protein. Results have indicated that species at highest risk of infection with SARS-CoV-2 include primates – particularly of the catarrhine (Old world) order (Damas et al., 2020) – and most of the carnivores, even- and odd-toed ungulates, pangolins and scaly anteaters (Praharaj et al., 2020). Medium to high probability was ascribed to pangolins, rabbits and cats; lower probability to chickens and turkeys; and rats, mice and ducks were rated as low to very low (Praharaj et al., 2020; X. Zhao et al., 2020). X. Zhai et al. (2020) noted that pigs and dogs, and even cattle and sheep appear to have ACE2 proteins capable of acting as a receptor for viral entry, and the apparent low susceptibility of infection in these species is likely related to the relatively low levels of ACE2 expressed in the respiratory tract. Notably, the Damas et al. (2020) model predicted domestic cats, tigers and golden hamsters to be only medium risk, and ferrets low risk. Contrary evidence of natural and experimental infection in these animals (see below) suggests that the utility of these models as predictive tools may be limited.
Given the conflicting results of predictive modelling studies and the large numbers of journal articles, pre-prints, press releases and media reports of varying quality that have flooded the literature in recent months, definitive answers regarding the infectivity and clinical picture of SARS-CoV-2 in animals have been difficult to find. To this end, this study aimed to review the current literature to identify animal species that have been conclusively shown to be permissive to SARS-CoV-2 infection and transmission, and to collate the latest information regarding natural and experimental infections of the virus in animals, in order to assess the potential role of animals in community transmission of COVID-19.