The initial outbreak of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in humans is widely thought to be of zoonotic origin. Zoonosis occurs when a pathogen that derives from animals jumps to, and successfully transmits between, human hosts.
A commonly subscribed to hypothesis during the early onset of the pandemic was a possible spillover event during close human contact with wild animals at a ‘wet market’ in Wuhan, China, where the virus was first detected in December 2019.
While subsequent phylogenetic evidence has largely problematized this initial thesis, most of the world’s scientific community is still fairly certain that SARS-CoV-2 had jumped to human hosts via wild animals. This was potentially through an intermediary host that contracted the virus from horseshoe bats (a common reservoir for betacoronaviruses like SARS-CoV-2).
Some investigations of the types of animals that are susceptible to this type of coronavirus – or that can act as zoonotic reservoirs – have already been undertaken. These have aimed both to try and get closer to understanding the precise origins of the present pandemic, but also to conduct risk assessments on potential future zoonotic spillovers of SARS-CoV-2 so that we may avoid these sorts of calamitous outbreaks in the future. Of particular concern is the virus’s ability to advantageously evolve within animal hosts and then jump back to humans through immunity-evading mutations.
This said, research into how the virus has affected wildlife species is currently limited. However, this may be crucial to determining whether wildlife species can serve as reservoirs for retaining the virus.
In new research attempting to address this issue, peridomestic mammals, which live in and around human habitations, have been investigated to ascertain how SARS-CoV-2 can affect species external to, but in close proximity with, the human population.
This study – conducted by researchers at the Colorado State University, USA, and the University of Queensland, Australia – has been published by the Centers for Disease Control and Prevention (CDC) within the Emerging Infectious Diseases journal.
Peridomestic animals may include wild and feral animals which reside near humans and can be seen as a key component for researching the epidemiology of SARS-CoV-2.
The importance of understanding how SARS-CoV-2 interacts with peridomestic mammals is underscored when assessing their associations with humans and the modification of their habitats due to human intervention. These species are at the highest risk of exposure to the virus from humans, which can be said even in pets such as cats.
Subsequently, if these species were to be susceptible to the SARS-CoV-2 virus, they may have the capacity to replicate and spread this to a high titer, which may lead to the virus being maintained in the species and could result in shedding and being transmitted back into humans. This could possibly lead to an outbreak with new viral variants and populations being led back into a pandemic.
Susceptibility to viruses
Mammals that can display peridomestic characteristics within urban and suburban environments can include wild rodents, cottontail rabbits, raccoons and striped skunks. Research has shown these species to shed influenza A viruses after experimental inoculations, suggesting their ability to harbor productive infections when exposed to other human infectious respiratory viruses (like SARS-CoV-2).
Protein analyses of amino acid residues of the angiotensin-converting enzyme 2 (ACE2) host cell receptor and the viral spike protein suggest that carnivores and wild rodents are within high-risk groups regarding susceptibility to the virus. However, when investigating which specific species is susceptible to SARS-CoV-2, the research is more complex.
Rodents are the largest and most diverse order of mammals which makes susceptibility to the virus more varied within the same species. Non-transgenic mice are predominantly found to be unsusceptible to infection. However, transgenic humanized mice and hamsters, as well as Syrian hamsters, are found to be highly susceptible to infections. One previous study found that Roborovki dwarf hamsters exposed to infection resulted in disease and death after three days.
The research, published in the CDC journal, assessed six common peridomestic rodents for their level of susceptibility to SARS-CoV-2, which included deer mice, wild-caught house mice, bushy-tailed woodrats, fox squirrels, Wyoming ground squirrels, and black-tailed prairie dogs. These animals are commonly found in the United States and have close contact with humans as well as human residences. Another three peridomestic mammals – cottontail rabbits, raccoons, and striped skunks – which may have less interaction with humans but are still commonly found were also assessed.
The research findings consisted of illustrating that peridomestic species such as deer mice, bushy-tailed woodrats and striped skunks are susceptible to being infected by SARS-CoV-2 and have the ability to shed the virus in respiratory secretions. However, other peridomestic mammals such as cottontail rabbits, fox squirrels, Wyoming ground squirrels, black-tailed prairie dogs, house mice and raccoons were found not to be susceptible to the virus.
The animals were all assessed daily for the temperament and clinical signs of disease before conclusions were drawn. The results of this study confirm previous research into the susceptibility of animals, such as deer mice. The researchers conclude that most wildlife exposed to the SARS-CoV-2 virus manifest with either no or mild symptoms of the clinical disease. Additionally, they either did not shed the virus or shed the virus for short periods of time.
The researchers acknowledge that while some rodents may be found to be potential reservoirs for the virus, the study has limitations of being unrepresentative due to the high doses provided to the animals. This may not be reflective of reality which relies on the available dose within nature, as this may be lower than what was provided in this study.
However, due to the susceptibility of some peridomestic animals, the researchers recommend the development of safety guidelines that can ensure the safety of humans and their pets. This can include risk assessing occupational hazards when working with or around susceptible animals in areas of contact such as barns and sheds. This may require personal protective equipment (PPE) to prevent exposure from any type of pathogen that these rodents may carry. Safety is always of paramount importance, and action should be cautioned when interacting with susceptible animals, especially during the ongoing pandemic.
This study has provided insight into how human and wildlife interactions can result in a continual circulation of SARS-CoV-2 in both animals and humans and the likelihood of susceptibility from this interaction.