Humans, followed by ferrets, and to a lesser extent cats, civets and dogs are the animals most likely to be infected with the novel coronavirus, according to a new study which analysed ten different species for their susceptibility to the virus which causes COVID-19. The findings, published in the journal PLOS Computational Biology, found that ducks, rats, mice, pigs and chickens had lower or no susceptibility to the coronavirus compared to humans, while cats, ferrets, civets, and dogs have documented cases of the SARS-CoV-2 virus infection.
"Knowing which animals are susceptible to SARS-CoV-2 helps us prevent building up animal reservoirs from which the coronavirus can re-emerge at a later date," said Luis Serrano, study co-author from the Centre for Genomic Regulation (CRG) in Barcelona, Spain. "Our findings offer a clue for why minks -- which are closely related to the ferret -- are being infected by the disease, which is probably made worse by their packed living conditions and close contact with human workers," Serrano said.
While the scientists also found potential susceptibility in cats, they said the felines do not co-exist with humans in the same conditions as other animals, possibly explaining why there are no known cases of people being infected by their pets. In the study, the scientists used computer modelling to test how the coronavirus uses its spike proteins, which protrude from the surface of the virus, to infiltrate the cells of different animals. Citing earlier research, they said the main point of entry on a cell's surface is the ACE2 receptor, which binds with the spike protein through a lock-and-key mechanism.
While there are many different variants of ACE2 within human populations and across different species, the scientists said the ACE2 receptor types in humans, ferrets, cats, dogs and civets have the highest binding affinities to the viral spike protein, while mice, rats, chicken and ducks have poor binding energy. However, the scientists said binding affinity alone is not enough to gauge a cell's susceptibility to infection.
The researchers also tested how efficient the coronavirus is at commandeering the cell machinery in different species once it enters, also known as the codon adaptation index. According to the scientists, the more efficient this process, the better the coronavirus can create the proteins it needs to replicate. They said humans, chickens, and ducks have the highest codon adaptation index, while the other species are worse adapted. Considering both binding affinity and this index, the researchers concluded that humans, followed by ferrets, cats, civets and dogs are the most susceptible animals to infection by coronavirus.
They also found that different human variants of ACE2 showed differences in stability and binding to the spike protein -- a sensitivity, which the scientists believe may underlie why some people suffer from severe COVID-19 symptoms. "We have identified mutations on the S-protein that dramatically reduces the capacity of SARS-CoV-2 to enter into the cell, protecting the host from catching COVID-19," said study co-author Javier Delgado from CRG. "We are now engineering mini-proteins from the human ACE2 protein to 'distract' the attention of the virus from entering cells and block an infection. Should new mutations of the viral spike protein arise, we could engineer new variants to block them," Delgado added. The researchers believe understanding SARS-CoV-2 infectivity across different species can better inform public health measures, helping reduce human contact with other susceptible animals, and avoiding the potential prolongment of the pandemic.
