Host barriers to SARS-CoV-2 demonstrated by ferrets in a high-exposure domestic setting

Abstract

Ferrets (Mustela putorius furo) are mustelids of special relevance to laboratory studies of respiratory viruses and have been shown to be susceptible to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and onward transmission. Here, we report the results of a natural experiment where 29 ferrets in one home had prolonged, direct contact and constant environmental exposure to two humans with symptomatic disease, one of whom was confirmed positive for SARS-CoV-2. We observed no evidence of SARS-CoV-2 transmission from humans to ferrets based on viral and antibody assays. To better understand this discrepancy in experimental and natural infection in ferrets, we compared SARS-CoV-2 sequences from natural and experimental mustelid infections and identified two surface glycoprotein Spike (S) mutations associated with mustelids. While we found evidence that angiotensin-converting enzyme II provides a weak host barrier, one mutation only seen in ferrets is located in the novel S1/S2 cleavage site and is computationally predicted to decrease furin cleavage efficiency. These data support the idea that host factors interacting with the novel S1/S2 cleavage site may be a barrier in ferret SARS-CoV-2 susceptibility and that domestic ferrets are at low risk of natural infection from currently circulating SARS-CoV-2. We propose two mechanistically grounded hypotheses for mustelid host adaptation of SARS-CoV-2, with possible effects that require additional investigation.

Predicting the zoonotic capacity of mammals to transmit SARS-CoV-2

“…Insofar as data limitations preclude perfect computational predictions of zoonotic capacity (e.g., limited ACE2 sequences, crystal structures, or species trait data), laboratory experiments are also limited in assessing true zoonotic capacity. For SARS-CoV-2 and other host-pathogen systems, animals that are readily infected in the lab appear to be less susceptible in non-lab settings (ferrets in the lab vs. mixed results in ferrets as pets [36,53,141]; rabbits in the lab vs. rabbits as pets [48,142]. Moreover, wildlife hosts confirmed to shed multiple zoonotic viruses in natural settings (e.g., bats, [143]) can be much less tractable for whole-animal laboratory investigations (for instance, requiring high biosecurity containment and very limited sample sizes in unnatural settings).…”

Natural and Experimental SARS-CoV-2 Infection in Domestic and Wild Animals

“…However, the number of households with ferrets is low compared to households with cats and dogs, and ferrets are rarely kept in large groups, in contrast to mink which are held on commercial fur farms. Interestingly, a recent study outlined a case in which two symptomatic COVID-19-positive individuals were in close and prolonged contact with 29 free-roaming pet ferrets, providing ample opportunity for natural transmission [ 173 ]. However, clinical samples failed to provide evidence of active ferret infection or seroconversion in this instance.…”

Mutation Y453F in the spike protein of SARS-CoV-2 enhances interaction with the mink ACE2 receptor for host adaption

“…It is conceivable that after transmission from humans to minks, the virus evolved further in mink to adapt to the new host, with efficient binding of SARS-CoV-2 S protein to mink ACE2 a prerequisite for ready transmission among minks. Of interest, a recent study reported the absence of natural SARS-CoV-2 human-to ferret transmission in a high-exposure setting, and genetic analysis suggested that infection of ferrets may require viral adaptation 32 . Here we demonstrate that the emergence of Y453F in SARS-CoV-2 spike significantly enhanced its interaction with other Mustela ACE2 orthologs—namely ferret and stoat—conferring a potential fitness advantage in Mustela species that could subsequently promote virus transmission and possible risk of animal-adapted virus with genetic alterations to spilling over into humans.…”