Within-host genetic diversity of SARS-CoV-2 across animal species

Abstract

Infectious disease transmission to different host species makes eradication very challenging and expands the diversity of evolutionary trajectories taken by the pathogen. Since the beginning of the ongoing COVID-19 pandemic, SARS-CoV-2 has been transmitted from humans to many different animal species, and viral variants of concern could potentially evolve in a non-human animal. Previously, using available whole genome consensus sequences of SARS-CoV-2 from four commonly sampled animals (mink, deer, cat, and dog) we inferred similar numbers of transmission events from humans to each animal species but a relatively high number of transmission events from mink back to humans (Naderi et al., 2023). Using a genome-wide association study (GWAS), we identified 26 single nucleotide variants (SNVs) that tend to occur in deer -- more than any other animal -- suggesting a high rate of viral adaptation to deer. Here we quantify intra-host SARS-CoV-2 across animal species and show that deer harbor more intra-host SNVs (iSNVs) than other animals, providing a larger pool of genetic diversity for natural selection to act upon. Within-host diversity is particularly high in deer lymph nodes compared to nasopharyngeal samples, suggesting tissue-specific differences in viral population sizes or selective pressures. Neither mixed infections involving more than one viral lineage nor large changes in the strength of selection are likely to explain the higher intra-host diversity within deer. Rather, deer are more likely to contain larger viral population sizes, to be infected for longer periods of time, or to be systematically sampled at later stages of infections. Combined with extensive deer-to-deer transmission, the high levels of within-deer viral diversity help explain the apparent rapid adaptation of SARS-CoV-2 to deer.