White-tailed deer milk exhibits SARS-CoV-2 neutralizing antibodies and synergistic mechanisms that contribute to rapid viral RNA degradation

White-tailed deer (WTD) represent the most significant SARS-CoV-2 wildlife reservoir in North America, yet the role of antiviral mechanisms in vertical transmission remains unexplored. We investigated SARS-CoV-2 antibody responses and viral stability in milk from lactating WTD and humans to characterize species-specific antiviral mechanisms. SARS-CoV-2 neutralizing antibodies were detected in milk and serum in WTD specimens using complementary immunoassays, providing the first evidence of humoral immune responses in wildlife milk. Despite antibody presence indicating prior SARS-CoV-2 exposure, viral RNA was undetectable in all WTD milk samples. This pattern aligns with observations in human milk, where viral RNA was also undetectable both during active infection (when nasal swabs were positive) and during antibody-positive periods following recovery. In vitro stability studies revealed striking species differences: all SARS-CoV-2 variants (A, B.1.1.7, BA.1.1.529) rapidly degraded in WTD milk within 30 min at physiological temperatures, while remaining mostly stable in human milk for up to 60 min. Biochemical characterization identified multifactorial degradation mechanisms in WTD milk, including 5-20 fold elevated mineral concentrations (sodium, magnesium, phosphorus, and potassium), enhanced protease activity, and increased lactoperoxidase levels. Individual mineral supplementation revealed variant-specific susceptibilities, with B.1.1.7 showing pronounced sensitivity to ionic stress. Mechanistic studies demonstrated synergistic effects between elevated ionic concentrations and proteolytic activity, with heat-labile and heat-stable degradation pathways contributing to viral inactivation. These findings reveal that WTD milk possesses intrinsic antiviral properties fundamentally different from human milk, representing an evolutionary adaptation that may impact viral persistence and transmission dynamics in wildlife populations. These findings reveal antiviral mechanisms in WTD milk that represent a previously unrecognized component of pathogen control in wildlife reservoirs, with important implications for understanding wildlife-pathogen interactions and zoonotic risk assessment. Author SummaryWhite-tailed deer (WTD) have become the primary wildlife reservoir for SARS-CoV-2, with millions of infected animals across North America. Despite this significance, the presence of protective antibodies and viral behavior in WTD milk remained unexplored. We collected milk samples from lactating WTD during hunting seasons to investigate whether WTD produce neutralizing antibodies similar to those found in human milk and to examine how the virus behaves in this biological fluid. Our analysis revealed that WTD milk contains antibodies capable of neutralizing SARS-CoV-2. When we compared viral stability between WTD and human milk, we observed that WTD milk rapidly degrades viral genetic material within 30-40 min, while the same virus remains stable in human milk for over an hour. We identified that WTD milk contains mineral concentrations 5-20 times higher than human milk, including elevated levels of sodium, magnesium, and potassium, along with enhanced enzyme activity that breaks down viral components. These findings indicate that WTD milk functions as a protective barrier rather than a transmission route. This has implications for understanding viral persistence in wildlife populations and assessing potential risks to human health. Our work demonstrates that deer milk possesses multiple biological defense mechanisms that may protect offspring from viral infections, contributing to our understanding of wildlife immunity and pandemic preparedness.