In wildlife, the natural reservoir host for most animal viruses, overt clinical infections are generally absent. In this paper it is hypothesized that for hundreds of millions of years viruses co-evolved with arthropods, fishes, amphibians, reptiles, birds, and mammals, in a friendly manner, by minimizing virus replication costs and host damage. This virus-host mutualism hypothesis may be viewed as diametrically opposite to the virulence-transmission hypothesis. Building on previous work, the transmission ecologies of 36 of the world main livestock viruses are examined in more detail. Viruses and organ systems are aligned on ecological grounds, in an outer- to inner-body fashion. The virus-host interplay changes from acute to persistent infection to ever more virus-host intimacy. From outer- to inner-body virus-host mutualism is on the increase. Virus-host antagonism increases from inner- to outer-body. Also explored is the role of the host domain in mutualism-antagonism evolution. For this, the virus evolution trajectory upon a host shift from wildlife to humans or livestock is examined. Again, epithelial viruses are contrasted to non-epithelial, fully internalized viruses. It is found that a-clinical enteric viruses of wild birds and bats, in humans and livestock evolve as respiratory or also enteric pathogens. Viral virulence is most prominent among respiratory and enteric poultry and pig viruses. Regarding the internalized viruses, it is found that upon a transfer from wildlife to humans or livestock as host, the virus evolves to return to the sylvatic trait profile, very gradually so. Collectively, the results of this study corroborate the notion that in natural ecosystems friendly virus-host relationships tend to be selected for. Evolution of viral virulence and other forms of pathogenic fitness, including immune-suppression, primarily occur in animal mass rearing and so are anthropogenic in nature. The crowd agents circulating in humans as host take an intermediary position.
Published Date: 2021-09-13;