The emergence and evolution of swine viral diseases: to what extent have husbandry systems and global trade contributed to their distribution and diversity?

Over the last 20 years, pig production has been characterised by a rapid increase in the volume of pig meat produced, greater intensification of the pig-rearing process and much greater international movement of products. There have also been many novel viral diseases that challenge the industry. Are these two developments linked and, if so, how? To understand how changes in the industry may influence the evolution of viruses, it is important to understand something of evolutionary theory. For RNA viruses, the concept of 'quasispecies' has moved solidly from theory to fact. Such viruses do not exist as a single entity, but as a 'cloud' of viruses, whose degree of diversity is influenced by a number of factors. Chief among these are the size and rate of the replicating population, along with the availability and diversity of susceptible hosts. A feature of RNA viruses is a high level of mutation, due to lack of capability to correct errors on the part of the host cell. Both in vivo and in vitro, RNA viruses have been shown to accumulate and fix these mutations, leading to bottleneck events and fitness loss, the phenomenon known as'Muller's ratchet'. Likewise, the opposite effect, fitness gain, can be achieved in an environment providing for high levels of replication and the generation of large populations of virus. This has been shown to be possible in vitro by high-volume passage. It is possible that the regular introduction of diverse viruses within large-scale pig production provides an in vivo equivalent that could drive quasispecies populations to increased fitness, and may explain why emergent viruses, either new to science or with new synergies and presentation, seem to be appearing more commonly.