It is often said: ‘Large-scale livestock farms are a source of potential misery when it comes to infectious animal diseases’. Almost everyone can imagine something about that. Keeping many animals together can be the prelude to massive outbreaks. Spread from livestock farming to livestock farming is a realistic scenario. The origin of the current bird flu (H5N1), which is causing worldwide problems, is attributed to a goose farm in China around 1996.
Modern livestock farms worldwide are now part of logistics chains at regional, national or even international level. This has created large-scale ecosystems in which animal diseases can manifest themselves relatively easily. This is about as far as the average consumer’s knowledge extends.
However, the underlying risk is much greater and more fundamental than the above representation. This has to do with the fact that such ecosystems, in addition to the spread, also seem to promote the evolution of viruses and bacteria. Within the argument below, I focus mainly on the evolution of viruses, assuming that the evolution of bacteria is influenced in roughly the same direction.
Spread rate of virus variants as the most important evolutionary characteristic
When talking about virus variants, this refers to the changed or mutated offspring of a certain virus strain. The virus variants that then manage to spread more quickly across their hosts than other virus variants ultimately come to dominate within the gene pool of a particular virus strain.
After all, natural selection concerns the more successful survival and propagation of certain genes. The rate of spread of a virus variant ultimately depends on its genes and thus forms the starting point for natural selection. In practical terms, this means that the less rapidly spreading virus variants are relatively more likely to encounter immunity already acquired by their hosts. As a result, the slower-spreading virus variants will eventually be completely displaced by the faster-spreading virus variants of a particular virus strain.
Mutated virus variants are usually worse off
When viruses multiply, this is sometimes accompanied by copying errors. This means that the original genetic material of a virus particle has been changed in more or less random places. We call these changes mutations. Viewed from the perspective of a newly emerged virus variant, the majority of such mutations have neutral or negative effects on the course of infection and the rate of spread. Random changes within a proven effective and complex functioning system more often have neutral or negative consequences than positive consequences.
Most mutations are dead ends under natural conditions
Most mutations therefore have neutral or negative effects on the course of infection and the rate of spread. This means that in an evolutionary sense they are in fact dead ends. Unless there are unnaturally high concentrations of hosts with reduced resistance. In that case, the evolutionary playing field is significantly expanded, as I argue in the upcoming paragraphs. And that is precisely the blueprint of modern livestock farming: a high concentration of hosts in non-optimal living conditions. The non-optimal living conditions mainly lead to stress and reduced resistance.
A high concentration of hosts with reduced resistance
A high concentration of hosts with reduced resistance is the ideal scenario for virus evolution. This allows new virus variants with an initially substandard spread rate to persist for substantially longer. Simply put: new virus variants have more time to acquire a potential accumulation of different mutations. Such an accumulation of different mutations can lead to the emergence of a coincidentally successful virus variant.
The cause of this basically consists of three factors. First of all, the immediate proximity of hosts initially offers less infectious variants sufficient opportunities for propagation. In addition, the immediate displacement of less successful virus variants by more successful virus variants under such conditions of reduced resistance is less absolute.
Finally, a high concentration of hosts with reduced resistance increases the opportunities for mixing virus genes (recombination) from different origins. For example, the genes of different virus variants or even different virus strains can recombine within one host into a coincidentally successful virus variant.
The above factors offer significantly more potential opportunities for the accumulation of different mutations. Possibilities that rarely or never arise in nature.
Immunocompromised HIV patient in South Africa
The following example is illustrative of the last two arguments: the meteoric rise of the relatively highly mutated Omicron variant of COVID-19 in South Africa. This is explained by some by the long-term proliferation of a corona infection in an immunocompromised HIV patient. Under such circumstances, in addition to recombination of different virus variants, accumulation of different mutations within one and the same host is a realistic scenario.
Accumulation of mutations and recombination offer additional evolutionary possibilities
Virus variants with an accumulation of different mutations can lead to evolutionary breakthroughs via essentially dead-end evolutionary paths. This also applies to the recombination of virus genes. Although the chance of such breakthroughs is extremely small, the possibilities regarding virus evolution nevertheless seem to be significantly increased.
After all, virus variants that do not spread optimally are dealt with in the wild. The possibilities for recombination of virus genes also appear to be much smaller in the wild – where collectively reduced resistance plays a marginal role.
Livestock farms have become evolutionary pressure cookers for animal diseases
Within the ecosystems of livestock farms, the accumulation of various mutations and recombination appear to lead to the emergence of potential virus variants that have no chance in the wild. Or they take much longer to develop at all. In that respect, livestock farms actually form a pressure cooker for virus variants that either never arise or only emerge after a much longer period of time. Something similar may apply to certain bacterial animal diseases.
Read more here what exactly the evolutionary strategy is behind the culling of poultry farms due to bird flu.
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