From ants to humans. How prevention affects the reproduction of viruses

To prevent the spread of a dangerous pathogen, both vertebrates and social invertebrates adopt various behavioral measures, aimed at minimizing its spread in the group and the consequent negative effects. In particular, Treatment of infected members of a group is a hallmark of collective defense against disease in a large assemblage of social animals. This, of course, is particularly evident in humans, where the treatment of infected subjects has a double purpose: the cure of the individual, but also the prevention of the spread of the infection, as we have all been able to observe well during the still ongoing pandemic. course.

And yet, despite being predictable, the effects of this sort of behavioral “social immunity” on the reproductive success of pathogens – that is, the effects capable of influencing their natural evolution, due to the selective pressure due to the contrasting social behaviors of the hosts – are still largely unknown. Using social insects, i.e. ants, and pathogenic fungi, a new study has for the first time very clearly identified the presence and type of effects that social behavioral immunity has on the evolution of pathogens.

Ants, like other social insects, have developed cooperative defenses against disease in addition to individual immunity. Specifically, individuals belonging to a nest clean colony members exposed to pathogens, removing and disinfecting infectious particles even before they can cause infection. Pathogens that infect their hosts by attaching themselves to and penetrating the body surface, such as the spores of many parasitic fungi, are then subjected to strong selection to counteract the negative impact that mutual grooming of their social hosts has on their likelihood of replication. . Notably, although cleaning uniformly reduces the number of spores capable of successfully infecting a host, the degree of this effect can vary even among closely related fungal species, depending on their specific properties.

This effect was proven by the authors of the study in question, who showed how different strains of parasitic fungi, when their spores are used to infect an ant, have different success rates depending on whether the ant exposed to the pathogens is then subjected to cleaning by its nestmates or not. If an ant is not cleaned up, one given fungal stock immediately succeeds, competing with the other; if, on the other hand, the normal behavior of “social immunity”, i.e. cleaning, is implemented, then all mushrooms decrease their success, but even the normally most disadvantaged strains are able to reproduce.

The behavior of prevention of infections by ants, that is, favors the maintenance of a community of pathogens at lower levels, but more varied: this, in the specific case, happens because the fungus that would normally be extinct in the absence of social cleaning behaviour, has much lower quantities of a chemical component in its membrane that the ants are able to identify, and which therefore disadvantages the strains more aggressive than pathogens. Also, under social immunity selection, pathogens are found to produce more, but less virulent, spores; this is because the same chemical, ergosterol, which is reduced favoring evasion of reciprocal cleaning behavior, is also positively correlated with spore pathogenicity. The set of these complex selective effects and Darwinian evolution, both on individual fungal strains and on the entire ecosystem of parasites that interact with each other, has indeed been found specifically on the model of pathogenic insects and fungi, but obviously appears not bound to that host-parasite system.

If we think, for example, of what happened after the large-scale application of the Sars-cov-2 detection tests, which favored those variants better able to evade them, we have a homologous example of the selection of pathogens with less ergosterol in the membrane by ants, which use that molecule to identify fungal spores. Similarly, the complex co-evolutionary effects that our defense behaviors against one pathogen have had on other pathogens are reminiscent of what was observed for different fungal strains in the work just described: Pathogen co-ecology results in adaptation not only of a single species targeted by treatments, but also of a myriad of different pathogen populationsfrom antibiotic resistant bacteria to the most diverse other infectious agents.

The concept of “one health”, too often used to connect our health to that of the ecosystem in which we live, must therefore be understood in an even broader way, in the sense of considering a large interconnected network of parasitic species, which respond in a coordinated way to our defensive behaviors towards any of them; and it is for this reason that broad-spectrum measures, such as those illustrated on these pages, are the ones we should most likely implement.