Enough with the story of the good virus: nature has fangs

Enough with the story of the good virus: nature has fangs
Enough with the story of the good virus: nature has fangs

As soon as there is a decline in the contagions, the story of the pathogen comes back, trying to live with us in order not to become extinct

There have been once again in recent days statements about the fact that the Sars-CoV-2 virus will necessarily have to evolve in a benign direction from a clinical point of view, otherwise it will be extinguished.. This is a hard-to-die preconception, according to which the damage inflicted on a host by a parasite would be disadvantageous for the parasite itself as, by causing the death of the host, it would decrease its own chances of survival, a bit like who burns their house. It is an evidently false and widespread prejudice, especially in the medical environment because, particularly in Italy, evolutionism is not sufficiently studied: the truth is that many parasites, even far more harmful from a clinical point of view than Sars-CoV- 2, maintain their pathogenetic characteristics unchanged, because the damage inflicted on the host does not in any way alter the number of copies that a parasite can leave of itself, which depend primarily on its ability to infect other hosts before a subject infected develop immunity or die.

The smallpox virus, for example, has not changed its lethality and its pathogenicity during millennia of co-evolution with humans.: smallpox epidemics inflicted serious losses in terms of mortality to the affected communities, after which the virus locally decreased, the population recovered and continued in a cycle that lasted until the use of vaccines, The same can be said for the various strains of Ebola emerged during the decades since this virus is known, for HIV, for hepatitis viruses or for non-viral parasites such as malaria: the balance with our species of these parasites has not prevented the maintenance of the serious consequences at all infection clinics. Let’s look only at malaria, which causes millions of deaths for the simple reason that even at values ​​of lethality or pathogenicity much higher than those of Sars-CoV-2 there is never such an overall scarcity of human beings as to disadvantage some of the most common strains pathogenic or more lethal currently in circulation.

This happens, for example, because the infection can lead to death in such slow times that it does not appreciably affect the possibility of transmission to other hosts, or because there can be many different host species, of which only one is man, which they act as a refuge until the local recovery of the human population, or because infectivity is so high as to compensate for the rapid death of the hosts, or because death occurs after the reproductive age of the host or for many other reasons.

Basically, there are many conditions in which even the massive death of human hosts does not affect the transmissibility of the parasite or the human population in such a way as to compromise the fitness of the parasite itself.. Apart from these considerations concerning the parasites that we can observe, that is, those with whom we have reached a more or less favorable coexistence for us (more favorable since we have found vaccine treatments and prophylaxis; less favorable for those human populations that do not have access to them) then there is a bias, that of the surviving observer. It is obvious that all the parasites we have observed so far have not extinguished our species, because otherwise we would not be here talking about it. However, this does not at all mean that there are no parasites capable of extinguishing their host, both because it is sufficient that they also have other hosts, and because extinctions occur, and both a host and a parasite could become extinct, without that this is in principle prohibited by some natural law. These are cases that we have been able to observe for other species, such as for example a land snail of the genus Partula, extinct by its parasite: natural evolution does not have in itself a mechanism that excludes extinctions, as we can moreover measure from extinctions that our species has caused, and indeed the extinction of species frees ecological niches that can be exploited by others.

Having made these considerations, we can reason about our future coexistence with Sars-CoV-2. It will become less clinically relevant? If, as I believe, it will happen, this will be due to the success of vaccines and any drugs, as has happened recently for many other species of viruses, not due to a nonexistent evolutionary trajectory. In particular, we are witnessing a lesson in evolutionary biology on a large scale: an initial leap of species led to the evolution of more infectious variants first (despite those who last summer already spoke of the emergence of clinically less relevant variants, only because he saw hospitals empty), as is expected when a virus has billions of guests at its disposal and can freely replicate. With the development of immunity, both natural and vaccine, it is possible for immunoevasive variants to emerge: some of the mutations on the spike protein, for example, are already adaptations to the human immune system, and their continuous and independent re-emergence in many parts of the world is proof. the adaptation of the virus to our immune system.

If less pathogenic variants emerge, this will be a simple accident, because some mutation that confers greater replicative capacity, or greater infectivity or greater immunoevasiveness, will be accidentally accompanied by characteristics that for example decrease the cytokine storm in response to the virus; however this is not a necessary process, until it is shown that some characteristic that induces the current pathogenicity and lethality does not favor replication, or infectivity, or immunoevasiveness of the virus that carries them.

How long will the virus still be able to mutate? We do not know, because we do not know what is called the “phenotypic space” at its disposal, that is, we do not know what all the mutations that could emerge are, which are not deleterious or are advantageous for its bearer; certainly, the fact that some mutations appear repetitively suggests that this space is not infinite, and if we are lucky it could be sufficiently small to be covered by the new generation vaccines in development, at least those that are also targeting parts of the virus little subject to mutations (where the space available for the virus is more restricted, because each mutation is deleterious).

For the reasons set out, please: let’s stop, whenever we find ourselves in a favorable epidemic phase, to fish out under other clothes the idea of ​​the virus that inevitably adapts to live with us: nature, as Darwin explained, has fangs and claws dripping with blood, and does not provide for any balance, but only a more or less stationary state until the next catastrophe.

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