The cross : To what extent are livestock activities a cause of virus transmission?
Daniel Marc: An infectious agent never arises out of thin air. Whether it was Ebola, HIV in the 20th century, or measles probably a few centuries ago, viruses always come from animals. It can come from direct contact with the first host, for example AIDS with chimpanzees. There are the contaminations which are made by an intermediate host, through which the virus adapts to humans, which is the case, for example, with coronaviruses (the 2003 SARS had thus adapted in the civet ). Finally, you can be infected by the bite of a vector, that is to say a stinging insect, like the Zika virus or yellow fever transmitted by mosquitoes.
→ INVESTIGATION. Covid-19: how investigators trace the virus
Therefore, it is logical that some zoonoses – diseases transmissible between animals and humans – have reached us through livestock, but this is far from the norm.
Can we quantify the number of epidemics in recent history that come from intensive breeding?
D. M. : Today, the fashionable discourse, which speaks a lot to the younger generations, aims to question intensive breeding. We want to believe that this is the cause of everything, including epidemics.
There are precedents, of course, but they are exceptions. I see basically two. The 2009 influenza A (H1N1) influenza pandemic, for example, was the result of more than ten years of viral assemblage between viral strains of avian origin, pig strains and a human strain that formed in pigs. It is in pig farms that it has been able to find a form that adapts to us and that it has spread in the population.
The other is the Nipah virus which killed around 100 people in Malaysia in 1998. It was also transmitted from bats to humans through pig farms. In this case, pig farms had been built on deforested areas that encroached on the natural habitat of bats.
Changing our agricultural model would therefore not have an impact on the epidemic risk?
D. M. : In my opinion, intensive or extensive breeding, that does not change anything. In both cases, it is the fact of being in contact with the animals that explains the transmission, not their number or their breeding conditions. We even have a paradox right now with the avian flu epidemic. It mainly affects the South-West because they practice the breeding of ducks in the open air and the virus comes from the wild world. On the other hand, intensive poultry farms, which are very numerous throughout Europe, are protected from it because they are confined.
→ ANALYSIS. Avian influenza, the preventive slaughter in question
One of the explanations is that we are seven billion human beings compared to two billion at the beginning of the 20th century. We have a much higher population density and the increase in the flow of goods and people allows infectious agents to spread in a few days on all continents, in particular by air transport.
That being said, an epidemic will remain an exceptional fact and difficult to predict, like a volcano which erupts or an earthquake. If we are to believe the written historical sources dating from before the twentieth century, we notice that there are three to four influenza epidemics per century, and this, well before the birth of our agricultural model.
I think we have to live with this risk and respond scientifically. We have already succeeded in eradicating viruses, such as smallpox (this was in 1978). This is an exceptional fact, which was only made possible by the will of all the actors concerned and absolute confidence in science and in vaccination.