The global Covid-19 pandemic has brought to light an older debate on the role of the transformations wrought by our species on its natural environment in the emerging infectious diseases. The impact of so-called intensive farming is, in particular, the subject of intense controversy. This joins a broader questioning of this farming method, because of its harm to animal well-being and the environmental pollution it generates.
However, few scientific arguments are available to support the hypothesis of a contribution of livestock intensification to emergence of pandemics.
Emerging diseases and zoonoses
First, we need to clarify what we call “emerging disease” and “intensive farming”. If it is commonly accepted that more than 60% of diseases in this case since the middle of the XXe century are of animal origin, the vast majority of the pathogens concerned have an animal population as their natural reservoir, i.e. they are transmitted mainly among animals and sporadically cause foci of infection in humans which do not give rise to circulation. lasting in the human population.
These diseases are called "zoonosis", a term that basically encompasses all diseases transmitted from animals to humans. Highly pathogenic avian influenza is probably the most publicized example. This emerging zoonosis has become permanently established in livestock populations, in this case poultry, in several countries in Asia and Africa, and is now threatening Europe and North America. These influenza viruses cause severe illnesses, but are not transmitted permanently in our country.
On the other hand, the emergence of a human disease involves a real species leap: the pathogen must adapt to this new host in such a way that its circulation is ensured outside of its initial animal reservoir. This is the case of the coronaviruses SARS-CoV-1 and SARS-CoV-2, respectively at the origin of the severe acute respiratory syndrome (SARS) pandemic, and Covid-19, which emerged in 2002 and 2019 respectively. probably from a wild animal tank. This is also the case with the human immunodeficiency virus (HIV), initially a zoonosis transmitted by great apes.
While several ancient human diseases do indeed result from jumps in species from a domestic animal reservoir – this is the case with measles and whooping cough in particular – this phenomenon remains extremely rare if we place it on the scale of the animal domestication, which spans tens of thousands of years.
The difficulty of characterizing intensive farming
The notion of livestock intensification is no less complex. If no official definition of intensive farming exists, it is commonly accepted that it refers to a set of changes in farming practices that have taken place since the end of the Second World War, first in the countries then industrialized in emerging and low-income countries, aiming to reduce the cost of production of foodstuffs of animal origin.
More specifically, intensification has reduced the need for agricultural land and the labor required to produce a given quantity of animal products. This gain in productivity has been achieved in several ways, notably through genetic improvement and mechanization.
Intensification is generally accompanied by an increase in the number of animals per farm, a specialization of farms in a given stage of production (genetic selection, reproduction, fattening), and their increasing integration into commercial circuits. complex and globalized.
Intensification, however, is not a uniform process and has taken on different modalities depending on the socio-economic and institutional contexts in which it has developed. The control of health risks in medium to large-scale commercial farms thus seems to be greatly influenced by the degree of support for farming practices by the State, farmers' access to loans and veterinary services, and by the measures of transparency put in place in the animal sectors.
The complex link between these two notions
Once the definitions have been laid down, there remains the essential question of the causal link between the intensification of animal husbandry and the appearance of new human diseases. It is clear that a demonstration seems impossible, given the rarity of the emergence of human diseases from domestic animal reservoirs and the relative novelty of the process of intensification of breeding.
Two examples of viral zoonoses, however, shed light on how intensive farming could, in the future, contribute to emergences:
● The first example is that of thehighly pathogenic avian influenza due to H5N1, identified in China in 1997 before its global spread from 2003. areas of occurrence of the H5N1 subtype combine high densities of domestic poultry and landscape features (wetlands) conducive to interfaces with wild aquatic birds. Studies carried out in different countries of Southeast Asia show the role of commercial farms and poultry trade networks as accelerators of the spread of the virus in animal populations, at different scales.
● Another eloquent example is that of theNipah encephalitis, transmitted by domestic pigs, which emerged in Malaysia in 1998. The combination of pig farming and large-scale mango cultivation promoted contact between fruit bats, the wild reservoir of the virus, and domestic pigs, and then virus transmission was favored by the concentration of pigs in large numbers on farms and the transport of live pigs via commercial networks.
A link between practices linked to intensive farming and the increase in virulence – that is to say the dangerousness of infections – of certain pathogens has also been documented, in particular for the Marek's disease virus which affects chickens and is not zoonotic.
The mechanisms of this adverse selection for more virulent forms of pathogens are complex. They are linked in particular to the grouping of animals in high density, to their shorter lifespan and to the selection of breeding stock on the basis of their productivity and not their resistance to disease – contrary to the selection mechanisms at work in the more extensive forms of farming.
More virulent pathogens in animals would, if transmitted to humans, be more likely to cause pronounced symptoms in our species as well. This is a hypothesis proposed to explain the severity of infections due to the H5N1 subtype of highly pathogenic avian influenza, but which has not been substantiated to date.
Finally, large-scale commercial exchanges of live animals promote the exchange of genetic material between distant viral strains, thus favoring the appearance of new strains capable of transmission to humans. These phenomena, called "viral recombination", are particularly common and feared in the case of avian and swine influenza.
Prepare to avoid disaster
As we can see, the absence of definitive proof of a link between the intensification of livestock farming and the risk of a pandemic does not exempt us from anticipating and preventing potential future risks.
Industrialized countries and emerging countries are, however, facing contrasting situations. In the former, intensive farming practices, which have been in place for a long time, are associated with systems aimed at keep a close eye on emerging and limit the spread of disease.
The second face a rapid expansion of large-scale livestock farming, to respond to the accelerated urbanization of the population and a growing demand for animal protein. This expansion must be accompanied by State services and acceptable transparency and health monitoring measures for rural populations.
These efforts will ensure, in the long term, that meeting the growing needs of populations in animal protein does not take place not at the expense of public health.
Alexis Delabouglise, Researcher, socio-economist of animal health, CIRAD; Francois Roger, Regional Director Southeast Asia, veterinarian and epidemiologist, CIRAD; Marisa Peyre, Deputy head of ASTRE research unit, epidemiologist, CIRAD et Matilda Paul, Professor of Epidemiology, National Veterinary School of Toulouse, Inrae