The global Covid-19 pandemic has brought to the fore an older debate about the role of transformations carried out by our species in its natural environment in the emergence of infectious diseases. The impact of so-called intensive agriculture is, in particular, the subject of intense controversy. This is part of a broader questioning of this method of creation, due to the impact on animal welfare and the environmental pollution it generates.
However, few scientific arguments are available to support the hypothesis of a contribution of the intensification of livestock to the emergence of pandemics.
Emerging diseases and zoonoses
First, we need to clarify what we call “emerging disease” and “intensive agriculture”. If it is commonly accepted that more than 60% of the diseases in this case since the mid-twentieth centuryand century are of animal origin, the vast majority of the pathogens in question have an animal population as their natural reservoir, that is, they are mainly transmitted between animals and sporadically cause foci of infection in humans that do not result in sustained circulation 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 permanently established itself in livestock populations, in this case birds, in several countries in Asia and Africa, and is now threatening Europe and North America. These influenza viruses cause serious illness but are not permanently transmitted in our country.
On the other hand, the emergence of a human disease involves a true species leap: the pathogen must adapt to this new host in such a way that its circulation is assured outside its initial animal reservoir. This is the case of the SARS-CoV-1 and SARS-CoV-2 coronaviruses, respectively at the origin of the severe acute respiratory syndrome (SARS) pandemic, and Covid-19, which emerged in 2002 and 2019, respectively. wild animal reservoir. This is also the case with the human immunodeficiency virus (HIV), originally a zoonosis transmitted by great apes.
While many ancient human diseases do indeed result from jumps in species from a reservoir of domestic animals – this is the case with measles and whooping cough in particular – this phenomenon remains extremely rare if we put it on the scale of animal domestication, which spans tens of thousands of people. years old.
The difficulty of characterizing intensive agriculture
The notion of livestock intensification is no less complex. If there is no official definition of intensive agriculture, it is commonly accepted that it refers to a set of changes in agricultural practices that have taken place since the end of the Second World War, first in industrialized countries in emerging and low-income countries, aimed at reducing the cost production of food of animal origin.
More specifically, intensification has reduced the need for farmland and the labor needed to produce a given amount of animal products. This productivity gain has been achieved in several ways, notably through genetic improvement and mechanization.
Intensification is usually accompanied by an increase in the number of animals per farm, a specialization of farms at a given stage of production (genetic selection, breeding, fattening) and their increasing integration into complex and globalized commercial circuits.
Intensification, however, is not a uniform process and has taken on different modalities depending on the socioeconomic and institutional contexts in which it developed. The control of health risks on medium and large commercial farms thus seems to be heavily influenced by the degree of support for agricultural practices by the State, the access of farmers to loans and veterinary services and the transparency measures put in place in the the animal sectors.
The complex connection between these two notions
Once the definitions have been established, the essential question of the causal link between the intensification of livestock farming and the emergence of new human diseases remains. Of course, a demonstration seems impossible, given the rarity of the emergence of human diseases from reservoirs of domestic animals and the relative novelty of the process of intensifying reproduction.
Two examples of viral zoonoses, however, shed light on how intensive agriculture may, in the future, contribute to emergencies:
● The first example is the highly pathogenic avian influenza due to H5N1, identified in China in 1997 before its worldwide spread from 2003. Areas where the H5N1 subtype appears combine high densities of poultry and landscape features (wetlands ) conducive to interfaces with wild waterfowl. Studies carried out in different Southeast Asian countries 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 telling example is Nipah encephalitis, transmitted by domestic pigs, which emerged in Malaysia in 1998. The combination of swine farming and large-scale mango cultivation favored contact between bats. pigs, so the transmission of the virus was favored by the concentration of pigs in large numbers on the farms and the transport of live pigs through commercial networks.
A link has also been documented between intensive farming practices and increased virulence – that is, the dangerousness of infections – of certain pathogens, notably 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 clustering of animals at high density, to their shorter lifespan, and to the selection of breeders based on their productivity rather than their resistance to disease – contrary to the selection mechanisms at work in the most extensive forms. of farming.
More virulent pathogens in animals, if transmitted to humans, would 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 proven so far.
Finally, large-scale commercial exchanges of live animals promote the exchange of genetic material between distant viral strains, thus favoring the emergence of new strains capable of transmission to humans. These phenomena, called “viral recombination”, are particularly frequent and feared in the case of avian and swine flu.
Prepare to avoid disasters
As we can see, the absence of definitive proof of a link between the intensification of livestock 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 first case, intensive agricultural practices, which have been in place for a long time, are associated with systems that aim to closely monitor the emergency and limit the spread of diseases.
The second face a rapid expansion of large-scale livestock, to respond to the accelerated urbanization of the population and the growing demand for animal protein. This expansion must be accompanied by State services and acceptable measures of transparency and health monitoring for rural populations.
These efforts will ultimately ensure that meeting populations’ growing needs for animal protein does not come at the expense of public health.
Alexis Delabouglise, researcher, socio-economist in animal health, CIRAD; François Roger, Regional Director for Southeast Asia, veterinarian and epidemiologist, CIRAD; Marisa Peyre, deputy head of the ASTRE research unit, epidemiologist, CIRAD and Mathilde Paul, Professor of Epidemiology, National Veterinary School of Toulouse, inrae
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Image credit: Shutterstock/CHIRATH PHOTO
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