Wildlife, a reservoir of pathogens
Because they harbour a great diversity of animal species, natural environments are a reservoir of pathogens to which domestic animals and humans are not accustomed. So in the event of direct or indirect contact with wildlife, transmission can occur. This type of contamination is a major problem, both economically – as shown by the avian influenza epidemics, which can sometimes only be controlled through the mass culling of domestic poultry – and in terms of public health, with the risk of emergence of new diseases for humans, such as the COVID-19 pandemic.
Not all transmissions result in the emergence of a disease
Cases of pathogen transmission from wild to domestic animals are common. It is only if the pathogen adapts to the new host and spreads among individuals of the newly infected species that there is the emergence of a disease, or a re-emergence if the disease reappears in a region from which it had been eradicated. Some of these emerging or re-emerging diseases only affect animals, such as African swine fever. Others, such as tick-borne encephalitis or rabies, can be transmitted to humans and are therefore zoonoses.
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Because RNA viruses (i.e. which use ribonucleic acid (RNA) as their genetic material) have a higher mutation rate than DNA-based viruses, bacteria or parasites, they are more likely to acquire a mutation that allows them to adapt from one species to another. This explains why most of the zoonoses that have emerged in recent years (influenza, coronavirus, Ebola, etc.) are RNA viruses.
Ecosystem disruption, a risk factor for transmission
The places with the greatest biodiversity are also those where pathogen diversity is highest. Environmental disruption is often behind disease emergence, as it allows contact between species that are not accustomed to living alongside each other. The Nipah virus in South-east Asia is a typical example of this: the virus, from the same family as measles, was transmitted from fruit bats to pigs, which then passed it on to humans. These flying mammals had found refuge in pig farms after colossal fires, a consequence of climate change, destroyed their habitat in the Malaysian rainforest. The Nipah virus has a specific mortality rate of over 40% in humans. The outbreak was contained through the slaughter of practically a million pigs.
Closer to home, the development of green belts to facilitate the movement of wild animals can increase the risk of spread of diseases such as tick-borne encephalitis, which is transmitted by these mites from rodents to humans. In other situations, biodiversity can, on the contrary, have a buffering effect, by diluting pathogens. Some species are not as competent as others in allowing replication of a pathogen; they are known as dead-end hosts. The pathogen will then be transmitted less than if the community only consisted of a few susceptible species. In systems where this dilution effect is present, therefore, the greater the diversity of species, the less likely humans or domestic animals are to encounter a contaminating species.