The ongoing Covid-19 coronavirus epidemic, which started in Wuhan at the end of last year, illustrates the threat posed by emerging infectious diseases not only to human and animal health, but also to stability. social, trade and the global economy.
However, there are many indications that the frequency of the emergence of new infectious agents could increase in the coming decades, raising fears of an impending global epidemiological crisis. Human activities are causing profound changes in land use as well as major changes in biodiversity in many places on the planet.
These disruptions occur in a context of increased international connectivity through human movement and trade, all against a background of climate change.
These are the optimal conditions for promoting the passage of pathogenic microorganisms from animals to humans. However, according to the WHO, the diseases resulting from such transmissions are among the most dangerous.
Identify new threats
Crimean-Congo hemorrhagic fever, Ebola virus and Marburg virus disease, Lassa fever, Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome (SARS), Nipah and henipaviral diseases, fever Rift Valley, Zika…
All of these diseases have in common the fact that they are included in the “Blueprint for priority diseases”, established by WHO in 2018.
The diseases listed here are considered emergencies on which research should focus. They pose a large-scale public health risk because of their epidemic potential and the absence or limited number of treatment and control measures currently available.
This list also includes a “disease X”: this enigmatic term designates the disease which will be responsible for an international epidemic of magnitude, caused by a pathogen currently unknown. WHO has no doubt that it can happen, and therefore calls on the international community to prepare for such a catastrophic scenario.
Currently, the response of public health authorities to these emerging infectious diseases is to “get ahead of the curve”, that is, to identify the environmental factors that may trigger the emergence. Unfortunately, our understanding of how new infectious threats surface is still limited.
But one thing is certain, animals will most likely be involved in the next epidemics. This is another common point of the diseases on this list drawn up by the WHO: all of them can be classified as zoonotic viral infections.
Animals widely implicated in new epidemics
In the past four decades, more than 70% of emerging infections have been shown to be zoonoses, that is, animal infectious diseases that can be transmitted to humans.
At its simplest, these diseases include a single host and a single infectious agent. However, often several species are involved, which means that changes in biodiversity have the potential to modify the risks of exposure to these infectious diseases linked to animals and plants.
As such, one might think that biodiversity represents a threat: since it contains many potential pathogens, it increases the risk of the appearance of new diseases.
Curiously, however, biodiversity also plays a protective role against the emergence of infectious agents. Indeed, the existence of a great diversity of host species can limit their transmission, by a dilution effect or by a buffering effect.
Loss of biodiversity increases transmission of pathogens
If all species had the same effect on the transmission of infectious agents, one would expect that a decline in biodiversity would similarly decrease the transmission of pathogens. However, this is not the case: in recent years, studies have concordantly shown that biodiversity losses tend to increase the transmission of pathogens, and the frequency of associated diseases.
This trend has been demonstrated in a large number of ecological systems, with very different host-agent types and modes of transmission. How is this situation explained? The loss of biodiversity can modify the transmission of diseases in several ways:
1) By changing the abundance of the host or vector. In some cases, a greater diversity of hosts may increase agent transmission, increasing the abundance of vectors;
2) By modifying the behavior of the host, vector or parasite. In principle, greater diversity can influence host behavior, which can have different consequences, whether it is an increase in transmission or a change in the evolution of virulence dynamics or pathways. of transmission. For example, in a more diverse community, the parasitic worm that is responsible for schistosomiasis (a disease that affects more than 200 million people worldwide) is more likely to end up in an inadequate intermediate host. This can reduce the probability of future transmission to humans by 25 to 99%;
3) By modifying the condition of the host or vector. In some cases, in hosts with high genetic diversity, infections can be reduced or even induce resistance, which in fact limits transmission. If genetic diversity decreases because populations decrease, the likelihood of resistance also decreases.
In this context, the ongoing loss of biodiversity is all the more worrying. Current estimates suggest, for example, that at least 10,000 to 20,000 freshwater species have disappeared or are at risk of disappearing. The rates of decline observed today compare with those of the major crises of the past, such as that which marked the transition between Pleistocene and Holocene, 12,000 years ago, and which was accompanied by the disappearance of the megafauna, including the woolly mammoth. was one of the emblematic representatives.
But the loss of biodiversity is not the only factor influencing the emergence of new diseases.
Climate change and human activities
It is the shift in the geographic footprint of the pathogens and / or the host they infect that leads to the emergence of new infectious diseases. As such, the increasing unpredictability of the global climate and local human-animal-ecosystem interactions, which are becoming increasingly close in certain parts of the planet, play a major role in the emergence of new infections in human populations.
Thus, the increase in average temperatures would have had a significant effect on the incidence of Crimean-Congo hemorrhagic fever, caused by a virus transmitted by ticks, as well as on the durability of the Zika virus, transmitted by mosquitoes in subtropical and temperate regions.
The consumption of bushmeat and the trade in animals, resulting from the growing demand for animal proteins, are also causing significant changes in the contact between humans and animals. Studies have shown that the SARS and Ebola outbreaks are directly linked to the consumption of infected bushmeat. In addition, Lassa fever and diseases caused by the Marburg and Ebola viruses thrive in West and Central Africa, where the consumption of bushmeat is four times that of the Amazon, which is richer in biodiversity. .
Another risk is the expansion of agriculture and livestock. In order to meet the ever increasing demand of human populations, new spaces must be conquered, by deforesting and clearing. However, we know that this reallocation of land can trigger the emergence of infectious diseases, by promoting contact with organisms that have so far been rarely encountered. In the islands of Sumatra, for example, the migration of fruit bats caused by deforestation due to forest fires has led to the emergence of Nipah disease among breeders and slaughterhouse workers in Malaysia.
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When the extraction of gold makes bacteria devouring flesh proliferate
Inevitable emergences
The relationships between the biodiversity of host species and that of parasites and pathogens are complex. By modifying the structure of communities, all of these environmental changes are likely to modify existing epidemiological patterns.
In this context, human populations can come into contact with an animal carrying a virus capable of contaminating them. A cycle of infections can then take place. It begins with sporadic cases of transmission from animals to humans, known as “chatter virus” (“viral chat”). Then, as cycles multiply, the emergence of human-to-human transmission becomes inevitable.
Once the epidemic has started, speed of response is essential. In addition to the necessary health measures, when there is insufficient time to conduct appropriate epidemiological studies, mathematical modeling can be of great help in quickly assessing the effectiveness of prevention, and anticipating the course of the disease.
But understanding the complexity of the interactions between the natural reservoir, the pathogen and the intermediate host (s) remains a major challenge when it comes to intervening quickly to stop the transmission of the disease. The example of COVID-19 illustrates this once again: more than two months after the first infections, the various animal links in the chain of transmission of the epidemic remain to be identified.
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