The ongoing Covid-19 coronavirus outbreak, which began in Wuhan late last year, illustrates the threat posed by emerging infectious diseases, not only to human and animal health, but also to social stability, trade and the global economy. These disruptions are occurring against a backdrop of increased international connectivity through human travel and trade, all against a backdrop of climate change.
There are many indications that the frequency of emergence of new infectious agents may increase in the coming decades, raising fears of an impending global epidemiological crisis. Indeed, human activities are leading to profound changes in land use and major disruptions in biodiversity in many parts of the world.
These are the optimal conditions for the transfer of pathogenic micro-organisms from animals to humans. According to the WHO, the diseases resulting from such transmission are among the most dangerous.
Identify new threats
Crimean-Congo haemorrhagic fever, Ebola virus and Marburg virus disease, Lassa fever, Middle East Respiratory Syndrome Coronavirus (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS), Nipah and hepnaviral diseases, Rift Valley fever, Zika
What all these diseases have in common is that they are all on the WHO's 2018 "Priority Disease Blueprint"list
The diseases listed here are considered emergencies on which to focus research. 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.
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This list also includes "disease X": this enigmatic term refers to the disease that will be responsible for a major international epidemic caused by a currently unknown pathogen. WHO is confident that it could happen, and therefore calls on the international community to prepare for such a worst-case scenario.
Currently, the public health authorities' response to these emerging infectious diseases is to "get ahead of the curve," i.e., to identify environmental factors that may trigger emergence. Unfortunately, our understanding of how new infectious threats emerge is still limited.
But one thing is certain, animals will most likely be involved in future outbreaks. For this is another common feature of the diseases on this list drawn up by the WHO: they can all be classified as zoonotic viral infections.
Animals heavily involved in new outbreaks
Over the past four decades, more than 70 % of emerging infections have been zoonoses, i.e. infectious animal diseases transmissible to humans. At their 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 alter the risks of exposure to these animal and plant-related infectious diseases.
In this respect, one might think that biodiversity is a threat: since it contains many potential pathogens, it increases the risk of the emergence of new diseases. Yet, curiously, biodiversity also plays a protective role against the emergence of infectious agents. Indeed, the existence of a wide variety of host species can limit their transmission, either through dilution or buffering effects.
Biodiversity loss increases the transmission of pathogens
If all species had the same effect on the transmission of infectious agents, one would expect that a decrease in biodiversity would similarly lead to a decrease in the transmission of pathogens. However, this is not the case: in recent years, studies have consistently shown that biodiversity losses tend to increase the transmission of pathogensand 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 can this be explained? Biodiversity loss can alter disease transmission in several ways:
1) By changing the abundance of the host or vector. In some cases, a greater host diversity may increase the transmission of agents by increasing vector abundance ;
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 an alteration in the evolution of virulence dynamics or transmission pathways. For example, in a more diverse community, the parasitic worm that causes bilharzia (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 likelihood of future transmission to humans by 25 to 99 % ;
3) By changing the host or vector condition. In some cases, in hosts with high genetic diversity, infections can be reduced or even induce resistance, thereby effectively limiting transmission. If genetic diversity is reduced because populations are shrinking, the likelihood of the emergence of resistance also decreases.
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In this context, the ongoing loss of biodiversity is all the more worrying. Current estimates suggest, for example, thatat least 10 000 to 20 000 freshwater species are extinct or are at risk of disappearing. The rates of decline observed today rival those of the great crises of the past, such as the one that marked the transition from the Pleistocene to the Holocene 12,000 years ago, which was accompanied by the disappearance of the megafauna, of which 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 geographical footprint of 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 some parts of the world, 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 haemorrhagic fever, caused by a tick-borne virus, as well as on the durability of the Zika virus, transmitted by mosquitoes in subtropical and temperate regions.
The bushmeat consumption and the animal trade, resulting from the increasing demand for animal protein, are also causing important changes in the contact between humans and animals. Studies have shown that outbreaks of SARS and Ebola were 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 bushmeat consumption is four times greater than in the Amazon,which is richer in biodiversity.
Another risk is the expansion of agriculture and livestock farming. In order to meet the ever-increasing demands of human populations, new areas must be conquered, through deforestation and land clearing. It is well known that this reallocation of land can trigger the emergence of infectious diseases,by encouraging contact with organisms that have hitherto been rarely encountered. For example, in the islands of Sumatra, the migration of fruit bats caused by deforestation due to forest fires has led to the emergence of Nipah disease among farmers and slaughterhouse workers in Malaysia.
The relationships between the biodiversity of host species and that of parasites and pathogens are complex. By modifying community structure, all these environmental changes may lead to changes in existing epidemiological patterns.
In this context, human populations may find themselves in contact with an animal carrying a virus capable of contaminating them. A cycle of infections can then be set up. It begins with sporadic cases of transmission from animals to humans, known as "chatter virus" ("viral chatter"). Then, as the cycles multiply, the emergence of human-to-human transmission becomes inevitable.
Once an outbreak occurs, the speed of response is paramount. In addition to rigorous health measures, when there is not enough time to carry out appropriate epidemiological studies, mathematical modeling can be of great help in rapidly assessing the effectiveness of prevention and anticipating the evolution of the disease.
But understanding the complexity of the interactions between natural reservoirs, pathogens and intermediate host(s) remains a major challenge when it comes to taking rapid action to stop disease transmission. The example of COVID-19 illustrates this once again: more than two months after the first infections, the various animal links in the epidemic transmission chain have yet to be identified.
Rodolphe GozlanResearch Director, Institute of Research for Development (IRD) and Soushieta JagadeshDoctoral Student, Institute of Research for Development (IRD)