The ongoing Covid-19 coronavirus epidemic, which started in Wuhan at the end of last year, illustrates very well the threat posed by emerging infectious diseases not only to human and animal health but also to social stability, trade and global economy.
However, many signs are suggesting 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 and biodiversity in many places on the planet.
These disturbances occur in a context of increased international connectivity through human movements and trade, against the backdrop of climate change.
These are precisely the optimal conditions to promote the transfer of pathogenic microorganisms from animals to humans. However, according to WHO, the diseases resulting from such transmissions are among the most dangerous.
Identifying new threats
Crimean-Congo haemorrhagic fever (CCHF), Ebola and Marburg viruses, Lassa fever, Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome (SARS), Nipah and henipaviral diseases, Rift Valley Fever (RVF), Zika…
All of these diseases appear on the “Priority diseases blueprint” list, established by WHO in 2018.
The diseases listed here are considered as emergencies on which research should focus. They represent a large-scale public health risk because of their epidemic potential and the absence or the limited number of treatment and control measures currently available.
This list also includes a “Disease X”: this cryptic term designates the disease which will be responsible for a major international epidemic, caused by a pathogen currently unknown. WHO does not doubt that such a disaster could occur, and therefore urges the international community to be prepared for such a worst-case scenario.
Currently, the response of public health authorities to these emerging infectious diseases is to "get ahead of the curve, i.e., to identify the environmental factors that may trigger their emergence. Unfortunately, our understanding of how emerging infectious threats surface is still limited.
But one thing is certain, animals will most likely be involved in future epidemics. This is another common feature shared by the diseases on the WHO list: they can all be classified as zoonotic viral infections.
Animals heavily involved in new epidemics
Over the last four decades, more than 70% of emerging infections have proved to be zoonoses, in other words, diseases that are transmissible from animals to humans.
In the simplest case, these diseases include one host and one infectious agent. However, several species are often involved, which means that changes in biodiversity can potentially alter the risks of exposure to these animal and plant-related infectious diseases.
One might think that biodiversity represents a threat: since it harbours many potential pathogens, it increases the risk of new diseases emerging.
Yet, surprisingly, 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, through dilution or buffering effect.
Biodiversity loss increases pathogen transmission
If all species had the same effect on the transmission of infectious agents, one would expect that a decrease in biodiversity would lead to a corresponding decrease in pathogen transmission. However, this is not the case: in recent years, studies have consistently shown that biodiversity losses tend to increase the transmission of pathogens and the frequency of associated diseases.
This trend has been demonstrated in a wide range of ecological systems, with very different host-agent types and modes of transmission. How can we explain this situation? Biodiversity loss can affect disease transmission in several ways:
- By changing the abundance of host or vector. In some cases, greater host diversity may increase the transmission of agents by increasing vector abundance;
- By altering the behaviour of the host, vector or parasite. In theory, greater diversity can influence host behaviour, which can have different consequences, whether it is an increase in transmission or an alteration in the evolution of virulence dynamics or transmission routes. 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 probability of future transmission to humans by 25 to 99 %;
- By changing the host or vector condition. In some cases, in hosts with high genetic diversity, infections can be reduced or even induce resistance, which effectively limits transmission. If genetic diversity is diminished because populations are declining, the likelihood of the emergence of new resistances also decreases.
In this context, the ongoing loss of biodiversity is even more alarming. Current estimates suggest, for instance, that at least 10,000 to 20,000 freshwater species are extinct or at risk of extinction. The rates of decline observed today rival those of the major crises of the past, such as the one that marked the transition from the Pleistocene to the Holocene 12,000 years ago, with the disappearance of megafauna, including the woolly mammoth, one of its 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 pathogens and/or the host they infect that leads to the emergence of new infectious diseases. As such, the increasing unpredictability of global climate and local human-animal-ecosystem interactions, becoming closer and closer in some parts of the world, play a major role in the emergence of new infections among 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, but also on the durability of the Zika virus, transmitted by mosquitoes in subtropical and temperate regions.
Bushmeat consumption and trade in animals, resulting from the growing demand for animal protein, are also causing significant changes in the contact between humans and animals. Studies have shown that both SARS and Ebola outbreaks were directly linked to the consumption of infected bushmeat. Also, Lassa fever and diseases caused by the Marburg and Ebola viruses are thriving in West and Central Africa, where bushmeat consumption is four times higher than in the Amazon, which is richer in biodiversity.
Another risk is the expansion of agriculture and livestock farming. To meet the ever-increasing demand of human populations, new areas must be conquered, through deforestation and land clearing. Yet it is known that this land re-allocation can trigger the emergence of infectious diseases, by encouraging contacts with organisms rarely encountered until now. For instance, in the islands of Sumatra, the migration of fruit bats caused by deforestation due to forest fires has led to the emergence of the Nipah disease among farmers and slaughterhouse workers in Malaysia.
The linkages between the biodiversity of host species and that of parasites and pathogens are complex. By modifying community structure, all these environmental changes have the potential to modify existing epidemiological patterns.
In this context, human populations may find themselves in contact with an animal carrying a virus that can contaminate them. A cycle of infections can then take place. It begins with sporadic cases of transmission from animals to humans, known as " viral chatter ". Then, as the cycles multiply, the emergence of human-to-human transmission becomes inevitable.
Once an epidemic has erupted, the speed of response is paramount. In addition to rigorous sanitary measures, when there is no time to conduct adequate epidemiological studies, mathematical modelling can be very helpful in rapidly assessing the effectiveness of prevention and anticipating the evolution of the disease.
However, 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 COVID-19 example illustrates this once again: more than two months after the first infections, the various animal links in the chain of transmission of the epidemic still need to be identified.
This article was originally published by The Conversation