Updated: Nov 1, 2021
While the boundary between humans and wilderness is fading and contacts with wildlife are more and more frequent, new diseases naturally transmit from animals to humans; such a disease is called a zoonosis. These zoonoses are all the more worrying as their risk of developing into a pandemic – an epidemic at the international scale – is high.
HIV in 1981, SARS-Cov-1 in 2003, the H1N1 flu virus in 2009 also known as the swine flu, MERS-CoV in 2012, Ebola in 2014, the Zika virus in 2015, SARS-Cov2 in 2019. These viruses, which have attracted and still attract considerable attention, worry scientists as well as the general population. Like myself, you probably wonder why these specific viruses cause pandemics while others do not? How did they appear in humans? What means are available to us to fight against these scourges? I will try to answer these few questions that I asked myself during my studies at the French Public Health Department.
These viruses are the cause of infectious diseases that naturally spread from animals to humans, also designated by the scientific term of zoonoses. The transmission can occur through direct contact with the animal carriers, through the intermediary of another vertebrate species or via an insect vector such as the infamous tiger mosquito. Rabies is an emblematic example of zoonosis: it mainly circulates in canine populations and is transmitted to humans by biting. However, viruses are not the only zoonosis-creating pathogens. For example, mosquito transmitted parasites of the Plasmodium genus are responsible for malaria epidemics in subtropical areas. Contrary to these well-known and long-studied diseases, the so-called emerging zoonoses were freshly detected in humans or recently started spreading to new geographic areas. Here we will focus on the former category of emerging zoonoses, the ones recently detected in humans.
First and foremost, how does zoonosis emerge? An infectious agent passing from a wild animal to humans is a quite rare phenomenon because numerous factors are simultaneously crucial, such as the closeness of the contacts between the animal reservoir and humans, the density of the human population and how adapted to humans the infectious agent is. Among the great diversity of pathogens that circulate in animals, some have the natural capacity to infect humans and can sporadically contaminate human beings. Should such a pathogen acquire the capacity to pass from human to human, it may start localized epidemics like MERS-CoV in Saudi Arabia. This virus is responsible for a respiratory syndrome probably transmitted to humans from bats via dromedaries. If the pathogen’s contagiousness becomes higher then the epidemic gains momentum, like what happened with Ebola 1. In some cases like with HIV, a pathogen can become specific to humans once it is transmitted to our species 2.
Secondly, how can a zoonosis cause a pandemic? Fortunately, not all zoonoses lead to a pandemic! Two factors are essential: the transmission capacity of the new pathogen within human populations and how these latter are structured 1, 3. In the case of MERS-CoV, the risk of developing a pandemic stays low because the virus is not easily transmitted between humans. However, the situation is not set in stone and one mutation or more could increase its contagiousness. In the case of SARS-CoV-1, the density and structure of the human population played a major part in the speed and magnitude at which the disease spread 3. Indeed, it rapidly spread from the airline hub that is Hong Kong to the rest of the world after a super-spreading event when a dozen travellers who were infected by the same individual disseminated the virus in their respective countries.
The emergence of zoonoses is not a novel phenomenon. Throughout the history of our species we acquired new viruses, bacteria and parasites when we domesticated farm animals. Let’s take measles for example. It comes straight from cattle and likely first spread to humans during the sixth century BC 4. Nowadays domestication cannot explain the increased number of zoonotic diseases. It is modern human activities that enable their emergence by increasing the risk of transmission to humans and quickening the rate of propagation in human populations. For example, deforestation alters ecosystems, which modifies the behaviour of wild animals and thus increases contacts between wildlife, domestic animals and humans like with the Lassa fever. In other cases, the risk of transmission from animal to humans was increased by bushmeat hunting, such as in the case of HIV and Ebola, or by intensive farming in the cases of the swine flu and SARS-CoV-1. In a more indirect way, human based climate change contributes to the geographic expansion of zoonotic insect vectors like for the emergence of dengue in Europe 3.
The complexity and diversity of emergence and propagation factors of zoonotic diseases require a perspective that integrates human health, animal health and the proper functioning of ecosystems. The World Health Organization developed the One Health multidisciplinary approach that creates bridges among various healthcare professionals (doctors, epidemiologists, virologists, etc.) to better understand and fight against zoonoses, particularly emergent zoonoses. For instance, surveillance programmes help to better understand and follow the propagation of bat viruses before they can spread to domestic animals or humans 5.
In a nutshell, emergent zoonoses are infectious diseases transmitted from animals to humans that were recently detected in humans or in new geographic areas. Numerous anthropic factors are responsible for the rise of these emergent zoonoses, which constitute a threat to humans and biodiversity. In order to respond to these challenges, the World Health Organization combines the expertise of several scientific domains to develop effective research and control programmes.
1. Wolfe ND et al (2007) Origins of major human infectious diseases. Nature. https://doi.org/10.1038/nature05775
2. Karesh WB et al (2012) Ecology of zoonoses: natural and unnatural histories. Lancet. https://doi.org/10.1016/S0140-6736(12)61678-X
3. United Nations Environment Programme, International Livestock Research Institute. (2020) Preventing The Next Pandemic: Zoonotic diseases and how to break the chain of transmission. https://www.unep.org/resources/report/preventing-future-zoonotic-disease-outbreaks-protecting-environment-animals-and
4. Düx A et al. (2020) Measles virus and rinderpest virus divergence dated to the sixth century BCE. Science http://doi.org/10.1126/science.aba9411
5. Gibb R et al (2020) Ecosystem perspectives are needed to manage zoonotic risks in a changing climate. BMJ https://doi.org/10.1136/bmj.m3389
To go further :
Arnaud Fontanet, Cours « Les pandémies » à la Chaire de Santé Publique du Collège de France, 2018-2019 https://www.college-de-france.fr/site/arnaud-fontanet/course-2018-2019.htm