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Monkey pox, Sras, Mers, Ebola, avian flu, zika, HIV and undoubtedly Covid-19… Zoonoses, diseases transmitted to humans by animals, have multiplied in recent years, raising fears of new pandemics .
What is a zoonosis?
Zoonoses are diseases or infections that are transmitted from vertebrate animals to humans, and vice versa. The causative pathogens can be bacteria, viruses or parasites.
The transmission of these diseases occurs either directly, during contact between an animal and a human, or indirectly through food or through a vector (insect, spider, mites, etc.).
Some end up becoming specifically human, like Covid-19.
According to the World Organization for Animal Health, 60% of human infectious diseases are zoonotic.
What types of diseases are involved?
The term “zoonoses” includes a wide variety of diseases: some affect the digestive system such as salmonellosis, others the respiratory system such as avian and swine flu as well as Covid, others still the nervous system such as rabies.
The severity of these diseases in humans varies greatly depending on the disease itself, the causative pathogen being able to be more or less virulent, but also depending on the infected person, who may present a particular sensitivity to these pathogens.
What animals are involved?
Bats act as a reservoir for many viruses that affect humans. Some have been known for a long time, such as the rabies virus, but many have emerged in recent decades: Ebola, the SARS coronavirus, Sars-CoV-2 or the Nipah virus, which appeared in Asia in 1998.
Badgers, ferrets, minks, or even weasels are often implicated in viral zoonoses, and in particular those caused by coronaviruses.
Other mammals (cattle, pigs, dogs, foxes, camels, rodents) also often play the role of intermediate host.
All the viruses responsible for major influenza pandemics had an avian origin, direct or indirect.
Finally, insects such as ticks are vectors of many viral diseases that affect humans.
Why has the frequency of zoonoses increased?
Appearing thousands of years ago, zoonoses have multiplied over the past twenty or thirty years.
In question, the intensification of travel, which allows them to spread more quickly.
By occupying increasingly large areas of the globe, humans also contribute to disrupting the ecosystem and promoting the transmission of viruses.
The intensification of factory farming thus increases the risk of the spread of pathogens between animals. Trade in wild animals also increases human exposure to the microbes they may carry. Deforestation increases the risk of contact between wildlife, domestic animals and human populations.
Should we fear a next pandemic?
Climate change will push many animals to flee their ecosystems for more livable lands, warned a study published in Nature in 2022. However, by mixing more, the species will transmit their viruses more, which will promote the emergence of new diseases potentially transmissible to humans.
Pandemics will “emerge more often, spread faster, kill more people“, warned in October 2020 the UN group of experts on biodiversity (IPBES).
The reservoir is huge: according to estimates published in the journal Science in 2018, there would be 1.7 million unknown viruses in mammals and birds, and 540,000 to 850,000 of them”have the ability to infect humans“.
But above all, the expansion of human activities and increased interactions with wildlife increase the risk of viruses capable of infecting humans “find“their host.