The WHO considers it “probable” that an animal served as an intermediary in the transmission of SARS-CoV-2, which confirms the role of these as reservoirs of viruses capable of infecting man. What species transmit viruses? Where could the next pandemics come from?
-Most human diseases
According to the World Organization for Animal Health (OIE), 60% of human infectious diseases are zoonotic, that is, they are found first in another animal.
This percentage even rises to 75% for new infectious diseases, according to a British study published in 2001, considered a reference on the subject.
Among the pathogens responsible for these diseases, one in six would be a virus, a third a bacterium and another third parasites. 10% are microscopic fungi, this study indicates.
-The bat, the ideal suspect?
Bats play a reservoir role for a large number of viruses that affect humans. They house them without getting sick themselves.
Some have been known for a long time, such as the rabies virus, but many emerged in recent years, such as Ebola, the SARS coronavirus, SARS-CoV-2 and the Nipah virus, which appeared in Asia in 1998.
Bats “have always been good reservoirs for many viruses, but before we had very little contact” with these species, Eric Fèvre, professor of veterinary infectious diseases at the University of Liverpool (United Kingdom) and International Livestock Research, told AFP. Institute (Kenya).
The reduction of tropical forests due to the advance of cities and cultivated areas, combined with the effects of climate change, bring these animals closer to inhabited areas and push them to “interact more and more with human populations,” he says.
-The ferret, the mink and the weasel –
Another family of mammals, the mustelids (badgers, ferrets, minks, weasels …), is often pointed out as responsible for viral zoonoses, and in particular those caused by coronavirus.
The civet or civet cat has been identified as the intermediate host for Severe Acute Respiratory Syndrome (SARS), which left 774 deaths worldwide in 2002-2003. Although the SARS coronavirus was found in some civets, it is not confirmed that this small carnivore close to the mongoose was the one who transmitted the virus to humans.
SARS-CoV-2 contamination of mink farms has shown that this species could be infected by human carriers. But the reverse case has not been proven.
-The pangolin, innocent?
At the beginning of the COVID-19 epidemic, this endangered animal was identified by Chinese researchers as the “possible intermediate host”, given the closeness of the genetic sequences of SARS-CoV-2 and a coronavirus that infects the pangolin.
Although this mammal is the natural host for many viruses, its role in the transmission of SARS-CoV-2 has not been established.
The study published on Monday by experts from the WHO and China did not clarify this point either.
“Among the viruses that come from these two mammals (bat and pangolin, ndlr) identified so far, none resembles SARS-CoV-2 enough to be considered its direct ancestor,” according to the experts.
“After a year, we continue with the same questions,” laments Serge Morand, a health ecologist at the French National Center for Scientific Research (CNRS), underlining that a genetic proximity would not suffice as a demonstration either.
“Ecological hypotheses are also necessary, which explain to us how a pangolin could come into contact with a bat: it was certainly not in a market,” he says.
-The other mammals
“From a historical point of view, our viral load comes essentially from farm animals”, emphasizes Serge Morand.
The measles virus, today fully human, arose from the adaptation of a virus in the Middle Ages that affected cattle.
The pig also often plays the role of intermediate host for influenza viruses and Nipah, for example.
This animal, sensitive to human viruses, is also conducive to recombinations. This is probably what happened during the H1N1 pandemic in 2009-2010, initially classified as “swine flu”, with an estimated balance of between 152,000 and 575,000 deaths. The virus strain would have arisen from a pig carrying both bird flu and human flu.
The rabies virus transmitted by infected dogs and foxes, different from that of bats, is responsible for the vast majority of the 59,000 annual deaths caused by this disease.
Among wild mammals, great apes have served as intermediate hosts for HIV (from the Simian Immunodeficiency Virus, or SIV) and Ebola, while the dromedary appears to be “a large reservoir host for MERS-CoV and an animal source of infection in man “, even if” the precise role that these animals play in the transmission of the virus and the exact mode of transmission are not known, “says WHO.
Rodents are also known to be the reservoirs of many viruses, including some responsible for epidemics in humans, such as Lassa hemorrhagic fever, endemic in several West African countries.
-Wild and domestic birds
The Spanish flu of 1918-1919, the “Asian” flu in 1957, the “Hong Kong” flu eleven years later, the H1N1 flu in 2009: all the viruses responsible for the major influenza pandemics were directly or indirectly of avian origin .
Two other strains of bird flu, H5N1 between 2003 and 2011, and then H7N9 from 2013, led to direct infections in Asia from infected birds, or in very rare cases of human-to-human transmission.
Wild birds can be the starting point for these epidemics, and farm birds very often play a role of “amplifying populations”, Eric Fèvre observes, because the density of the breeding places of “genetically very similar” birds makes them very “receptive” to the virus.
Then the mutations can favor their passage to humans, as in the case of the H5N8 virus, present in many European farms for some months, and which was detected in Russia in seven workers at a poultry processing plant.
-Mosquitoes and ticks –
Although the word “zoonosis” refers to vertebrate animals, insects such as mosquitoes and arthropods such as ticks are vectors for numerous viral diseases that affect humans.
The tick primarily transmits Crimean-Congo hemorrhagic fever, while mosquitoes carry viruses responsible for yellow fever, chikungunya, dengue, Zika, and even West Nile virus and Rift Valley fever.
-The next pandemic
In October 2020, the UN Biodiversity Expert Group (IPBES) warned that pandemics would “arise more often, spread more rapidly and kill more people.”
Firstly, because the reservoir is immense: according to estimates published in the journal Science in 2018, there would be 1.7 million viruses unknown to mammals and birds, and between 540,000 and 850,000 would have the “capacity to infect human beings.”
But above all because the expansion of human activities and increasing interactions with wildlife increase the risk that viruses capable of infecting people will find a host.
“We do not know when, how or where” the next pandemic will arise, sums up Serge Morand, for whom first of all it would be necessary to urgently rethink “our link with wild and domestic animals”.
fig / fmp / mar-app / zm