The links between human and environmental health have become clearer than ever with the COVID-19 pandemic. So far, scientists believe that the most likely hypothesis for the origin of SARS CoV-2 is transmission through an intermediate host animal.
60% of human infectious diseases are zoonotic, that is, they are found first in another animal, according to the World Organization for Animal Health (OIE).
This percentage even rises to 75% for new infectious diseases, according to a British study published in 2001, considering 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.
What species transmit viruses? Where could the next pandemics come from?
The usual suspects
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, 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 at the International Livestock, told AFP. Research 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.
Another family of mammals, the mustelids (badgers, ferrets, minks, weasels …), is often identified 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.
has shown that this species could be infected by human carriers. But the reverse case has not been proven.
“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 2009-2010 H1N1 pandemic, 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.
Birds are also suspicious
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.
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 flu 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 such epidemics, and farm birds very often play a role as “amplifying populations”, Eric Fèvre observes, because the density of “genetically very similar” bird farms 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 in seven workers in a poultry processing plant.
With information from AFP and EFE
