According to the World Organization for Animal Health, 60% of infectious diseases are zoonotic
The World Health Organization announced today that the transmission of the coronavirus to humans through an animal that has played the role of host is a "possible to very probable" case, confirming that many animals are a reservoir of viruses that can infect humans. Which animals transmit these viruses? Where can other pandemics come from?
According to the World Organization for Animal Health, 60% of infectious diseases are zoonotic. That figure rises to as high as 75% for new infectious diseases, according to a 2001 British study.
Among the pathogens responsible for these diseases, one in six are a virus, a third a bacterium, another a third parasitic worms and about 10% tiny fungi, according to the study.
Bats: the ideal suspect?
Bats have the role of a reservoir of a large number of viruses that affect humans, they are their hosts without getting sick themselves.
Some viruses have long been known, such as rabies, but many others have emerged in recent years: Ebola, the SARS coronavirus, SARS-CoV-2, or the Nipah virus, which appeared in 1998 in Asia.
Bats "have always been good reservoirs for many viruses, but in the past we had little contact with them," explains Eric Febre, a professor of animal infectious diseases at the University of Liverpool and the International Livestock Research Institute in Kenya.
"Reducing rainforests, expanding cities and arable land combined with the effects of climate change are bringing these animals closer to residential areas and forcing them to 'interact more and more with humans,'" he said.
Weasels, mink and ferrets
Another family of mammals, mustelids (which include ferrets, weasels, mink and badgers) are often blamed for transmitting zoonotic infectious diseases, mainly those caused by coronaviruses.
Muscovy is considered to be the host of the virus that caused the acid, respiratory syndrome (SARS), from which 774 people died in the period 2002-2003. Although SARS has been identified in some of these animals, it has not been confirmed that they transmitted the virus to humans.
Mink infection in farms with the SARS-CoV-2 virus has shown that this species can be infected by humans, but the opposite has not been proven.
Pangolin: innocent?
At the beginning of the pandemic covid-19 This endangered animal was considered by Chinese researchers to be a "potential host", given its similarity in the genetic sequence of SARS-COV-2 and a coronavirus that infects pangolins. But his role in its transmission covid-19 it is not clear.
The report published today by WHO experts in collaboration with Chinese scientists did not help to clarify the situation.
"Among the viruses derived from these two mammals (bats and pangolins) that have been identified so far, no one is similar enough to SARS-CoV-2 to be considered a direct ancestor," the experts said.
Other mammals
"Historically, our viral load has come mainly from farmed animals," said Serge Moran of the National Center for Scientific Research (CNRS).
The measles virus originated in the Middle Ages from the adaptation of a virus that infected cattle.
Pigs often have the role of host for influenza viruses or Nipah.
This animal is sensitive to human viruses, while in pigs it is observed that a combination of viruses occurs. This was probably the case with the H1N1 pandemic in 2009-2010, originally called "swine flu", which killed an estimated 152.000 to 575.000 people: the strain of the virus first appeared in a pig that was a host avian influenza virus and at the same time a human influenza virus.
The rabies virus transmitted by dogs and foxes, which is different from that seen in bats, is responsible for the vast majority of the 59.000 deaths annually recorded worldwide from the disease.
Large monkeys have served as hosts for HIV and Ebola, and a camel has been shown to be a "host of MERS-CoV and a source of infection in humans," although the specific role that these animals play in transmitting the virus but and the mode of transmission is not known ", points out the WHO.
Wild and domestic birds
The Spanish flu of 1918-19, the Asian flu of 1957, the "Hong Kong flu" eleven years later, the H1N1 flu in 2009: all of these viruses came directly or indirectly from birds.
Two other strains of avian influenza, H5N1 from 2003 to 2011 and H7N9 from 2013, caused the disease in humans who came in direct contact with infected birds and in rare cases in humans.
Wild birds can be the starting point for these epidemics, and pets often play the role of a booster population, Eric Febre observes.
Mutations in the virus can then be transmitted to humans, as was the case with the H5N8 virus, which has been present in European farms for a few months and was detected in seven poultry workers in February.
At present this virus is not transmitted from person to person, it needs to be in direct contact with a sick bird or its feces to infect someone.
But other mutations may make it possible to transmit it from person to person.
Mosquitoes and ticks
The term zoonotic diseases also refers to insects, such as mosquitoes, or arthropods such as ticks, which carry many communicable diseases from which humans are infected.
Mosquitoes, for example, transmit the virus responsible for yellow fever, dengue fever, Zika virus or West Nile virus, among others.
In October 2020, a team of UN experts on biodiversity (IPBES) warned that pandemics "will occur more frequently, spread faster, kill more people."
Firstly because the pool is huge: according to estimates published in the journal Science in 2018, there are 1,7 million unknown viruses in mammals and birds and 540.000 to 850.000 of them "can infect humans".
But mainly the spread of human activities and the increased interaction with wildlife increase the risk of viruses capable of infecting humans to find their host.
"We do not know when, how or where" the next pandemic will come from, concludes Serge Moran. According to him, in this context, the most urgent is to "reconsider our relationship with wild and domestic animals."
