For several weeks, our eyes have been focused on an invisible enemy who attacks the health of millions of people, takes the lives of too many of them, upsets our social organization, destroys the world economy and paralyzes The political life.
From memory, never a phenomenon, whatever it is, will have destabilized human society as much as the SARS-CoV-2 virus and the Covid-19 pandemic that it causes. But where was this enemy, so formidable until now, hidden for having escaped the vigilant and inquisitive gaze of scientists for so long?
And here reappear in front of the scene these small flying mammals, bats, true protective wings of deadly viruses and refuges dreamed of many legends and superstitions. A bat, of the genus Rhinolophus, is indeed suspected of harboring the virus responsible for the Covid-19 pandemic, and of having discharged it, directly or indirectly via a pangolin, into the respiratory system of humans, its new favorite place.
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A contrasting image in the collective imagination
Winged nocturnal animals, bats have always been associated with the devil, characters and satanic feasts in Western societies, generating fear and repulsion. The devil, represented with bat wings, Dracula, the vampire, and the Halloween party are the best-known examples.
Conversely, in Asia, and more particularly in China, bats have evoked happiness and longevity since very ancient times. But – and this is known thanks to the considerable advances in scientific technologies and also thanks to the disturbances of our environment linked to human activities (global warming, deforestation, intensive agriculture) – bats have gradually become veritable reservoirs of new pathogenic viruses for humans.
Reservoirs of many emerging viruses, vectors of zoonoses
The place of zoonoses (these animal-transmitted diseases) in public health continues to grow. Now estimated that about 75% of emerging diseases have emerged since the beginning of the XX th century fall into this category.
All animal species, whether wild or domestic, harbor an impressive amount of virus without developing symptoms, hence the term “reservoirs”. This is particularly the case of bats that naturally harbor without being sick many viruses, some of which have proven to be particularly pathogenic for humans, causing deadly epidemics all over the world.
Several coronaviruses have thus been detected in insectivorous bats of the genus Rhinolophus, mainly in Asia. Among them, SARS-CoV, MERS-Cov and now SARS-CoV-2 have been responsible for massive epidemics of the severe acute respiratory syndrome.
Although direct transmission from the bat to humans is almost certain, the involvement of other intermediate animal species – civets or pangolins – is envisaged. Thus, on the basis of strong similarities between the genome of the virus responsible for the Covid-19 epidemic and that of a virus isolated in a pangolin, the hypothesis of the implication of these small scale mammals in the chain of transmission between bats and humans has been transmitted.
Take the example of the rabies virus, responsible for thousands of thousands of cases each year. Even if the reservoir is the bat, the transmission of the virus to humans most of the time passes through wild or domestic carnivores, such as the fox or the dog. Or again, the Hendra and Nipah viruses, which caused, in the 1990s, epidemics of encephalitis in Australia and Malaysia with a mortality rate of between 40 and 60%.
Here too, contamination to humans has been relayed by other animal species, horses for Hendra and pigs for Nipah. Finally, the famous Ebola and Marburg viruses have African fruit bats as their reservoir. Human victims are said to have been contaminated either directly from bats, or during the handling of chimpanzees or dead gorillas.
Characteristics particularly favorable to zoonotic viruses
In total, more than 60 viruses have been detected from organs, blood or bat droppings, a much higher number than in other animal species.
Several exceptional characteristics predispose these animals to the harbor and then transmit such a high number of viruses.
First, the order of Chiroptera (bats, therefore) includes more than 1,200 species or about 20% of all mammals. This is the 2 th largest taxonomic order among mammals after the rodents.
Bats are also among the oldest mammals – the majority of species are thought to have formed over 100 million years ago. The profusion of species, as well as their age, have resulted in a great genetic diversity between the species, which has made the bed of the extraordinary viral diversity observed in these animals.
Secondly, bats are characterized by a great diversity of size (some are 2 meters wide while others weigh barely 3.5 grams), diet (insectivorous, frugivorous and even hematophagous) and life (some are sedentary and others are migratory).
This biological diversity has enabled them to colonize very varied geographic areas and ecosystems. Through multifaceted contacts with the inhabitants and specific animals of each part of the world, this ubiquitous presence has undoubtedly contributed to the extreme diversity of viruses harbored by these animals.
Third, bats have physiological characteristics that are conducive to long-term virus persistence. On the one hand, they have a longevity of several decades, up to 40 years for certain species ( Myotis lucifugus for example), which is exceptional for small animals. This longevity partly explains why these animals are infectious for long periods.
On the other hand, species from temperate regions hibernate for several months in cold seasons. The state of hibernation is accompanied by hypothermia, a decrease in the heart rate (up to 1 heartbeat per minute) and a general slowing of the metabolism which would contribute to the persistence of the virus in the body. Finally, the immune defenses of bats are essentially supported by a very effective innate immunity which would intervene significantly in the control and durability of viral infection.
Fourth, most species have a gregarious lifestyle and live in colonies of hundreds, even thousands, of individuals, optimal conditions for widespread of viruses within colonies. In addition, colonies often include several different species of bats, which increases viral diversity following numerous interspecies transfers.
Finally, bats often live in contact with human populations. Many species stay in natural lodgings frequented by human beings (caves, cellars, the foliage of trees) or places created by men (roofs of houses, abandoned my roof spaces…). Likewise, frugivorous species frequently consume the fruits of fruit trees grown in villages.
The inhabitants of the tropical forest regions of Africa and Asia regularly hunt and consume these animals. Thus, the handling of hunted or captured animals, the arrival in the abundance of these animals on fruit trees in villages and their proximity during their stays in environments frequented by human communities are all sources of exposure conducive to transmission virus.
Rigorous monitoring to prevent epidemics
Although bats are reservoirs rich in pathogenic viruses for humans, they nevertheless occupy a precious ecological niche within our planet and play a primordial, even vital, role in the functioning of the biosphere.
It is therefore urgent and imperative to increase research aimed at regularly identifying and characterizing the viruses harbored by these flying mammals, and elucidating the genetic, environmental and anthropological modalities and mechanisms of their transmission to humans, the only means of proposing and implementing implements epidemic prediction and prevention strategies.