Bats, a never-ending source of viruses posing a threat to human health?

There is nothing in living memory to compare with the SARS-CoV-2 virus and the ensuing Covid-19 pandemic in terms of the way it has destabilised our society.
So where has this fearsome enemy been hiding all this time, eluding the gaze of our ever-watchful scientists?

The answer has once again turned the spotlight onto a certain flying mammal: the bat, an airborne repository of lethal viruses and the subject of countless legends and superstitions. As it happens, a bat belonging to the genus Rhinolophus is the prime suspect for the source of the virus causing the current Covid-19 pandemic. Directly or indirectly (perhaps with the help of a pangolin), that bat has unleashed a previously unknown coronavirus on the human respiratory system, where it is now flourishing.

A mixed reputation in the collective imagination

As a winged creature of the night, in Western cultures the bat long been associated with the devil and satanic rituals, making it a figure of fear and repulsion. The devil is often depicted with bat wings, and popular culture is awash with images derived from that tradition: from Dracula to vampire movies and Hallowe’en.
In Asia, on the other hand, and particularly in China, bats have been regarded as symbols of well-being and longevity since ancient times. But – as a result of the rapid pace of scientific and technological development, and the severe environmental disruptions caused by human activity (climate change, deforestation, intensive agriculture etc.) – bats have gradually become well-stocked reservoirs of new viruses with potentially deadly effects for humans.
 

Reservoirs of new viruses, vectors of zoonoses

The intersection of zoonoses (diseases transmitted by animals) and human health is a subject of increasing importance. Estimates suggest that around 75% of all new diseases which have emerged since the turn of the 20th century fall into this category.

All species of animals, wild and domesticated, carry an impressive quantity of viruses without displaying any symptoms, hence why we refer to them as “reservoir hosts.” This is particularly true of bats, which naturally carry a large number of viruses without falling ill, some of which have proven to be particularly dangerous to humans and led to deadly epidemics all over the world.

Several types of coronavirus have been detected in insectivorous bats belonging to the Rhinolophus genus, primarily in Asia. Strains detected have included SARS-CoV, MERS-Cov and now SARS-CoV-2, all responsible for widespread epidemics of acute respiratory disease.

Although the hypothesis of a direct transmission from bat to human seems virtually certain, we cannot exclude the possibility that other, intermediary species such as civets or pangolins may have been involved. Based on the close genomic similarities between the virus responsible for the Covid-19 epidemic and a similar virus detected in a pangolin, it has been suggested that this small scaly mammal may be one link in the chain of transmission between bats and human beings.

Take rabies as an example, a virus which has claimed thousands of victims every year for centuries. Even though bats are the reservoir hosts for this virus, in the majority of cases it is transmitted to humans via the intermediary of wild or domesticated carnivores, such as foxes or dogs. The Hendra and Nipah viruses offer further examples: in the 1990s, both provoked epidemics of encephalitis in Australia and Malaysia with mortality rates of between 40 and 60%.

Once again, in these cases humans were contaminated by other species of animal: horses in the case of Hendra, and pigs for Nipah. Last but by no means least, the famous Ebola and Marburg viruses are also carried by fruit-eating bats in Africa. Human victims of these viruses seem to have been contaminated either directly by bats, or indirectly while handling dead chimpanzees or gorillas.

Characteristics particularly conducive to the spread of zoonotic viruses

All in all, more than 60 viruses have been detected in the organs, blood or excrement of bats, a much higher number than that found in other animal species.
Bats have a number of exceptional physical characteristics which make them unusually well-suited to hosting and transmitting such a large number of viruses.

Firstly, the biological order Chiroptera (i.e. bats) contains more than 1,200 species, approximately 20% of all mammals. This makes it the 2nd largest taxonomical order in the mammalian class, behind rodents.

Bats are also among the oldest extant mammals – the majority of known species are believed to have formed over 100 million years ago. The great number and age of extant bat species has led to the development of substantial genetic diversity between the species, accounting for the extraordinarily diverse array of viruses they host.

Secondly, bats display immense diversity in terms of their size (the largest species can have wingspans of up to 2 metres, while the smallest weigh barely 3.5 grams), their diet (fruit-eating, insectivorous, blood-sucking) and their lifestyles (some species are sedentary, others migrate). 
 

This biological diversity has enabled bats to thrive in a vast array of ecosystems and geographical areas. Coming into regular contact with people and animals from all parts of the world, their ubiquity has undeniably played a role in making bats the carriers of an incredibly diverse selection of viruses.

Thirdly, bats have certain physiological characteristics conducive to harbouring viruses in the long term. For one, they have a long lifespan (some species, such as Myotis lucifugus, may live to the age of forty and beyond), which is highly unusual for such small animals. This longevity goes some way to explaining why bats are capable of infecting other animals over a long period of time.

Furthermore, bat species living in temperate regions hibernate for several months during the winter. This hibernation sees them enter into a state of hypothermia, accompanied by a drop in heart rate (as little as one beat per minute) and a general slowing down of the metabolism which helps to preserve any viruses they may be carrying. Bats also have a very high level of innate immunity, which plays a significant role in the spread and durability of viral infections.

It is also important to remember that most species of bats live in colonies containing several hundreds or even thousands of individuals, providing the ideal conditions for widespread sharing of viruses. Moreover, these colonies often comprise multiple species of bat, boosting viral diversity through repeated inter-species transfers.

Last but not least, bats often live in close proximity to humans. Many species live either in natural locations also frequented by humans (caves, grottoes, forest canopies) or man-made spaces (eaves of houses, lofts, disused mineshafts etc.). Fruit-eating species often live off the fruit of trees cultivated for agricultural purposes.

People in the tropical forest regions of Africa and Asia regularly hunt and eat bats. The handling of dead or captured bats, their presence in large numbers in fruit trees which feed whole villages, and their general proximity to humans in many contexts and communities are all potential sources of exposure conducive to the transmission of viruses.

Rigorous monitoring to prevent epidemics

Although bats are formidable reservoirs of viruses which are potentially pathogenic to humans, they nevertheless occupy a precious ecological niche and play a key, even vital, role in the functioning of the biosphere.

It is therefore an urgent priority to expand our research efforts in order to regularly identify and profile the viruses hosted by these flying mammals, elucidating the genetic, environmental and anthropological conditions and mechanisms of their transmission to humans. Only then can we hope to develop and implement effective strategies for predicting and preventing epidemics.