Several viral epidemics are recent (Ebola, SARS, COVID-19) have all as the source of the virus found in the natural state in bats. But how do these animals manage to host these viruses without being themselves sick ?
60% to 75% of infectious diseases affecting humans are initially transmitted from animals such as birds (influenza), rodents (plague), primates (AIDS) or mosquitoes (Lyme disease, west Nile virus, Zika).
Bats are another important reservoir of infectious agents, and not the least of it : several of the viruses most devastating to have infected humans are indeed present in the natural state in these mammals, whether hemorrhagic fevers (Ebola, Marburg), of encephalitis, acute (Hendra, Nipah) or severe respiratory infections (SARS, MERS).
The virus SARS-CoV-2 responsible of the current pandemic of COVID-19 also originates initially from a bat bat (iron horse), with possibly a pangolin as intermediate host(1). Normally, these animals are not in contact with humans and there is little risk that these viruses can infect ; however, the changes in the ecosystems caused by deforestation, large-scale agriculture and urban sprawl have, in many cases destroyed their natural habitats and increased the risk of direct contact with humans.
The poaching and sale of wild game (wet markets) that may also increase the likelihood of human infection, and the example of the COVID-19 shows how quickly these infections can spread across the globe.
The presence of these dangerous viruses do, however, have any impact on the survival of these animals. With nearly 1,400 different species (or about one-quarter of all the mammal species present), the bats are a group of mammals, an extremely diverse mix that is perfectly adapted to life on Earth since its appearance it has approximately 60 million years ago. In addition, bats generally live much longer than other mammals of similar size, some species have a life expectancy of up to 30 to 40 years.
How can we explain this exceptional longevity, despite the presence of virus very dangerous ?
The answer lies in the immune system very particular to these animals, surprisingly linked to the flight !
Bats are the only mammals able to fly (some species of which can attain speeds approaching 160 km/h !) and must considerably increase their metabolism to provide sufficient energy for active flight.
This super-metabolism generates considerable amounts of metabolic waste (free radicals) that damage cells. Fragments of DNA escape also of the cells during the flight, and the presence of this genetic material in the blood of bats mime in some way a viral infection. Normally, these two phenomena should trigger a strong inflammatory response and immune, but the bats have rather developed a series of adaptations that attenuate these responses and prevent the development of inflammation uncontrolled, with consequent damage to the vital organs.
In other words, the adaptations that have allowed bats to tolerate tissue damage caused by the theft would allow them to coexist with viruses, without generating inflammatory responses to disproportionate(2). Another advantage of this inflammatory response is attenuated is that it allows you to slow down for several diseases closely linked to inflammation, such as atherosclerosis, arthritis, neurodegenerative diseases or cancer, and therefore extend the duration of life in good health.
In sum, even if the bats are the direct cause of many viral infections life-threatening, including COVID-19 present, the study of these animals can give us valuable clues on how to effectively fight these viruses, in particular by controlling the inflammation excessive. The understanding of the mechanisms used by bats to tame the inflammation could therefore pave the way for the development of new drugs to treat two main threats to human health in the Twenty-first century, is the viral pandemics and chronic diseases related to aging.
(1) Xiao K et al. Isolation of SARS-CoV-2-related coronavirus from the Malayan pangolins. Nature 2020; 583: 286-289.
(2) Gorbunova V et al. The world goes bats: living longer and tolerating viruses. Cell Metab. 2020; 32: 31-43.