It's no coincidence that some of the most serious viral diseases in recent years—SARS, MERS, Ebola, Marburg, and new SARS-CoV-2—all originated from bats. Recently, a new study published on eLife by Cara E Brook's group at the University of California, Berkeley found that bats' strong immune response to the virus may cause the virus to replicate faster, so when the virus spreads to mammals with normal immune systems (such as humans), can cause fatal damage.
Some bats, including those considered to be a source of human infection, have an immune system that is always ready to defend against the virus and can quickly isolate the virus from cells. Although this may protect bats from high viral loads, it also encourages these viruses to reproduce faster in the host. This makes bats a unique host for rapid reproduction and high virus transmission.
Although bats can tolerate these viruses, when these bat viruses enter other animals that lack a fast-reacting immune system, they quickly knock down new hosts, leading to high lethality. Researchers point out that damaging bat habitats seem to put pressure on them to release more viruses in their saliva, urine and feces, infecting other animals.
Mike Boots, a disease ecologist and UC Berkeley professor of integrative biology, said that bats can be special in hosting viruses, and many viruses come from bats, which is not random. Bats are not closely related to us, so they don't host a lot human viruses. But this study demonstrates how the bat's immune system overcomes toxicity. As the only mammals that can fly, bats increase their metabolic rate in flight, making them twice as large as rodents in running.
Researchers explained that strenuous physical activity and high metabolic rates often lead to higher tissue damage, which is due to the accumulation of active molecules, mainly free radicals. But in order to be able to fly, bats seem to have evolved a physiological mechanism that effectively eliminates these harmful molecules. This is probably why bats live longer. Some bats can live for 40 years, while rodents of the same size can only live for two years. This rapid suppression of inflammation may have another benefit: suppression of inflammation associated with antiviral immune responses. A key skill in the immune system of many bats is the immediate release of a signal molecule called interferon alpha, which tells other cells to "ready to fight" before the virus invades.
Brook and Boots created a simple bat immune system model and modeled their experiments on a computer. Researchers have noticed that many bat viruses are transmitted to humans through animal vectors. SARS coronavirus is transmitted to humans through the Asian palm civet, MERS virus through camels, Ebola virus through gorillas and chimpanzees, Nipah virus through pigs, Hendra virus through horses, and Marburg virus through African green monkeys. Nevertheless, these viruses are extremely toxic and lethal when they finally enter the human body.