Why are bats carrying viruses so deadly to humans?

It is no coincidence that in recent years, some serious viral infections – SARS, MERS, Ebola, Marburg, and the new type of coronavirus pneumonia – have all originated in bats. On February 3, 2020, research teams such as Cara E. Brook and Mike Boots of the University of California, Berkeley published an issue in eLife magazine entitled “Accelerated virals in bat cell lines, with implic An article for zoonotic emergence.

They found that some bats, including those thought to be the original source of human infection, have special immune systems against the virus. Bats react quickly to infected viruses and disengage the virus from cells. The bat’s powerful immune response to the virus in turn drives the virus to replicate faster and increases its infectiousness. This characteristic makes bats a host library for highly transmitted viruses. When the virus spreads into mammals ( such as humans ) that lack rapid resilience, it can cause fatal damage.

Why are bats carrying viruses so deadly to humans?

The study’s lead author, Cara E. Some bats can produce powerful antiviral reactions and use anti-inflammatory reactions to balance them, Says Brook. The human immune system, on the other hand, produces a wide-ranging inflammatory response to the virus, a threat that bats’ immune systems cleverly avoid.

If the bat’s habitat is destroyed, it appears to put pressure on them, the researchers said, potentially causing them to release more viruses in saliva, urine and feces, infecting other animals. Brook also points out that if the threat to bat habitats increases, it may increase the incidence of human and animal disease.

According to Mike Boots, a disease ecologist at the University of California, Berkeley, bats are special in storing viruses. Because many viruses come from bats, this is not random. Bats are not closely linked to humans, so we don’t want them to carry many human viruses.

The researchers say intense physical activity and high metabolic rates can lead to the accumulation of active molecules, mainly free radicals, which can cause serious damage to tissues. Bats are the only mammals that can fly. During the flight, bats increase their metabolic rate, making them twice as likely as rodents of similar size. But in order to be able to fly, bats appear to have evolved a physiological mechanism that effectively removes any harmful molecules that produce inflammation, which may be the reason for the bat’s longevity. Some bats can live for 40 years, while rodents of the same size may live for up to two years.

This rapid suppression of inflammation may have another benefit: inhibiting inflammation associated with the antiviral immune response.

A key defense mechanism for the bat’s immune system is the trigger release of interferon-a signal molecules, which tell other cells to “fight” before the virus invades.

How does bats’ immune response systems affect the evolution of virus-carrying? Why is the spread of the virus carried by bats deadly to humans?

In the study, Brook and Boots conducted in vitro and in vitro experiments, respectively.

First, Brook tested the cells of two bats and a monkey (as a control). The Egyptian fruit bat is the natural host of the Marburg virus, which attacks the virus directly before transcribing the interferon-a gene, which contains high concentrations of interferon. The host of the Australian black fox, the Hendra virus, responds slightly faster to Egyptian fruit bats, using transcriptional interferon-a RNA to fight viral infections. The African Green Monkey (Vero) cell line does not produce any interferon at all.

When the Ebola virus and the Marburg virus invade cells, the different reactions of these cell lines are surprising. Green monkey cells were quickly killed by the virus, but due to early warning of interferon, some Egyptian fruit bat cells managed to isolate themselves from the virus. In Australian black fox cells, the immune response is more successful, and the rate of viral infection is much slower than that of the Egyptian fruit batcell. In addition, due to the release of interferon, these bats can prolong the duration of infection.

Research by Brook et al. shows that surviving cells can reproduce, provide new targets for viruses, and form an incubation period for infection throughout the life cycle of bats.

At the same time, Brook and Boots built a model of the bat immune system outside the body and modeled their experiments. When a powerful interferon system is present, Brook points out, it protects cells from viral infections and causes the virus to persist in the host body. This results in a higher replication speed for the virus, but without harming the host. But humans do not have this antiviral mechanism.

In summary, the study suggests that having a powerful interferon system will help these viruses survive in the host body. In the process of fighting the virus, the bat’s immune system quickly produces interferon-a for early warning and direct attacks on the virus, which can significantly reduce the virus concentration and avoid cell damage. The interferon produced by bats also has a long-lasting immunity that protects cells from infection over the long term. However, this also promotes rapid replication of the virus and makes it more contagious. Humans lack this rapid immune protection mechanism and their ability to fight inflammation effectively, so when the virus spreads to humans, it can lead to a high fatality rate.

Now, Brook and Boots’ team are designing an evolutionary model of disease in bats to better understand the virus’s infection with other animals and humans. Brook believes it is important to understand the trajectory of infection in order to be able to predict the occurrence and spread of a virus infection.