Virus World

It’s no coincidence that some of the worst viral disease outbreaks in recent years — SARS, MERS, Ebola, Marburg and likely the newly arrived 2019-nCoV virus — originated in bats. A new University of California, Berkeley, study finds that bats’ fierce immune response to viruses could drive viruses to replicate faster, so that when they jump to mammals with average immune systems, such as humans, the viruses wreak deadly havoc. Some bats — including those known to be the original source of human infections — have been shown to host immune systems that are perpetually primed to mount defenses against viruses. Viral infection in these bats leads to a swift response that walls the virus out of cells. While this may protect the bats from getting infected with high viral loads, it encourages these viruses to reproduce more quickly within a host before a defense can be mounted. This makes bats a unique reservoir of rapidly reproducing and highly transmissible viruses. While the bats can tolerate viruses like these, when these bat viruses then move into animals that lack a fast-response immune system, the viruses quickly overwhelm their new hosts, leading to high fatality rates.

“Some bats are able to mount this robust antiviral response, but also balance it with an anti-inflammation response,” said Cara Brook, a postdoctoral Miller Fellow at UC Berkeley and the first author of the study. “Our immune system would generate widespread inflammation if attempting this same antiviral strategy. But bats appear uniquely suited to avoiding the threat of immunopathology.” The researchers note that disrupting bat habitat appears to stress the animals and makes them shed even more virus in their saliva, urine and feces that can infect other animals. “Heightened environmental threats to bats may add to the threat of zoonosis,” said Brook, who works with a bat monitoring program funded by DARPA (the U.S. Defense Advanced Research Projects Agency) that is currently underway in Madagascar, Bangladesh, Ghana and Australia. The project, Bat One Health, explores the link between loss of bat habitat and the spillover of bat viruses into other animals and humans.

“The bottom line is that bats are potentially special when it comes to hosting viruses,” said Mike Boots, a disease ecologist and UC Berkeley professor of integrative biology. “It is not random that a lot of these viruses are coming from bats. Bats are not even that closely related to us, so we would not expect them to host many human viruses. But this work demonstrates how bat immune systems could drive the virulence that overcomes this.”....

Published in Ecology Epidemiology and Global Health (03 February, 2020):

https://doi.org/10.7554/eLife.48401

Genetic sequences of viruses isolated from the scaly animals are 99% similar to that of the circulating virus — but the work is yet to be formally published. Researchers in Guangzhou, China, have suggested that pangolins — long-snouted, ant-eating mammals often used in traditional Chinese medicine — are the probable animal source of the coronavirus outbreak that has infected more than 30,000 people and is wreaking havoc worldwide. Scientists say that the suggestion, based on a genetic analysis, seems plausible — but caution that the researchers’ work is yet to be published in full. “This is an extremely interesting observation. Although we need to see more details, it does make sense as there are now some other data emerging that pangolins carry viruses that are closely related to 2019-nCoV,” says Edward Holmes, an evolutionary virologist at the University of Sydney, Australia.

The identity of the animal source of the coronavirus, named nCoV-2019, has been one of the key questions that researchers have been racing to answer. Coronaviruses are known to circulate in mammals and birds, and scientists have already suggested that nCoV-2019 originally came from bats, a proposal based on the similarity of its genetic sequence to those of other known coronaviruses. But the virus was probably transmitted to humans by another animal. The coronavirus that caused severe acute respiratory syndrome, or SARS, spread from bats to civet cats to humans. Now, the South China Agricultural University in Guangzhou says that two of its researchers, Shen Yongyi and Xiao Lihua, have identified the pangolin as the potential source of nCoV-2019 on the basis of a genetic comparison of coronaviruses taken from the animals and from humans infected in the outbreak and other findings. The sequences are 99% similar, the researchers reported at press conference on 7 February.

Previously, researchers have noted that coronaviruses are a possible cause of death in pangolins1, and that nCoV-2019 and coronaviruses from pangolins use receptors with similar molecular structures to infect cells. Even before today’s announcement, pangolins were a good candidate for being an intermediate species for the virus, so it’s very interesting that the researchers have found such a close sequence, says David Robertson, a computational virologist at the University of Glasgow, UK....