Wednesday 26 February 2020

Coronavirus and bats

Blog by Tom August. Tom studied diseases in bats for his PhD, with a focus on coronaviruses in UK bat species. He now works as a computational ecologist at the UK Centre for Ecology and Hydrology.

Researchers believe that the recent outbreak of coronavirus disease in China - COVID-19 - originated in wild bats in China, just as the SARS outbreak did back in 2002. So, what is the coronavirus, why does it seem to have come from bats, and what should we be doing to stop this happening again?

If you are old enough to be reading this you have probably been infected by a coronavirus before. Members of this group of viruses cause many cases of the common cold around the world, and much like the flu, coronaviruses are always circulating the human population. It turns out the same is true in bats. Bat populations around the world, including those in the UK, have been found to host coronavirus, and just like in humans these infections don’t seem to cause them much harm.

So how come COVID-19 is making people so sick?

Coronaviruses, like a number of other viruses, are able to jump the species barrier. In the vast majority of cases the virus is not able to survive in the new host, but occasionally they do, and it's these pioneers into a new species that typically cause more severe disease than the coronaviruses that normally infect that species. This is what happened in the case of SARS and what we think has happened in the case of COVID-19.

(c) AddAlissa Eckert, MS, Dan Higgins, MAM caption
This illustration, created at the Centers for Disease Control and Prevention (CDC), reveals ultrastructural morphology exhibited by coronaviruses. 

Since humans don’t typically come into contact with bats, spillover of diseases from bats to humans tend to come via an intermediate host, typically animals kept by humans. This could be animals kept for food, such as livestock or wild animals hunted for meat, or animals kept for other reasons, such as horses. Bats infect the intermediate host, which then in turn (typically in crowded conditions) spread it to other individuals, and on to humans who handle these animals. During the SARS outbreak the intermediate hosts are thought to have been palm civets, the intermediate host for COVID-19 is currently unknown.

To stop spillovers happening we need to: 1) reduce the level of disease in the wild host (in this case bats), and 2) reduce the chance they will pass on the virus to livestock or other wild animals, and in turn us.

Wild animal populations under stress tend to have higher levels of infection. Habitat destruction is known to reduce food availability for bats, which can lead to malnourishment and higher levels of infection. At the same time habitat destruction can force bats out of their natural habitat and into urban settings bringing them into contact with humans, livestock and wildlife they wouldn’t normally be in contact with. Habitat destruction is thought to have been a contributing factor to the spill over of Hendra and Nipah, two other viruses that have found their way from bats into humans.

Agricultural intensification has led to dense populations of captive animals that humans have regular contact with. This has contributed to a number of virus spillover events, including ‘swine flu’ and ‘bird flu’. In the case of coronaviruses, both SARS and COVID-19 have been linked to markets in China selling live animals. These 'wet markets' often host a range of animals that can be bought alive or butchered, offering conditions favourable for the spread of disease between species and to humans.

Habitat destruction and agricultural intensification are just two of the many factors that cause spillover of coronaviruses from bats to humans. However, by supporting healthy wild bat populations and maintaining their natural habitats we can help to reduce the level of disease in wild populations, and keep them from close contact with livestock and humans.

Information correct as of Wednesday 26th February 2020.