As bats and humans cross paths more viruses are making the jump from bat to people. China’s latest scare is the latest coronavirus to affect humans likely to have its origins in bats.
The outbreak of a brand new virus in China has put humans’ relationship with bats under the spotlight again.
As the human population grows, so too are our interactions with bats, either through people and domestic animals sharing bat habitats, or increased hunting of bats for meat.
The novel coronavirus discovered in China has not officially been linked to bats, but given its similarity to SARS (Severe acute respiratory syndrome) there’s a fair chance it may be another coronavirus which has jumped from bat to animal to human.
Zoonotic diseases are infectious diseases caused by bacteria, viruses and parasites that spread between animals and humans. This encompasses anything from cow poo in water making people sick, to a bite from a dog or monkey with rabies or a flea with the plague.
The species which carries the most diseases though is the bat. There are around 60 diseases bats have which can be passed to humans either directly or indirectly through another animal.
These include coronaviruses, hantaviruses, lyssaviruses, SARS coronavirus, rabies virus, nipah virus, lassa virus, Henipavirus, Ebola virus and Marburg virus.
So far the outbreak of the new coronavirus has killed four people, infected around 200 and travelled beyond China with cases reported in Japan, Thailand and South Korea. Another person has been isolated and tested in Australia. Several countries are screening passengers at borders in an attempt to stop the spread of infection.
Massey University professor David Hayman specialises in zoonotic disease and said one reason for the upswing in diseases spread from bats to humans is more exposure.
“It’s simply a numbers game. The number of humans on the planet and the number of opportunities for human – bat contact are increasing.”
He said it’s not definitive the new virus has come from a bat.
“It is definitely related. Ultimately it will have its origins in a bat, it’s just whether it’s come from another species between the bat and the people.”
Furry, flying bundles of viruses
Bats have been found to harbour up to 137 diseases in total, with 61 of those able to affect humans. One theory why they’re so chock-full of disease is because they tend to live in colonies in close quarters making disease spread inevitable. They also appear to have viruses for a long time.
What’s curious is why bats don’t die from the diseases they carry.
One study suggests bats only suffer mildly from the viruses they carry because they leak DNA when they fly.
Flying is incredibly hard work – far harder than exercise other mammals do. Due to the effort expended bats leak DNA into cells. With most mammals this leaked DNA would be confused as a virus and would be attacked by the immune system. To avoid attacking their own tissue the researchers believe bats have evolved to have a milder reaction to viruses.
It’s thought this mild reaction allows viruses and bats to co-exist.
Hayman said there is debate around the topic but it’s clear their immune response to viruses is different to humans.
“They don’t mount an immune response that allows them to completely clear the virus but nor does the virus ever replicate to very high levels.”
Sharing their germs with other species
There are several ways a disease carried by a bat can be passed to another animal. Fruit that a bat has bitten and spread saliva on may be eaten by other species. Urine or droppings on grass can be eaten, or the bat itself might be eaten by another animal.
In Australia, fruit bats passed the Hendra virus to horses through urine and droppings. The outbreak was small but deadly. Of the seven people who caught the disease from infected horses, four died.
MERS (Middle East Respiratory Syndrome) also appears to have originated in bats, before jumping to camels and then humans.
Hayman said bat viruses didn’t necessarily need to mutate in another animal to infect humans. With SARS it was thought the disease was passed to humans by people eating civet cats which had caught the virus from bats.
“People thought it had to go through the civet to have a mutation so that it could affect humans and there were mutations which only appeared in the people and the civets.”
He said subsequent research showed this might not be the case. Bat meat was also sold for consumption in wet markets, where’s they’re often killed on-site and there’s a chance the virus jumped directly from bats to humans.
The current coronavirus outbreak in China was initially centred around a seafood market which also sold other animals. It’s unlikely the virus would have jumped from bats to sea species.
Why are so many bat viruses emerging now?
“These viruses have been in the bats for millennia, and don’t cause a problem but what we are doing in the environment is leading to increased opportunity for viruses to spread,” said Hayman.
In Malaysia, agricultural expansion in the form of intensive pig farming occurred next to orchards. Fruit bats attracted to the orchards infected the pigs and the nipah virus emerged.
On the Hendra virus in Australia, Hayman points out this is an artificial contact created by human activity.
“It seems we’ve changed the habitat with loss of trees and we’ve introduced horses to Australia and horses seem susceptible to the hendra virus that’s in the fruit bats.”
With a bigger human population and more habitat encroachment the statistical likelihood of disease spillover increases. Hayman said scientists have viewed disease outbreaks as inevitable.
“It’s like throwing the dice. When there is that magical moment where you’ve got an infected bat that has contact with a susceptible person and they actually shed [virus] and that person or animal gets infected and starts replicating those cells, that’s lots of chance events.
“One of the reasons people like me still do this work is that we know infections are still there, they’re circulating in these animals and at some point somewhere another SARS-like virus was going to happen. It was a question of when.”
Should we kill all bats to be safe?
Killing all the bats is a really bad idea.
Bats are sometimes described as a cornerstone species. They pollinate flowers and help disperse seeds. Their droppings can be an important fertiliser for plants. They also eat large numbers of insects. In Madagascar, bats pollinate the flowers of the baobob tree, in Mexico they pollinate agave, the key ingredient in tequila.
Slaughtering mass numbers of bats could cause entire ecosystems to collapse.
Stopping an outbreak
Viral infections can’t be treated with antibiotics. There are some anti-viral medications but in the case of coronaviruses such as SARS and MERS these have been largely ineffective. In most cases only the symptoms of the infection can be treated.
The best cure is not catching it in the first place. While it may sound like silly advice, this is exactly how SARS was eliminated. People exposed to the virus were quarantined and the outbreak ended.
The civet cats suspected in SARS as the go-between animals passing the virus from bats to humans were also culled, as were pigs during the nipah virus outbreak.
For Ebola, another deadly disease with bat origins, vaccines have been developed. One was approved for use by the United States in December. Work did start on a vaccine for SARS but no vaccine has been licensed.
There is not much of a business case to engineer a vaccine for a disease which might not reappear.
In the short term, Hayman thinks the best thing which can be done is to improve efforts at prevention of spread. With some viruses such as Ebola this might mean better healthcare infrastructure.
Another answer to human susceptibility to viruses with bat origins could lie in the bats themselves. Humans might not be about to learn how to fly, but scientists are looking at why bats are almost impervious to so many viruses. Their ability to repair themselves after flight is one aspect – and a lack of inflammation.
“Some people are beginning to say ‘what can we learn from bats to help us prevent disease?’.”
