The risk of Hendra virus being passed from bats to horses – increasing the risk to humans – has escalated in Australia in recent decades.
When viruses spillover from animals to humans there can be serious consequences. SARS Cov-1, SARS Cov-2 (which causes COVID19) and Hendra viruses have all been linked to this kind of event.
A team of researchers, led by conservation biologist Dr Peggy Eby, from the University of New South Wales, and wildlife disease ecologist Dr Alison Peel from Griffith University, analysed 25 years of data on land use change, bat behaviour and Hendra virus spillover events in Australia. Their research is published in Nature.
The team documented 63 Hendra events from bats to horses from 1996 to 2020. They found the frequency increased from 2006 onwards, with spillovers detected in 80% of years.
Horses are the intermediary hosts from which Hendra can pass, or spillover, to humans.
While flying foxes are known to carry the Hendra virus, it does not appear to cause disease in the animals. For horses and humans however, the virus is severe, having fatality rates of 75% and 57% respectively.
The researchers linked the spillover risk to habitat loss and native vegetation clearance. They believe this has led flying foxes to spend more time in urban and agricultural areas, due to the lack of native food, particularly flowering eucalypts, over winter.
“The loss of habitat is causing bats to adopt some behaviours they’ve previously only used for short periods of time during climate-driven food shortages, adapting them to more permanently persist in our urban and agricultural environments,” Peel told Cosmos.
The behaviour increases the likelihood of contact between bats and horses. In addition, bats which are spending more time in these environments are excreting higher rates of Hendra, she says.
By 1996, more than 70% of winter habitat for flying foxes had already been cleared in Queensland. A further third of the remaining habitat has since been cleared.
These land use changes are also upping the ante when it comes to conflicts between flying foxes and humans.
Eby says, “that same process … where the bats were reducing their nomadic behaviors, moving into particularly urban areas to feed, reducing the size of the roosting groups, and increasing the number of roosts that they’re using in those areas, leads to conflict with humans.”
“Suddenly, people find that the lovely creek line at the end of the street contains a roost of flying foxes. People don’t adapt easily to the presence of flying foxes in their urban landscapes.”
It’s important to remember that the bat behaviours are a survival mechanism, she says.
“Those key winter habitats that we’re losing through ongoing clearing are fundamental to the survival of the animals. So these are animals under considerable stress.
“To break their long-held evolved behaviours of roosting in large groups, tracking pulses of eucalypt flowering, and changing them so dramatically, is a clear sign of severe stress on the animals themselves,” she says.
One way to mitigate the risk of Hendra virus spillover, is to protect and restore the bats’ native winter habitats, and encourage more native flowering.
“Flying foxes track nectar flow in native forests over vast distances and play a critical role in pollinating our native trees. When large areas of eucalypts burst into flower, flying foxes congregate in roosts of 100,000 individuals (or more) to enjoy the abundant food,” Eby said.
At times of abundant eucalyptus flowering, no Hendra virus spillovers were detected.