A long-standing hypothesis about the origin of pandemic viruses has been confirmed in practice. Man-made changes in the environment bring people more often into contact with dangerous bat viruses. This is the conclusion reached by a team led by Peggy Eby from the University of New South Wales, who analyzed data from the years 1996 to 2020 on land use, bat behavior and hendra outbreaks in horses in Australia. Horses are the intermediate hosts from which the virus jumps to humans. According to the study now published in "Nature", there is a direct link between climate, land use and outbreaks of hendravirus, which kills more than half of infected people.
Hendravirus carriers are flight dogs from the Pteropus genus who do not develop themselves - the virus for horses and people is extremely fatal. 84 out of 112 infected horses died since the first appearance of the virus, and four out of seven infected people. The disease breaks out when the flight dogs use the feed trough of horses or sleep in horse stables - they probably transmit the virus by saliva or faeces. People then stick to the sick horses. However, the virus is very little contagious. So far, no transmissions are known from person to person.
However, Hendra is a good model for bat viruses because the disease is too deadly for outbreaks to go undetected. In addition, the host organisms are well known here. The study of the team around Eby focuses on the subtropical regions of Australia, where the flying foxes migrate over long distances between flowering eucalyptus trees, their most important food source. While many trees bloom and provide nectar in summer, food sources are much rarer in winter. Weather-related hunger phases of a few weeks occur again and again. In these, the flying foxes temporarily divide into small groups and eat fruit in gardens and agricultural regions until trees bloom again.
However, this was only the same until 2002, as the team reported. In this phase, neither humans nor horses on the Hendravirus suffered in the examined region. However, that changed dramatically in the years after. The bats occurred more and more in small groups and near human dwellings, and horses were increasing with Hendra more and more. From 2006 there were outbreaks in 80 percent of the years. The background is that the behavior of the bats changed: if they only used human feed sources for a few weeks, large parts of the population later remained most of the time in this emergency mode.
The cause was a shortage of food, as the team around EBY reports. Even before the examination began, around 70 percent of the habitats of the flight dogs were destroyed, and the destruction continued continuously. Another third was destroyed by this rest by 2018. As a result, the already rare sources of feed are becoming increasingly scarce in winter and are becoming more and more apart. A kind of tipping point was apparently reached in 2002. Since then, the emergency program has been the norm - the bats are distributed in small groups in people inhabited by people where there are fruit.
But the animals are by no means happy with their role as a successor: as soon as the eucalyptus suddenly blooms in one of the remaining forests, the animals return to their original source of feed. In such years, no horses are infected with the Hendravirus, as the working group notes. On the one hand, the study confirms general warnings from experts that destruction and use of the habitats of wild animals make new zoonoses more likely. On the other hand, it brings up specific factors that promote outbursts in certain years and regions, for example el-Niño years. Last but not least, the effect may also be part of the explanation of an increasing trend of larger outbursts of once rare zoonoses such as monkeys or Ebola.
