For more than 50 years, Latin American countries have been killing vampire bats to limit the spread of rabies. These flying hemlocks are common hosts for the virus that causes the disease, which kills hundreds of cattle each year and costs local farmers in southern Peru about $170,000 a year in losses.
Killing wildlife uses vampiricide, a deadly poison that spreads to up to 15 bats for each treated animal. But there’s one thing people haven’t considered – bats aren’t going to hang around and get poisoned.
“The intervention was well-intentioned in trying to manage an infectious disease, but it backfired because we didn’t understand how the animals would react,” says Daniel Streicker, professor of viral ecology at the University of Glasgow and senior author. of a new study published today in Advances in Science. Rather than slowing rabies, culling in areas with high virus circulation caused bats to fly more frequently across the landscape, accelerating the spread of the disease.
It’s all about timing
The study authors first studied how culling affected bat population size and, in turn, the risk of rabies transmission to animals living in a large area of southern Peru. While the culling reduced the bat population, it did not completely stop the hungry biters. About 59 percent of farms in the region reported at least one animal bitten by a vampire bat three months after the slaughter.
The team also looked at how culling might affect the circulation of the rabies virus. By sequencing viral genome sequences, they reconstructed how the virus has spread across the landscape through space and time.
Killing affected the speed of the virus’ spread. If a cull took place in an area with active viral circulation, then the viruses seem to spread more quickly. Bats are social creatures, Streicker explains, and seeing their group members die from poison likely caused them to fly away to avoid death. This increases the chances that bats can carry the virus over longer distances.
However, in areas without recent rabies cases, the virus takes longer to spread. The findings show that it’s all about context. If culling is done proactively in areas before the virus arrives, it can slow the spread to new areas, allowing areas to remain rabies-free for longer. “We can’t say that rabies would never happen, just that maybe it would happen more slowly,” says Streicker.
Is slaughter the only strategy?
Vaccination is an alternative method to contain rabies, but Stryker says people in Latin America have historically been reluctant to get vaccinated. People often get the rabies vaccine after exposure to the virus, which is less effective. Another issue is that some remote areas in Latin America make it difficult to deliver vaccines.
Vaccination of animals is another option for controlling the transmission of rabies. But immunizing every animal is expensive for farmers, especially those living in areas that have had little to no rabies problems.
Some locals have tried to take matters into their own hands, lighting roosters to destroy potentially sick birds or cutting down trees that might harbor other rabies-infected creatures. However, researchers do not recommend these methods because they can harm animals that are not infected with the virus.
Other ways to slow down rabies
Vampire bats are not the only bat species that carry rabies, but they are probably the main spreaders. These bloodhounds need to feed almost every night, which has a high risk of exposing their victims to rabies. And the grazing of cattle and other livestock has given the bats a nearly limitless food source, Streicker says.
Instead of vaccinating humans and cattle, one approach is looking at vaccines for vampire bats. Bats are not easy to vaccinate – they are isolated wild animals and it would be almost impossible to catch every one. A successful rabies vaccine requires delivery technology to reach sufficient numbers of wild animals. Stryker and his team are currently working on a self-propagating vaccine that would be administered to a bat. From there, the vaccine will be able to spread to other bats on its own.
Another approach is reproductive suppression, in which bats consume blood containing coumestrol, an organic compound that acts as a bat birth control. Ongoing research shows that coumestrol affects the fertility of male and female vampire bats, reducing their population. Ultimately, Streicker says, containing rabies in a bat population requires all three strategies: vaccination, reproductive suppression and preemptive culling in areas where the virus has not yet arrived.