Common vampire bat (Desmodus rotundus). (iStock)

For nearly three decades now, invasive wild pigs have been spreading throughout Brazil, uprooting native plants, damaging soil quality and destroying crops. There are even reports of the pigs attacking livestock. But while they’re considered a nuisance by most, at least one group seems to be benefiting from them: Brazil’s residents vampire bats.

A new paper, published this week in the journal Frontiers in Ecology and the Environment, lends support to the idea that vampire bats have a taste for pig blood. And the scientists are cautioning that this might be a bigger problem than it seems. They’re concerned that as the pig populations continue to grow, so will the vampire bats — and this could mean an uptick in the spread of rabies to humans.

In other words, it’s a classic example of how one human action — introducing a species that becomes an invader and colonizes new ecosystems in a destructive way — can have cascading and damaging effects that ultimately come back around and hurt humans, themselves.

“Vampire bats…carry infectious disease, a lot of infectious disease,” said Felipe Pedrosa, a Ph.D. student at Sao Paolo State University and one of the new paper’s authors. “And one of these diseases is rabies.”

Typically, vampire bats are known for preying mostly on livestock. But they’ll feed on wild animals as well, if they happen to be more accessible, and even humans. In 2005, a spate of attacks on humans in Brazil made international headlines by causing 23 rabies deaths in two months and leading to more than 1,300 people seeking medical treatment for rabies.

The bites typically happen at night while people are sleeping, and are most common in children, who tend to sleep more soundly and are less likely to be wakened by the bats. While 2005 was an unusually alarming year for rabies scares, generally human infection with rabies remains rare.  

Today, the incidence of rabies infections in vampire bats varies by location — it tends to be anywhere from about 1 to up to 10 percent, according to the authors of the new paper. Some farmers routinely vaccinate their livestock against the disease, but the feral pigs, which can also carry rabies, are another story and “may therefore pose a serious threat by spreading the disease,” the authors write.  

For the new paper, the researchers analyzed thousands of photographs and videos used to monitor wildlife in Brazil’s Pantanal region, a tropical wetland area mostly occupying the Brazilian state of Mato Grosso do Sul, and the Atlantic Forest, which runs down the Atlantic coast. They found that, in addition to preying on livestock like cattle, the bats also feed on wild animals including tapirs, deer and feral pigs. The videos and photos from the Pantanal region suggested there was about a 2 percent chance that a pig might be attacked by a vampire bat on any given night. In the Atlantic Forest, this chance rose to 11 percent.

Pedrosa pointed out that previous studies have suggested vampire bats may even have a preferential taste for pig blood. A study published last year in Science, for instance, used a genetic technique to investigate what species the bats prey on most frequently. The results indicated that vampire bats were about seven times more likely to prey on pigs than one would expect would happen by chance alone — in other words, the bats were likely actively seeking out the boars.  

The researchers suggest this may be happening in part because of human-induced changes to the Brazilian landscape. Large swaths of former pastureland in southeastern Brazil have recently been converted for the cultivation of sugarcane, they’ve pointed out, meaning there are fewer cows to prey on — but plenty of pigs.

There have been feral pigs in Brazil for up to 200 years, research suggests, when a few domestic pigs escaped and went wild in the Pantanal region. But a large-scale, country-wide invasion can be traced back to the 1990s, when wild boars were imported from Europe and Canada for use in high-quality meat products. In Brazil, many farmers bred these boars with the domestic pigs that already existed in the country. Eventually, the government stopped permitting the importation of wild boars, and many of the interbred pigs were released — accidentally or intentionally — into the wild.

The feral pigs cause enough ecological and agricultural damage as it is, but now the authors of the new study are concerned that their continued spread could boost bat populations in some areas and contribute to a spike in rabies infections in people. This could happen in a variety of ways, they’ve suggested. While vampire bats have been known to bite sleeping humans and infect them directly, bushmeat hunters — and their hunting dogs — could also be exposed through contact with infected pigs.

And rabies isn’t the only concern either, Pedrosa added. Vampire bats are known reservoirs for a handful of other infectious diseases as well, including several viruses that can cause serious respiratory illness in humans.  

That said, vampire bats are part of the native ecology in Brazil, and they’ve also been helpful in certain modern medical advances. Vampire bat saliva contains properties that prevent blood from clotting so the animals can feed — it’s similar to the effect observed in leeches. Scientists have been able to study and incorporate these properties into anticoagulant drugs that can help prevent or break up blood clots in at-risk patients.  

The Brazilian government has attempted to tackle the rabies problem in the past by encouraging the vaccination of livestock and introducing bat culls, although some scientists have suggested that the culls have done little to halt the disease, and in some cases may have even encouraged its continued spread. In light of the new concerns surrounding feral pigs, Pedrosa suggests a different approach.  

“Hashtag ‘kill the pig,’ ” he said. The Brazilian government has established a program allowing the killing of feral pigs, he noted, but added that rigorous restrictions on the purchase of firearms has kept the number of participants fairly small so far. He and other scientists are currently involved in helping federal environment agents come up with better plans to address the pig problem in the future.

In the meantime, “vampire bats feeding on the constantly spreading feral pigs may therefore be viewed as a potential risk to wildlife, livestock and humans,” the researchers write.