In 2010, a 19-year-old migrant farmworker from Mexico arrived at a sugarcane plantation in Louisiana unknowingly carrying a deadly virus. His symptoms were mild at first: fatigue, shoulder pain, and numbness in one of his hands. As his condition worsened, he was admitted to a hospital in New Orleans.
There, he spiked a fever. His lungs filled with fluid. His pupils became fixed and dilated, and he was soon unresponsive. Doctors suspected swelling in his brain, and ran a test that showed antibodies to the rabies virus in his blood. Rabies is almost always fatal in humans if left untreated, and in this case, it was already too late; the man died shortly after. Postmortem testing revealed that the virus was in his brain tissue, and public health officials later learned he had been bitten by a vampire bat before leaving Mexico.
The case marked the first rabies death in the United States due to a vampire bat. Though these bats don’t currently live in the US, their territory in Latin America has been slowly expanding northward. They thrive in warm, humid areas where temperatures do not dip below 50 degrees Fahrenheit. As average temperatures increase with climate change, experts predict that the common vampire bat, or Desmodus rotundus, is likely to cross the US southern border in the next several years. In recent years, the species has been documented within about 30 miles of Texas.
A new study published in the journal Ecography last week found a strong relationship between changes in climate over the past 100 years and the gradual expansion of vampire bats to the northern hemisphere. “We expect invasion of vampire bats to US soil between five and 20 years in the future,” says Luis Escobar, an assistant professor of wildlife conservation at Virginia Tech. Other climate models have also predicted their move into the southern parts of Texas and Florida. As the bat’s territory spreads, so will the variant of rabies it carries.
Escobar says that vampire bat rabies isn’t necessarily a bad thing—it helps control their numbers, and that may benefit the greater ecosystem. “Rabies can reduce populations of bats from 10 to 80 percent. Imagine if we had too many vampire bats because we didn’t have this virus,” he says. Because bats are social animals that tend to roost together and form colonies, rabies spreads easily among them. But the disease never wipes them out. “Rabies has been in bats for a very long time,” he says.
The problem is when the virus spills over to domestic animals or humans. Many animals can carry rabies, including raccoons, foxes, skunks, and dogs. In the US, human cases are rare, only one to three a year. Contact with bats is increasingly the main cause, although most bats—even rabid ones—rarely bite people. They only strike when they feel threatened.
But vampire bats represent a new threat because they feed on the blood of other animals. Their usual victims are livestock, and occasionally wild mammals and birds. Using their sharp front teeth, they make a small incision in their victim’s skin and lap up a teaspoon or two of blood with their tongues. The bites don’t kill, but if a vampire bat is carrying rabies, the disease eventually will.
Vampire bats are a particular menace to the cattle industry in Latin America. “There's a lot of cattle on landscapes where they exist,” says Toni Piaggio, a research biologist with the US Department of Agriculture’s National Wildlife Research Center, who has conducted genetic analyses on vampire bats to confirm their northward spread. “It’s probable that vampire bats have been able to survive in areas where they couldn't in the past, because humans have put so many cattle on the landscape.”
In Mexico, vampire bat rabies costs the livestock industry more than $46.7 million per year, according to a 2020 USDA report. And there’s the risk to human health. Infected cattle can spread rabies to people who come into contact with them. “Our real concern is about people being exposed to rabies through livestock,” says Mike Bodenchuk, Texas director of the USDA’s Wildlife Services division, and an author on that report.
The USDA’s National Rabies Management Program has been anticipating the vampire bat’s eventual arrival. According to a government report released in September, officials have inspected 500,000 cattle at livestock sales, dairy farms, feedlots, and ranches in Arizona, New Mexico, Texas, and Florida since 2016 for evidence of vampire bat wounds. So far, no bites have been found. The agency has also been carrying out a campaign on both sides of the border to educate ranchers and livestock producers on how to spot the bites and signs of rabies.
Bodenchuk says the wounds can often be found around the neck or tail. Because animals keep bleeding for a while after being bitten, dried blood can be a telltale sign. Other signs are neurological: The virus travels to the brain and spinal cord, so infected cattle become disoriented and can’t move their hindquarters. They can become aggressive and charge at people.
In the US, cattle owners are taking notice of the vampire bat’s northward spread. “This bat species causes a lot of concern in agriculture due to its ability to transmit diseases, injure livestock, and cause infections. Rabies is the most obvious issue because of livestock welfare and potential to infect humans,” says Gary Joiner, a spokesperson for the Texas Farm Bureau. “It’s a difficult situation that we’d like to address as soon as possible, so vigilance is crucial.”
In Latin America, governments have long used poison to cull vampire bats and prevent rabies transmission. This can be effective in places where rabies hasn’t already been detected. But a study published earlier this year found that poisoning can backfire in areas where rabies is circulating, because the surviving bats tend to flee, carrying the virus further.
In some parts of Latin America, including Colombia and Mexico, livestock owners regularly vaccinate cattle against rabies. The pharmaceutical company Boehringer Ingelheim manufactures an injectable vaccine, called Imrab, for dogs, cats, horses, cattle, sheep, and ferrets. The company also makes an oral version that is used to vaccinate raccoons and other land-dwelling wild animals.
In the US, vaccinating cattle against rabies isn’t common, but it may be the best option to prevent the spread of the virus once vampire bats arrive. “They’re not going to come across the border by the millions,” Bodenchuk says. “It's going to be a slow trickle for a while. But landowners will want to consider whether or not to vaccinate their animals.”
Some researchers are trying to develop vaccines for bats. One approach is applying a jelly-like dose to wild bats that have been caught, which are then released back to their natural habitats. The bats would ingest the gel and spread it to others in their colony when they groom each other.
But Escobar says efforts to vaccinate bats should proceed with caution. “We don't know what the ecological effects of disrupting the circulation of this virus in bats are going to be,” he says. Vaccinating the bats could mean there will be more of them, because rabies won’t thin their populations. And even if they aren’t able to get rabies, they can still carry other diseases that they could pass on. Plus, their bites will still be a nuisance to ranchers, because they can weaken livestock and make them vulnerable to other infections.
Vaccine testing in the wild also raises ecological questions for bat species that are in decline. While vampire bats aren’t threatened, others are, and many of them are helpful members of their ecosystems. Most bats are insectivores that eat mosquitoes and other agricultural pests, or act as pollinators and seed spreaders.
“They have an important role, whether they pose a human health risk or not,” Piaggio says. “If we got rid of everything that posed a human health risk, then there wouldn't be anything left.”
