According to a recent study, rabies-infected vampire bats don’t typically behave in stereotypically “rabid” ways; rather, affected male bats tend to withdraw socially and scale back on the customary practice of grooming each other before succumbing to the disease.
The research is one of the few attempts to understand how rabies infection impacts behavior in one of the species most responsible for rabies outbreaks in humans and livestock in Latin America. The study was the first to observe how rabies affects vampire bat social behavior. Most of the time, the virus is passed from one species to another through direct contact between the saliva of infected vampire bats and the punctured skin of livestock, other animals, or, very rarely, humans.
Gerald Carter, the study’s lead author and an assistant professor of evolution, ecology, and organismal biology at The Ohio State University, thinks that vampire bats in the roost may spread disease to each other through their grooming behavior, which includes licking and chewing and takes up to 5% of their active time.
Carter noted that, “despite that possibility, no studies had previously attempted to quantify changes in grooming behaviors in vampire bats infected with rabies.” Even though vampire bats live in close quarters in the roost, it’s possible that the fact that they tend to stay away from other bats when they’re sick, which has been seen in previous research, makes it less likely that rabies will spread from one bat to another.
Sebastian Stockmaier, an Ohio State University President’s Postdoctoral Scholar who works in Carter’s lab, and Elsa Cárdenas-Canales, a PhD candidate at the University of Wisconsin-Madison who is now a postdoctoral scholar in pathobiological sciences, were the co-first authors of the study.
The study was recently published in the journal Biology Letters.
Prior studies in a range of animal species and people have demonstrated that rabies infection can result in either “furious” symptoms of increased aggression or “paralytic” symptoms of lethargy and paralysis. It is anticipated that the angry reaction will increase transmission to other hosts.
The 40 male common vampire bats that were a part of a larger sample of bats used to test a rabies vaccine candidate were observed by the researchers in this study using infrared surveillance cameras set up in a lab. Before being exposed to a coyote variant of the rabies virus, groups of bats were housed together in cages for four months and given one of three treatments—oral vaccination, topical vaccination, or placebo.
The team started quantifying behaviors from three one-hour sample periods each night one day after the challenge, noting the presence or absence of grooming or aggression. To determine behavior rates, the team examined the resulting 18,808 behavioral samples.
According to Stockmaier, we were generally interested in how social behaviors, such as allogrooming and aggression, that may be related to rabies transmission changed when vampire bats were infected.
All of the vampire bats displayed low levels of aggression, and rabid vampire bats gradually reduced the amount of grooming they gave and received in comparison to their healthy cagemates. About 12 days after getting the vaccine for the virus, the effect got stronger as the bats’ deaths got closer.
The decrease in grooming could have been caused by a general immune response that made the bats sick and lethargic, which made them less social, or it could have been caused by a rabies infection that caused the central nervous system to stop working properly.
The last symptoms of either the paralytic or aggressive forms of rabies manifest before an animal dies, according to Cárdenas-Canales, suggesting that disease transmission can take place without any overt behavioral changes.
She explained that every bite from a bat during feeding or combat could potentially spread an infection. “In some cases, asymptomatic bats fly, forage, and interact normally while having infectious saliva,” she said. By observing how frequently bats with advanced disease spit out the rabies virus and are fed by others, we could learn even more about the transmission of the disease.
The fact that male vampire bats can be aggressive towards each other calls into question the widely held belief that an animal with rabies makes it more likely to spread the virus by making it more aggressive.
According to Stockmaier, if the host is naturally aggressive or likely to bite other animals for food, rabies may not need to manipulate the behavior of its host. This needs to be verified.
The behavioral effects of rabies are highly variable and may vary by viral strain, which is another explanation for why increased aggression has not been observed. Three observational studies did note signs of increased aggression, and these were all in vampire bats that were naturally exposed to rabies in the wild. The researchers noticed that in six other cases, bats that had been infected with variants from other populations or species did not show clear signs of “furious” rabies.
The National Science Foundation, the U.S. Geological Survey, the Global Health Institute, and the Institute for Regional and International Studies at UW-Madison all provided funding for this project.
