The first glimpse of how rabies affects the social behavior of vampire bats

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Vampire bats infected with the rabies virus are not likely to act stereotypically “rabid”, according to a new study – instead, infected male bats tend to withdraw socially, reducing the common habit of grooming before dying of the disease.

The study was the first to observe how rabies affects the social behavior of vampire bats, and one of the few research efforts to understand how rabies infection affects the behavior of one of the species most most responsible for rabies epidemics in humans and livestock in Latin America. The virus is usually transmitted to other species by direct contact between the infected saliva of vampire bats and the broken skin of livestock or other animals (and, rarely, humans) which they bite to feed on blood. .

In the roost, vampire bats could become infected through licking and chewing which is the grooming behavior they engage in for up to 5% of their active time, said study lead author Gerald Carter. and assistant professor of evolution, ecology and biology of organisms at Ohio State University.

“Despite this possibility, no previous studies have attempted to quantify changes in the grooming habits of rabies-infected vampire bats,” Carter said. “It could be that the tendency of vampire bats to withdraw from social activity when sick, as we have seen in our previous work, reduces the likelihood of transmission of rabies to their groupmates. despite living in close quarters within the roost.”

The study was conducted by the co-first authors Sebastien Stockmaieran Ohio State University President’s postdoctoral researcher in Carter and Elsa Cá’s labrdenas-Canales, then a doctoral student and now a postdoctoral researcher in pathobiological sciences at the University of Wisconsin-Madison.

The research was published recently in the journal Biology Letters.

Previous research in various animal and human species has shown that there are two observed responses to rabies infection: “furious” symptoms of increased aggression, or “paralytic” symptoms that lead to lethargy and paralysis. The furious response should increase transmission to other hosts.

In this work, the researchers outfitted a lab with infrared surveillance cameras to observe 40 male common vampire bats that were part of a larger sample of bats used to test a candidate rabies vaccine. Groups of bats given three treatments – oral vaccination, topical vaccination or placebo – were placed together in cages for four months before being challenged with a coyote variant of the rabies virus.

A day after the challenge, the team began quantifying behaviors from three one-hour sampling periods each night, recording the absence or presence of grooming or aggression. The team analyzed the resulting 18,808 behavioral samples to estimate behavior rates.

“We were generally interested in how social behaviors that might be relevant to rabies transmission — allogrooming and aggression — changed when vampire bats were infected,” Stockmaier said.

All bats showed low levels of aggression, and compared to their healthy cagemates, rabid vampire bats showed a reduction over time in giving and receiving care. The effect was first observed around 12 days after virus inoculation and strengthened closer to the death of the bats.

Researchers can’t be sure what caused the decrease in grooming – whether it resulted from a general immune response that made the bats sick and lethargic, and therefore less social, or from nervous system dysfunction. center due to rabies infection.

VSaRdenas-Canales noted that the signs of the aggressive or paralytic forms of rabies are the last to appear before the animal dies, suggesting that disease transmission can occur without any obvious change in behavior.

“In some cases, asymptomatic bats fly, feed and interact normally while having infectious saliva – making every bite, while feeding or fighting, potentially infectious,” she said. “We could learn even more by measuring how often bats in late stages of disease shed the virus in their saliva and are fed by others, and what this might imply for rabies transmission. “

The findings, in male vampire bats, which can be aggressive towards each other, actually raise a question about a common idea regarding rabies infection – that the virus manipulates its host into becoming aggressive. to improve the chances of viral transmission.

“Rabies might not have to manipulate its host’s behavior if the host is frequently aggressive anyway or likely to bite other animals for food,” Stockmaier said. “That remains to be tested.”

Another possible explanation for the lack of increased aggression is that the behavioral effects of rabies are highly variable and may differ by viral strain. The researchers noted that bats infected with variants from other populations or other species did not show clear “furious” rabies in six other cases, but three observational studies recorded signs of increased aggression. , and these were all about vampire bats that were naturally susceptible to rabies in the wild.

This work was supported by the National Science Foundation, the US Geological Survey and the Global Health Institute and the Institute of Area and International Studies at UW-Madison.

Carter is an affiliate of the Smithsonian Tropical Research Institute. Additional co-authors include Eleanor Cronin of Ohio State, Tonie Rocke of the US Geological Survey-National Wildlife Health Center, and Jorge Osorio of UW-Madison.

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