I have no expertise in epidemiology or disease ecology, but it’s hard to ignore these important topics if you study vampire bats, which cause deadly rabies outbreaks throughout Latin America. Preventing rabies outbreaks is one of the most important reasons that scientists study vampire bats. Surprisingly, though, few studies look closely at how rabies actually changes the behavior of vampire bats. The general assumption is that they would probably become aggressive, more likely to bite each other, and then eventually become paralyzed and die. But what does that process look like?
Elsa Cárdenas Canales was a PhD student at University of Wisconsin- Madison (now a postdoc), who was working with Tonie Rocke and Jorge Osario at the USGS National Wildlife Health Center. She contacted me about keeping captive vampire bats in the lab for a study testing an oral rabies vaccine that could block rabies transmission. If successful, wild vampire bats smeared with the oral vaccine could vaccinate each other against rabies, which could help prevent outbreaks in livestock and people.
I was curious about how the rabid bats’ social behavior actually changes. How do rapid vampire bats behave? After some discussion, we decided to collaborate on a project to look at changes in their behavior after the subjects were experimentally infected with rabies, which was necessary to test the vaccine. Elsa marked the bats individually using forearm bands and we installed infrared surveillance cameras.
The collaboration on my end was led by Basti Stockmaier, who was recently awarded a Presidential Postdoctoral Scholarship from OSU, and who is interested in more generally in how sickness behavior and pathogens impact social behavior. Basti, with help from Eleanor Cronin, a dedicated undergraduate in our lab, scored many of the behaviors of the bats while being blind to the experimental treatments. As expected, we found that the rabid male vampire bats decreased their social grooming relative to the control group, prior to death.
However, we did not detect any increase in aggression in the rabid bats, as one might expect. Why? Perhaps higher aggression was too rare to detect given the number of bats and observations we had: absence of evidence is not evidence of absence. Perhaps rabid vampire bats don’t actually become as aggressive as often as other species; perhaps they don’t need to be more aggressive to transmit the virus since they already bite other animals so frequently and can spread it socially in other ways. Perhaps the behavioral effects of rabies differ by the viral strain: Basti noted that in 6 studies where vampire bats were infected with lab variants of the rabies virus (that were sampled from foreign populations or other species like coyotes), the bats did not seem to become more aggressive, but they did in 3 other cases where they were naturally exposed to the virus in the wild. At this point, we can only speculate without a larger sample of observations.
Reference: Cárdenas Canales EM*, Stockmaier S*, Cronin E, Rocke T, Osorio JE, Carter GG. 2022. Social effects of rabies infection in male vampire bats (Desmodus rotundus). Biology Letters. http://doi.org/10.1098/rsbl.2022.0298 (*co-first authors)
I expect we might learn much more about behavioral changes with rabies in vampire bats from the ever-impressive work happening in Daniel Streicker‘s lab. His team has studied vampire bat rabies on a landscape scale and is developing exciting and potentially revolutionary virally-vectored vaccines (a rabies vaccine attached to another virus) that will continuously spread from vaccinated to unvaccinated vampire bats over multiple generations (see video below). Testing these vaccines might also involve captive trials where the behavioral changes could be observed.