Previously, Daniel Streicker and colleagues argued that vampire bat control methods (targeted poisoning) might actually increase rabies rates in vampire bats. I wrote about that here. The team has since published a new model to examine hypotheses regarding why culling vampire bats does not reduce, and worse may even increase, rabies rates in vampire bats. Here’s the article and a press release.
Surprisingly, most of the vampire bats that are exposed to rabies develop resistance. This doesn’t mean that they are lifelong carriers, but rather that they have fought the infection and have been, in effect, immunized. Previous lab studies have found that most of the bats (50 to 90%) exposed to the rabies virus in a lab setting will die, but the authors estimate that only 10% of wild bats bitten by rabid conspecific actually develop a lethal infection. From an evolutionary standpoint, this outcome probably helps the virus from going extinct by wiping out a whole bat colony before the virus can be transmitted. From the virus point of view, it has to get into from one bat to another bats it kills the host, and it has to get from one colony to another. Therefore, the maintenance of the rabies virus over time at high levels relies heavily on dispersal of bats between roosting sites. This is key to why killing as many vampire bats as possible at a site doesn’t effectively get rid of rabies.
But why might culling vampire bats actually lead to higher rates of rabies? One explanation was that the poisoning (which is spread by social grooming) disproportionately targeted adults more often than young, and the adults had developed more resistance than younger bats. I became dubious about whether this made any sense. The authors explicitly based this explanation on the assumption that juveniles are less likely to groom adults, and thus less likely to expose themselves to poison-treated adults. However, in contrast to this claim, juveniles and subadults lick the fur of adults more than adults lick each other. In a roost, young vampire bats should therefore be exposed to vampiricide more, not less, which removes a key assumption for this “selective adult culling hypothesis”. I suggested that either a different mechanism explains the negative relationship between culling and rabies, or that this correlation was spurious.
But there’s another explanation that makes more sense to me. Male bats guard roosts like territories to get access to females. If culling removes these male bats, than males from other areas are more likely to move in. Likewise, killing a group of females could open up a roost for new residents. In other words, culling could increase dispersal, which could increase rabies transmissions.
Culling could also increase dispersal if bats are more likely to leave an area after many of their conspecifics have died. Either scenario seems plausible to me.
Overall, the study highlights the complexity of host-pathogen dynamics and makes the important suggestion that killing vampire bats– even using targeted methods such as poison spread by social grooming– is probably not a good way of fighting rabies in Latin America. This reveals why the task of rabies control is so daunting. It’s also good news for me, because having seen these vampire bat control methods, I am not a fan. I would very happy to see them retired.
But I don’t see vampire bat control disappearing completely anytime soon. Even without rabies, vampires would still be considered an agricultural pest. Farmers and ranchers in Latin America still see 20-30 bites on a single cow. They are like wolves and coyotes to a sheep rancher or deer and groundhogs to a vegetable farmer. Except they can fly right over your fences.