December 3, 2013
Efforts To Stem Vampire Bat Rabies May Be Making It Worse
redOrbit Staff & Wire Reports - Your Universe Online
Culling vampire bat colonies to slow the spread of rabies in Latin America is ineffective, and could even have the opposite effect, according to research currently appearing in the online edition of the journal Proceedings of the National Academy of Sciences.
Vampire bats are known to transmit rabies throughout Latin America and are said to be responsible for thousands of livestock deaths and even a handful of human fatalities every year, the researchers explained. Experts have used several different methods, including explosives and poison, in an attempt to keep the population under control, but in most cases those efforts have proven unsuccessful.
Investigators from the University of Michigan and the University of Georgia had been conducting a long-term field study of vampire bats in Peru, the results of which they reported last year. Now, those same researchers have taken that data and combined it with computer models of rabies transmission and information from infection studies involving captive vampire bats. Their efforts show that population control attempts not only fail to contain the virus, but in some instances can actually increase its spread by forcing infected bats into nearby healthy colonies.
“The findings suggest that geographic coordination of vampire bat control efforts in Latin America – taking into account the interconnectedness of seemingly isolated colonies – might reduce transmission to humans and domestic animals,” researchers from the University of Michigan said Monday, adding that the newly published paper “also establishes that rabies is usually not lethal among vampire bats.”
Senior author Pejman Rohani, a population ecologist and epidemiologist at the university, added that the previous paper “demonstrated that bat colony size wasn't a predictor of rabies prevalence, which indicated that culling hadn't reduced transmission,” and that the new paper used models encompassing “alternative assumptions regarding this system,” identifying “an important role of movement between colonies” and then using “the best-fitting model to examine what happens under culling, especially if the cull is indiscriminate, rather than targeting infected bats specifically.”
Rohani said that culling was once again proven ineffective, but that the newly developed model shed some light as to why that is the case. He and his colleagues developed four mathematical models of rabies transmission, with each one representing an alternative hypothesis for the biology of rabies infection. Those models were tested against the University of Georgia-led field study of rabies exposure in wild Peruvian bats.
“Thousands of computer simulations were run, and the most successful models demonstrated that a single, isolated vampire bat colony cannot maintain the rabies virus over time. Frequent movement of infectious bats between colonies is needed to keep the rabies virus at levels consistent with the field observations,” researchers said.
The researchers found that “the critical role of immigration between bat colonies predicted by our analysis indicates that current culling practices, often reactive to outbreaks in livestock or haphazardly implemented, are unlikely to eliminate VBRV (vampire bat-transmitted rabies virus).”
“While programs targeting specific colonies may limit local spillover from bats to humans or domestic animals, regional viral persistence will likely remain unaffected due to high connectivity between bat colonies," Rohani said. “Moreover, if culling increases movement due to freeing up space or disturbance-mediated dispersal, culling could, perversely, have the opposite of the intended effect on rabies transmission.”
The study authors report that this type of phenomenon had also been observed in controlled badger populations in the UK, where the disruption and dispersal of the creatures led to increased tuberculosis transmission in nearby cattle. Furthermore, the new study also found that the overwhelming majority of rabies virus exposure among vampire bats was nonlethal, ultimately immunizing the rodents and helping to prevent colony extinction.