Trying to eliminate Latin vampire bats colonies disperses them, study says

Culling vampire bat colonies to stem the transmission of rabies in Latin America does little to slow the spread of the virus and could even have the reverse effect, according to University of Michigan researchers and their colleagues.

Vampire bats transmit rabies virus throughout Latin America, causing thousands of livestock deaths each year, as well as occasional human fatalities. Poison and even explosives have been used since the 1960s in attempts to control vampire bat populations, but those culling efforts have generally failed.

Last year, a team of university researchers and their University of Georgia colleagues reported the results of a long-term vampire bat field study in Perú. Now, the same team has combined the field findings with new computer models of rabies transmission and data from infection studies using captive vampire bats to show that culling has minimal effect on containing the virus, and can, in some cases, actually increase its spread by driving infected bats into neighboring 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. The team’s new paper, scheduled for online publication in the Proceedings of the National Academy of Sciences Monday, also establishes that rabies is usually not lethal among vampire bats.

“In the paper last year, we demonstrated that bat colony size wasn’t a predictor of rabies prevalence, which indicated that culling hadn’t reduced transmission,” said university population ecologist and epidemiologist Pejman Rohani, senior author of the paper

Developing effective control strategies for vampire bat-transmitted rabies virus in Latin America requires an understanding of the mechanisms that have allowed the highly virulent pathogen to persist despite control efforts, said the paper. But understanding the persistence has proved elusive, despite recognition of the virus and its health risks since the early 1900s, it added.

To determine those persistence mechanisms, Rohani and colleagues created four mathematical models of rabies transmission, each representing an alternative hypothesis for the biology of rabies infection.

Then they tested the models against data from the University of Georgia-led field study of rabies exposures in wild vampire bat colonies across Perú. That study tracked rabies exposures in individually marked Desmodus rotundus vampire bats from 17 colonies in four regions of Peru between 2007 and 2010 and yielded the most complete dataset on rabies exposure patterns ever collected for any bat species, according to the authors of the paper.

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.

The critical role of immigration between bat colonies predicted by their analysis indicates that current culling practices, often reactive to outbreaks in livestock or haphazardly implemented, are unlikely to eliminate vampire bat-transmitted rabies virus, the researchers said.

The bat study’s other main finding is that the vast majority of rabies virus exposures among vampire bats are nonlethal and actually immunize the bitten bat, thus helping to prevent colony extinction and sustain the virus.

The probability of a vampire bat developing a lethal infection upon exposure to rabies is around 10 percent, much lower than the 50-to-90 percent mortality rate seen in previous experimental studies that involved inoculating vampire bats with rabies virus, according to the researchers.

In Latin America, coordinated efforts to eliminate human rabies transmitted by dogs began in 1983 and led to a roughly 90 percent reduction in human and canine rabies, according to the U.S. Centers for Disease Control and Prevention. Since 2000, vampire bats have been the leading cause of human rabies there, especially in remote areas of the Amazon region in Perú, Ecuador and Brazil, according to the CDC.

Of more than 1,200 species of bats worldwide, only three are vampires, and all three live in Latin America, according to Bat Conservation International. Two of the species feed primarily on the blood of birds and one— the common vampire bat, D. rotundus — prefers mammals, especially livestock.

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