Rabid Vampire Bats
Vampire bats are bats whose food source is blood, a dietary trait called hematophagy. The very concept because of how humans perceive blood sucking varmints creates an image of horror. Only 0.5% of bats carry rabies. However, of the few cases of rabies reported in the United States every year, most are caused by bat bites. However, no vampire bat species are known to live in the United States. The highest occurrence of rabies in vampire bats occurs in the large populations found in South America. A new study of rabies in vampire bats in Peru has found that culling bats—a common rabies control strategy—does not reduce rates of rabies exposure in bat colonies, and may even be counterproductive. The findings may eventually help public health and agriculture officials in Peru develop more effective methods for preventing rabies infections in humans and livestock, according to a team of scientists from the United States and Peru led by Daniel Streicker, a postdoctoral associate at the University of Georgia Odum School of Ecology.
Rabies is a serious problem in South America. Human deaths are increasingly recognized in the Amazon rainforest, and cattle die from the disease each year by the thousands. While rabies is present in most bat populations in South America, vampire bats—the only bat species that feed on mammals' blood—are responsible for the majority of human and livestock infections. The encroachment of cattle farming and human settlements into areas with existing vampire bat populations has only exacerbated the problem.
Rabies is a viral disease that causes acute encephalitis (inflammation of the brain) in warm-blooded animals. It is zoonotic (i.e., transmissible from animals to humans), most commonly by a bite from an infected animal. For a human, rabies is almost invariably fatal if post-exposure prophylaxis is not administered prior to the onset of severe symptoms. The rabies virus infects the central nervous system, ultimately causing disease in the brain and death.
Since the 1970s, efforts to control the spread of rabies in Peru have focused on culling vampire bats using poison or even explosives, on the assumption that if the size of the bat colony could be reduced enough, rabies virus would die out in that colony. Streicker, Rohani and colleagues set out to learn whether this was true. They also wanted to learn how the impacts of human activities, such as agriculture and culling practices, affect the rabies/vampire bat system.
The researchers set up 20 sampling sites across Peru, where they took repeated blood samples from vampire bats over a 40-month period. They also collected data on the size of the bat colonies and how often bats were culled for each site.
What they learned surprised them. The virus was present in every colony, regardless of its size, nearly every time they sampled.
"We found that rabies is there no matter what, and the size of the bat colony didn't predict the proportion of bats that were exposed to the virus," Streicker said. "That's important because if there's no relationship between bat population density and rabies, then reducing the bat population won't reduce rabies transmission within bats."
When they looked at the effect of culling vampire bats on rabies prevalence, they had another surprise.
"We detected something that's a little bit worrying," Streicker said. "In areas that were sporadically culled during the course of the study, we saw an increase in the proportion of bats exposed to rabies.
"Colonies that were culled regularly had slightly lower rabies exposure rates, and those that were never culled had the lowest rates of all. The next thing we have to do is understand the mechanisms for why this happens."
The researchers outlined several theories to explain higher rabies exposure in the sporadically culled bat colonies. Most culling is done by spreading an anticoagulant paste on captured bats that are then released. When its roost-mates groom the treated bat, they ingest the paste and die.
"There's some experimental evidence that bats that are exposed repeatedly to rabies may develop a degree of immunity," Streicker said. "When you kill off the adult bats that may be immune, you're making space for susceptible juvenile bats (which the team found to have higher rates of rabies exposure than adults).
"There's also something called the vacuum effect—when the adults are removed, individuals from neighboring roosts might move in to the colony to fill the vacated space."
This kind of disturbance-induced dispersal has been shown to enhance the transmission of other wildlife diseases, such as bovine tuberculosis in badgers, and the same could be true for vampire bats.
Streicker is hopeful that the insights gained from the team's research, which will continue for two more years, will help Peru's public health and agriculture officials to implement science-based strategies for controlling rabies.
Rohani added: "The emergence of several highly virulent zoonotic diseases in humans, including SARS, Nipah encephalitis, and Ebola and Marburg hemorrhagic fevers, has revealed bats as an important source of infection for humans and domesticated animals. Moreover, these emergence events have unveiled fundamental gaps in our basic understanding of virus transmission and wild bat populations."
For further information see Rabies.
Vampire Bat image via Wikipedia.