IMAGE: FURRYSCALYMAN VIA FLICKR
In at least one bat species, hibernation keeps the rabies virus at bay, according to a new mathematical model published Monday (June 6) in PNAS. Their annual winter slumber allows the big brown bats (Eptesicus fuscus) to survive till the warmer months, when the sick animals can infect a new crop of baby bats.
“What’s really exciting is that it ties the seasonal behaviors of these animals to how they interact with the disease and how their populations persist in the presence of the disease,” said Paul Cryan, a research biologist with the U.S. Geological Survey who was not involved with the study.
Scientists had previously modeled how rabies infects animals that don’t hibernate, such as skunks or raccoons. But those models couldn’t capture the summer spike in rabies deaths that occurs after bats come out of hibernation, which helps big brown bats survive colder months when their insect food source dries up.
To understand how winter dormancy affected transmission, Colorado State University biological modeler Dylan George, and his colleagues modified existing rabies transmission models to account for hibernation.
To calibrate their model, they used lab data on rabies disease progression, as well five years of overall hibernation and mortality data gleaned from the comings and goings of about 4,000 tagged bats to roughly 160 maternity roosts in Fort Collins, Colorado. They checked the prediction of their model against bat rabies cases reported to the Centers for Disease Control.
In the model, when bats went into cold storage, the virus did as well. That’s because a hibernating bat’s body temperature drops to match the outside temperature, causing viral replication to halt. “Hibernation for the virus is almost like putting it in freeze-frame or putting it into Carbonite,” said George, who is now with the US Department of Defense.
As a result, even though the rabies virus normally kills bats in about six days in the lab, infected bats could stay alive through the long, cold winter. That enabled infected bats to wake up from hibernation and pass along the disease to young, immunologically naïve bats in the spring and summer.
Hibernation also seems to stabilize the survival of both rabies and bats within a colony. When George and his coauthors removed hibernation from the model, bats—gregarious, social creatures—passed on the disease and died off so quickly that populations were rapidly decimated and they went extinct.
The study also showed the power of using modeling to enhance field work on wildlife, said James Wood, a veterinary epidemiologist at Cambridge University who was not involved with the study.
The model could be extended to study other bat populations, and may also have implications for the spread of human diseases that also live in bats, including SARS and Ebola, George said.