Every year, in mid-September, big brown bats throughout Colorado head for their favourite roosts, where they will spent the winter in hibernation. But some of the bats won’t sleep alone – they are carrying the rabies virus, and it will also hibernate through the winter in its slumbering host.
The rabies virus is a killer. Infections are almost always fatal, and around 55,000 people around the world succumb to the virus every year. Dogs are the leading carriers, but in North America, vaccination programmes have effectively eliminated dog rabies. Bats are another story – they are far more difficult to vaccinate and they have overtaken man’s best friend as the leading cause of American rabies.
Now, Dylan B. George from Colorado State University has shown that the rabies virus, by hibernating alongside the big brown bats, gets a free pass to the next generation.
In bats, the rabies virus isn’t always lethal; some of them develop antibodies that neutralise the virus and render them immune. Come the winter, these survivors take off for hibernation roosts. Their metabolism slows to a crawl and their body temperatures drop. These cooler temperatures also slow the development of the rabies virus so it ends up hibernating along with its host.
In early spring, the bats slowly rouse from their winter naps. For a while, the females are prone to shutting their bodies down again, but in the mean time, they form tight-knit colonies. In such close groups, viruses like rabies are easily transmitted from one bat to another. When the bats wake properly, they fly off to form maternity colonies. Again, they cluster in close quarters and they give birth to their pups. That gives rabies a chance to infect an entirely new generation of hosts, whose immune defences haven’t been trained against the virus yet.
George modelled all of this using data from a five-year project that tracked around 15,000 big brown bats in roosts around the city of Fort Collins. He used this big bat census to create a mathematical model, which simulated the rise and fall of the rabies virus in these hosts.
The model explained why rabies shows up in bats with a distinctive seasonal trend, peaking between spring and autumn, when the cycle of infection begins again. The model also showed why hibernation is so important for both the bats and the virus. When George took it out of his simulation, he found that the rabies virus quickly cut through the bat populations, causing them to crash. By hibernating, the bats manage to save themselves until they can raise a new generation – something that also benefits the virus.
These results have implications for understanding and controlling rabies, but they have a broader importance. Bats are natural reservoirs for many deadly viruses including rabies, Ebola, henipaviruses, and coronaviruses such as SARS. George thinks his model can be easily adapted to predict how new emerging diseases will behave in bat hosts.
Reference: George, Webb, Farnsworth, O’Shea, Bowen, Smith, Stanley, Ellison & Rupprecht. 2011. Host and viral ecology determine bat rabies seasonality and maintenance. http://dx.doi.org/10.1073/pnas.1010875108