As bats hibernate through the winter, so does rabies

By Ed Yong | June 7, 2011 9:00 am

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.


Comments (7)

  1. The CDC website seems to imply that raccoons are the most frequently reported species that shows up positive for rabies, followed by bats. Bat bites can sometimes go unnoticed, and sometimes the source of rabies is not known. I know that visitors to foreign countries are often reported to have come into contact with rabid dogs and did not get treatment (and then return to the United States, infected). I’m just curious – where is the information that bats are the leading cause of rabies, and what exactly does that mean? Not trying to be a jerk, I’m just wondering where to find that information.

    I’m vaccinated for rabies because I used to work with wildlife. It’s such a feared disease, and I know the vaccine is rare and cost-prohibitive, but you’d think we’d just vaccinate ourselves and be done with it. Cats and dogs are abusively vaccinated every single year (I was vaccinated 12 years ago – once, and my titer is still sky-high) even elderly, indoor-only cats over the age of 20. Quite frankly, it disgusts me.

    I also wonder if that one young woman, who survived rabies and worked in a church that was known to have many bats, gained some type of immunity through the aerosolized bat guano. I know that sounds odd, but she’s the only known unvaccinated survivor of the disease and the anesthetic-antiviral combination has apparently not worked on anyone since.

  2. This CDC page gives numbers:

    I would imagine it’s that in the USA, people always seek medical treatment after being bitten by a racoon. So while “leading cause of rabies” is slightly ambiguous, more humans die of bat acquired rabies, regardless of how likely an individual animal is to be rabid.

    I bet if rabies vaccination was routine, you’d have at least a few dozen nasty allergic reactions per year that might be blamed on the vaccine… and you’d potentially have people get bitten by an animal and not seek prophylactic medical care, due to making assumptions about their vaccination status (or vaccination uptake). So you’d have to have the vaccine be terribly safe, cheap and effective to reduce the 1-2 deaths per year figure. Better tetanus vaccine coverage would save more lives…

  3. @Emmy – The paper says, “However, BRV variants are currently the leading
    source of rabies in humans in the United States.” BRV = bat rabies virus.

  4. I have am going to be interviewing a bat expert, Rob Mies, from the Organization for Bat Conservation for my podcast on Thursday – and I was going to ask him about rabies in bats. I didn’t realize that it’s the leading cause of deaths by rabies in the U.S.

    As much as I want to get the message out about bat conservation (get people to install bat houses, for example), I’m fearful this is something that could make people shy away from doing it.

  5. An interesting story though, about rabies hibernating. Another great post, Ed!

  6. Matt Gruner

    How well is rabies transmitted in bat populations affected by the white-nose syndrome (caused by a fungus) currently decimating the brown bat population. My understanding is that hibernation is disrupted in the bats resulting in a precipitous depletion in energy reserves and death before the end of summer. How would widespread disruption of hibernation affect the development of rabies within a bat population.

  7. Merlin D. Tuttle

    Unfortunately, your recent article on bats, rabies and other diseases is highly misleading and will contribute to the further demise of bats at a time when whole populations are being decimated by
    White Nose Syndrome. Bats are the primary source of the rarest public health-reported disease in America: rabies. Since record keeping began, rabies from bats has averaged less than two human cases per year in the U.S. And most of what has been written about bats and other dread diseases, such as SARS and Ebola, is little more than hypothetical speculation. For people who simply avoid handling sick bats they find, the risk of contracting a disease from one is miniscule relative to the risks we take for granted on a daily basis. For example, dog attacks kill more than 10 times as many Americans annually as die from anything associated with bats, not to mention that 10,000 of us die annually from spouse attacks. When 1.5 million free-tailed bats began moving into newly created crevices beneath the Congress Avenue Bridge in Austin, Texas in the early 1980’s, public health officials warned they were rabid and would attack humans. When people panicked, the bats were nearly eradicated. Fortunately, through Bat Conservation International, we helped citizens overcome their fears and learn to live safely with the bats, and these bats are now the city’s most popular tourist attraction. They consume 15 tons of insects nightly, bring in 12 million tourist dollars each summer, and in more than 30 years, we’re still waiting for the first person to be attacked or harmed. A recent publication in Science estimated an annual value of U.S. bats to agriculture alone at nearly 30 billion dollars. It’s time to place benefits versus risks in perspective!


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