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Research / Discovery

Bats provide clues to rabies outbreaks

January 20, 2009

The bat has long been a symbol of the night in Western culture, as well as the primary animal player associated with fictional characters of the night like vampires and superheroes. But it wasn't until the 1950s that bats became associated with something more real and a little bit scarier than vampires - rabies.

Rabies continues to take lives

Though rabies is readily prevented with a vaccination protocol, the disease continues to takes tens of thousands of lives worldwide, and new outbreaks in the United States are resulting in calls for increased research into the natural reservoirs of rabies, including bats.

Researchers at the College of Veterinary Medicine and Biomedical Sciences, with funding from the National Institutes of Health and the National Science Foundation, are working to understand the prevalence of rabies in bat populations, and how human-bat interactions impact the incidence of rabies.

Urban ecosystems

CSU researchers are working with the United States Geological Survey and the Centers for Disease Control and Prevention to understand rabies transmission in urban ecosystems.

“We don’t see big outbreaks of rabies in bat populations with big die-offs,” said Dr. Richard Bowen, a professor in the Department of Biomedical Sciences and lead investigator on the four-year grant. “But we know that bats are a natural reservoir for a large number of zoonotic pathogens, including rabies. Our studies try to quantify the level of rabies exposure in bats and determine the impact of that exposure in the local bat population.”

Students key in tracking bats, collecting samples 

Bats are natural reservoirs in part because of their high mobility, broad distribution, and social behavior. Many species also appear to have a high tolerance for harboring pathogens without developing disease. Dr. Bowen’s research team, which involved many students in the Professional Veterinary Medical Program, would go to out at night to catch bats in urban areas in and around Fort Collins, bring them back to the lab to collect blood samples and oral swabs, and implant PIT tags that would relay movement as the bats flew under reader placed above the entrance of their roosts.

“During the study, we collected tons of information on bat movement and biology,” said Dr. Bowen, of the approximately 2,500 bats tracked in Fort Collins. “As far as rabies was concerned, we thought of it as invariably fatal, but found that in some roosts, up to 25 percent of bats had antibodies to rabies virus, indicating that they had been exposed, but had not developed rabies disease. We did not, however, find evidence that bats can carry rabies for a long time as there is a sustained die-off, but not in what we see with an outbreak. It seems to be an endemic disease managed in the population.

Fort Collins bats migrate up Poudre Canyon

”One interesting result of the work was the migration of bats out of Fort Collins each year in the early fall. No one knew precisely where they went, but radiotracking studies revealed that many of the bats migrate up Poudre Canyon each year to hibernate. One bat was located roughly 70 km from Fort Collins.

Dr. Bowen also is involved with a separate study looking at raccoon rabies which, once confined to the southeast United States, is now a significant problem in the Northeast and spreading to the Midwest.

Working with the National Wildlife Research Center based in Fort Collins, the research team is testing vaccine-laden baits to determine how long immunity lasts in wild raccoon populations once the bait is ingested. The work is showing some protection at up to 18 months after the vaccine is administered.

Expanding epidemic in raccoons

“Raccoon rabies was pretty rare in the Northeast, but there has been an expanding epidemic since the mid-1970s,” said Dr. Bowen. “The virus variant was believed to have been brought north by hunters transplanting raccoons to help replenish the declining northern population.

Unfortunately, some of the transplanted raccoons were apparently infected with rabies virus. Rabies is commonly diagnosed in raccoons from densely populated regions (including New York, Pennsylvania, and Virginia), so the interaction between humans and raccoons is of great concern.”

Assume every bat is infected with rabies

Of course, one advantage of being bit by a rabid raccoon is that you know you’ve been bit and can seek treatment (if you can consider that an advantage). Almost all rabies deaths (22 recorded deaths from 1980-1997) in the United States are attributed to bat bites and, in a number of cases, the victim did not know they had been bitten by an infected bat.

Because rabies is ubiquitous in bat populations, Dr. Bowen said, individuals need to assume that every bat is positive and react appropriately (avoiding contact, safe capture, reporting to health authorities, etc.)

Human deaths from rabies continue in Asian and Africa

Thanks to Louis Pasteur and Emile Roux, who developed the first vaccine for rabies in 1885, most Americans and their companion animals remain relatively free from the threat of rabies. But this is not the case in other parts of the world. Almost all 55,000 annual human deaths caused by rabies are in Asia and Africa, with the largest number of cases reported in India.

Dr. Bowen, who also is involved with bat studies in Mexico, said it continues to be an underappreciated but important health threat, with fundamental studies still needed on how rabies affects the brain, as well as continued studies on bat populations and their ability to harbor infectious diseases including not only rabies, but severe acute respiratory syndrome (SARS) virus, as well as the ebola and Nipah viruses.

Bats harbor scary pathogens

“Bats are becoming a hot topic because they harbor some pretty scary pathogens,” said Dr. Bowen. “People are wondering what’s going on with bats. But it’s easy to see that with more than 600 species of bats, adaptation to a diversity of ecosystems, including urban ecosystems, we’re going to pick up some of their pathogens. Bats play an important and vital role in the environment as insectivores and pollinators, but we need to better understand their role as reservoirs of infection so that we can better protect ourselves from the diseases that are a part of their world and ours.”

Originally published in the College of Veterinary Medicine and Biomedical Sciences Insight newsletter, Fall 2008.