Professor Offers Insight Into How Viruses Jump From Hosts

HIV-AIDS, SARS, Ebola, avian influenza, swine flu, and rabies -- these are emerging infectious diseases where the viruses have jumped from one animal species into another and now infect humans. This is a phenomenon known as cross-species transmission (CST) and scientists are working to determine what drives it.

Gary McCracken, a professor at the University of Tennessee, Knoxville, and department head of ecology and evolutionary biology, is one of those scientists and has made a groundbreaking discovery into how viruses jump from host to host. His article, "Host Phylogeny Constrains Cross-Species Emergence and Establishments of Rabies Virus in Bats," will appear in the Aug. 6 edition of Science and will be featured on the issue's cover.

It has been a long-held belief that rapid mutation is the main factor that allows viruses to overcome host-specific barriers in cellular, molecular or immunological defenses. Therefore, it has been argued that viruses emerge primarily between species with high contact rates.

McCracken and his colleagues now report that CST may have less to do with virus mutation and contact rates and more to do with host similarity.

"That innate similarity in the defenses of closely related species may favor virus exchange by making it easier for natural selection to favor a virus' ability to infect new hosts," McCracken explained.

McCracken performed his research with former UT Knoxville PhD student Amy Turmelle who now works with the Centers for Disease Control (CDC) and Maarten J. Vonhof, a former post-doctoral scholar at UT Knoxville, who is now with Western Michigan University. Other colleagues include CDC Rabies Team Members Ivan Kuzmin, Charles Rupprecht and Daniel Streicker, who is also with the University of Georgia.

The team made their discovery by analyzing hundreds of rabies viruses in 23 species of bats. In the United States, there are at least 45 different species of bats and many different strains of rabies. Not coincidentally, the CDC collects rabid bats after humans or their pets or livestock may have been exposed to the virus -- adding nearly 2,000 bats annually to its database. McCracken and his colleagues used this database to document the cases in which a rabies virus jumped from one species of bat to another. They verified the cases by genotyping both the viruses and the bats.

The researchers documented over 200 examples of CSTs and analyzed the best explanations for CSTs, such as geographic range, behavior, ecology and genetic relatedness. The study found that the majority of viruses from cross-species infections were tightly nested among genetically similar bat species.

"It turns out, the most important factor in cross-species transmission is how closely related the bat species are," McCracken said. "Our study demonstrates that rapid evolution can be insufficient to overcome phylogenetic barriers at two crucial stages of viral emergence: initial infection and sustained transmission."

This discovery may have significant implications for public health authorities as they try to track where the next infectious disease will emerge. The team's research provides a model for how such diseases transfer from host to host.

"Although CST events are the source of infectious diseases that kill millions of people each year, the natural reservoirs of viruses in wild animals and how they cross species barriers are poorly known and difficult to observe. In this study, rabies in bats serves as a model to understand events that are critical to public health concerns worldwide," McCracken said.