Cross-Species Transmission Plays Key Role in Viral Evolution

Viruses cross between host species more frequently than previously thought, and this seems to play a key role in virus evolution, a new study suggests. Although the researchers found cross-species transmission to be common among all the viruses they studied, it occurred most frequently among the DNA viruses, including hepadnaviruses, polyomaviruses, poxviruses, papillomaviruses, and adenoviruses.

Jemma L. Geoghegan, PhD, from the University of Sydney, Australia, and colleagues published the results of their study online in PLOS Pathogens.

According to senior author, Edward C. Holmes, PhD, FAA, there has been much recent interest among researchers as to whether it is possible to predict aspects of disease emergence: “Can we second guess what types of virus might emerge, from what animal hosts, and in what parts of the world, to cause a novel human disease?”

As a consequence, Professor Holmes and his team decided to examine one aspect of this—namely, whether some virus families are better able to jump across species boundaries and emerge in new hosts than others.

The researchers analyzed gene sequence data for viruses from 19 families that infect a range of hosts such as mammals, birds, fish, amphibians, reptiles, insects, and plants. They then compared the evolutionary histories of these viruses and their host species.

Unexpectedly, the researchers found that all of the virus families studied were able to jump species boundaries on a regular basis. “The frequency of host jumping was generally so frequent among all the virus families that it was really impossible to come to clear prediction of what viruses pose the greatest risk and are most likely to jump species boundaries and emerge in new hosts,” said Professor Holmes in an interview with Contagion^®.

Nevertheless, he emphasized an important general finding in their study: RNA viruses (viruses with RNA genomes, including Zika, Ebola, and influenza viruses) were, on average, better able to jump species boundaries than those with DNA genomes (such as herpes and pox viruses) were. “This does make sense as most of the virus epidemics we know about are caused by RNA viruses,” explained Professor Holmes.

Overall, the high frequency of cross-species virus transmission found in this study makes it difficult for scientists to predict what virus might emerge next, he stressed.

Because their data suggest that sampling more viruses increases the odds of detecting host-jumping events, Professor Holmes added that their next main goal is to survey as much of the viral universe—the virosphere—as possible. “As we sample more, I am confident that we will find many more cases of host jumping,” he said.

“It is clear that we are only just scratching the surface of the known viruses that are present in nature. My guess is that we have sampled much less than 1%,” Professor Holmes concluded. “Clearly, to get a better understanding of virus biodiversity and evolution we need to sample more. This may also reveal viruses that could emerge in humans.”

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.