A new study has detailed how initial demographic, ecological and genetic processes simultaneously shaped the spread of rabies virus in North American raccoons.
A raccoon is a mammal native to the Americas.
Although raccoons are common throughout North America, their impact as a rabies host before the 1970s was limited to the southeastern United States, particularly Florida. However, this situation changed dramatically in 1977 when a raccoon-specific rabies virus variant (RRV) was detected in West Virginia. The RRV later spread quickly along the mid-Atlantic coast, and by 1999 infiltrated thousands of square kilometers.
The study was conducted by a team of researchers including Leslie Real at at Emory University.
As part of the study, researchers analysed 30 years of data of the particular outbreak, which began in the mid-1970s. The Centres for Disease Control and Prevention (CDC) had been stockpiling viral samples from the outbreak since 1982, giving scientists a treasure trove of genetic data ripe for analysis.
"Together these data offer a rare chance to examine how the demographic and spatial processes of spread and population expansion over 30 years have shaped viral evolution on a geographic scale. Landscape features, such as rivers and mountains, can have a pronounced effect on the rate of rabies' spread and may therefore affect viral dynamics on a large scale," Real said.
Researchers found that that because mountain ranges make for a poor raccoon habitat especially at higher elevations, raccoons did not cross the Appalachian mountain chain during the first part of the outbreak, which clearly limited the virus's westward expansion. The Allegheny Mountains appear to have slowed the virus's expansion to the north.
The study area ranged from North Carolina to Vermont, as far east as Chesapeake Bay, and westward into Tennessee and Ohio.
"These results provide important insights into the geographic scale of rabies persistence and will be increasingly important in understanding the epidemiology of rabies and other emerging zoonotic diseases, those diseases that can be transmitted between animals and people, in a geographic context. We can then use these insights to predict where and when zoonotic disease outbreaks will occur so we can target surveillance and intervention," Real said.
"Our study demonstrates the combined evolutionary and population dynamic processes characterizing the spread of a pathogen after its introduction into a susceptible host population," Real said.
During invasion, emerging pathogens, such as rabies, ebola and hantavirus, undergo rapid evolution while expanding their numbers and geographic range. Yet, it is difficult to demonstrate how these processes interact.
The findings of the study were published online in the Proceedings of The National Academy of Sciences.