Contributed by Guest Blogger: D. Patel ’14

Deadly human diseases including HIV Aids, swine flu and rabies are infectious diseases where the viruses have jumped from one animal species into another and now infect humans too. This is a phenomenon known as cross-species transmission (CST). Understanding this process is the key to predicting and preventing future outbreaks.
The scientists who researched CST and wrote this paper made a groundbreaking discovery into how viruses jump from host to host. They used and thought of rabies as an ideal system because it occurs across the country, affects many different host species, and is known to mutate frequently. Although cases of rabies in humans are rare in the U.S., bats are a common source of infection. Hence, the study was based on and narrowed down to CST events among different bat species.
To determine the rate of CST, a large dataset containing hundreds of rabies viruses from 23 North American bat species was used. Population genetics tools were used to quantify how many CST events were expected to occur from any infected individual and the cases were verified by genotyping both the viruses and the bats.
The study showed that depending on the species involved, a single infected bat may infect between 0 and 1.9 members of a different species; and that, on average, CST occurs only once for every 72.8 transmissions within the same species. This means that the majority of viruses from cross-species infections were tightly nested among genetically similar bat species.
It is a long-held belief that CST depends on virus mutation and contact of the host with other species. However, this study showed that CST may have more to do with host similarity. The 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.
Whether other factors (like evolution of viruses) are enough to overcome the genetic differences between hosts remains questionable. However, the basic knowledge gained through the study is key to developing new intervention strategies for diseases that can jump from wildlife to humans.