Evolution may favor flu mutations that increase mammalian transmissibility
A team of scientists led by Thomas Friedrich from the University of Wisconsin-Madison School of Veterinary Medicine used deep sequencing to identify low-frequency mutations occurring when the avian flu virus grows in and transmits between mammals. The team found that natural selection acts strongly on hemagglutinin, the structure the flu virus uses to attach to and infect host cells.
While all flu viruses originally came from birds, the virus evolved over time to adapt to other animals like humans. Natural selection favored viruses that contained mutations allowing them to more readily infect new host cells. Every time a virus infects a cell, a mutation occurs somewhere on the viral genome.
"You might think they all have the same sequence, but they don't," Friedrich said. "We found that this diversity increases over time in essentially all infected individuals we examined."
Using the data from a transmission study in ferrets, the researchers found that mutations present in approximately six percent of the viruses infecting one ferret could be transmitted to another ferret. The findings suggest that even rare mutations can be transmitted if they have an evolutionary advantage.
Friedrich said that avian flu viruses capable of infecting humans likely already exist in nature at very low frequencies. He said it was possible that there are viruses out there only a mutation away that have yet to encounter a susceptible host.