One unique feature of viruses is their ability to mutate, to evade the immune system or infect new hosts. Viruses can mutate through a few basic mechanisms: antigenic drift, antigenic shift, reassortment and recombination.
Antigenic Drift
As the name implies, antigenic drift refers to the “drifting” of antigenic proteins over time. This occurs through natural point mutations in the genome, as the virus replicates without any proof-reading. When the mutations allow a new protein to be produced, the virus may be better suited to survive in its environment. While other strains of virus may die, the mutated virus will be able to replicate and produce more virus, locking in that mutation.
Antigenic Shift and Recombination
Antigenic shift occurs when two or more strains of the same virus combine. By combining multiple strains’ virulence factors, a new strain can be produced that has increased infectivity or access to new host species. This tactic is commonly used by influenza viruses. Another type of antigenic shift is recombination, which occurs exclusively in DNA viruses. With recombination, the DNA strand is broken and reattached in a new configuration, potentially allowing for new virulence factors to be expressed.
Reassortment
Reassortment only occurs in RNA viruses, due to the segmented nature of their genome. In reassortment, segments of the genome are moved around to produce a new RNA strand that may produce new virulence factors.
Zachary JF. Pathologic Basis of Veterinary Disease, Sixth Edition.
