Where's the evolution?
We're not even sure if viruses are alive — can they evolve? Definitely! To evolve by natural selection, all an entity needs is genetic variation, inheritance, selection, and time, all of which viruses have in spades. And this is the concern. The avian flu virus evolves rapidly and could easily evolve into a form that can be passed from human to human.
The current outbreak involves a flu strain called H5N1, which we already know from occasional bird-to-human transmissions can be deadly to humans. H5 and N1 represent forms of viral proteins that our bodies use to recognize and attack the virus. Some flu strains, such as H1N1, are relatively common in humans; many people's immune systems can recognize and attack these strains. This reduces the number of human carriers and thus, the risk that this strain will cause a serious pandemic. Unfortunately, people's immune systems do not yet have any ability to recognize the H5N1 strain, leaving us extremely vulnerable to it. Luckily, H5N1 is not adapted to human hosts and does not have the genes that would allow it to be passed easily person to person. But evolution may change that.
Viruses evolve quickly, in part because they acquire genetic variation in multiple ways. Sometimes viruses acquire genetic variants through random mutation, much as human populations do. However, viruses have a much higher mutation rate than humans and produce a high number of genetic variants as they reproduce. The more genetic variants, the higher the odds that one of them carries a useful mutation that selection can act upon. This increases the rate at which viruses evolve. Through random mutation and subsequent selection, an H5N1 virus could slowly evolve into a form better adapted to human-to-human transmission.
The most worrisome possibility, however, is that an H5N1 virus could acquire genes for human-to-human transmission directly from a human flu strain. Unlike humans, many viruses can easily incorporate ready-made genes from other viruses into their genomes. This is a possibility anytime a host is infected with two different viral strains. A human infected with a typical, non-lethal human flu virus and H5N1 avian flu could serve as a mixing vessel for the two viruses, resulting in a flu strain with the deadly properties and unrecognized proteins of H5N1 but with human transmissibility genes.
Each case in which a human is infected by H5N1 from a bird is another opportunity for the virus to adapt to human hosts via random mutation or by acquiring genes directly from other viruses. This explains why governments participate in programs to cull infected birds: the fewer infected birds, the fewer infected humans, and the fewer chances for the evolution of a pandemic-causing H5N1 strain.
A global flu pandemic is a very real possibility. However, the situation is not hopeless. Policy makers, health organizations, and scientists are working together to find ways to forestall an epidemic and lessen the impact, should one occur. For example, scientists have used computers to model the evolution of flu viruses and have found that preventatively administering antiviral drugs near the beginning of pandemic could slow the evolution of the virus into a fully transmissible form and buy us more time to develop and produce vaccines. |
