|
Evolution and the avian flu
November/December 2005, updated August 2008
|
The warnings are dire. The economic cost for developed countries alone is estimated at 550 billion dollars, and the projected worldwide death toll ranges between 2 million and 150 million people. The very real specter behind these warnings is, of course, avian flu. As the virus spreads through bird populations, governments have heeded the warnings of health officials and begun to cull infected flocks. More than 150 million birds have been killed so far, with further control efforts looming. However, less than 200 human cases of avian flu have been identified thus far. Why the global concern over localized outbreaks? Currently, the virus can only spread bird-to-bird and, in rare instances, bird-to-human — but biologists warn that the virus could easily "change" to pass from human to human, sparking a deadly, global pandemic.
|
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. |
 |
|
News update, August 2008
Since we published this report in November/December 2005, avian flu has faded from newspapers, though the pandemic itself still threatens. The number of human cases of bird flu has dropped somewhat in the past two years, but the virus has spread among the poultry of countries like Indonesia, Bangladesh, Vietnam, and Egypt — which has dashed any hope we might have of eradicating the disease entirely. The H5N1 virus is here to stay.
Once the dangers of pandemic avian flu were recognized, medical researchers focused their attention on vaccines that could protect us from the infection. Of particular value would be a vaccine that could be produced in mass quantities ahead of time and stored in preparation for an outbreak. But bird flu, like many other viruses, presents a challenge in this regard. The problem is one of genetic variation. For the reasons described above — the virus's high mutation rate and ability to trade DNA with other viruses — a huge number of genetically distinct variants of avian flu circulate at any given time. And this can thwart vaccine developers. To work most effectively, a vaccine must be closely matched to the viral strain it is meant to provide protection from. So which of the avian flu strains currently in circulation should be the basis for the vaccine? There's no surefire way to know which strain will be the first to evolve the ability to jump from human to human — at least not until after the pandemic actually starts. And after that happens, it will likely take several months to produce and begin to distribute the vaccine. That's the bad news.
The good news involves vaccine adjuvants — substances that can be added to a vaccine to make it more effective. Adjuvants can stretch our vaccine supply, since each person can be protected with a smaller amount of vaccine. And just as importantly, new research suggests that adjuvants may improve the effectiveness of imperfectly matched vaccines against a target virus. This makes stockpiling vaccine ahead of time more practical. Plans are in the works to produce and store 100 million doses of vaccine matched to earlier H5N1 strains — with the hope that this vaccine, perhaps along with an adjuvant, will offer some protection against any avian flu strain that does evolve into a global threat.
|
|
|
Read more about it
News and journal articles:
Understanding Evolution resources:
For teachers
Discussion and extension questions:
- What role does evolution play in a potential avian flu pandemic?
- Why are health workers more concerned about a bird flu epidemic than illness caused by a normal human flu virus?
- How could the avian flu change from a bird-to-bird strain to a human-to-human strain?
- Why do viruses evolve quickly?
- How are viral evolution and human evolution different? How are they the same?
- Compare and contrast the role of natural selection in producing antibiotic resistant bacteria (see Battling bacterial evolution: The work of Carl Bergstrom) and in producing a pandemic-causing H5N1 viral strain.
Related lessons and teaching resources:
- Teach the basics of natural selection. In this classroom activity for grades 9-12, students learn about variation, reproductive isolation, natural selection, and adaptation through this version of the bird beak activity.
- Teach about another application of evolutionary theory in medicine. In this classroom activity for grades 9-12, students learn why evolution is at the heart of a world health threat by investigating the increasing problem of antibiotic resistance in menacing diseases such as tuberculosis.
- Teach about viruses and evolution. In this classroom activity for grades 9-12, students learn about natural selection in rabbits by observing the effects of a virus on the Australian rabbit population.
References
- Brahmbhatt, M. (2005, September 23). Avian influenza: Economic and social impacts. Washington DC: The World Bank Group.
Retrieved November 7, 2005 from The World Bank Group.
- Butler, D. (2008, July 9). Whatever happened to bird flu? Nature. doi:10.1038/news.2008.945
Retrieved July 18, 2008 from Nature.
- Centers for Disease Control and Prevention. (2005). Avian influenza (bird flu).
Retrieved November 7, 2005 from CDC.
- Stephenson, I, Bugarini, R., Nicholson, K. G., Podda, A., Wood, J. M., Zambon, M. C., and Katz, J. M. (2005). Cross-reactivity to highly pathogenic avian influenza H5N1 viruses after vaccination with nonadjuvanted and MF59-adjuvanted influenza A/Duck/Singapore/ 97 (H5N3) vaccine: A potential priming strategy. Journal of Infectious Diseases. 191:1210-1215.
- United Nations. (2005, September 29). WHO: Mutated bird flu could kill up to 150 million people.
Retrieved November 7, 2005 from USA Today.
- World Health Organization. (2005). Avian influenza.
Retrieved November 7, 2005 from WHO.
|
