The flu has been dangerously prominent
in the Western hemisphere in the recent winter months, and the notion that the
situation could grow worse troubles scientists and other people around the
world. There are several types of flu viruses, ranging from H1N1 to H15N9,
depending on the various surface proteins. Avian flu begins in birds and infects
others of the same species. Every now and then, a viral strain evolves to where
it is able to attach to cells of a different species. H5N1 (hemagglutinin and neuraminidase), the most
widespread and deadliest known avian flu virus, is one that has spread to
humans but is unable to spread amongst the human species. Even though it has been roaming in mostly
Egypt and Asian nations (Figure 1), H5N1’s evolutionary ability brings the possibility of
a global H5N1 pandemic.
How bad is H5N1? How will the
infamous virus evolve in the future? H5N1’s paradoxical traits have kept questions
like these in constant debate. Although
widespread in about 63 countries, among birds and many other animal populations,
the viral disease is difficult to detect. In spite of being fatal in at least
half of the people infected, about 364 out of 615 people since 2003, the virus has
also been of no consequence to thousands of people in the virus’s presence.
Despite continuous evolution since the late 1990’s, H5N1 is not yet capable
of airborne transmission… Concerns about the future evolution of H5N1 led two scientific teams to conduct research on how H5N1 would behave in ferrets and the effects of the strain’s mutations. The research led by Dr. Ron A. M. Fouchier and his colleagues in the Erasmus Medical Center was carried out by introducing three mutations and transferring the virus through ten generations of ferrets by means of nasal infection. “The genetically modified A/H5N1 virus acquired mutations during passage in ferrets, ultimately becoming airborne transmissible in ferrets. None of the recipient ferrets died after airborne infection with the mutant A/H5N1 viruses” (Herfst et al., 2012). On the other hand, Dr. Yoshihiro Kawaoka’s experiment in the University of Wisconsin-Madison utilized the H5N1 spike gene and inserted it into H1N1 from the 2009 swine flu. A viral strain with four mutations infected but did not kill the ferrets that breathed it in. The results from both research groups indicated that H5N1 lost its lethality as it became more contagious but shockingly proved how mutations could evolve and render H5N1 airborne transmissible.
Obviously, natural selection plays a large and significant role in “molding” H5N1’s functional characteristics. How the virus will mutate to form a fair tradeoff between replication speed and infection rate (Figure 2 – more deadly viruses are usually located farther down the respiratory tract and thus spread to new hosts more slowly) is something we can only predict and hope for the best. Still, with our prior flu experiences and our own evolutionary capabilities, we can stand our guard and prepare for the fight against the evolution of H5N1, no matter how it turns out.
Figure 1. Map of Asia, Europe,
Africa, and Australia from the World Health Organization reports the areas
where H5N1 have been found.
Figure 2. Two images of human
respiratory system compare the effects of H1N1 to those of H5N1. Source:
wpclipart.com
Works Cited:
S. Herfst et al., “Transmission of influenza A/H5N1
virus via aerosol or respiratory droplets between ferrets”.
Science. 336, 1534-1541 (22 June 2012). Web. 18 February 2013.
San-Pei Lee
Word Count: 552
Word Count: 552
Works Cited:
Yong, Ed. “Influenza:
Five Questions on H1N1”. Nature. 486,
456–458 (28 June 2012). Web. 19 February 2013.
Zimmer, Carl. “The Future Evolution of Bird Flu”. Phenomena: The Loom, 7 February 2013. Web.
17 February 2013.
When you say "the virus has also been of no consequence to thousands of people in the virus’s presence," do you mean that many people are exposed to the virus (it is in their blood stream) but don't react or get sick from it, or are you referring to the fact that many of the people haven't gotten sick from it simply because it is not spread through the air? I'm assuming that the virus is spread through mucus or other body fluids, sharing a glass of water, sharing needles, etc. is that correct?
ReplyDeleteConsidering the lethality of the H5N1 strain I was curious whether or not a vaccine produced using the current virus (the one that can not be transmitted through the air) would yield appreciable immunity should the virus mutate so as to be transmissible through the air. Presumably there would only be a small change in the genetic code of the virus and therefore antibodies produced against the ancestral H5N1 with a vaccine should be somewhat effective.
ReplyDeleteShould we redefine the status of viruses? They're classified as nonliving because they cannot autonomously reproduce and do not really react to stimuli, but here you use the terms "evolution" and "natural selection." The viruses that survive therefore are more likely to reproduce, and their genetics change over time. Or maybe we should redefine natural selection since a virus can't really survive if it was never alive, but certain mutations do promote reproduction.
ReplyDeleteAll of you have brought up interesting, important points!
ReplyDeleteThe H5N1 virus can indeed spread through other means (not air) but is primarily prevalent in wild birds. There is debate about the severity of H5N1 infection in humans, but many scientists believe that a relatively small percentage of exposed humans become infected (some just have antibody responses), and of those infected, most display mild or no symptoms.
There is currently an approved vaccine for H5N1. Of course, there is always the problem of viruses mutating so that vaccines will have to be changed to "match" the current strain, and the cycle repeats. However, no matter what, vaccines can reduce disease severity and increase people's readiness for a possible pandemic.
Scientists constantly argue over whether viruses are living or non-living. In my opinion, the definition of "virus" is more of a theory that will undergo changes as more research is done, as with other things in the field of biology. I believe that not everything can be rigidly defined in certain categories, and viruses may be something in between living and non-living, since they are clearly not merely ordinary non-living objects, like rocks for example. Evolution and natural selection certainly occur as can be proven with the vaccines that change annually to accomodate the ever-changing flu.
The issue of whether a virus is living or non-living is a complicated issue I think (biologically controversial, ethically? probably not). The problem with using the term "evolution" on virus is that the virus is being selected. We have discussed in class that virus DNA/RNA replication process is highly vulnerable to errors, which create both beneficial/detrimental strain depending on the environment (vaccines, host immune system). Certain strain happens to survive the current environment, so it is not like the virus is using a mechanism to specifically encounter host evolution.
ReplyDeleteRemember that if the virus causes a fatal disease, it could run against the survival of the virus itself. Virus can evolve to be less malignant over time in order to keep the host alive for a longer time (=more opportunities to reproduce and spread).
It always interesting to read about the status of viruses as "living or not" especially so when talking about such a common and sometimes scary virus such as the flu. Although some form of selection certainly occurs with viruses a great deal of the selection process is left up to chance and the host the virus is in. The most virulent strain of a virus to ever arise will not infect anyone if it arises in too small and sparse of a population.
ReplyDeleteIf Hardy-Weinberg equilibrium were reached, this would mean that evolution was not occurring. No selection, no mutation, no migration, no genetic drift, and random mating are conditions that must be met to reach Hardy-Weinberg equilibrium. In the case of H5N1, selection, mutation, and migration all occur, which suggests that evolution is occurring even though H5N1 is not living. The argument that evolution is occurring in the H5N1 virus provides support for defining viruses as being alive.
ReplyDeleteI find it surprising that H5N1 lost its lethality as it became more contagious. Do you know if this is a common trend among viruses? Are there any unique characteristics of H5N1 that allowed it to become airborne?
ReplyDeleteI actually did my post on the evolution of viruses because of how humans have interacted with them and the H1N1 virus was one of my examples! I think that its really interesting that you talked about how the viruses living in human bodies without actually making them "sick." Did you ever consider the idea that the virus might just be using the human as a temporary host and as a way to get to other humans more quickly or maybe if the human's immune system is strong, the virus stays dormant to prevent the trigger of antibodies and transfers through things that you may not see as a threat such as a sneeze or a cough.
ReplyDeleteI am so happy to have received so many responses to my blog on H5N1!
ReplyDeleteI know that trade-offs are quite prominent in evolution. Although viruses evolve, they never become perfect and lethality vs. contagious level is an example. However, the question of what characteristics H5N1 will acquire throughout time is what scares scientists. To be airborne, a virus has to have the structure/function to travel (due to being higher in the respiratory tract)from one person to another and to latch onto the host more easily. The hypothesis that viruses use humans as temporary hosts is a probable one, but viruses just need hosts to "live" on, so I do not really think that viruses "think" in terms of "temporary" or "permanent".
It was interesting to learn that as H5N1 accumulated more favorable mutations, like becoming airborne transmissible, its lethality also decreased. Do you think there is any explanation for this tradeoff? Why is it that as a virus gains advantageous mutations, it also becomes less potent? Is this some sort of evolutionary consequence? Great article, San Pei!
ReplyDelete