As we enter another autumn season the memories of summer begin to fade and our minds turn towards the impending winter as the leaves begin to crisp and fall. This time of year also marks the appearance of flu-shots and news stories about the newest and scariest strains of influenza virus. Recently,  there have been multiple reports in the US  about a new form of swine flu that has been circulating at county fairs as people and pigs enter close proximity (see Related Articles after the jump).

Why is a strain of flu that infects pigs or dangerous or even relevant for people? As it turns out, from the point of view of the influenza virus a pig, bird, or human are not terribly different. Furthermore, pigs are also susceptible to certain strains of bird flu just like humans and can be multiply infected with different strains that could be human, avian, or porcine in origin. What this means is that pigs are an ideal breeding ground for reassortant  viruses and their subsequent antigenic shift, and this is where things get dangerous.

Click through to find out more about these potentially dangerous reassortant flu viruses…

In order to understand why the influenza virus is capable of reassortment we have to understand some basic influenza virology first. Influenza is a RNA virus whose genome is composed of eight different segments. These are not true chromosomes though; each short segment only codes for one protein. What is important about the segments is that it is possible for a cell to be infected with more than one strain of influenza virus at a time and the segments can get mixed up, or “reassorted” into different genetic configurations in the new viruses produced from these multiply-infected cells. Only two strains replicating in a cell can result in over 2⁸, or 256, distinct different progeny viruses. This process results in a phenomenon known as “antigenic shift” because the parts recognized by our immune systems (the antigen) can drastically change and leave us with no immunity to these new viruses. This is a much more problematic scenario than antigenic drift, or the slow accumulation of mutations over time which our immune systems are much more capable of handling as some cross-reactivity of existing antibodies may provide some immunity to these slightly mutated strains.

More importantly, these new reassortment viruses may be more pathogenic in humans than either original strain and capable of causing a modern epidemic or even pandemic.  Furthermore, reassortment viruses can circulate in pig and avian populations where they can further mutate before jumping back into humans. This means that surveillance of wild and domestic birds as well as domestic pigs must be undertaken regularly so that we can be aware of any potential new viruses in these populations before they make the jump to humans.

Human, bird, and pig populations do live in very close quarters in some parts of the world. An example of this would be the live-animal open air markets present in areas such as Hong Kong and Guangxi, China. Here it is easy to find live pigs, birds, and various other species in very close proximity with humans. Furthermore, at these locations it is not uncommon to process the animal, making them hotbeds for viruses making the jump from animals to people.

Surveillance for novel forms of influenza is underway in these populations and the data is interesting. A paper published in late 2011 details the surveillance of swine in both Hong Kong and Guangxi for H3N2 influenza¹.  This group showed that not only is H3N2 circulating in these animal populations, but that some of these viruses had reassorted with the 2009 H1N1 pandemic strain of influenza. This is a prime example of potential pandemic strains developing right alongside humans in densely populated regions. A second paper that came out in 2012 details the emergence of another swine H3N2 virus that had undergone genomic reassortment with the 2009 pandemic H1N1 strain and was actively circulating in pig farms in the United States². Additionally, birds are able to contract these new strains of flu and thanks to large global migratory patterns stains of flu originating in Asia or America can circulate to other parts of the globe quickly.

Birds aren’t the only way that these new flu strains can spread rapidly. Thanks to the wonders of modern transit someone can become infected in Hong Kong and within 24 hours be anywhere in the world. This fact was captured well in the 2011 film “Contagion” in which a new virus begins in Asia, and within a matter of days spreads globally with significant health and social consequences. Furthermore, this film captured the social unrest and breakdown well, showing that our health is not the only aspect of our lives that suffers during and outbreak of this scale and severity. These scenes of accumulating piles of garbage and of hospitals set up in gymnasiums are modern echoes of the social disruption that occurred during the 1918 flu pandemic, when in many places there was a shortage of medical staff, space, care, and even coffins. In this way the effects of a virus reach outside of our own individual bodies and impact the functioning of society as a whole.

For these reasons it is important to be aware of influenza strains circulating in pig and bird populations. Continued surveillance is necessary to identify and prepare for these potentially pandemic strains before they cross into our own population with significant consequences for humanity.

  Related articles:

• Pig Flu Virus Strain Shown to Have Pandemic Potential (scientificamerican.com)
• New swine flu virus could infect people (sciencenews.org)
• CDC: U.S. Swine Flu Cases Up, Avoid State Fair Pigs (news.brevardtimes.com)

References:

1.           Fan, X. et al. Emergence and dissemination of a swine H3N2 reassortant influenza virus with 2009 pandemic H1N1 genes in pigs in China. Journal of virology 86, 2375–8 (2012).

2.           Liu, Q. et al. Emergence of novel reassortant H3N2 swine influenza viruses with the 2009 pandemic H1N1 genes in the United States. Archives of virology 157, 555–62 (2012).