Thursday, September 09, 2010

When Pigs Flu



# 4884



For the second time in a month we’ve a study on reassortant influenza viruses in pigs.  The first one I wrote about in August in a blog called EID Journal: Asymptomatic H5N1 In Pigs and again in earlier this month in A Bold Bird Flu Headline From Japan.


This study was also featured in a New Scientist article earlier this week called Bird flu jumps to pigs.


To recap briefly:


Chairul Nidom of the Institute of Tropical Disease, Airlangga University in Indonesia - along with colleagues in Japan - have been conducting surveillance of Indonesia’s pigs since 2005.


They found that between 2005 and 2007, 7.4% of pigs tested asymptomatically carried the H5N1 virus.  Phylogenic analysis showed at least 3 separate introductions into the pig population.


Of particularly concern, in one sample researchers found an adaptation of the virus to human-like receptor cells via an Ala134Ser mutation (swaping alanine with serine) at position 134.


The study can be viewed in the CDC’s  EID Journal at:

DOI: 10.3201/eid1610.100508

Nidom CA, Takano R, Yamada S, Sakai-Tagawa Y, Daulay S, Aswadi, D, et al. Influenza A (H5N1) viruses from pigs, Indonesia. Emerg Infect Dis. 2010 Oct;[Epub ahead of print]

Influenza A (H5N1) Viruses from Pigs, Indonesia


Which brings us now to the second study, this time out of China, that appears in the PLoS One  Journal this week called:


Reassortant between Human-Like H3N2 and Avian H5 Subtype Influenza A Viruses in Pigs: A Potential Public Health Risk

Yanlong Cong, Guangmei Wang, Zhenhong Guan, Shuang Chang, Quanpeng Zhang, Guilian Yang, Weili Wang, Qingfeng Meng, Weiming Ren, Chunfeng Wang, Zhuang Ding


Essentially what they found was that during the surveillance period between  2007 and 2008 H3N2 influenza viruses were regularly detected from domestic pigs in Jilin Province, China.





Phylogenetic analysis showed two distinct lineages:


  • One that closely matched the contemporary strain of human-like H3N2 viruses
  • And another consisting of double-reassortant viruses containing genes from human H3N2 viruses and avian H5 viruses.


It’s a long and informative study (with a lot of technical detail on how it was conducted), but their conclusion reads:



The present study reports for the first time the coexistence of wholly human-like H3N2 viruses and double-reassortant viruses that have emerged in pigs in Jilin, China. It provides updated information on the role of pigs in interspecies transmission and genetic reassortment of influenza viruses.


As we’ve talked about many times before, pigs have long been believed to be ideal `mixing vessels’ for influenza because they possess both avian-like (SAα2,3Gal) and human-like (SAα2,6Gal) receptor cells in their respiratory tract.


That means pigs are susceptible to human, swine, and avian strains of flu.  And they are capable of being infected by more than one flu virus at a time.


This raises the possibility that two flu viruses could swap genetic material inside a pig, and create a hybrid (reassortant) strain.  One that could go on to infect humans.


mixing vessel


This is basically how the 2009 H1N1 pandemic virus evolved, although it took multiple gene swaps over a decade or longer before it finally emerged into the human population.


We know that reassortments happen, but only rarely do they result in a biologically fit virus capable of causing a pandemic.  Most hybrids are evolutionary dead-ends and die out within the host.



Source: FAO

But as global pig production grows to meet the demands of a hungry world we have a rapidly escalating global pig population - playing host to numerous strains of human, swine and avian influenzas – all of which creates increasing opportunities for a new virus to emerge.


Crowded factory farming techniques, the transport of live pigs across long distances, and a lack of biosecurity controls and surveillance at many pig farms around the globe all compound the risk.


For more insight on these practices, you may wish to view Michael Greger’s eye-opening video Flu Factories.


Admittedly, it may be years or even decades before another `swine flu’ emerges and threatens the human population. The timing of these events is impossible to predict.


But surveillance studies like the two mentioned above show that nature’s laboratory is vast, open 24/7, and actively engaged in genetic experimentation.


Which means that it isn’t a matter of if a new virus will emerge. 


It is probably just a matter of when.

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