# 5581
Not that I’m trying to promote paranoia, but a recurring theme in this blog is that nature’s laboratory is open 24/7, and that it is constantly trying out new genetic combinations looking for an evolutionary advantage.
We see this with growing antimicrobial resistance in bacteria (see here, here, and here), emerging anti-viral resistance in influenza viruses (here and here), and even resistance in mosquitoes to DEET repellant (see From the `Nature Bats Last’ Dept).
If you build a better mousetrap, nature will begin work on constructing a better mouse.
Viruses - which generally leave behind some degree of post-infection immunity - must change over time to evade that immune response, else they would run out of susceptible hosts.
Nowhere is that more apparent than in the world of influenza, where flu viruses mutate at an astonishing rate, and thus are able to spark global epidemics every year.
Adding to this constantly evolving pool of human influenzas are rare introductions of new, `novel’ viruses that can jump from other species (usually avian or porcine).
In The (Swine) Influenza Reassortment Puzzle last December, I wrote about the 19 (now 20) detected human infections by novel swine viruses in the United states. This from the CDC report":
Reported Human Infections with Swine Origin Influenza Viruses (SOIV) in the United States since 2005
Of the 19 human cases reported since 2005, 12 have been trH1N1 viruses, six have been trH3N2 viruses and one has been a trH1N2 virus. All 19 persons infected with swine viruses recovered from their illness. Twelve cases occurred in children (persons younger than 19) and 7 cases occurred in adults. In 15 cases, exposure to swine has been identified.
The H1N1 virus that sparked the 2009 pandemic was a descendent of a triple reassorted H1N1 swine flu virus that first appeared in American swine herds in 1998. It apparently bounced around in swine herds for a decade before finding the right genetic mutations to adapt to humans.
But H1N1 isn’t the only swine flu virus out there. Known Swine influenza A viruses include H1N1, H1N2, H3N1, H3N2, and H2N3.
But for sheer diversity of influenza viruses, we look to avian species for the mother lode.
Ducks, geese, and other aquatic birds are believed to be the natural reservoir – and the ultimate source – of all influenza A viruses.
While we’ve been focused on the H5N1 avian flu virus for a number of years, other avian strains (like the H7s, H9s, and H11s) have demonstrated the ability to infect humans as well.
Which brings us to this dispatch from the CDC’s EID Journal from the People’s Republic of China, that tells us of the discovery of a new reassortment of the avian H5 virus in domestic ducks.
Novel Reassortant Highly Pathogenic Avian Influenza (H5N5) Viruses in Domestic Ducks, China
Min Gu, Wenbo Liu, Yongzhong Cao, Daxin Peng, Xiaobo Wang, Hongquan Wan, Guo Zhao, Quangang Xu, Wei Zhang, Qingqing Song, Yanfang Li, and Xiufan Liu
Abstract
In China, domestic ducks and wild birds often share the same water, in which influenza viruses replicate preferentially. Isolation of 2 novel reassortant highly pathogenic avian influenza (H5N5) viruses from apparently healthy domestic ducks highlights the role of these ducks as reassortment vessels. Such new subtypes of influenza viruses may pose a pandemic threat.
Undoubtedly, influenza reassortments like these happen all the time and outside the view of scientists. Most are viral flashes in the pan, are unable to compete with more biologically fit flu viruses, and so we rarely learn of them.
But occasionally, the right genetic combination will be generated, and a new emerging virus is born.
The authors of this study write:
Ducks have been considered "Trojan horses" for influenza (H5N1) because of their pivotal role in virus propagation and evolution (11–13).
In our study, the 2 reassortant influenza viruses (008 [H5N5] and 031 [H5N5]) and their 3 possible parent viruses (108 [H5N1], 909 [H5N1], and 013 [H6N5]) were all isolated from apparently healthy domestic ducks.
We speculate that domestic ducks may serve as reassortant vessels for creating new subtypes of influenza viruses. In view of the practice of raising ducks in a free-range system, these novel strains could be transmitted to other domestic poultry and even humans.
There is evidence that these subtype H5N5 viruses have been transmitted to terrestrial poultry (Zhao et al., unpub. data). Thus, the role of domestic ducks in the influenza virus ecosystem should not be neglected.
Systematic surveillance should be instituted to identify emerging HPAI (H5N5) viruses and to reduce their potential threat to animal and human health.
Whether the H5N5 virus has evolutionary `legs’, and will ever pose a threat to poultry or humans, is unknown at this time.
All we really know is that it is a novel reassortment that was detected in two healthy ducks in eastern China in December 2008 and January 2009, and that it is highly pathogenic in poultry and moderately pathogenic in mice.
But what this does show is that without good systematic global surveillance of human, avian, and swine populations, we have no way of knowing what other emerging viruses might be fluing just under our radar.
