During summer months in North America, when we talk about influenza, we often talk about swine and swine variant flu strains that have sparked small outbreaks of human disease among fairgoers and pig farmers over the past few years (see Keeping Our Eyes On The Prize Pig).
Like humans, pigs are primarily affected by H1, H2 & H3 flu viruses, although studies have shown they are not immune to infection by other subtypes, including avian H5N1 (see EID Journal: Asymptomatic H5N1 In Pigs).
Swine possess both avian-like (SAα2,3Gal) and human-like (SAα2,6Gal) receptor cells in their respiratory tract, which scientists believe can facilitate a `bridging’ between avian and human strains. And being susceptible to avian, human, and swine flu strains, makes pigs a likely `mixing vessel’ for influenza reassortment.
Since pigs can be infected by more than one flu virus at the same time, it is possible for two viruses to swap genetic material (reassort), resulting in a new hybrid strain.
Curiously, what is often a fatal infection in humans and poultry, is usually subclinical in pigs. While good news for hog farmers, the authors of that EID study warned that the virus `can replicate undetected for prolonged periods, facilitating avian virus adaptation to mammalian hosts.’
Iowa, which is embroiled in the biggest HPAI (H5N2) outbreak in American history, also happens to be the nation’s largest producer of hogs.
According to the USDA’s Census of Agriculture (2012): The top three producers – Iowa, North Carolina, and Minnesota – together accounted for 55 percent of the value of U.S. hog and pig sales and 56 percent of the 66 million hog and pig end-of-year inventory in 2012.
All of which leads to the unavoidable question: Are pigs susceptible to the North American HPAI H5N2 virus?
Yesterday, AgriNews carried a reassuring reporting, entitled HPAI no threat to pigs, but vets urge biosecurity vigilance, with quotes from Dr. Bill Hollis, District 5 director on the board of the American Association of Swine Veterinarians :
“It’s not currently expected to infect or to create disease in people or pigs,” he said.
Hollis said that while pigs can become infected with influenza virus, at the moment, there is no evidence that herds have contracted the HPAI H5 virus.
“We don’t think it would create clinical disease. We don’t expect clinical disease in people or pigs. But we don’t want to risk any transmission,” he said.
Producers are urged to continue strict biosecurity protocols and to pay attention to water supplies.
At this point in time, Dr. Hollis is absolutely correct: We’ve seen no evidence that American Swine herds have contracted the H5N2 virus.
The two caveats I would add here are that swine infection would probably be subclinical and therefore unlikely to be detected without lab testing, and we don’t know how much active surveillance (PCR & Serological testing) has been done in swine herds in the affected areas over the past couple of months.
The University of Minnesota’s Swine Disease Eradication Center has a slightly less sanguine assessment, in the following notice on their front page:
NOTICE: PIGS AND HIGH-PATH AVIAN INFLUENZA
Highly pathogenic avian influenza (HPAI) has recently been diagnosed in the US in Minnesota, Missouri, Arkansas, Oregon, Washington and Idaho. Birds in the affected flocks have experienced high mortality and measures have been put in place to quarantine and depopulate the affected flocks. Contact with wild birds is considered the most likely source of infection for domestic poultry.
Pigs are susceptible to infection with all influenza viruses, including HPAI. Pigs are usually sub-clinical and evidence of infection is usually only seen by serology. However, there are reports indicating that pigs can also develop the disease.
To date, there is no evidence that the new strains of HPAI (H5N2 or H5N8 subtypes) have infected pigs in the US. However, producers should be diligent about their biosecurity practices. Avian influenza viruses are highly contagious, extremely variable and wide-spread in birds. Preventing introduction of birds into swine facilities, avoiding contact with wild birds and bird droppings in general, and avoiding non-chlorinated surface water should be emphasized.
With the further caveat that not all H5N2 viruses are created equal, and you can’t necessarily predict how a new reassortant strain will behave based on an older strain, we do have some additional evidence of the ability of HPAI H5 viruses to infect pigs, with a potential for reassortment.
Jin-Hong Lee, Philippe Noriel Q. Pascua, M.S. Song, Y.H. Baek, C.J. Kim, .W. Choi
Journal of Virology (Impact Factor: 4.65). 02/2009; 83. DOI: 10.1128/JVI.02403-08
ABSTRACT Due to dual susceptibility to both human and avian influenza A viruses, pigs are believed to be effective intermediate hosts for the spread and production of new viruses with pandemic potential. In early 2008, two swine H5N2 viruses were isolated from our routine swine surveillance in Korea.
The sequencing and phylogenetic analysis of surface proteins revealed that the Sw/Korea/C12/08 and Sw/Korea/C13/08 viruses were derived from avian influenza viruses of the Eurasian lineage. However, although the Sw/Korea/C12/08 isolate is an entirely avian-like virus, the Sw/Korea/C13/08 isolate is an avian-swine-like reassortant with the PB2, PA, NP, and M genes coming from a 2006 Korean swine H3N1-like virus. The molecular characterization of the two viruses indicated an absence of significant mutations that could be associated with virulence or binding affinity.
However, animal experiments showed that the reassortant Sw/Korea/C13/08 virus was more adapted and was more readily transmitted than the purely avian-like virus in a swine experimental model but not in ferrets. Furthermore, seroprevalence in swine sera from 2006 to 2008 suggested that avian H5 viruses have been infecting swine since 2006.
Although there are no known potential clinical implications of the avian-swine reassortant virus for pathogenicity in pigs or other species, including humans, at present, the efficient transmissibility of the swine-adapted H5N2 virus could facilitate virus spread and could be a potential model for pandemic, highly pathogenic avian influenza (e.g., H5N1 and H7N7) virus outbreaks or a pandemic strain itself
And then there’s this, from a 2010 PLoS One study:
Reassortant between Human-Like H3N2 and Avian H5 Subtype Influenza A Viruses in Pigs: A Potential Public Health Risk
Published: September 7, 2010
Human-like H3N2 influenza viruses have repeatedly been transmitted to domestic pigs in different regions of the world, but it is still uncertain whether any of these variants could become established in pig populations. The fact that different subtypes of influenza viruses have been detected in pigs makes them an ideal candidate for the genesis of a possible reassortant virus with both human and avian origins. However, the determination of whether pigs can act as a “mixing vessel” for a possible future pandemic virus is still pending an answer. This prompted us to gather the epidemiological information and investigate the genetic evolution of swine influenza viruses in Jilin, China.
Influenza surveillance of pigs in Jilin province, China revealed that H3N2 influenza viruses were regularly detected from domestic pigs during 2007 to 2008. Phylogenetic analysis revealed that two distinguishable groups of H3N2 influenza viruses were present in pigs: the wholly contemporary human-like H3N2 viruses (represented by the Moscow/10/99-like sublineage) and double-reassortant viruses containing genes from contemporary human H3N2 viruses and avian H5 viruses, both co-circulating in pig populations.
It is quite possible – perhaps even likely - that the North American reassortants of HPAI H5 will never pose a health threat to pigs, humans, or any other non-avian species. So far, we’ve seen no evidence that they can, and that is certainly encouraging.
But there is also much we don’t know about these viruses – including whether they can asymptomatically infect pigs or other mammals – and how and when they might reassort or evolve, and what effect that might have on their behavior in the months and years to come.
The only true constant with influenza is that it is constantly changing.
Which is why – while H5N2 doesn’t appear to be ready for swine time - when it comes to predicting what a virus will, or won’t do, it is probably best to never say `never’.