During the summer and early fall we normally see a drop off in the number of avian flu reports - at least in the temperate regions of the Northern Hemisphere - but in its place we often see a rise in human infections with swine variant flu viruses; H1N1v, H1N2v and H3N2v.
The CDC describes Swine Variant viruses in their Key Facts FAQ.
Between 2005 and 2009, 1 or 2 cases were reported each year, although the real number is believed much higher. Most cases were mild, and indistinguishable from seasonal flu, and so very few cases were tested.
What is a variant influenza virus?When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant influenza virus.” For example, if a swine origin influenza A H3N2 virus is detected in a person, that virus will be called an “H3N2 variant” virus or “H3N2v” virus.
In 2010 that number jumped to 8, and in 2011 to 12.The next year (2012), more than 300 cases were reported across 10 states (see CID Journal: H3N2v Outbreaks In United States – 2012) - nearly all associated with attendance at state and county fairs. Indiana reported the most cases (n=138), followed by Ohio (n=106).
After 2012, things settled down for a few years, with only a handful of cases reported between 2013 and 2015. In 2016 that number rose to 22 cases, with 18 of those - all fair attendees - diagnosed with swine-variant H3N2v in two states; Michigan and Ohio during the month of August.
In October of 2016 the MMWR: Investigation Into H3N2v Outbreak In Ohio & Michigan - Summer 2016 revealed that 16 of the 18 cases analyzed belonged to a new genotype not previously detected in humans.Last year (2017), the United States reported 67 human infections, once again mostly linked to agricultural exhibits at state and county fairs. This was the second highest yearly total reported since surveillance began in 2005, only surpassed by 2012.
Since the influenza subtypes that commonly circulate in swine (H1, H2 & H3) are also the same HA subtypes as have caused all of the human pandemics going back 130 years (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), when swine variant viruses jump to humans, it gets our attention.
Pigs are viewed as excellent `mixing vessels' for influenza viruses, due to having both mammalian α2,6 receptor cells and avian-like α2,3 receptor cells, and having frequent contact with humans and birds. They are often infected with human seasonal flu, along with their own swine flu viruses.
While most swine variant infections have been mild or moderate, a couple of deaths have been reported since 2012, along with a number of hospitalizations. The CDC takes these zoonotic infections seriously, and their Influenza Risk Assessment Tool (IRAT) lists H3N2v as having moderate pandemic potential.
The problem of swine-variant infections extends far beyond the United States, with occasional reports of serious illnesses from Europe, Asia, and South America. Testing and surveillance is very limited, however, and we are far from having a complete picture.Two and a half years ago, Chen Hualan - director of China's National Avian Influenza Reference Laboratory - gave an interview to the Chinese News Agency Xinhua where she pegged the EA (Eurasian Avian-like) H1N1 swine virus (EAH1N1) as having perhaps the greatest pandemic potential of any of the novel viruses in circulation.
Avian-like H1N1 swine flu may "pose highest pandemic threat": studyPNAS: The Pandemic Potential Of Eurasian Avian-like H1N1 (EAH1N1) Swine Influenza) by Hualan et al. that described the Prevalence, genetics, and transmissibility in ferrets of Eurasian avian-like H1N1 swine influenza viruses.
WASHINGTON, Dec. 28 (Xinhua) -- The Eurasian avian-like H1N1 (EAH1N1) swine flu viruses, which have circulated in pigs since 1979, have obtained the ability to infect humans and may "pose the highest pandemic threat" among the flu viruses currently circulating in animals, Chinese researchers said Monday.
In June of 2016, in Sci Rpts: Transmission & Pathogenicity Of Novel Swine Flu Reassortant Viruses we looked at a study where pigs were experimentally infected with one of these Eurasian-Avian H1N1 swine influenza viruses and the 2009 H1N1pdm virus.
Researchers generated 55 novel reassortant viruses spread across 17 genotypes, demonstrating not only how readily EAH1N1 SIV can reassort with human H1N1pdm in a swine host, but also finding:
`Most of reassortant viruses were more pathogenic and contagious than the parental EA viruses in mice and guinea pigs'.A few months later, in EID Journal: Reassortant EAH1N1 Virus Infection In A Child - Hunan China, 2016, we reviewed the case report on a 30-month old child from Hunan Province, who was infected with one of these reassortant EAH1N1 - H1N1pdm viruses.
While EAH1N1 is the prime swine-origin virus of concern in China, it isn't alone in that regard (see Front. Microbiol.: A Novel H1N2 Reassorted Influenza Virus In Chinese Pigs).
The emergence of novel swine viruses is truly a global concern, with new strains continuing to pop up in North America, South America, Europe and Asia.Their ability to reinvent themselves through reassortment with human, avian, and swine viruses, the shipment of live pigs nationally and internationally, and the growth in the global pork industry in general, pretty much ensures this trend will continue.
While past performance is no guarantee of future results, as county and state fair season begins this month, we'll be watching for any signs of increased swine variant infections over the summer and fall.Some additional past blogs include:
Lastly, the CDC has published guidance documents for animal exhibitors, venues, and visitors to agricultural exhibits and sponsors a Public Health Youth Agriculture Education Program.