Thursday, August 17, 2017

EID Journal: Transmission Of Swine H3N2 To Humans At Agricultural Exhibits - Michigan & Ohio 2016


Given this week's news of a third Ohio county fair shutting down their pig barn due to swine flu, and reports of at least 15 infections among fair goers in the past month, this week's publication in the EID Journal of a report on last year's H3N2v outbreak in Ohio & Michigan is well timed.
You may recall that just over a year ago (Aug 6th, 2016) the Michigan: DOH Announced 2 H3N2v Cases Connected To Muskegon County Fair, followed a week later by  CDC FluView: 4 H3N2v Cases Reported (2 in Michigan , 2 in Ohio).
By the end of August, 18 H3N2v cases -  all linked to fair attendance - were identified across two states.  In October of last year, in MMWR: Investigation Into H3N2v Outbreak In Ohio & Michigan - Summer 2016,  the CDC published their investigation into this cluster, the largest reported since 2012.
The `headline' of that report was that 16 of the 18 cases analyzed belonged to a new genotype not previously detected in humans.
Since the influenza subtypes that commonly circulate in swine (H1, H2 & H3) are similar to those that have caused all of the human pandemics going back 130 years (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), we pay close attention whenever a swine flu virus manages to jump - even in a limited way - to humans.  

All of which serves as prelude to today's EID Dispatch.
Volume 23, Number 9—September 2017


Influenza A(H3N2) Virus in Swine at Agricultural Fairs and Transmission to Humans, Michigan and Ohio, USA, 2016

Andrew S. BowmanComments to Author , Rasna R. Walia, Jacqueline M. Nolting, Amy L. Vincent, Mary Lea Killian, Michele M. Zentkovich, Joshua N. Lorbach, Sarah E. Lauterbach, Tavis K. Anderson, C. Todd Davis, Natosha Zanders, Joyce Jones, Yunho Jang, Brian Lynch, Marisela R. Rodriguez, Lenee Blanton, Stephen E. Lindstrom, David E. Wentworth, John Schiltz, James J. Averill, and Tony Forshey


In 2016, a total of 18 human infections with influenza A(H3N2) virus occurred after exposure to influenza-infected swine at 7 agricultural fairs. Sixteen of these cases were the result of infection by a reassorted virus with increasing prevalence among US swine containing a hemagglutinin gene from 2010–11 human seasonal H3N2 strains.

Influenza A virus infects many animal species. Zoonotic transmission allows for the introduction of novel influenza A virus strains to the human population, which has the potential to cause the next influenza pandemic. Swine exhibitions at agricultural fairs have emerged as a source for amplification of swine-lineage influenza A virus; these unique swine–human interfaces have generated most human infections with variant influenza A virus in the United States (1).

During July–August 2016, outbreaks of variant H3N2 virus (H3N2v) were reported in Ohio and Michigan, and 18 zoonotic influenza A virus infections were detected (2). All persons identified with H3N2v infections during these outbreaks reported swine exposure while attending >1 of 7 fairs in Ohio or Michigan. We examined the role of exhibition swine in the transmission of this reassortant influenza A virus, which contained a hemagglutinin gene from 2010–11 human seasonal H3N2 strains.


Variant influenza infections in humans continue to occur through contact with exhibition swine; often, the cases are in swine exhibitors with close and prolonged swine exposure. The concurrent detection of genetically identical influenza A viruses from exhibition swine across 2 states illustrates the rapidity with which this virus, and potentially other pathogens, can move within the highly mobile exhibition swine population. In addition to the zoonotic risks of influenza A virus, this pattern serves as a warning of possible dissemination of other emerging or high-consequence diseases in swine.

 Management practices common in the exhibition swine industry (i.e., frequent exhibition and relaxed biosecurity) facilitate the rapid dissemination of influenza virus across a large geographic landscape (14). Collaboration between animal and public health officials facilitated this investigation. Methods to control intraspecies and interspecies influenza virus transmission during swine shows have been outlined by the National Association of State Public Health Veterinarians (

The recovery of human-like H3 influenza A viruses from exhibition swine supports previous studies demonstrating that the US commercial swine herd can serve as an influenza A reservoir for the much smaller exhibition swine population, which is more accessible to humans. Within the US commercial herd, the proportion of H3 isolates containing human-like H3 nearly doubled to 46% in spring and summer 2016 (data not shown).

Whereas human-like H3s have been circulating, reassorting, and becoming more prevalent in the commercial swine population since 2012, introduction and expansion of the human-like H3 reassortant influenza A viruses in exhibition swine facilitated documented zoonoses from this genotype.
The path traversed by this human-like H3, from initial introduction from humans to swine until the zoonotic transmission events of 2016, demonstrates how novel viruses can be generated and maintained in animal populations and, subsequently, can infect humans through specific ecologic niches like swine exhibitions or live-animal markets (15). Therefore, continued surveillance in swine populations is imperative for detecting novel influenza A viruses that threaten swine and human health.

Dr. Bowman is an assistant professor at The Ohio State University, where he leads a team researching the ecology and epidemiology of influenza A virus in swine and avian populations. He and his team are working to control influenza A virus in swine populations and reduce influenza A virus transmission across the swine–human interface.

Of particular interest, while widespread illness in pigs was only rarely reported, surveillance  revealed an average prevalence of influenza A in fair pigs of  77.5%. Today's study cautions that this  suggests `that subclinical influenza A infections in pigs remain a threat to public health (3).'
We saw similar findings in a  2012 study (see EID Journal: Flu In Healthy-Looking Pigs), that reported that it isn’t always possible to identify pigs carrying an influenza virus based simply on their appearance. 
A few months later, in Asymptomatic Pigs: Revisited, we looked at a pair of studies from Ohio State University -  one that found a surprisingly large percentage of flu infected swine to be asymptomatic - and another that established just how closely linked the human and swine variant strains of influenza that summer really were.
In both cases, the lead author was Andrew Bowman - then a  Ph.D. candidate in veterinary preventive medicine at Ohio State - who is the lead author of today's dispatch.
For a recent recap of swine flu and swine variant influenza studies, you may wish to revisit:

A Reminder About The `Other' Novel Flu Threat

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