A couple of months ago in EID Journal: Flu In Healthy-Looking Pigs we looked at a report indicating that it isn’t always possible to identify pigs carrying an influenza virus based simply on their appearance.
A dispatch from the EID Journal reported that nearly 1 in 5 healthy-looking pigs they tested at the Minnesota State Fair during the 2009 H1N1 influenza pandemic were actually infected with a flu virus.
This came out during the midst of several swine variant flu outbreaks this summer, involving several hundred people across 10 states. Viruses apparently contracted from pigs on display at local fairs (see MMWR: H3N2v Related Hospitalizations In Ohio – Summer 2012).
While fair officials were turning away pigs with overt signs of infection, this report suggested that perhaps those measures might not be sufficient.
Today, a pair of studies from Ohio State University - one that finds a surprisingly large percentage of flu infected swine to be asymptomatic - and another that establishes just how closely linked the human and swine variant strains of influenza this summer really were.
In both cases, the lead author is Andrew Bowman, a Ph.D. candidate in veterinary preventive medicine at Ohio State.
First, from the EID Journal, research that found more than 80% of the pigs that tested positive for influenza at the Ohio State fair between 2009 and 2011 showed no signs of illness.
Subclinical Influenza Virus A Infections in Pigs Exhibited at Agricultural Fairs, Ohio, USA, 2009–2011
Author affiliations: The Ohio State University, Columbus, Ohio, USA
Agricultural fairs are associated with bidirectional, interspecies transmission of influenza virus A between humans and pigs. We examined pigs exhibited at agricultural fairs in Ohio during 2009–2011 for signs of influenza-like illness and collected nasal swab specimens from a representative subset of these animals.
Influenza virus A was recovered from pigs at 12/53 (22.6%) fairs during the 3-year sampling period. Pigs at 10/12 (83.3%) fairs from which influenza virus A was recovered did not show signs of influenza-like illness. Hemagglutinin, neuraminidase, and matrix gene combinations of the isolates were consistent with influenza virus A concurrently circulating among swine herds in the United States.
Subclinical influenza virus A infections in pigs at agricultural fairs may pose a risk to human health and create challenges for passive surveillance programs for influenza virus A in swine herds.
Ohio State University has published a lengthy press release that discusses both papers.
Studies: Pigs Look Healthy But Test Positive for Flu at Fairs; Flu Transmission Seen Between Pigs and Humans
COLUMBUS, Ohio – More than 80 percent of pigs that tested positive for influenza A virus at Ohio county fairs between 2009 and 2011 showed no signs of illness, according to a new study.
Ohio State University researchers tested 20 pigs each at 53 fair events over those three summers and found at least one flu-positive pig at 12 fairs – almost a quarter of fairs tested.
The influenza strains identified in pigs in this study include H1N2 and H3N2 viruses – strains that have been circulating in pigs since 1998. In 2011, all of the H3N2 and H1N2 isolates found in pigs at the fairs contained a gene from the 2009 pandemic strain of H1N1, which is similar to the H3N2v strain causing human illness this year.
Though this finding alone is no cause for panic, it does show how quickly influenza viruses can change, said Andrew Bowman, lead author of the study and a Ph.D. candidate in veterinary preventive medicine at Ohio State.
In a second study led by Bowman, researchers compared the genomic sequences of influenza A viruses recovered in July 2012 from pigs and people. The analysis, showing a greater than 99 percent genetic similarity among the viruses, confirms that pigs and humans were infected with the same virus, indicating interspecies transmission.
This second study appears in the journal Emerging Microbes & Infections.
Although the timing of the illnesses (and initial sub-typing) in humans and swine at the Ohio State Fair last July strongly suggested interspecies transmission, a >99% genomic match pretty much erases all doubt.
Molecular evidence for interspecies transmission of H3N2pM/H3N2v influenza A viruses at an Ohio agricultural fair, July 2012
Evidence accumulating in 2011–2012 indicates that there is significant intra- and inter-species transmission of influenza A viruses at agricultural fairs, which has renewed interest in this unique human/swine interface.
Six human cases of influenza A (H3N2) variant (H3N2v) virus infections were epidemiologically linked to swine exposure at fairs in the United States in 2011. In 2012, the number of H3N2v cases in the Midwest had exceeded 300 from early July to September, 2012.
Prospective influenza A virus surveillance among pigs at Ohio fairs resulted in the detection of H3N2pM (H3N2 influenza A viruses containing the matrix (M) gene from the influenza A (H1N1) pdm09 virus). These H3N2pM viruses were temporally and spatially linked to several human H3N2v cases.
Complete genomic analyses of these H3N2pM isolates demonstrated >99% nucleotide similarity to the H3N2v isolates recovered from human cases. Actions to mitigate the bidirectional interspecies transmission of influenza A virus between people and animals at agricultural fairs may be warranted.
While the vast majority of the swine-to-human flu transmissions this year have involved the H3N2v virus, as you can see from the following table, small number of H1N1v and H1N2v infections have been reported as well.
Given the limits of surveillance, testing, and reporting – we really don’t know what the normal `background rate’ of these types of novel flu infections are in humans.
The smattering of reports since 2005 indicate they may be fairly rare, but are certainly not unheard of.
The majority of human swine variant flu infections this summer were relatively mild (out of 300+ infections, 16 were hospitalized, and 1 died), but it is always of concern anytime a novel influenza virus jumps from another species to humans.
It's possible that sporadic infections and even localized outbreaks among people with this virus will continue to occur.
While there is no evidence at this time that sustained human-to-human transmission is occurring, all influenza viruses have the capacity to change and it's possible that this virus may become widespread.
So far, the severity of illnesses associated with this virus in people has been similar to the severity of illnesses associated with seasonal flu virus infections. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not.
CDC is closely monitoring human infections with all novel influenza viruses, including H3N2v viruses, and will provide more information as it becomes available.
With further evidence of asymptomatic influenza infection in pigs, fair officials and public health agencies will need to decide on the best policies to limit future exchanges (in either direction) of flu viruses between people and pigs.