Influenza A virus continues to be one of the most problematic respiratory pathogens in the swine industry, as it causes cough, sneezing, fever and secondary infections and has an impact on growth and feed conversion.
The subtypes H1N1, H1N2 and H3N2 are endemic in swine herds, and the virus is year-round, with seasonal peaks.
IAV is also an evolving virus, which Gustavo Lopez says makes it very difficult to control.
"Typical of RNA viruses, IAV sustains mutations, and because it's a segmented virus, it can have gene reassortment, with other circulating influenzas that give emergence to new strains," the University of Minnesota doctoral candidate and research assistant says. "We have evidence of these events. We had the human influenza, avian and classical swine; they reassorted, changing internal segments, which gave origin to the swine triple-reassortant that further on had another reassortment and gave origin to the 2009 pandemic influenza called the swine flu."
While swine influenza virus infections in people are not common, Lopez points out bidirectional transmission of IAV can occur and be significant, resulting in pandemics.
It has been documented that swine workers have a higher risk of influenza exposure than the normal population (Gray et al.). But how frequently does IAV transmission occur between pigs and people on-farm?
That was the question Lopez set out to answer when he recently set up a surveillance system at the worker-swine interface to study influenza transmission between pigs and people, evaluate influenza positivity in swine farmworkers before and after work, and identify risk factors associated with influenza detection in farmworkers.
Seven sow farms in the Midwest were selected for the two-year study. All farms had a history of IAV in their herds.
Sixty-six workers voluntarily enrolled in the study, with 60 completing the entire testing period. The majority of participants were male (65%) and the majority of participants said they had not been vaccinated (65%).
For two days per week for eight weeks, participants were asked to self-collect a nasal swab before entering the farm and at the end of the workday during the human flu season each year.
Every time they took a sample, participants were also asked to take their body temperature using a disposable thermometer, and answer a short questionnaire regarding their activities performed that day and if they were experiencing any influenza-like symptoms. Pigs at those farms were also sampled at three points throughout the study.
A polymerase chain reaction was done on the nasal swab samples of the pigs to target the matrix gene specifically for swine, then for virus isolations on Madin-Darby canine kidney cells.
The human nasal swabs were first tested by PCR provided by the Centers for Disease Control and Prevention that targets both human and swine influenza.
Those who tested positive were further tested with another subtyping PCR to identify whether the IAV detected was an H3, pH1 (pandemic H1) or H1 subtype.
Results in pigs, people
Five of the seven farms’ pigs were IAV-positive at least once. Four of the farms tested positive during each of the three sampling points.
Six out of the seven farms (65%) had at least one worker test positive for influenza. Out of the 60 participants, 33 tested positive once (55%), and out of the 1,814 human samples collected, 58 tested positive (3.2%). Lopez says they were able to subtype only nine samples — five of them to the H3 and four of them to the pandemic H1.
"An interesting finding, when we were looking at some risk factors, is that the samples collected after work were two times more likely to test influenza-positive than the morning or before work samples," Lopez says. "The number of workers that tested influenza-positive at least once before work was 16 out of 60, while after work [it was] 24 out of 60."
Another noteworthy discovery was that several workers that tested negative before work tested positive after work — but then negative the next morning.
"We think those are exposures to the swine influenza that is circulating in the pigs," Lopez says. "We need to do further testing of the samples to really conclude that, but it’s a good indication."
Lopez says the study also found several people had consecutive positive samples, but he says he thinks those are more likely to be infections than exposure, and further work needs to be done with cycle threshold values and whole genome sequencing to really assess that.
On the two farms with negative pigs, there were no positive samples from the workers after work. "Although there were only six participants in these farms — we would have liked to have more, but it is an interesting finding, nevertheless," he says.
The reporting of clinical signs, such as fever, were also not good predictors of influenza positivity, Lopez says.
For example, 18 people reported temperatures higher than 100 degrees F, but none of them tested influenza-positive. For the 20 samples that were positive before entering the farm, those participants did not have a fever.
Lopez says the team also looked to see if there was an influence from human seasonal vaccination or area of work on-farm, and found there to be no significant difference between vaccination status of the workers or the area of work, and the likelihood of testing IAV-positive.
"Our results indicate that a number of farmworkers may test influenza-positive when reporting to work, even though they don't have clear symptoms of influenza infection — not even a fever reported by a thermometer," Lopez says. "We also found evidence of workers' exposure to influenza while working in swine farms; and additional analysis is needed to evaluate the bidirectional transmission of influenza between the farmworkers and pigs, and its implication in human and animal health."