Evidence of influenza A infection and risk of transmission between pigs and farmworkers

Today, we share a recent study by the Torremorell’s lab published in open access, in the Zoonoses and Public Health journal this past month. Led by Dr. Gustavo Lopez, this work focuses on the interface between the pigs and farmworkers when considering influenza A infection.

Objectives of the study

  1. Assess if influenza A virus could be detected in swine farmworkers prior to and after work
  2. Identify risk factors associated with IAV detection in the workers
  3. Characterize the genetic sequences of the IAV detected in both the workers and the pigs.


Farmworkers from seven farrow-to-wean farms in the Midwest were enrolled in this study. Participants self-collected a total of up to 32 nasal swabs (twice a week for eight weeks). At each farm, 30 nasal swabs were collected from due-to-wean piglets at the beginning, middle, and end of the sample collection period in the farm workers. All samples were then tested by Rt-PCR for influenza A either individually from farmworkers or by pools of 3 for pigs. A subset of samples was sent out for whole-genome sequencing.


Five farms were found to have positive PCR results in their swine samples. Out of the 1,814 nasal swabs collected on farmworkers, 58 tested positive for influenza A and 33 individuals (out of 64) tested positive at least once. Six farms had at least one employee tested positive. 2.2% and 4.2% of the samples were positive when collected prior to and after work, respectively.

Photo by mohamed hassan from PxHere

Three samples that tested positive prior to the workday (out of 20) belonged to one farmworker who reported at least one influenza-like symptom, and from whom a 2009 pandemic-like H1N1 (1A.3.3.2) was detected. Eight pre-workday positive samples originated from workers who had received the seasonal influenza vaccine. None of the samples collected from people with a body temperature > 100.4°F tested positive by rRT-PCR. Lastly, in this study, farmworkers were twice as likely to test positive for influenza A.

Read the entire publication on the journal’s website.


Interspecies transmission of influenza A virus (IAV) between pigs and people represents a threat to both animal and public health. To better understand the risks of influenza transmission at the human–animal interface, we evaluated 1) the rate of IAV detection in swine farmworkers before and after work during two human influenza seasons, 2) assessed risk factors associated with IAV detection in farmworkers and 3) characterized the genetic sequences of IAV detected in both workers and pigs. Of 58 workers providing nasal passage samples during 8-week periods during the 2017/18 and 2018/19 influenza seasons, 33 (57%) tested positive by rRT-PCR at least once. Sixteen (27%) workers tested positive before work and 24 (41%) after work. At the sample level, 58 of 1,785 nasal swabs (3.2%) tested rRT-PCR positive, of which 20 of 898 (2.2%) were collected prior to work and 38 of 887 (4.3%) after work. Although farmworkers were more likely to test positive at the end of the working day (OR = 1.98, 95% CI 1.14–3.41), there were no influenza-like illness (ILI) symptoms, or other risk indicators, associated with IAV detection before or after reporting to work. Direct whole-genome sequencing from samples obtained from worker nasal passages indicated evidence of infection of a worker with pandemic 2009 H1N1 of human-origin IAV (H1-pdm 1A 3.3.2) when reporting to work, and exposure of several workers to a swine-origin IAV (H1-alpha 1A 1.1) circulating in the pigs on the farm where they were employed. Our study provides evidence of 1) risk of IAV transmission between pigs and people, 2) pandemic H1N1 IAV infected workers reporting to work and 3) workers exposed to swine harbouring swine-origin IAV in their nasal passages temporarily. Overall, our results emphasize the need to implement surveillance and transmission preventive protocols at the pig/human interface.

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