Abstract

Influenza has a worldwide distribution and is a reoccurring problem for the hog production industry. At this human-animal interface on pig farms there is ample opportunity for pig-to-human and human-to-pig (interspecies) influenza transmission. Influenza can cause significant clinical disease within pigs and potentially be transmitted to the workforce. We conducted a semi-quantitative risk assessment using the World Health Organization of Animal Health’s (OIE) risk assessment framework with the aim of evaluating the risk and uncertainty of IAV entering a single US indoor pig growing farm and an interspecies transmission event occurring during a single, six-month production cycle. We evaluated two risk pathways. The first risk pathway involved pigs being the initially infected species and transmitting to swine workers; the second risk pathway involved swine workers as the initially infected transmitting to pigs on swine farm. A literature review of published, grey literature, and surveillance reports from the Centers of Disease Control and Prevention (CDC) and United States Department of Agriculture (USDA) were used to inform this semi-quantitative risk assessment. We evaluated influenza transmission across two risk pathways: 1. What is the likelihood that based on current conditions on a single typical hog grower-finisher facility in the Midwest (USA), during a single production cycle, at least one hog becomes infected with influenza virus (either H1N1, H3N2, or H1N2) [step 1a] and that at least one worker becomes infected as a result [step 1b] and that the worker develops symptoms [step 1c]? And 2. What is the likelihood that, based on current conditions on a single typical hog grower-finisher facility in the Midwest (USA), during a single production cycle, at least one worker becomes infected with influenza virus (either H1N1, H3N2, or H1N2) [step 2a] and that at least one pig becomes infected as a result [step 2b] and that the pig(s) develop(s) symptoms [step 2c]? The combined risk assessment for at least one worker being infected with influenza outside of the pig growing facility, infecting at least one pig within the facility, and at least one pig developing influenza symptoms within a single production cycle was estimated to be Extremely Low for H1N1 and H3N2 during a single production cycle. Scenario analyses where separate influenza control measures were assumed (i.e., implementing vaccinating sows, mass vaccinating incoming pigs or improved PPE adherence) showed no reduction in the combined risks. However, when implementing all three influenza control methods, the combined risk could be reduced to Extremely Low for both pathways. Overall, there was limited empirical data assessing the risk of interspecies influenza transmission on an indoor pig growing facility and therefore all results from this research had a high level of uncertainty. Using a fully quantitative, multi-species transmission model we evaluated the effects of vaccination, isolation of infected pigs, and changes to workforce routine (ensuring workers moved from younger pig batches to older pig batches) during a single production cycle on an indoor hog growing unit containing 4000 pigs and two workers. The absence of control practices resulted in 3,957 pigs [0 – 3971] being infected and a 0.61 probability of workforce infection. Assuming incoming pigs had maternal-derived antibodies (MDAs), but no control measures were applied, the total number of infected pigs reduced to 1 [0 – 3958] and the probability of workforce infection was 0.25. Mass vaccination (40% efficacious) of incoming pigs also reduced the total number of infected pigs to 2362 [0 – 2374] or 0 [0 – 2364] in pigs assumed to not have MDAs and have MDAs, respectively. Changing the worker routine by starting with younger to older pig batches, reduced the number of infected pigs to 996 [0 – 1977] and the probability of workforce infection (0.22) in pigs without MDAs. In pigs with MDAs the total number of infected pigs was reduced to 0 [0 – 994] and the probability of workforce infection was 0.06. All other control practices alone, showed little improvement in reducing total infected pigs and the probability of workforce infection. Combining all control strategies reduced the total number of infected pigs to 0 or 1 with a minimal probability of workforce infection (<0.0002 – 0.01). These findings suggest that non-pharmaceutical interventions can reduce the impact of influenza on swine production and workers when efficacious vaccines are unavailable. Both models suggest that implementing a single influenza control measure is insufficient to reduce the risk of influenza transmission on pig production facilities and a multi-layered approach should be tailored to each individual farm.