Influence of wind direction on the relationship between proximity to pig farms and risk of infection with MRSA CC398 among persons without known contact to livestock: a Danish nationwide population-based study.

Background: Livestock-MRSA (methicillin-resistant Staphylococcus aureus) can cause infections in persons without known contact to livestock, but the route of transmission is unclear. We investigated whether the risk of livestock-MRSA infection among persons with no known contact to livestock is associated with the number of pig farms near the home, and whether this association is affected by the upwind/downwind location of the farms.

Methods: Register-based case-control study of 518 persons from Denmark with clinical infections with livestock-MRSA in 2016-2021 and no known exposure to livestock, and 4,944 matched controls. Distances and angles from home addresses to all pig farms within a distance of 25 km were calculated, and compared with the mean wind direction in the area.

Results: The mean number of pig farms within 13,127 m of the home address was 3.3 [0.3; 6.3] higher for cases (60.5) than controls (57.1), with a larger difference for farms upwind than downwind. The primary analysis showed that the livestock-MRSA exposure from a downwind farm was 59% [40%; 178%] of the exposure from an upwind farm, but the difference disappeared after confounder adjustment. In a post-hoc analysis, cases were surrounded by more pig farms at 50 - 6,250 m from the home address, and in the interval 1,250-6,250 m the difference was only seen in the upwind directions.

Conclusion: The risk of livestock-MRSA infection among persons without known livestock contact was influenced by the number of and distance to pig farms. In an exploratory post-hoc analysis, but not the main analysis, the risk was also influenced by mean wind direction.