The effect of the discontinued use of zinc oxide on antimicrobial usage in Danish pig farms

In the Danish pig industry, zinc oxide, used for prevention of E. coli-related post-weaning diarrhoea in weaners, was banned in 2022 due to environmental concerns. This epidemiological study aimed to estimate the effect of the discontinued use of zinc oxide on antimicrobial usage (AMU) for weaners and finishers in Danish farms. Using farm and prescription data from 2018 to 2023 from national databases, we fitted a linear mixed-effect model to a three-level nested dataset, consisting of monthly average standardized AMU (Defined Animal Daily Doses per pig-day), on a farm (n = 4020), overseen by a veterinarian (n = 146). The most significant effect of the zinc oxide ban was seen for weaners, where AMU increased by ∼5 % on average in the first 5 months post-discontinuation, followed by a ∼17 % increase after > 5 months. A long-term effect was also seen for weaners on farms that did not use zinc oxide in the year preceding the ban, where AMU increased by ∼19 % on average > 5 months after the ban was enforced, although from a lower pre-ban AMU. The between-farm variation indicated that the effect of the absence of zinc oxide on the prevalence of post-weaning diarrhoea was significantly influenced by individual farm management practices. The random variation in AMU between farms overseen by the same veterinarian was ∼10 times higher than the estimated variation between veterinarians, indicating that antimicrobial prescriptions made by veterinarians are specific to the disease status in each farm. The estimated autocorrelation in AMU between consecutive months indicates that the constant amount of antimicrobials needed to manage the production is linked to non-varying management practices in individual farms.