Sources of Salmonella infection in pigs in the Netherlands: Epidemiological risk factors and subtyping-based source attribution

Effective Salmonella control in pigs requires understanding how the pathogen is introduced and spreads on farms. This study aimed to identify potential sources of Salmonella infection in pigs using both epidemiological and microbiological methods. A cross-sectional study was conducted on 148 fattening pig farms in the Netherlands. Porcine fecal samples were collected and tested for the presence of Salmonella, and a questionnaire on farm characteristics was completed. Risk factors significantly associated with Salmonella presence at farm-level were identified through logistic regression analysis. For source attribution based on microbial subtyping, serotyped Salmonella isolates obtained from the Dutch national laboratory surveillance system (2016–2023) were used. Pig isolates (n = 494) were attributed to nine putative sources (n = 4171 isolates) using the modified Dutch (mDM) and modified Hald (mHM) models. A farm-level prevalence of 9.5 % (14/148; 95 % CI 5.3–15.4 %) was found. Cleaning the pens between batches (odds ratio [OR] 0.139, 95 % CI 0.030–0.625) and moving piglets at a later age to the fattening unit (in weeks, OR 0.538, 95 % CI 0.290–0.998) were significantly associated with reduced odds of Salmonella presence, whereas the presence of cattle on the farm increased the odds (OR 4.860, 95 % CI 1.346–17.544). Both mDM and mHM attributed most pig isolates to other pigs (mDM: 43.0 %; mHM: 72.6 %), followed by cattle (mDM: 22.6 %; mHM: 11.0 %) and broilers (mDM: 9.2 %; mHM: 5.0 %), as sources. This study indicates pig-to-pig transmission as the main source of Salmonella in pigs and suggests that cattle may significantly contribute to Salmonella introduction into pig herds on multi-species farms.