Risk factors for Salmonella spp in Portuguese breeding pigs using a multilevel analysis

Salmonella is the second most frequent cause of foodborne illness in the European Union (EU), so EU enforced legislation to achieve a reduction in Salmonella prevalence in the swine sector. To set the reduction target each country carried out a baseline survey to estimate Salmonella prevalence. The aim of our study was to identify risk factors for the presence of Salmonella in breeding pigs based on the data of the Baseline Study for Salmonella in Breeding Pigs in Portugal. In total, 1670 pen fecal samples from 167 herds were tested by culture and 170 samples tested positive. Along with the collection of the samples a survey was applied to collect information about the herd management and potential risk factors. Multilevel analysis was applied to the data using generalized linear mixed models and a logit link function. The outcome variable was the presence/absence of Salmonella in the pen fecal samples. The first level was assigned to the pen fecal samples and the second level to the herds. The results showed significant associations between Salmonella occurrence and the factors (p < 0.05): maternity pens versus mating pens (OR = 0.39, 95%CI: 0.24-0.63), feed from external or mixed source versus home source (OR = 2.81, 95%CI: 1.19-6.61), more than 10 animals per pen versus 10 animals per pen (OR = 2.02, 95%CI: 1.19-3.43), North Region versus Alentejo Region (OR = 3.86, 95%CI: 1.08-13.75), rodents control (OR = 0.23, 95%CI: 0.090-0.59), more than 90% of boars homebred or no boars versus more than 90% of boars from an external source (OR = 0.54, 95%CI: 0.3-0.97), semen from another herd versus semen from insemination centers (OR = 4.47, 95%CI: 1.38-14.43) and herds with a size of 170 or more sows (OR = 1.82, 95%CI: 1.04-3.19). This study offers very relevant information for both the Portuguese veterinary authorities and the pig farmers currently developing control programmes for Salmonella. This is the first study providing evidence for semen and boars source as risk factors for Salmonella in breeding pigs.