Occurrence and antimicrobial resistance of Salmonella isolates in farrow-to-finish pig farms in Bulgaria

Salmonella spp. is an important zoonotic and foodborne pathogen. It is spread worldwide and represents a public health risk. Pigs are a significant reservoir and are frequently subclinical carriers. The aim of this study was to detect the occurrence and antimicrobial resistance of Salmonella isolates being the five most important for public health Salmonella serovars in fattening pigs in Bulgaria. The isolation of Salmonella spp. was carried out according to EN ISO 6579-1 for the detection of Salmonella bacteria in feces and Salmonella serotyping following the Kauffmann-White scheme. All confirmed S. enterica isolates were analysed by the disk-diffusion method for susceptibility to 14 antimicrobials. Salmonella enterica was detected in 13 out of 32 tested farms in Bulgaria. The overall percentage of Salmonella positive pooled fecal samples was 6.8% (43 of 630 samples). The highest occurrence was present in fattening pigs aged between 121-180 days (16.3%; 20/123), followed by dry sows (6.5%; 4/62) and gilts (4.8%; 12/248). About 75% of the isolated strains belonged to three serotypes: Salmonella Infantis (41.9%), Salmonella Give (16.3%) and Salmonella Typhimurium monophasic (16.3%). All the tested isolates were resistant to Tilmicosin (100%), 88.4% to Ampicillin, followed by 69.8% to Tiamulin, 25.6% to Amoxicillin and Chlortetracycline. Multidrug resistance was recorded in 62.8 % of the tested strains. This study reports data regarding the circulation of the most important for public health five Salmonella serovars (S. Enteritidis, S. Typhimurium, S. Typhimurium monophasic, S. Infantis and S. Derby) in farrow-to-finish pig farms in Bulgaria and represent 74.4% of the total Salmonella spp. isolates. This phenomenon has critical effects for the health of consumers and therefore represents a key "one health" issue.