Immediate after birth transmission of epidemic Salmonella enterica Typhimurium monophasic strains in pigs is a likely event

Abstract

Sir, Salmonellosis remains an important foodborne disease to date and Salmonella enterica subsp. enterica serovar 4,[5],12:i:2, also known as monophasic Salmonella Typhimurium, has been rising as a cause of infection in humans and is widely spread among certain animal populations, namely in the pig reservoir. – 3 Little is known on the dynamics of transmission of this serovar and, as such, our aim in this study was to determine the likelihood of immediate after birth transmission of monophasic Salmonella Typhimurium from sow to piglet. At one large industrial pig herd in Portugal, 10 sows and 7 piglets from each sow’s litter were randomly chosen and sampled at birth. Salmonella was isolated according to the protocol described in ISO 6579:2002 Annex D and serotyped based on the Kauffmann–White–Le Minor scheme. The genus and the absence of the second-phase flagellar antigen fljB were confirmed by PCR, as recommended by the European Food Safety Authority (EFSA) Panel on Biological Hazards. To determine the likelihood of a sow carrying monophasic Salmonella Typhimurium transmitting Salmonella to her offspring, Fisher’s exact test was employed using R software (http://www.r-project.org). Susceptibility to 17 antimicrobials was determined using the disc diffusion and broth microdilution methods (VetMIC Stordjur, National Veterinary Institute, Uppsala, Sweden), and interpreted according to CLSI guideline M31-A3. CLSI M100-S21 susceptibility criteria were used for nalidixic acid. For neomycin, recommendations from the veterinary working party of the Antibiogram Committee of the French Society for Microbiology were followed. All isolates were screened for blaTEM, blaSHV, blaOXA-1, blaCTX-M, aadA, tet(A), tet(B), floR, sul1, sul2, dfrA1, qnrA, qnrB and qnrS using PCR. Plasmid extraction was performed using standard protocols and the size was determined after linearization with S1 exonuclease. Clonality was assessed by PFGE with XbaI restriction, according to the Pulsenet protocol. The patterns were analysed by BioNumerics software using the unweighted pair group method with arithmetic mean and the Dice similarity coefficient. The definition of a pulsed-field type cluster was based on a similarity cut-off value of ≥80% and a subtype on a value of ≥97%. A total of 10 sows and 70 piglets were sampled, yielding 6 sows and 17 piglets positive for Salmonella 4,[5],12:i:2. Among the positive animals, three sows and one or more of the respective piglets carried monophasic Salmonella Typhimurium (named as families A, B and D) (Table 1). All offspring from the other three positive sows were negative and one positive piglet descended from a sow negative for monophasic Salmonella Typhimurium. All Salmonella 4,[5],12:i:2 isolates from sows and their respective piglets were further characterized. Two resistance patterns were identified: 17 strains were resistant to ampicillin, neomycin, nalidixic acid, sulphonamides, streptomycin and tetracycline, and 2 strains were additionally resistant to amoxicillin/clavulanate. As for the resistance genes identified, all strains harboured blaTEM, tet(B) and sul2, except for a single piglet isolate that was negative for sul2. The plasmid linearization yielded a single small plasmid of 5.6 kbp for all isolates. All isolates belonged to a unique pulsed-field type cluster. Furthermore, 11 strains shared a similar profile representing a subtype with .97% similarity when a 2.5% band tolerance setting was used (see Figure S1, available as Supplementary data at JAC Online). There was a higher probability of a sow positive for monophasic Salmonella Typhimurium having a positive offspring with the same serovar (OR1⁄416.10, P1⁄40.001). Accordingly, the probability of a piglet having monophasic Salmonella Typhimurium was 16 times higher if it belonged to a positive sow. Several researchers have reported a high Salmonella prevalence in sows and their respective piglets early in life. Beyond the food safety risk when the sow ultimately enters the food chain, information on vertical transmission from the sow to her offspring is scarce. One study reported the contradictory fact that Salmonella serovars isolated from a sow often differed from those isolated from her piglets. In this study, the similarity of the resistance patterns, the antimicrobial resistance genes detected, the plasmid profiles and the PFGE types support the transmission of monophasic Salmonella Typhimurium from sow to piglet during or immediately after birth as a means of Salmonella colonization of the newborn’s gut. To the best of our knowledge, this constitutes the first report of mother-to-piglet horizontal transmission of monophasic Salmonella Typhimurium in pigs. The antimicrobial resistance core profile was similar to