Emerging Threat of Multidrug Resistant (MDR) Extended-Spectrum beta-Lactamases (ESBL)-Producing Salmonella enterica in Hospitalized Equine

Objectives: Salmonella enterica is a well-recognized zoonotic pathogen. Although ESBLs are endemic worldwide, data on ESBL-producing Salmonella (ESBL-S) in horses is lacking. We aimed to investigate ESBL-S in hospitalized equine.

Methods: Rectal swabs for ESBL-S gut colonization were collected prospectively from hospitalized horses in the Koret-School of Veterinary Medicine (2015-2016). Swabs were enriched and streaked on CHROMagarESBL plates, and presumptive Salmonella colonies were isolated, identified and subjected to antibiotic susceptibility (Vitek-2). Clinical ESBL-S isolates during the study period were also included. ESBL phenotype was confirmed (EUCAST), and ESBL genes were identified by multiplex-PCR and sequencing. Serovars were determined serologically (Kaufmann-White-Le-Minor). WGS was performed for each serovar (Illumina HiSeq) and ESBL-S plasmids were sequenced.

Results: Twelve ESBL-S enterica isolates were isolated from eight foals and four horses (eight colonizing and four clinical). Isolates showed an identical MDR profile, with co-resistance to trimethoprim/sulfamethoxazole and aminoglycosides, and susceptibility to quinolones. Three S. enterica serovars were identified: Cerro (n=7); Havana (n=3) and Liverpool (n=2). Equine colonization with Salmonella Cerro clustered in time and isolates shared >95% similarity suggesting an outbreak. All ESBL-S isolates carried a transferrable plasmid-encoding bla_CTX-M-3 ESBL. 10/12 isolates (83.3%) shared the same plasmid, designated pSEIL3, a 86.4kb IncL/M-type plasmid, suggesting an inter-serovar horizontal plasmid dissemination. Conjugation experiments demonstrated transferability of pSEIL-3 from Salmonella to Escherichia coli J53 and to Klebsiella pneumoniae. The pandemic plasmid, pSEIL-3 showed 99% identity to pCTX-M3 from Citrobacter freundii.

Conclusions: This is an alarming report on the emergence of ESBL-P Salmonella enterica in hospitalized equine, associated with gut-colonization and foal morbidity. Both clonal expansion and horizontal plasmid transmission mediated the spread of ESBL-S. These findings demonstrate the necessity of active surveillance of ESBL-S in the veterinary hospital due to the potential zoonotic reservoir and highlights the clinical severity of this pathogen on foals’ health.

Key words: Salmonella enterica, Zoonosis, Endemic plasmid, ESBL, Antibiotic resistance spread, Horse