EMERGENCE AND SPREAD OF THREE DIFFERENT MULTIDRUG-RESISTANT SALMONELLA SEROVARS AMONG HORSES IN ISRAEL: A ZOONOTIC THREAT?

Background: Salmonella enterica is a well-recognized zoonotic pathogen and a major cause for human gastroenteritis. Israel is endemic for multidrug-resistant (MDR) enteric pathogens due to the production of extended-spectrum-beta-lactamases (ESBLs). In a nation-wide veterinary antibiotic resistance surveillance study we aimed to investigate ESBL-producing (ESBL-P) Salmonella in equine.

Study Design: A prospective study for gut-carriage of ESBL-P Enterobacteriaceae was carried out in the Koret-School of Veterinary Medicine (2015-2016). Rectal swabs from hospitalized horses were inoculated onto BHI-broth, streaked onto CHROMagarESBL plates and sub-cultured to obtain pure cultures. ESBL-P Salmonella isolates recovered from clinical specimen of hospitalized horses during the study period were also included. Initial identification performed on SS-agar-plates and polyvalent agglutinating serum. ESBL-production was confirmed with clavulanate-combination-disc assay. Salmonella serovars were determined by serology according to the Kaufmann-White-Le Minor scheme. ESBL-encoding plasmids were transformed and compared, and ESBL genes were identified using multiplex-PCRs and sequencing.

Results: 12 Salmonella ESBL-P MDR isolates were identified. Isolates were resistant to all cephalosporins, Trimethoprim/sulfamethoxazole and aminoglycosides, and susceptible to carbapenems and quinolones. Isolates were classified into three different serovars: Cerro (group K), n=7; Havana (group G), n=3; Liverpool (group E4), n=2. Colonization of horses with Salmonella Cerro clustered in time (March-May 2016) suggesting nosocomial transmission. Four/12 ESBL-P Salmonella (33.3%) caused infections in foals (1-umblicus, 1-infected joint, and 2 cases of diarrhea). All isolates carried ESBL-encoding plasmids carrying a CTX-M-1-type ESBL. Interestingly, a high-molecular-weight ESBL-encoding plasmid (~95kb) was found in all three serovars suggesting inter-species horizontal transfer.

Conclusions: This is the first world report on the emergence of ESBL-P Salmonella among equine population. Three unique Salmonella serovars were identified, associated with horse gut-colonization and foal morbidity. These findings highlight the potential zoonotic reservoir of ESBL-P Salmonella in horses and the risk of dissemination to humans and the environment.