Prevalence, antimicrobial resistance characteristics and virulence genes of Streptococcus suis in pigs in upper northern Thailand

This study aimed to investigate the prevalence, virulence genes and antimicrobial resistance of Streptococcus suis from pigs in Upper-Northern Thailand. A total of 768 nasal swabs were obtained from pigs at the municipal slaughterhouses in five provinces including Phayao (n=202), Nan (n=190), Chiang Rai (180), Chiang Mai (n=130), and Mae Hong Son (n=66) in 2018-2019. The prevalence of S. suis varied from 3.0%-9.4%, of which the highest prevalence was in Chiang Rai (9.4%) and Mae Hong Son (9.1%). Of all 59 isolates confirmed to be S. suis, serotype 8 (50.8%) was most commonly identified, followed by serotype 10 (3.4%), 2 (1.7%) and 9 (1.7%). The only one S. suis serotype 2 originated from Phayao. Twenty-five isolates (42.4%) could not be typed. The sly gene was the most frequent virulence genes, while mrp-sly-arcA-hyl (33.9%) was the most common virulence gene profile. All the isolates carried arcA, but none were positive to epf. The S. suis isolates exhibited high resistance rates to all antimicrobial agents tested (more than 50%) and all were multidrug resistant (100%). The most common resistance pattern was CLI-CTC-ERY-GEN-NEOOTC-TET-TIL-TYL (5.1%). The isolates were positive to tetM (32.3%) , tetO (30.5%) and mefA (8.47%). In conclusion, the results confirm the important role of subclinical carrier pigs of S. suis and emphasize the need for routine detection of S. suis in clinically healthy pigs.