Emergence of livestock-associated MRSA clonal complex 398 in Thailand's swine industry

Livestock-associated MRSA (LA-MRSA) clonal complex (CC) 398 is primarily found among pigs across Europe. However, CC398 has also been sporadically identified in several Asian countries, including Thailand. This study aimed to examine the drivers contributing to the emergence of LA-MRSA CC398 in Thailand's swine industry.

Whole-genome sequencing data of 18 LA-MRSA CC398 isolates from pigs, retail pork and swine workers in Thailand were analysed. Additionally, a total of 1197 qualified genomes from various sources in 24 countries were retrieved from Pathogenwatch. Subsequently, two SNP-based phylogenetic trees were reconstructed using maximum likelihood (n = 1215) and Bayesian inference (n = 168).

Our findings revealed that Staphylococcus aureus isolates with the CC398-SCCmec Vc (5C2&5)-t034 genotype were closely related to European isolates, including those from Denmark. In contrast, S. aureus isolates belonging to the CC398-composite SCCmec-t034 formed a phylogenetically distinct cluster.

This study reports that the CC398-SCCmec Vc (5C2&5)-t034 genotype identified in Thai pigs may have originated from Danish swine populations, possibly introduced into Thailand through international livestock movement. The CC398-composite SCCmec-t034 genotype was also detected in domestic pigs; however, it appears to have evolved independently from the CC398-SCCmec Vc (5C2&5)-t034 genotype. Bayesian phylogenetic analysis estimates that the common ancestor of the composite genotype emerged earlier than that of the CC398-SCCmec Vc (5C2&5)-t034 genotype, indicating a separate evolutionary origin.