Kednapat Sriphairoj

Lactococcosis is an important bacterial disease affecting farmed fish worldwide and is primarily associated with Lactococcus garvieae, Lactococcus petauri, and Lactococcus formosensis. In Thailand, information on L. garvieae infection in tilapia remains limited, particularly regarding genome-based identification, virulence determinants, and antimicrobial resistance profiles. This study characterized two L. garvieae isolates, AAHM-LG2501 and AAHM-LG2509, recovered from a lactococcosis outbreak in cage-cultured Nile tilapia (Oreochromis niloticus) in Ubon Ratchathani province, Thailand. Both isolates exhibited typical phenotypic characteristics of L. garvieae, including Gram-positive cocci, alpha hemolysis, positive capsule staining, and positive carbohydrate fermentation. Whole-genome sequencing confirmed both isolates as L. garvieae, with genome sizes of approximately 1.95 Mb and a G + C content of 38.9%. Genome-based taxonomic analysis supported species identification based on dDDH and ANI values, and both isolates were assigned to sequence type ST95 and serotype I. Virulence factor analysis identified 288 virulence-associated genes representing 97 virulence factors across 14 functional categories. Capsule-associated genes were prominent, together with genes involved in heme uptake, adhesion, hemolysis, stress survival, biofilm formation, and host adaptation. Ten capsule biosynthesis genes, including cpsABCFGKO, cps4A, and cps4I, as well as LPxTG cell wall anchor protein genes, were detected. Antimicrobial susceptibility testing showed resistance to nalidixic acid, oxolinic acid, and oxacillin, while reduced inhibition zones were observed for enrofloxacin and sulfamethoxazole-trimethoprim. Genome analysis identified predicted antimicrobial resistance determinants, including lsaD, vanT, vanY, and mdtA. Resistance-associated protein variants were detected in gyrA and gyrB, suggesting that target alteration may contribute to fluoroquinolone resistance. Overall, this study provides genome-level evidence of virulence and antimicrobial resistance determinants in L. garvieae from Thai tilapia and highlights the importance of whole-genome sequencing for accurate diagnosis, epidemiological surveillance, and disease management in aquaculture.

Background/Objectives: Clariid catfishes are a cornerstone of Southeast Asian aquaculture, where intensive breeding and interspecific hybridization have significantly decreased genetic diversity. To address the limited understanding of growth-governing molecular determinants, we characterized the genomic architecture and evolutionary dynamics of the myogenic factor 5 (Myf5) gene, a master regulator of myogenesis, in 511 individuals from 17 catfish populations in Thailand. Methods: A total of 511 individuals representing 17 catfish populations were analyzed. A 334-bp fragment of exon 1 of the Myf5 gene was sequenced using Illumina short-read technology. Genetic diversity indices were calculated to evaluate sequence variation, population structure, and selective constraints acting on the locus. Results: Sequence analysis of a 334-bp segment of exon 1 using Illumina short-read technology identified 17 haplotypes and 92 variable sites, which revealed moderate allelic richness despite low overall nucleotide diversity (π ≈ 0.008). Evolutionary analyses indicated that the Myf5 locus is governed by stringent purifying selection (ω = 0.063), wherein site-specific constraints preserve the stability of the α-helix folding architecture, which is essential for myogenic regulation. Population genetic analyses revealed substantial differentiation among taxa (56% among-population variation), while Bayesian clustering successfully resolved a hierarchical structure (K = 2) that distinguished Clarias gariepinus, Clarias macrocephalus, and Clarias batrachus. These findings demonstrate that species-specific Myf5 haplotypes serve as diagnostic markers for identifying parental species and assisting in the identification of F₁ hybrid stocks. Conclusions: Overall, this study provides the molecular markers for species identification, broodstock authentication, hybrid detection, and sustainable genetic management of Thai Clariid resources, which are critical for balancing production efficiency with the conservation of native biodiversity in Southeast Asia.

Lactococcosis caused by Lactococcus garvieae is an emerging threat to warmwater aquaculture, yet evidence integrating field outbreaks with robust molecular confirmation and controlled virulence testing remains limited for Thailand’s cage-cultured tilapia. From May to October 2025, acute mortality events were investigated in cage-cultured Nile tilapia (Oreochromis niloticus) in a reservoir in Ubon Ratchathani Province, Thailand. Suspected outbreaks were defined by abrupt daily mortality exceeding 5% accompanied by septicemia-like clinical signs. Water quality during sampling covered the following ranges: temperature 28.6–31.9 °C, pH 6.5–7.0, salinity 0.02–0.03 ppt, electrical conductivity 0.036–0.046 mS/cm, TDS 22.20–26.50 mg/L, total alkalinity 17.0–34.0 mg/L as CaCO₃, total hardness 12.0–60.0 mg/L as CaCO₃, dissolved oxygen 6.5–7.0 mg/L, and NH₃ were below the limit of detection. Full-length 16S rRNA tissue profiling revealed strong tissue partitioning: blood microbiomes were consistently dominated by Lactococcus and L. garvieae at the species level, whereas gills showed higher richness and mixed communities with multiple opportunistic taxa. Culture isolation was more reliable from blood than gills, yielding 16 Gram-positive, catalase-negative isolates (AAHM-LG2501–AAHM-LG2516) that clustered within the L. garvieae clade in near full-length 16S rRNA phylogenetic analysis and were separated from closely related Lactococcus lineages. A representative blood isolate (AAHM-LG2501) showed dose-dependent virulence in controlled challenges, with an LD₅₀ of ~1.05 × 10⁵ CFU/fish by intraperitoneal injection and an LC₅₀ of ~1.20 × 10⁶ CFU/mL by immersion. Histopathology supported systemic dissemination, with injection producing more consistent multi-organ lesions than immersion, particularly in head kidney, liver, and spleen, while gills exhibited route-associated epithelial and vascular alterations. Together, these findings confirm L. garvieae as a major etiological agent of septicemic outbreaks in cage-cultured tilapia in Thailand and support a practical surveillance framework prioritizing blood sampling, molecular confirmation, and risk-based monitoring to guide biosecurity and vaccine-oriented prevention.

Background/Objectives: North African catfish (Clarias gariepinus) and bighead catfish (Clarias macrocephalus) play crucial roles in Thai aquaculture. Although significant growth disparities exist among these species, the genetic factors underlying these differences are still unknown. This study aimed to identify GH1 gene polymorphisms, in North African and bighead catfish populations across Thailand and Laos. Methods: Sequencing, phylogenetic, and clustering analyses were performed to assess genetic diversity, selection patterns, and lineage differentiation of catfish partial GH1 fragment. Results: Six alleles of the studied fragment of GH1 gene were identified; they differed at 33 variable sites within intron 2, located between the conserved regions at the 3′ end of exon 2 and the 5′ end of exon 3. At the population-level, GH1 exhibited low heterozygosity (mean H_o = 0.043 ± 0.023; H_e = 0.059 ± 0.028). Bayesian clustering analyses identified two distinct genetic clusters, corresponding to North African and bighead catfish, apart from the bighead population in Laos, indicating their distinct genetic origins. Evidence of purifying selection was observed in both species. Phylogenetic analysis indicated the presence of lineage-specific alleles in the GH1 gene. Conclusions: These findings provide valuable insights into GH1 polymorphisms in commercially important catfish species and may help to develop future breeding programs aimed at enhancing aquaculture productivity.

Background/Objectives: Understanding variabilities in the Major Histocompatibility Complex class I (MHC I) gene is essential for evaluating immunogenetic diversity in clariid catfish. MHC I plays a critical role in immune defense by presenting endogenous antigens to cytotoxic T cells. Therefore, we aimed to investigate the genetic diversity, selection patterns, and phylogenetic relationships of MHC I alleles in three important clariid catfish species (Clarias gariepinus, Clarias macrocephalus, and Clarias batrachus) across wild and hatchery populations in Thailand. Methods: Targeted next-generation sequencing of a 174 bp fragment partial exon 6 of MHC I-UAA gene was performed, along with phylogenetic analyses, neutrality tests and dN/dS analyses. Results: Overall, 91 novel alleles were identified in 674 individuals, all of which were novel (100% novelty), with none matching existing reference sequences, thereby revealing extensive variation in population-specific variants. Phylogenetic analyses revealed allele sharing among species, which was consistent with balanced selection. Neutrality tests and dN/dS analyses provided evidence of both purifying and diversifying selection, with episodic positive selection detected at multiple codon sites associated with the antigen-binding α1 domain. Distinct selection patterns among populations, influenced by local environmental conditions and human pressures, along with high allele richness, are reflected in the diversity of immunogenetic variations. Conclusions: These findings provide critical insights into immune adaptation and highlight the potential of MHC I as a functional marker for genetic monitoring. Although a causal relationship between MHC I polymorphism and disease resistance is debated, studies suggest associations with pathogen survival, indicating future implications for aquaculture breeding and conservation, particularly in marker-assisted selection for broodstock management in Thailand.