Retailer-Level Heterogeneity in Aeromonas Loads among Nile Tilapia at Thai Wholesale Markets

Abstract

Aeromonas spp. are opportunistic pathogens commonly found in aquatic environments and are capable of causing gastrointestinal and systemic infections in humans. Nile tilapia (Oreochromis niloticus), a widely consumed freshwater fish in Thailand, may act as a reservoir for these bacteria. Although contamination in aquaculture and retail markets has been reported, data on retailer-level variability in wholesale markets remain scarce. This study investigated the prevalence, bacterial load, and species composition of Aeromonas spp. in fresh Nile tilapia sold by individual retailers at two wholesale markets in Pathum Thani Province, Thailand. A total of 32 fish were collected from 19 vendors at Talaad Thai Market (n = 26) and Talaad Rangsit Market (n = 5) between 29 May and 1 June 2024. Intestinal tissue samples were processed according to ISO 6887-1 guidelines, cultured on Aeromonas Selective Agar, and identified using standard biochemical tests. Colony counts (CFU/mL) were log₁₀-transformed and analyzed with non-parametric statistical tests. Aeromonas spp. were detected in 100% of samples. Median bacterial load was significantly higher in Talaad Thai (6.40 log₁₀ CFU/mL) than in Talaad Rangsit (5.86 log₁₀ CFU/mL) (p = 0.031), with significant heterogeneity among vendors (p = 0.012). Among 42 isolates, A. hydrophila was most common (26.2%), followed by A. caviae (9.5%), A. eucrenophila (7.1%), A. sobria (4.8%), and A. popoffi (2.4%); half of the isolates were unclassified to species level. The universal contamination and presence of pathogenic species highlight the need for Aeromonas surveillance in Thai fish markets. Vendor-level hygiene interventions, molecular species identification, and antimicrobial resistance profiling are recommended to strengthen food safety in aquaculture distribution.