Molecular detection and cox1-based phylogenetic analysis of Strongyloides stercoralis in fresh vegetables from lower northern Thailand.

Strongyloides stercoralis is an intestinal nematode with worldwide distribution, including Thailand, and contamination of vegetables poses a potential public health risk. Previous studies largely relied on light microscopy, often reporting nematode larvae without species identification. This study investigated the presence of S. stercoralis in fresh vegetables from the lower northern region of Thailand using both morphological and molecular methods, including cox1-based phylogenetic analysis. A total of 90 fresh vegetable samples were collected from farms across three provinces. In each province, 10 vegetable types; Chinese morning glory, lettuce, dill, green onion, celery, Chinese chives, Mustard green, coriander, gotu kola, and Chinese mustard were collected. Detection of S. stercoralis was performed by examining morphological characteristics under a microscope using the sedimentation technique. Molecular detection was carried out via PCR targeting the cox1 gene, followed by sequencing and phylogenetic analysis. Of 90 samples, 47.78% were positive, with the highest contamination found in green onion (100%), lettuce (88.89%), and Chinese morning glory (66.67%). Molecular detection by PCR targeting the cox1 gene (primers StrCoxAfrF/StrCoxAfrR) confirmed seven positive samples (23.33%) from four vegetable types: Chinese morning glory, lettuce, green onion, and Mustard green. Sequencing showed 99-100% identity with S. stercoralis, and phylogenetic analysis using the Neighbor-Joining method on the seven positive samples from three provinces revealed genetic affinity with strains from neighboring countries. These findings underscore the need for comprehensive public health strategies for surveillance, treatment, and prevention of S. stercoralis transmission through contaminated vegetables.