Occurrence of Gastrointestinal Parasites in Fecal Samples from Free-Grazing Ducks in Northeastern Thailand.

Intestinal parasitic infections remain a persistent concern in tropical agricultural systems, where close interactions among animals, humans, and the environment facilitate parasite transmission. Free-grazing ducks, commonly raised in rice-field ecosystems in Thailand, may be exposed to a wide range of intestinal parasites. This cross-sectional study examined 312 fecal samples collected from five selected free-grazing duck flocks in northeastern Thailand from October to November 2025. All fecal samples were examined using the modified formalin-ethyl acetate concentration technique. Parasites were identified based on egg and larval morphology. Positive parasite detections were found in 168 samples, yielding an overall percentage of positive fecal samples (%PFS) of 53.8%. The %PFS across the five provinces ranged from 42.5% to 63.5%. Among the positive fecal samples, the most frequently detected parasites were unidentified nematodes (38.1%), trichurid-like eggs (30.3%), strongyle-type eggs (25.0%), echinostome-like eggs (21.4%), and Ascaridia spp. (18.4%). Among the positive samples, small operculated trematode (SOT) and hymenolepidid-like eggs were detected in 3.0% and 1.2%, respectively; however, these findings may represent spurious passage. Single morphotype detection predominated, while co-detection of multiple morphotypes occurred less frequently. These findings document a diverse assemblage of intestinal parasite morphotypes in fecal samples from free-grazing ducks in northeastern Thailand, potentially reflecting exposure through aquatic and soil-associated routes.