The hidden burden: morphological, histological, and pathological features of Paradujardinia halicoris in dugongs (Dugong dugon) from the Andaman coast of Thailand

Abstract

Paradujardinia halicoris is a large gastrointestinal nematode that is commonly reported in dugongs (Dugong dugon). Although often considered non-pathogenic, heavy infections have been associated with significant gastrointestinal pathologies. However, detailed studies on the parasite’s morphology, histology, and pathological effects remain limited, particularly in dugongs in Thai waters. This study aimed to investigate the prevalence, morphological features, and pathological impact of P. halicoris on stranded dugongs along the coast of Thailand. Between January 2019 and May 2025, 166 Dugong carcasses were necropsied following standardized postmortem protocols. The gross lesions and parasite burden were recorded for each case. The recovered nematodes were preserved and examined using stereomicroscopy, scanning electron microscopy (SEM), and histological techniques to characterize their external and internal morphology. Pathological lesions were evaluated for severity and anatomical distribution, and parasitic occurrence was analyzed in relation to the biological data. Overall, 62.0% (103/166) of necropsied dugongs were infected with gastrointestinal nematodes, with the highest prevalence observed in juveniles and sub-adults. P. halicoris was predominantly localized in the stomach but also extended into the intestines, with worm burdens ranging from 4 to 687 individuals. Gross lesions included mucosal erosion, ulceration, hemorrhage, abscess formation, and intestinal obstruction. SEM revealed detailed surface structures such as lip morphology, interlabia, amphids, and sex-specific features. Histological examination clearly demonstrated the internal structures including the esophagus, reproductive tract, musculature, and pseudocoelom. This study provides the first detailed account of P. halicoris pathology, host biology, and parasitic morphology in Thai dugongs, emphasizing the importance of parasitic surveillance for marine mammal health and conservation.