First report of Thaparocleidus caecus infection in the high-value native Malaysian pangasius catfish Pangasius nasutus.

Abstract

This study documents the first disease outbreak associated with Thaparocleidus caecus infection in Pangasius nasutus, representing a new host record for the parasite in Malaysia. In January 2025, an investigation was initiated following persistent mortality among the broodstock of P. nasutus. The affected fish exhibited clinical signs including respiratory distress, refusal to feed, erratic swimming behaviour, flashing, and scraping against the tank bottom. Gill inspection revealed operculum erosion, pale gill colouration, and signs of gill hyperplasia. Histopathological examination of the gills revealed severe epithelial hyperplasia of the secondary lamellae, lamellar necrosis, aneurysms of the capillary vessels, epithelial lifting (intraepithelial oedema), and massive leucocyte infiltration. Wet-mount examination identified the monogenean as T. caecus based on its sclerite morphology and morphometric assessment. PCR amplification and sequencing of the 28S rRNA gene further supported the species identification, with the obtained sequence forming a monophyletic clade with the reference sequences. The outbreak resulted in a cumulative mortality rate of 52% among the broodstock, with estimated losses of USD 1,300. These findings highlight that Thaparocleidus infection poses a significant threat to P. nasutus aquaculture and conservation, especially during hatchery operations.