Intestinal helminths in farmed pacu (Piaractus mesopotamicus): Hematological and histopathological effects.

Biosecurity in aquaculture is essential to reduce disease burden and ensure sustainability. Piaractus mesopotamicus (pacu) is a key species in South American aquaculture, yet parasitic threats to its health remain understudied. We analyzed intestinal helminth and assessed their effects on the health of 34 farmed pacu reared in intensive and semi-intensive systems in Santa Fe, Argentina, using parasitological, hematological, and histopathological approaches. Three parasite taxa were identified: the digenean Dadaytrema oxycephala (∼65 % prevalence; mean abundance: ∼3 parasites/fish), the nematode Rondonia rondoni (3 % prevalence; one host with 119 parasites), and the acanthocephalan Echinorhynchus jucundus (100 % prevalence; mean abundance: ∼235 parasites/fish). E. jucundus caused macrocytic hypochromic anemia (low erythrocyte count and hemoglobin, high MCV) and severe intestinal lesions (muscle penetration and chronic inflammation). D. oxycephala had marginal hematological effects. Infestations likely originated in land-based ponds, where zooplankton hosts thrive in the absence of technical biosecurity measures. Despite the high parasite burden, infected fish maintained stable body condition, suggesting a degree of physiological tolerance. However, the risk of nutrient malabsorption and market rejection remains. We propose: (1) prevention measures such as ≥ 15-day quarantine, pond sterilization, zooplankton monitoring in semi-intensive systems, water quality control in intensive systems, and necropsy of samples in cases of low feed conversion; (2) corrective action via targeted anthelmintic treatment or culling during outbreaks (>50 parasites/gut). This study identifies E. jucundus as an emerging pathogenic parasite and highlights the need for tailored biosecurity protocols adapted to different production systems, promoting the sustainable culture of pacu.