Challenging Host Specificity: Malacosporean Parasite Dynamics— <i>Tetracapsuloides bryosalmonae</i> Beyond Salmonids—An Infection Experiment

Proliferative Kidney Disease (PKD) is a severe parasitic disease of salmonids caused by the malacosporean endoparasite Tetracapsuloides bryosalmonae. In the fish intermediate host, the parasite is disseminated via the circulatory system and ultimately reaches the kidney, where it matures before releasing infective spores into the environment through urine. Disease severity in salmonids is strongly temperature dependent, with prolonged exposure to temperatures above 15°C promoting parasite proliferation and pathology. We hypothesised that non-salmonid fish species better adapted to warmer conditions may serve as additional intermediate or reservoir hosts in the parasite life cycle. In a controlled laboratory infection experiment, minnow (Phoxinus phoxinus) were exposed to infectious malacospores released from T. bryosalmonae-infected bryozoans, the final host in the parasite cycle. Infection was confirmed by Sanger and Nanopore deep amplicon sequencing, and spore excretion in urine was demonstrated by eDNA. Notably, minnow showed no renal pathology and no mortality. These findings expand the recognised host range of T. bryosalmonae and suggest that minnow may contribute to parasite persistence in river systems, particularly under warming conditions that extend the transmission season and increase infection pressure.