Branchial Arteritis in Rainbow Trout, Oncorhynchus mykiss (Walbaum, 1792): A Novel Pathology Associated With Environmental and Infectious Stressors.

The frequency and impact of non-infectious cardiac and vascular diseases in salmonids have increasingly been recorded in the recent literature. Among others, case collections reported enlarged and round-shaped heart, vessels misalignment, epicarditis, coronary and ventral aorta arteriosclerosis, and myocardial fibrosis, degeneration, and necrosis, highlighting their potential influence on fish wellness, performance, and mortality and raising considerable attention in the field. In the present cases, peculiar histopathologic lesions involving the branchial arteries, bulbus arteriosus and ventral aorta developed after intense rainfalls in rainbow trouts. The morphological changes encompassed a spectrum of inflammatory arterial lesions of varying severity, ranging from mild inflammation to severe arteritis with fibrinoid necrosis and thrombosis, thus differing substantially from those previously reported. The context is further compounded by the presence of a severe and systemic form of proliferative kidney disease in a few subjects. Given the importance of cardiovascular pathology in farmed salmonids, these novel and unusual findings are relevant for understanding the spectrum of cardiovascular lesions potentially affecting these species. Moreover, the impairment of cardiovascular and respiratory function likely entailed by these lesions could compromise fish welfare by reducing their ability to cope with stressful events or adverse climatic environmental conditions.