Endocardiosis or myxomatous degeneration of the cardiac valves is a well-described age-related change in humans and dogs. Lesions consist of polypoid nodular proliferations of loose extracellular matrix and valvular interstitial cells, most commonly affecting the mitral valve. This entity has not been previously described in fish. Herein we report the appearance, location, and occurrence of valvular and mural endocardiosis in a retrospective survey of aging laboratory zebrafish. Endocardiosis was present in 59 of 777 fish (7.59%), most commonly affecting the sinoatrial (34 fish; 57.6%) and atrioventricular (33 fish; 55.9%) valves. Lesions were more common in fish raised in recirculating water systems and fed commercial diets (52/230 fish; 22.6%) versus flow-through systems with fish fed semi-purified diets (4/234; 1.71%). Lesions were overrepresented in fish heterozygous for a mutant smoothened allele (34/61 fish, 55.7% vs 17/168, 10.1% wild type). There was no association between endocardiosis and intestinal carcinoids. Valvular endocardiosis is a significant age- and husbandry-related background finding in zebrafish and should be considered in the design and interpretation of research studies.
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