Laboratory transmission of the monogenean Acolpenteron ureteroecetes infecting the posterior kidneys of largemouth bass: Time course and pathology

Acolpenteron ureteroecetes infections in the kidneys of largemouth bass Micropterus salmoides have been reported, but the time course of infection and progression of pathology in experimentally infected fish remain unknown. We exposed 299 naïve juvenile largemouth bass at 19.8°C to A. ureteroecetes-infected largemouth bass via recirculating water without direct contact. For 7 months, prevalence and density were determined monthly in squashes of posterior kidney (20 exposed fish, 20 nonexposed fish), and histopathology was assessed in 5 fish from each group. Prevalence increased steadily from months 1 (5%) to 4 (85%), thereafter remaining relatively stable. Mean density of infection doubled monthly (month 1, 0.1 individuals/2 cm ² squash; month 7, 15.1 individuals/2 cm² squash). Eggs were first observed at month 3, and mean density increased markedly from month 4 to month 5 (2.9-15.3 eggs/2 cm² squash). Histopathology showed damage in renal collecting ducts that got progressively worse between 5 and 7 months. The infected ducts were dilated, had a hyperplastic epithelium, and were surrounded by chronic inflammation, including eosinophilic granular cells and varying degrees of fibrosis. Eggs within granulomas were present in the interstitium; this response is newly reported. The infection system developed in this study provides a reproducible and consistent source of infected individuals that will facilitate further study of the parasite and potential treatments.