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Abstract

Cardiac pathology associated with the infection of Oncorhynchus mykiss Walbaum with Apatemon gracilis Rud. 1819.

Abstract

Groups of 5 small, 0+, uninfected rainbow trout were infected with A. gracilis by immersion in a suspension of cercariae for 30 min. All groups were infected with 250 cercariae/fish except those killed after 12 days and 8 weeks, which were infected with 150 and 800 cercariae/fish, respectively. The groups of fish were killed at 12 days and 7, 8, 10, 11 and 12 weeks pi: the coelomic and pericardial cavities were opened and the hearts were examined. Small indentations, the precursors of lesions, appeared in the outer surface of the compact myocardium, just beneath the epicardium, 8 weeks pi. Metacercariae and cysts accumulated posteriorly, in the dorsal and ventral 'corners' of the pericardial cavity, or anteriorly, around the junction between the bulbus arteriosus and ventral aorta. Histologically the pathological reaction consisted of a layer of fibroblasts and macrophages, through which ran many blood vessels. 10 weeks pi, both encysted and unencysted metacercariae were present in the pericardial cavity. Macrophages were present in the pericardial fluid indicating that an influx of monocytes into the cavity had occurred. 11 weeks pi there was considerable accumulation of host tissue, around both the heart itself and the adherent cysts. Fibrogranulomatous response tissue surrounded the cysts which had become attached to the epicardium; lymphocytes were also present. 12 weeks pi, the thickened, fibrosed pericardial layer remained and much fibrogranulomatous tissue had accumulated in the sulcus between the ventricle and bulbus arteriosus. Fine blood vessels had proliferated across the surface of the host reaction tissue. Larger and more distinct lesions had formed perpendicular to the surface of the compact myocardium. It is considered that infection of farmed rainbow trout with A. gracilis may pose a more serious problem than has been previously thought.<new para>ADDITIONAL ABSTRACT:<new para>The chronological development of gross pathological and histopathological changes associated with the experimental infection of farmed rainbow trout, O. mykiss, with 250 cercariae of A. gracilis was investigated. Fish were killed 12 days and then 7, 8, 10, 11 and 12 weeks pi and their hearts examined. 12 days pi, unattached, free metacercariae (97 × 33 µm) were present in the pericardial fluid but no pathological changes were seen. At 7 weeks pi, metacercariae (731 × 568 µm) were still unencysted but there was an increased accumulation of fluid in the pericardial cavity in some cases. By 8 weeks pi, clumps of metacercariae (498 × 329 µm) and encysted metacercariae, surrounded by host tissue, were adhered to the ventricle surface. At 10 weeks pi, both encysted (494 × 404 µm) and unencysted (502 × 321 µm) metacercariae were present in the pericardial cavity. Macrophages were present in the pericardial fluid indicating that an influx of monocytes into the cavity had occurred. Considerable accumulation of host tissue had occurred by 11 weeks pi and all metacercariae (511 × 314 µm) were encysted. 12 weeks pi, all metacercariae (511 × 314 µm) were encysted and much fibrogranulomatous tissue had accumulated in the sulcus between the ventricle and bulbus arteriosus. Fine blood vessels had proliferated across the surface of the host reaction tissue. The present findings are believed to indicate that the infection of farmed rainbow trout with A. gracilis may pose a more serious problem than was originally thought.