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Marine fish culture, both in hatchery and grow-out systems, is prone to parasitic infestations, which lead to fish health impairment and generally high mortality rates. The most frequent disease in these cultures, amyloodiniosis, is caused by the dinoflagellate Amyloodinium ocellatum, the parasite considered to inflict the most considerable damage on commercial marine fish ventures. In recent years, the yellowtail Seriola dorsalis cultured in Baja California has undergone recurrent parasitic infections. Thus, the objective of the present work was to evaluate the effects of a parasitic infection (A. ocellatum) in juvenile yellowtail in terms of mortality, gill histology, and blood parameters. Fish exposed to parasitic infection exhibited 100% prevalence, with mean intensity of 766 ± 500 parasites per fish (grand mean ± SD). Gill histological analyses indicated damage characterized by inflammation, epithelial detachment, hyperplasia, fusion of secondary lamellae, telangiectasia, and proliferation of mucous cells. Regarding blood parameters, red blood cell count, mean corpuscular hemoglobin concentration, and hemoglobin, glucose, and triglyceride concentrations were significantly higher (P < 0.05) in infected fish (parasite prevalence of 100% with a mean intensity of 882.19 ± 265.05 parasites per fish) than in healthy ones. Also, mean corpuscular volume, total protein, albumin, and globulin were significantly lower (P < 0.05) in infected fish than in healthy fish. No differences were found in the hematocrit, mean corpuscular hemoglobin, and white blood cell count (P > 0.05). This study demonstrated that A. ocellatum infection caused severe gill damage, affecting gas exchange efficiency, which resulted in blood parameter changes and, consequently, high mortality rates in a short-term period.
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