Effects of symmetric triazinone (toltrazuril) on developmental stages of Enteromyxum scophthalmi parasitizing turbot (Scophthalmus maximus L.): A light and electron microscopic study

In recent years, a new parasite that causes severe losses has been detected in farmed turbot (Scophthalmus maximus) in Northwestern Spain. Dead fish showed emaciation and cachexy caused by severe necrotizing enteritis that affected all portions of the digestive tract. The parasite was classified as a myxosporean and named Enteromyxum scophthalmi. This study tested toltrazuril (symmetric triazinone) in order to determine whether this commercial compound can be used as an effective treatment for fish affected by E. scophthalmi. To this end, we used turbot experimentally infected with E. scophthalmi and treated with toltrazuril, and at various time points after exposure and treatment, their digestive tracts were assessed histologically. We found that the drug induced generalized vacuolization in the cytoplasm of parasitic cells, with destruction of mitochondria and enlargement of endoplasmic reticulum cisternae and of the perinuclear spaces. Despite the apparent pathological changes shown by the parasite, the particular dose, route and duration of toltrazuril administration used in this study did not ameliorate the clinical progress of the disease in turbot infected by E. scophthalmi.