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Abstract
In February, 2008, 12000 fries of monosex red tilapia hybrid (Oreochromis SPP) of 0.2 g and 12000 fries of monosex Nile tilapia (Oreochromis niloticus) of 0.5 g were imported from a hatchery in South East Asia and stocked in concrete raceways at an agriculture farm in the Barka Region about 80 km from Muscat. Three days after stocking, mortality problems were observed. The preliminary inspection revealed abnormal flashing movements of the fish with body scratching against the walls. Faint grayish white discoloration was observed externally on the skin and fins with focal hemorrhagic areas. Examination of skin and gill biopsies demonstrated the presence of a large number of small sized monogenea parasitic worms attacking the tissue of these body regions. Histopathological observations revealed tissue reactions against the parasites and demonstrated a severe dermatitis of the skin, lamellar oedema and hyperplasia of the branchial tissue. A concomitant treatment regime using mebendazole, salt and formalin was practiced for three successive days and repeated three times every week together with parallel management procedures to enhance water quality during treatment. After treatment, a reduction of mortality was noticed within three days and completely ceased after one week at which time biopsy examinations revealed the absence of any parasitic agents. There were no deaths reported among the Nile tilapia, which were raised on the same farm.