A total number of 827 fish samples (oreochromis niloticus) were collected from different fish farms at Dakahlia province during a period extended from December 2001 to November 2002. The prevalence varied from 61.44% in may to 17.46% in December. Infection rate seemed to be increased with the increases of water temperature. The obtained encysted metacercariae were isolated and identified as clinostomatid (in branchial cawity), diplostomatid (in the eyes). Euclinostomatid (in the kidney tissue) and Heteroph d ( in the muscular tissue, gills, liver and heart).
There were no pathognomonic clinical signs associated with encysted metacercariae infestations except in Diplostomatid, which infested fish represented as loss of body weight, dark discoloration of the skin, uni or bilateral eye cataract and black spots on the skin in case of Heterophiid infection. Histopathological alterations were detected in fish samples especially in liver, eyes, heart, kidney, gills and branchial cavity of O. in loticus.
Treatment trials of Nile tilapia (O. niloticus) naturally infected with black spot disease ( Heteroph d ) cultured in earthen ponds with heavy loaded with fresh water snails at their bottoms. Copper sulphate and Bayluccid, 37 were used as chemical molluscicides at a dose of 0.25 mg/liter of ponds bottom after drainage. The obtained results showed that Bayluccid was more effective in eradication of snails and lowering black spots numbers of infected fish from 73+- 8 to 31+-10 than copper sulphate, which reduces black spots from 78+-5 to 57+-9, but Bayluccid had a negative effect on fish viability and reflexes post-exposur