Encysted metacercaria (EMC) in muscles of fresh water fish (Tilapia spp.) may be hazardous to consumers or fish eating mammals. The present study was undertaken to investigate the prevalence and distribution of different EMC in Oreochromus niloticus. A total of 150 specimens of Oreochromus niloticus were collected from different localities at Alexandria Governorate. The overall prevalence of infection with EMC was 58.6% (88 out of 150 examined fishes). Microscopic EMC were 57.3%, where macroscopic EMC were 1.3%. The prevalence was higher in summer (79.48%) followed by spring (72.72%) and autumn (65.62%) then winter (45.71%). The average number of EMC per gram of fish tissues was highest in trunk region (69) followed by tail (58) and lowest in head region (39). Six species of EMC were identified, the prevalence of Heterophyes spp. ( H. heterophyes and H. equalis) was 37.3%, Pygidiopsis geneta (16.7%), Haplorchis yokogawai (14%), Prohemistomum vivax (10.6%), Clinostomum tilapiae (1.3%) and Euclinostomum heterostomum (0.6%). Chemical composition of Oreochromus niloticus muscles infected with encysted metacercariae and control (healthy) muscles revealed that there is no obvious changes. Effect of freezing on the infectivity revealed that all EMC were killed at -10°C/14 days and at -30°C / 24 hrs. Zoonotic importance and impact of infection in fish as a source of high quality protein was discussed.
Key words: metacercariae , Tilapia nilotica , public health significance
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