Nile tilapia, Oreochromis niloticus, is the main cultured freshwater fish species of Saudi Arabia, KSA. Earlier studies have reported Aeromonas hydrophila to be the most predominant Aeromonas spp seen in tilapia aquaculture so, 560 Nile tilapia were randomly collected from two private freshwater farms at Al-Qatif, Eastern province, Saudi Arabia, during a period of January 2015 until December 2015. Bacterial isolates were recovered and biochemically identified using the API-rapid-identification system as A. hydrophila and A. sobria and confirmed genetically by sequencing of targets in the 16S rDNA and gyrB gene regions as A. veronii. A phylogenetic tree was also constructed and placed isolates of the present study in the A. veronii group. To evaluate the pathogenicity of isolated strains to Nile tilapia, an experimental infection challenge was done and histopathological alteration in both naturally and experimentally challenged fish were investigated and resulted in prominent clinical signs, gross pathological lesions and 78% mortality. The Antibiogram profiling for the strains isolated were also, done. Our results suggest that A. veronii is the predominant Aeromonas spp. in cultured O. niloticus in Eastern Province of Saudi Arabia

The Atlantic little tunny, Euthynnus alletteratus, is widely distributed in temperate and tropical waters of the Atlantic Ocean, Black and Mediterranean Seas. In this study, wild-caught little tunny from Egypt, were found to be naturally infected with trypanorhyncha metacestodes, and the overall prevalence rate of infection was 38.7%. The blastocysts were either loosely attached to the mesentery of infected fish, or firmly attached and deeply embedded within the hepatic parenchyma. These encysted plerocerci are identified as Callitetrarhynchus gracilis (Trypanorhyncha, Lacistorhynchidae) based on its morphological and molecular characterization. The morphological characteristics of C. gracilis including scolex shape; the bothridia groove; the presence of frontal glands; the length of post-larval (appendix); metabasal armature; the existence of 'Chainette' and satellite hooks of different size were studied and described by Light and Scanning electron microscope. The phylogenetic analysis of lsrDNA gene of plerocerci confirmed the identification of the species to be deeply embedded in genus Callitetrarhynchus. The histopathological examination revealed severe pathological changes in the affected organs, including necrosis, inflammatory reactions, fibrosis and migratory tracts of the parasitic larvae together with marked visceral organs adhesions. To the best of our knowledge, this is the first report describing the detection of C. gracilis in little tunny collected from the Abu Qir landing site in Alexandria, Egypt.