الفهرس 
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Abstract
Incidences of mortalities in cultured Nile tilapia (Oreochromis niloticus) have had a negative impact on the maintenance of many tilapia hatcheries and farms for the past few years. The socioeconomic structure of Egyptian aquaculture is severely affected due to fish loss, feed waste and loss of job opportunities. There is little information about pathogenic fish viruses in Egypt although members of twelve virus families have been identified worldwide in wild and farmed fish species causing drastic losses. Efforts were directed to investigate the possible viral causes of these massive infections. Therefore, the development of the synthetic positive control standard is crucial for the survey of viruses causing mortality in cultured tilapia as a rapid, affordable, and reliable molecular diagnostic assay. It was synthesized in silico allowed the amplification of fragments from nervous necrosis virus (NNV) RNA 2 and segment 9 of tilapia lake virus (TiLV) that reported to be threat to tilapia aquaculture. TilV RNA was not detected in any of the extracts tested using RT-PCR. NNV strains phylogenetically related to red-spotted grouper NNV were detected in three tilapia hatcheries in the period of investigation (2018-2019). Clinical signs of naturally infected fish were consistent with classical NNV infections (abnormal swimming behavior and skin darkening). Electron micrographs revealed virions in the retina and brain. NNV RNA was also detected in the brood stocks’ brains and ovaries indicating the vertical transmission possibility. Experimental infection of NNV-free fries resulted in 100% morbidity and mortality. Since this is the first report of VNN-infection in African tilapia, different aspects of viral epidemiology and possible ways to break the epidemic wave and prevent future interspecies transmission events are discussed. VNN does not constitute a public health hazard because it is not transmitted from fish to humans. The disease is not a notifiable disease according to the OIE, and therefore it does not affect international fish trade (export and import between countries).