Emergence of viral nervous necrosis is associated with mass mortality in hatchery-reared tilapia (Oreochromis niloticus) in Egypt

Mass mortalities in Egyptian hatchery-reared Nile tilapia (Oreochromis niloticus) fries have negatively impacted the livelihood of fish farmers for the past 3 years due to fish loss (up to 70%), feed waste, and loss of job opportunities. An investigation into possible viral causes of massive die-offs in tilapia during 2018–2019 identified nervous necrosis virus (NNV) strains phylogenetically related to red-spotted grouper NNV. Clinical signs and gross pathology of naturally infected fish were consistent with classical NNV infections. Electron micrographs revealed virions in the retina and brain. NNV RNA was also detected in the brood stocks’ brains and ovaries. Experimental infection of NNV-free fries resulted in 100% morbidity and mortality. Since this is the first report of VNN-outbreaks in African tilapia, different aspects of viral epidemiology and possible ways to break the epidemic wave and prevent future interspecies transmission events are discussed.