Syncytial Hepatitis of Tilapia (Oreochromis niloticus L.) is Associated With Orthomyxovirus-Like Virions in Hepatocytes

Using transmission electron microscopy (TEM), the presented work expands on the ultrastructural findings of an earlier report on “syncytial hepatitis”, a novel disease of tilapia (SHT). Briefly, TEM confirmed the presence of an orthomyxovirus-like virus within diseased hepatocytes, but not within endothelium. This was supported by observing extracellular and intracellular (mostly intraendosomal), 60-100nm round virions with a trilaminar capsid containing up to 7 electron dense aggregates. Other patterns noted included enveloped or filamentous virions and virion-containing cytoplasmic membrane folds, suggestive of endocytosis. Patterns atypical for orthymyxovirus included the formation of syncytia and the presence of virions within perinuclear cisternae (suspected to be the Golgi apparatus). The ultrastructural morphology of SHT-associated virions is similar to that previously reported for Tilapia Lake Virus (TiLV). A genetic homology was investigated using available RT-PCR probes for TiLV and comparing clinically sick with clinically normal fish and negative controls. By RT-PCR analysis, viral nucleic acid was detected only in diseased fish. Taken together, these findings strongly suggest that a virus is causally associated with SHT, that this virus shares ultrastructural features with orthomyxoviruses, and that it presents with partial genetic homology with TiLV (190nucleotides).