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

Abstract

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 the diseased hepatocytes but not within the endothelium. This was supported by observing extracellular and intracellular (mostly intraendosomal), 60-100 nm 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 the 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 the available reverse transcriptase polymerase chain reaction (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 it presents with partial genetic homology with TiLV (190 nucleotides).

Detection of Tilapia Lake Virus in Clinical Samples by Culturing and Nested Reverse Transcription-PCR

“…To further examine the developed nested RT-PCR, we applied it to detect TiLV RNA in liver samples preserved in RNAlater reagent, which were taken from South American tilapia showing signs of syncytial hepatitis (11,12), or from healthy controls. This test was performed in a blinded way using the following procedure: the presence or absence of TiLV RNA in the samples was tested by RT-PCR (12) at St. George's University, Grenada.…”

“…High mortalities were also observed in naive tilapia exposed to an isolate of TiLV (10). Tilapia mortality, suspected of having a viral etiology, has also been described in Ecuador (11,12). Although variations in pathological presentation have been described (where lesions were focused in the central nervous system or in the liver, in Israel or Ecuador, respectively), sequencing the whole genome of TiLV revealed that tilapia in the two countries were infected with almost identical viruses (4).…”

Production‐level risk factors for syncytial hepatitis in farmed tilapia (Oreochromis niloticus L)

“…Sixty‐seven asymptomatic fish were also obtained from ponds without excess mortality. Specific RT–PCR assays were performed accordingly (del‐Pozo et al., ).…”

“…Presence of SHT was defined as excess mortality between 28,000 and 40,000 deaths per 100,000 fish per production cycle, weak to strong positive band signals on RT–PCR for the orthomyxo‐like virus correlating with low (asymptomatic fish) to high viral loads (classical clinical signs), necrotizing hepatitis and syncytia giant cell formation (Bacharach et al., ; Ferguson et al., ; del‐Pozo et al., ). A modified Poisson regression model was constructed and used to determine the production‐level risk factors for the presence of SHT with the dependent variable, excess mortality being dichotomously categorized as a present or absent variable.…”

“…The identity of the associated virus in all of these distinct pathological presentations was shown to be an RNA virus (Eyngor et al., ; Tsofack et al., )‐related genetically (Bacharach et al., ) and morphologically to the Orthomyxoviridae family (del‐Pozo et al., ). There is now considerable information about the transboundary distribution, clinical presentation, gross pathology, histopathology and ultrastructural pathology of the disease, as well as characterization of the associated Orthomyxo‐like virus of tilapia (Bacharach et al., ; Dong et al., ; Eyngor et al., ; Ferguson et al., ; del‐Pozo et al., ; Tsofack et al., ), but we are aware of no work on epidemiological characteristics associated with its incidence and distribution. Knowledge of the epidemiological factors for SHT may contribute to the development of prevention and control strategies that should minimize the spread of the associated virus.…”