Inapparent infection cases of tilapia lake virus (TiLV) in farmed tilapia

Tilapia farming has been affected by a newly discovered Orthomyxovirus-like, tilapia lake virus (TiLV), which has caused considerable economic loss to farmers. Currently, mortality-associated TiLV infections have been reported in tilapia farms in Israel, Ecuador, Colombia, Egypt, Thailand, Chinese Taipei, India and Malaysia. In this study, sets of samples collected from clinically healthy adult and fingerling tilapia with no signs of diseases or mortality were randomly diagnosed for TiLV. The tissue samples were examined by semi-nested RT-PCR, histopathology, and in situ hybridization (ISH). Unexpectedly, individual organs (liver, kidney, spleen, brain, and heart) of the tested adult fish (2/2) and liver of the fingerlings (9/19) exhibited positive results in the second step RT-PCR, indicating a low viral load of TiLV in the fish tissues. Sequencing analysis of 250-bp amplicons revealed 97.2% identity to the prototype strain from Israel. Histopathology was investigated in the adult fish specimens and pathological features resembling syncytial hepatitis were observed while ISH yielded no detectable signal. Unlike previous reports, this study revealed cases of inapparent or subclinical infections of TiLV in tilapia. Underlying factors and mechanisms between host and virus resulting in inapparent infection require further scientific investigation.