Dual infections of tilapia parvovirus (TiPV) and tilapia lake virus (TiLV) in multiple tilapia farms: Their impacts, genetic diversity, viral tropism, and pathological effects

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Tilapia culture is an essential protein-rich food production sector worldwide. The emergence of pathogens is sometimes associated with fatal outbreaks in tilapia, which dramatically slow production and result in severe economic losses. While tilapia lake virus (TiLV) infection has been associated with fatal diseases in tilapia, the novel tilapia parvovirus (TiPV) has been recently identified and reported to be associated with mass mortality events in tilapia farming. In this study, we identified the coinfection of a novel TiPV and TiLV in multiple independent tilapia farms in Thailand, thus causing significant losses. The full-length genomes of the TiPV were characterized, and a phylogenetic analysis was performed, indicating the genetic diversity of the TiPV and multiple amino acid mutations found in the structural protein VP1. Using the in situ hybridization (ISH) technique, TiPV was localized in the gills, heart, brain, liver, pancreas, spleen, intestine, kidney, eyes, and muscles of tilapia, with evidence of cellular tropism. The TiLV localization was confirmed using a dual ISH/immunohistochemistry protocol and transmission electron microscopy, which indicated a potential association with pathological alterations. Both TiPV and TiLV were successfully propagated in tilapia brain cells and the fish cell line E-11. This study also provided an indication of the potential cellular roles of the pathological features of TiPV coinfection with TiLV in red hybrid tilapia. Since mutations in the parvovirus structural protein led to virulence and extensive infection in susceptible hosts, several point mutations in the VP1 gene with amino acid signatures of TiPV found in tilapia in Thailand warrant future observations. Although the synergism between TiPV and TiLV coinfection remains undetermined, the contributory role of these two viruses requires intensive focus. Further investigations should lead to a suitable strategy for disease control.