The last decade has seen the emergence of three novel viruses in Asian seabass, Lates calcarifer. They are scale drop virus (SDV) first described in 2012, Lates calcarifer herpesvirus (LCHV) reported in 2018 and recently a novel birnavirus in 2019. While molecular detection tools for these virus infections are not lacking, there is a paucity of information on the origin of these viruses and their contribution to disease observed in the field. In this study, we present the pathology observed in association with detection of these viruses, and correlate pathology to viral loads in specific organs. Fish from both sea cages and land based nursery tanks are included in this study. These diseases are difficult to distinguish based on clinical signs or gross pathology.
Preliminary studies suggest varying severity of splenic changes in fish tested positive for SDV by PCR. In presumably early cases of SDV infections, there is mild to moderate lymphocytic-monocytic infiltration of splenic ellipsoids to more progressive stages with multifocal to extensive coalescing splenic necrosis, often involving entire segments of spleens and resembling infarcts. Some SDV PCR positive fish with early splenic changes were clinically healthy, that is, did not show external signs of disease. SDV positive clinically diseased fish usually have severe renal glomerular necrosis.
LCHV PCR positive fish had remarkable liver pathology, ranging from atrophied lobulated livers to marked loss of hepatic acini leaving behind remnants of exocrine pancreas in what is normally hepatopancreas (liver). Extensive renal tubular necrosis and attenuated tubular epithelium are observed in LCHV PCR positive fish.
The pathology associated with the novel Lates birnaviral infection is currently unclear. It is also unclear the exact contributions of these novel viral infections to pathology observed in co-infections. The authors hoped that we will have more information by WAS 2022 in Singapore, as this is a study in progress.