Pathogenesis and immune response of Nile tilapia (Oreochromis niloticus) exposed to Tilapia lake virus by intragastric route.

Tilapia lake virus (TiLV) is regarded as one of the most important pathogens in tilapia aquaculture worldwide. Despite this, little is known regarding disease pathogenesis and immune responses to infection. The main objective of this study was to investigate the tissue distribution, histopathological changes, and immune response of fish exposed to TiLV. Nile tilapia (Oreochromis niloticus) maintained at 25 ± 2 °C were challenged with TiLV via intragastric-gavage. At 0.5, 1, 3, 5, 7, 10 and 15 days post-challenge (dpc), six fish per treatment were euthanized and subjected to complete necropsy. TiLV exposed fish presented 45% cumulative mortality at the end of the study. Gross lesions included cutaneous petechiae and ecchymoses, scale losses, skin ulcers, and exophthalmia. Mild multifocal hepatocellular degeneration and necrosis was observed as early as 3 dpc occasionally accompanied by syncytial formation, intracytoplasmic inclusion bodies, and inflammatory infiltrates of lymphocytes at subsequent time points. Necrosis of epithelial cells of the gastric glands and intestinal glands was also observed as early as 5 dpc. Intestinal samples showed reactive in situ hybridization signals as early as 1 dpc. No other lesions were observed in the brain or other organs. Histological changes were associated with viral dissemination and disease progression, as evidenced by increased TiLV detection in the intestine, gills, liver and spleen. Highest TiLV abundance was detected 7 dpc in gills, intestine, and liver showing an average of 6 LOG genome equivalent per ng of total RNA. Different transcript abundance was detected for the pro-inflammatory cytokine interleukin-1ß and interferon-induced myxovirus resistance protein gene in the mucosal sites (gills and intestine). Interferon regulatory transcription factor 3 transcript was more abundant in systemic organs (liver and spleen) while the expression in gills and intestine showed mixed expression at different time points. On the other hand, transforming growth factor ß expression patterns differed amongst the tissues with a trend towards downregulation of the gene in liver and gills, and a trend towards upregulation in the spleen and intestine. Overall, these results demonstrate the intestinal routes as a main port of entry for TiLV, which subsequently spreads systematically throughout the fish body.

Intragastric and intracoelomic injection challenge models of tilapia lake virus infection in Nile tilapia (Oreochromis niloticus L.) fingerlings.

To gain a better understanding of the pathogenesis of tilapia lake virus (TiLV) infections in Nile tilapia (Oreochromis niloticus), fingerlings were challenged with a single dose of 1 × 104  TCID50 /fish of TiLV utilizing intracoelomic/intraperitoneal (ICch ) or intragastric (IGch ) routes. Acute mortalities were present in both groups, reaching 70 and 40% in ICch and IGch after 10 days, respectively. Challenged fish presented erratic swimming, lethargy, anorexia, exophthalmia and cutaneous petechiae and ecchymoses. Histological changes in challenged groups included syncytial formation, intracytoplasmic inclusion bodies and multifocal hepatocellular degeneration and necrosis. In addition, multifocal areas of mild proliferation of glial cells and lymphocytic perivascular cuffing were observed in the brain of exposed challenged groups. TiLV RNA was detected in gills and faeces of challenged fish using quantitative reverse transcriptase-PCR, as well as in the tank water holding challenged fish. Moreover, TiLV RNA was detected in scrolls obtained from formalin-fixed paraffin-embedded tissue blocks from challenged fish. Results from this study suggest that IG methods represent an additional method to study the pathogenesis of the disease in this species, as it results in infection and diseases as in naturally occurring cases and does not bypass important mucosal immune responses as injectable routes do.