Weight-dependent susceptibility of tilapia to tilapia lake virus infection

Tilapia lake virus (TiLV) causes severe anaemia and systemic disease in tilapia

Tilapia lake virus disease (TiLVD) is an emerging disease in tilapia that is associated with mass mortality affecting global tilapia aquaculture. In this study, red hybrid tilapias (Oreochromis spp.) were experimentally infected by intracoelomic injection with Tilapia lake virus (TiLV) to gain a better understanding of the clinicopathological changes during infection. Pale bodies and gill were observed in infected fish after 7 days of post-challenge (dpc) associated with severe anaemia. Further haematological analysis in TiLV-infected fish revealed decreased levels of haemoglobin and haematocrit at 3 dpc. Common pathological findings included pale and friable liver, pale intestine with catarrhal content, and dark and shrunken spleen in TiLV-infected fish at 7 dpc and 14 dpc. Histologically, reduced numbers of red blood cells and accumulation of melano-macrophage centre in the spleen were found in infected fish at 3 dpc, and severe lesions were more commonly observed at 7 and 14 dpc. Lymphocyte infiltration, syncytial cell formation and multifocal necrotic hepatitis were the prominent pathological findings in the liver of infected fish. The severity of pathological changes was associated with TiLV-infection with higher viral loads and with the expression pattern of pro-inflammatory cytokines and antiviral genes, including interferon regulatory factor 1 (irf1), interleukin (il-8), radical s-adenosyl methionine domain containing 2 (rsad2) and mx. Our study provides a comprehensive analysis of the haematological profile and pathological changes in tilapia during TiLV infection. Overall, lesions present in various organs, together with alteration of host immune response in TiLV-infected fish, indicate the systemic infection of this virus. The knowledge gained from this study improves our understanding of how TiLV causes pathological and haematological changes in tilapia.

Chitosan nanoparticle immersion vaccine offers protection against tilapia lake virus in laboratory and field studies

Tilapia lake virus (TiLV), an enveloped negative-sense single-stranded RNA virus, causes tilapia lake virus disease (TiLVD), which is associated with mass mortality and severe economic impacts in wild and farmed tilapia industries worldwide. In this study, we developed a chitosan nanoparticle TiLV immersion vaccine and assessed the efficacy of the vaccine in laboratory and field trials. Transmission electron microscopy showed that the inactivated vaccine had a particle size of 210.3 nm, while the nano inactivated vaccine had a spherical shape with a diameter of 120.4 nm. Further analysis using fluorescent staining and immunohistochemistry analysis revealed the mucoadhesive properties of the nanovaccine (CN-KV) through fish gills. We assessed the efficacy of an immersion-based TiLV nanovaccine using a cohabitation challenge model. The fish that received the nanovaccine showed better relative percent survival (RPS) at 68.17% compared with the RPS of the inactivated virus vaccine (KV) group at 25.01%. The CN-KV group also showed a higher TiLV-specific antibody response than the control and KV groups (p < 0.05). Importantly, under field conditions, the fish receiving the CN-KV nanovaccine had better RPS at 52.2% than the nonvaccinated control group. Taken together, the CN-KV nanovaccinated fish showed better survival and antibody response than the control and KV groups both under laboratory control challenge conditions and field trials. The newly developed immersion-based nanovaccine is easy to administer in small fish, is less labor-intensive, and allows for mass vaccination to protect fish from TiLV infection.

Tilapia Lake Virus (TiLV) from Vietnam is genetically distantly related to TiLV strains from other countries

Tilapia Lake Virus (TiLV) is reported as a threat to tilapia aquaculture in 16 countries on four continents with outbreaks causing up to 90% mortality. This research is one of the first studies on TiLVs from Vietnam. We propagated successfully one TiLV isolate HB196-VN-2020 from a diseased tilapia sample using an E-11 cell line and evaluated its virulence in two different weights of red hybrid tilapia and three serial 10-fold diluted viral titers. Smaller fish (4.5 ± 1.98 g) were proved to be more susceptible to TiLV infection at the viral titre of 9.1 × 10⁵ TCID₅₀ fish^-1 than larger fish (20.8 ± 7.5 g) with the mortalities of 92.5% and 12.5%, respectively. Reassortant detection analysis revealed seven potential reassortment events among 23 TiLV genomes, indicating the mixed infection of multiple TiLV isolates at the farms and the fish movement among different regions. Seven maximum likelihood phylogenetic trees based on the individual segments or the concatenated coding regions of some segments showed the genetically distant relationship of the Southern Vietnamese isolate RIA2-VN-2019 with the 21 reference isolates, and suggest the different origins of two Vietnamese TiLV isolates (RIA2-VN-2019 and HB196-VN-2020). However, additional sequences from various sampling locations and times are required to better understand the impacts of genetic diversity and reassortments on the evolution, migration and natural selection of TiLVs in Vietnam and other countries.

Assessing the effect of probiotics on tilapia lake virus-infected tilapia: Transmission and immune response

Probiotics have been used to alleviate disease transmission in aquaculture. However, there are limited studies on probiotic use in modulating tilapia lake virus (TiLV). We assessed commercially available probiotic supplements used in TiLV-infected tilapia and performed mortality and cohabitation assays. We developed a mechanistic approach to predict dose-response interactions of probiotic effects on mortality and immune gene response. We used a susceptible-infected-mortality disease model to assess key epidemiological parameters such as transmission rate and basic reproduction number (R₀ ) based on our viral load dynamic data. We found that the most marked benefits of probiotics are significantly associated with immune system enhancements (~30%) and reductions in disease transmission (~80%) and R₀ (~70%) in tilapia populations, resulting in a higher tolerance of farming densities (~400 fold) in aquaculture. These findings provide early insights as to how probiotic use-related factors may influence TiLV transmission and the immune responses in TiLV-infected tilapia. Our study facilitates understanding the mode of action of probiotics in disease containment and predicting better probiotic dosages in diet and supplements to achieve the optimal culturing conditions. Overall, our analysis assures that further study of rationally designed and targeted probiotics, or mechanistic modelling is warranted on the basis of promising early data of this approach.