The occurrence of viral diseases that cause very high mortality can disrupt aquaculture production. This recently happened in Nile tilapia aquaculture with the emergence of a disease caused by tilapia lake virus (TiLV), which has dramatically affected tilapia farms around the world. In areas where the virus is endemic, three strategies can be used to limit the losses caused by infection: 1) improved biosecurity, 2) vaccination programmes, 3) selective b reeding to increase resistance. To explore the third strategy, we investigated the resistance to TiLV in three genetic strains of tilapia reared in Germany. We used two strains originating from Nilotic regions (Lake Mansala (MAN) and Lake Turkana (ELM)) and one from an unknown region (DRE).
Materials and methods
Nile tilapia juveniles were infected with TiLV by intraperitoneal injection or cohabitation. Immune responses were measured using a Fluidigm array and correlated with viral load and pathological changes.
Infection by injection resulted in 100% fish mortality in all three strains. However, when using cohabitation , we found that the ELM strain did not develop clinical signs of infection and had almost 100% survival. The other two strains showed severe clinical signs and a much lower survival rate of 29.3% for the DRE strain and 6.7% for the MAN strain. Disease resistance in tilapia from the ELM strain correlated with a lower viral load in both mucosal and internal tissues. The lower viral spread was associated with a stronger mx1-based antiviral response in the early phase of infection in the ELM strain. In addition, lower pro-inflammatory responses in the resistant strain may further contribute to its protection against disease-associated pathology .
Obtained results suggest the possibility of using TiLV -resistant strains as a cost-effective ad hoc solution to the TiLV challenge. However, it is important to note that fish of the resistant strain still had a significant viral load in the liver and brain 28 days after infection and could become persistent virus carriers, potentially transmitting the virus to naive populations. Therefore, the resistant strain should be used as part of an integrated approach that includes biosecurity, diagnostic and vaccination measures as appropriate.