The occurrence of viral diseases that cause very high mortality can disrupt aquaculture production. This has recently occurred 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. TiLV has reached global distribution, while the host range remains to be defined to better assess the threat posed by the virus. Therefore, the aim of this study was to evaluate the potential of TiLV to infect salmonids in a range of water temperatures that can be reached during summer heatwaves in continental Europe. Further investigations were carried out in canonical and non-canonical hosts to determine if resistance to the TILV is possible.
The susceptibility of several cell lines was assessed at different temperatures to find the optimal conditions for virus replication. The susceptibility of juvenile rainbow trout, brown trout and Atlantic salmon larvae to infection with TiLV was studied in infection experiments based on cohabitation with infected tilapia or intraperitoneal injection of the virus at elevated water temperatures of 20°C and 25°C. In addition, two Nile tilapia strains from Nilotic regions (Lake Mansala (MAN) and Lake Turkana (ELM)) and one from an unknown region (DRE) were used in infection experiments at 25°C. Immune responses were measured using a Fluidigm array and correlated with viral load and pathological changes.
TiLV was able to infect salmonid cells in vitro over a wide temperature range from 15°C to 25°C. Infection experiments showed that the susceptibility of rainbow and brown trout to the virus was low, considering the ability of the virus to enter the organism. Exposure of these fish to the virus by cohabitation did not result in high levels of virus in the liver and brain. However, the permissiveness, i.e. the ability of the virus to replicate in the body of the fish, is high because i.p. injection of TiLV resulted in high levels of virus replication in the internal organs of rainbow trout, brown trout and Atlantic salmon. Similarly, injection of the virus resulted in high permissiveness in all three Nile tilapia strains. However, when we used infection by cohabitation, we found that the ELM strain was resistant to the disease, showed no clinical signs of infection and had almost 100% survival. 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.
To summarise: The bad news is that TiLV has some pathogenic potential in salmonids, which could theoretically be enhanced by climate change and anthropogenic activities. The good news is that TiLV-resistant Nile tilapia strains can be used as a cost-effective ad hoc solution to the TiLV challenge. However, it is important to note that fish of the resistant strain become persistent carriers of the virus and can potentially further transmit the virus. Therefore, the resistant strain should be used as part of an integrated approach that includes biosecurity, diagnostic and vaccination measures as appropriate. Further studies should determine which factors in the mucosal barrier lead to resistance in canonical and non-canonical hosts.