Tilapia, Oreochromis niloticus, is the second most cultured fish, after carp. Tilapia is hardy and easy to rear, making it attractive to the aquaculture industry. However, the tilapia lake virus is a major threat to the fish as it causes high mortality. Tilapia lake virus epidemics have been reported from different parts of the world in recent years.
The virus is a single stranded RNA virus, like SARS-CoV 2, which is doing the rounds on human populations now. Such viruses are capable of switching hosts.
Does co-culturing carp fishes like Rohu, Labeo rohita, with tilapia pose a threat of tilapia lake virus infection to rohu?
Scientists at the ICAR-National Bureau of Fish Genetic Resources, Lucknow, were concerned. To check, they brought healthy juveniles of rohu and tilapia from their farm and cultured them in fibre-reinforced plastic tanks in their wet lab.
They refreshed a part of the 200 litres of water in each tank daily and monitored the temperature of the water, dissolved oxygen and pH regularly to assure a healthy environment.
Then they tested the fish randomly for the tilapia virus, to ensure that the fish were free of infection and injected the supernatant from infected cell lines into the peritoneum of 30 fish each of rohu and tilapia.
Ten each were kept separate for studying mortality patterns. The remaining 20 fish of rohu and tilapia were kept for quantifying the virus at different time points.
Every three days, post infection, the scientists collected the liver, brain and intestines of three fishes from each of the two groups and checked the viral load using quantitative real time polymerase chain reaction.
The liver and intestine of rohu showed negligible amounts of virus on the sixth day. And by the ninth day, the virus was undetectable in the liver. The brain showed no virus at all. The liver of tilapia however, was found to have about 40 million viruses by the sixth day which reduced to about five million by the end of the 12 day experimental period. The intestines also showed a similar pattern: the viral load increased to about six million in six days and then decreased to three million. The brain had nearly 8 million viruses by the sixth day and showed only minor variation till the end.
Rohu did not exhibit any signs of infection and no mortality was seen during the 12-day experimental period. On the other hand, tilapia showed signs of infection from the sixth day and started dying from the 7th day onwards. Mortality in tilapia was 80%.
Scientists at the ICAR-National Bureau of Fish Genetic Resources now feel reassured: their results show that the tilapia lake virus does not affect rohu. Even if there is a tilapia lake virus epidemic, the rohu harvest will not be affected – good news for the aquaculture industry!
Aquaculture, 515: 734567 (2020); DOI: 10.1016/j.aquaculture.2019.734567
Babasaheb Bhimrao Ambedkar University
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