Kit gives ray of hope to shrimp farmers

MANGALORE: In the year 1994, shrimp farmers were in a crisis following the outbreak of a mysterious disease which killed their entire stocks within a fortnight.
It was discovered that it was basically due to the combination of several factors, which increased stress levels and finally led to viral, bacterial and protozol infection of cultured shrimp.The virus that attacked the prawn was named ‘white spot virus’ (WSV) which was caused by one of several baculoviruses.
It affects a wide range of shrimp sizes, usually in ponds. Diseased shrimp exhibit shell spotting and sometimes a reddish colour. This virus has an incubation period of three to five days. Direct transmission occurs through several vectors including contaminated water, decomposing faecal matter or tissue, cannibalism of dying shrimp (in hatcheries).
The gravity of the situation can be gauged from the fact that the estimated losses due to the virus on shrimp exports is in excess of Rs 600 crore annually.
Though there is no cure for the virus, Mangalore’s College of Fisheries (CoF) has developed an indigenous kit which not only detects WSV in an early stage, but also another virus — monodon baculovirus — in a single test. The developed technology is the most advanced in the world.
I. Karunasagar, head, department of microbiology, CoF, who developed the kit, told The Times of India the kit uses DNA amplification technique which is capable of detecting a single virus particle. Moreover, the beauty of the technology is the DNA sample from broodstock can be taken without killing the animal. Thus live animals can be tested for the presence of virus and if negative, they can be used in hatcheries.
He observed that since there is no known treatment for virus diseases, the best strategy for health management is avoidance of the virus. The virus can come into aquaculture system through infected broodstock, infected larvae and through carrier animals.
Therefore, to prevent the entry of the virus into aquaculture system, it is necessary to check the broodstock/ larvae for virus and this is where the kit comes into play.
The kit has been developed by CoF, with financial assistance from the Department of Biotechnology. It detects WSV by a DNA-based technique called polymerase chain reaction (PCR).
Karunasagar said the technology enables shrimp farmers to select a virus-free seed for stocking. The PCR technique enables not only detection of the virus but also gradation of infection.
He said the available Taiwanese and Thai kits were not as sensitive they used ‘semi-nested’ method — which can lead to errors. His kit used ‘truly nested’ method, which took more time but gave accurate results every time.