(A popular article appeared in the Science
and Technology edition of
Indian shrimp farming which is passing through a troubled phase continues to be crippled due to a serious viral disease in the grow out ponds, which is known as White Spot Disease. Although it is possible that medicinal plant extracts, nutrient supplements, and pro-biotics can improve rearing success, there is no scientific cure available at present. With the sufficient knowledge of SEMBV (Systemic Ectodermal and Mesodermal Baculovirus), the causative virus, a number of immediate measures that could help to substantially improve the current situation can be used. The following package of preventive measures could be adopted to keep the virus out of the production system. Pond preparation by disinfection and elimination of potential carriers by fine screens installed at inlets. Use of reservoir, as the virus does not seem to remain infectious for more then few days when free in sea water, a simple process of storage can prevent infection, so long as no carriers are present. Refusal of fresh feed inputs especially crabs and other crustaceans, and disinfection of infected ponds before discharge into outlet canals are also to be strictly implemented. Now, very importantly, monitoring broodstock, post larvae and pond reared shrimp using DNA probes or gene probes, as the post larvae are fairly strongly implicated as a possible route of SEMBV transmission.
The limitations of conventional diagnostic methods (histopathology, bioassays etc.)in detecting the virus in a meaningful timeframe and inability to diagnose asymptomatic infection made way for DNA based diagnostics. These sensitive and highly specific detection methods are now commercially available for detecting all known shrimp viruses. These easy- to- use diagnostic kits can be handled by any farm or hatchery technician at the field level. This can be used as a non-lethal means of testing broodstock by using selected tissue from live animals.
The gene probe dot blot format uses either haemolymph or macerated tissue
dotted on a nylon membrane. The probe is a double stranded DNA that is
specific to certain sequences of the virus and has been modified by being
linked with the reporter chemical, digoxigenin (DIG). The probe is boiled
to separate the two strands of DNA and, when added to the sample, will
bind specifically to the target viral DNA. Anti-DIG antibodies conjugated
with an enzyme, alkaline phosphatase, are added. If the virus is present,
the antibodies bind to the DIG. The bound alkaline phosphatase ultimately
reacts with the substrate and dye compound to cause a colour change, signaling
the presence of the virus. The added advantage of gene probe is the interpretation
of final result, which is very difficult in the case of histopathology
and requires expertise to interpret. But any hatchery or farm technician
can interpret the end result of gene probe test.