Viability of white spot syndrome virus (WSSV) in sediment during sun-drying (drainable pond) and under non-drainable pond conditions indicated by infectivity to shrimp

摘要

White spot syndrome virus (WSSV), is a highly virulent rapidly replicating large, enveloped, double-stranded DNA (dsDNA) virus, causing an estimated losses of over US$6 billion since its emergence in 1992. As part of the better management practices (BMPs), ploughing, tilling and sun-drying of shrimp culture ponds are advocated for prevention and control of this disease. Despite this, instances of outbreaks of white spot disease (WSD) recur, causing loss to shrimp farmers. Some studies have indicated that WSSV DNA is reported to persist for over 20 months in sediments as detected by PCR. Since mere detection of virus using PCR methods does not indicate its viable nature and ability to infect, information on its viability outside the host in water and pond sediment is necessary. Hence, in this study, the viability of WSSV in seawater and shrimp pond sediments under experimentally simulated drainable and non-drainable pond conditions was examined by shrimp infectivity experiments. WSSV with an initial viral load of 1000 virions mL(-1) was found to be viable for a period of 12 days in seawater of 27 ppt salinity, pH of 7.5 at 29-33 degrees C as revealed by its ability to infect juvenile shrimp, whereas, in shrimp pond sediment (with initial viral load of 211,500 copies g(-1)), the virus was viable and infective up to 19 days despite sun-drying. In the case of non-drainable conditions, WSSV (753,600 copies g(-1)) remained infective for a period of 35 days. Although the sediment samples tested nested PCR positive after 19 days of sun-drying and 40 days under water-logged conditions, shrimps did not develop WSD, suggesting that WSSV was not viable. Over a period of time after 21 days under sun-drying and 40 days under non-drainable experimental conditions, due to reduction in viral load, sediments were positive only by nested PCR, and by this time, viability of WSSV was almost lost as revealed by shrimp infectivity. Hence, PCR testing of shrimp farm sediment before starting culture as one of the BMPs may help in ensuring biosecurity from WSSV. The information generated here would help in the improvement of better management practices (BMPs) with regard to pond preparation protocols for shrimp aquaculture