Seasonal variation in white spot syndrome virus-positive samples in broodstock and post-larvae of Penaeus monodon in Thailand

Records of PCR test results for white spot syndrome virus (WSSV) were reviewed in Thailand from 1998 to 2000 for wild Penaeus monodon broodstock purchased by hatcheries and for post-larvae (PL) brought by farmers to diagnostic laboratories for testing. Samples for PCR comprised DNA extracts from the last pleopod dissected from broodstock females after the first spawning and DNA extracts from whole, homogenized PL. There was a consistent pattern of fluctuation in percentage of WSSV-positive broodstock and PL. In broodstock, the fluctuation pattern was similar each year, with a low percentage (0 to 6%) from January to May and a higher percentage (6 to 18%) for the rest of the year, with a peak from September to November. The fluctuation pattern for PL was similar but offset with peaks and troughs occurring approximately 2 mo after those for the broodstock. The peak percentages of broodstock-positive samples were roughly constant from year to year, but those for PL decreased progressively in magnitude from 1998 to 2000. Examination of a small number of hatcheries in 2000 revealed that the percentage of WSSV-positive PL samples was significantly lower for hatcheries that routinely discarded WSSV-PCR-positive wild broodstock when compared to hatcheries that did not.