Longitudinal disease studies in small-holder black tiger shrimp (Penaeus monodon) ponds in Andhra Pradesh, India. II. Multiple WSSV genotypes associated with disease outbreaks in ponds seeded with uninfected postlarvae

A longitudinal study was conducted from January to August 2007 in a cluster of 61 small-holder shrimp (Penaeus monodon) ponds at Mallampudi (16°25′29″ N, 81°19′13 ″ E) in the Krishna District of Andhra Pradesh, India. Exhaustive PCR testing of postlarvae collected from ponds at the time of seeding detected no evidence of white spot syndrome virus (WSSV) infection. However, during grow-out, disease outbreaks occurred in 42 of the ponds (68.9%) in which the mean and median crop durations were 42.8 days and 40.5 days, respectively. Only three of the 61 ponds (4.9%) were harvested after more than 120 days of culture. Of 41 ponds sampled at harvest, 35 (85.4%) were WSSV-positive by PCR, including 27 of 28 (96.4%) disease outbreak ponds, of 17 which were graded as heavy or moderate infections. Eight of 13 (61.5%) normal harvest ponds sampled were WSSV-positive at the time of harvest, of which seven (53.8%) were graded as light or very light infections. WSSV genotype analysis was conducted on samples from 35 ponds using the ORF94 variable number tandem repeat (VNTR) marker. In total, 20 different genotypes from TRS1-TRS25 (1 to 25 repeats) were detected. Multiple TRS genotypes were detected in 27 of the 35 ponds sampled (77.1%) and 73 of the 146 individual shrimp sampled (50.0%). The most evident temporal and spatial associations of WSSV genotypes with disease outbreaks were the dominance of genotype TRS18 in seven ponds located on the eastern side and genotype TRS8 in eleven ponds in the central region of the study area. The study indicated a high risk of exposure to WSSV infection during grow-out and that multiple WSSV genotypes were circulating simultaneously in the farming area. The spatial and temporal pattern of WSSV genotype distribution suggested transmission of infection within two clusters of ponds.