Longitudinal disease studies in small-holder black tiger shrimp ( Penaeus monodon) farms in Andhra Pradesh, India. III. A complex dynamic of WSSV infection and WSSV genotype distribution in farmed shrimp and wild crustaceans

Abstract

A longitudinal study of white spot syndrome virus (WSSV) infection and disease was conducted from January to August 2007 in a cluster of 12 small-holder shrimp ( Penaeus monodon) ponds near Mallampudi in the Krishna District of Andhra Pradesh, India. Seven of the ponds had been sampled during the previous crop (Walker et al., 2011a); five adjacent ponds had not been sampled previously. Samples of mud were collected from the bottom of each pond at the commencement of the study. At intervals of approximately 10 days, farmed shrimp were sampled from the ponds and wild shrimp, crabs and plankton were sampled from inside ponds and from canals outside each pond. Of the 375 samples collected, 216 (57.6%) were WSSV-positive by Taqman PCR. The overall prevalence of WSSV infection varied significantly amongst sample types and was higher in farmed shrimp (76.0%) than for any other sample category (56% for crabs and plankton, 47% for wild shrimp, 25% for sediment). A wave of WSSV infection in plankton and wild crustaceans occurred across the study site, commencing at day 10, intensifying at day 20, subsiding at days 30 and 40, and passing by day 50. By day 60, only 2 ponds remained operational. WSSV-infection in plankton and wild crustacean samples collected from inside and outside these ponds again increased with a second peak at day 70. The pattern of WSSV infection in farmed shrimp appeared to follow the first wave of infection in plankton and wild crustaceans, with heavy viral genetic loads detected in most samples collected from days 30 to 80, including the final samples collected from 11 of the 12 ponds. Genotype analysis using the ORF94 variable number tandem repeat (VNTR) marker identified a very wide range of concurrently circulating WSSV genotypes (TRS1–TRS33) with multiple genotypes commonly detected in individual samples of all categories. Genotype TRS18, which had been associated with disease outbreaks in the previous crop, was not detected commonly in plankton or wild crustaceans. Although TRS18 was the most commonly detected genotype in farmed shrimp samples, it did not appear to be the cause of white spot disease outbreaks. There were several examples of simultaneous heavy infections with the same genotype in shrimp from several ponds but there was no clear pattern of association of a single WSSV genotype or a constellation of genotypes with disease outbreaks or pond abandonment.