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Walker P.J.,CSIRO | Gudkovs N.,CSIRO | Mohan C.V.,A+ Network | Raj V.S.,Central Institute for Brackishwater Aquaculture | And 6 more authors.
Aquaculture | Year: 2011

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. © 2011.

Walker P.J.,CSIRO | Gudkovs N.,CSIRO | Pradeep B.,Karnataka Veterinary, Animal and Fisheries Sciences University | Stalin Raj V.,Central Institute for Brackishwater Aquaculture | And 7 more authors.
Aquaculture | Year: 2011

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. © 2011.

Walker P.J.,CSIRO | Gudkovs N.,CSIRO | Padiyar P.A.,A+ Network | Stalin Raj V.,Central Institute for Brackishwater Aquaculture | And 8 more authors.
Aquaculture | Year: 2011

A longitudinal study was conducted from January to August 2005 in small-holder black tiger shrimp (Penaeus monodon) ponds in the West Godavari District of Andhra Pradesh, India (16°25' N, 81°19' E). The study involved 457 ponds owned by low-income farmers participating in a better management practice (BMP) programme. Disease outbreaks occurred in 16.6% of ponds. There was significant spatial clustering of disease outbreaks with 31 (40.8%) of the 76 recorded disease outbreaks occurring in a single village block. Bivariate analysis indicated a 1.6-fold higher likelihood of disease outbreaks from nursery-stocked ponds but this was not significant in multivariate analysis due to the confounding effect of pond location. There was evidence of increasing prevalence of WSSV infection during grow-out. WSSV was detected in 5.9% of 119 batches of postlarvae tested at stocking, 38.2% of 34 juvenile batches collected at the time of transfer to grow-out ponds, and 47.0% of 336 pond stock tested at normal harvest or crop failure. WSSV was detected in 43 of 59 (72.9%) disease outbreak ponds tested and 115 of 277 (41.5%) non-outbreak ponds tested. Heavy WSSV infection was detected at harvest in 116 of the 336 (34.5%) of the ponds tested, including 78 ponds for which no outbreak was recorded. Duration of crop was recorded for 431 ponds with a mean of 117.0. days and a range of 20 to 176. days. Median duration was significantly shorter for disease outbreak ponds (68.5. days) compared to non-outbreak ponds (119.0. days). Duration of crop also varied according to WSSV detection levels at harvest, with median duration for ponds classified as heavy WSSV infection (108.5. days) significantly shorter than for ponds classified as either light WSSV infection (116.0. days) or WSSV-negative (116.5. days). The study indicated a high risk of WSSV infection during grow-out but a relatively low incidence of disease despite a high prevalence of heavy WSSV infection in non-outbreak ponds. © 2011.

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