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White spot disease (WSD) is a pandemic disease caused by a virus commonly known as white spot syndrome virus (WSSV). Several risk factors for WSD outbreaks have been suggested. However, there have been very few studies to identify risk factors for WSD outbreaks in culture systems. This paper presents and discusses the risk factors for WSSV infection identified during a longitudinal observational study conducted in a Vietnamese rice-shrimp farming system. A total of 158 variables were measured comprising location, features of the pond, management practices, pond bottom quality, shrimp health and other animals in the pond. At the end of the study period WSSV was detected in 15 of the 24 ponds followed through the production cycle (62.5%). One hundred and thirtynine variables were used in univariate analyses. All the variables with a p-value ≤ 0.10 were used in unconditional logistic regression in a forward stepwise model. An effect of location was identified in both univariate and multivariate analyses showing that ponds located in the eastern portion of the study site, closer to the sea, were more likely to test positive for WSSV by 1-step PCR at harvest. Ponds with shrimp of a smaller average size 1 mo after stocking tended to be positive for WSSV at the end of the production cycle. Average weight at 1 mo was also highlighted in multivariate analyses when considered as either a risk factor or an outcome. Other risk factors identified in univariate analyses were earlier date of stocking and use of commercial feed. A number of variables also appeared to be associated with a reduced risk of WSSV at harvest including the presence of dead post larvae in the batch sampled at stocking, presence of Hemigrapsus spp. crabs during the first month of production, feeding vitamin premix or legumes, presence of high numbers of shrimp with bacterial infection and the presence of larger mud crabs or gobies at harvest. No associations were detected with WSSV at harvest and stocking density, presence, or number or weight of wild shrimp in the pond. The multivariate model to identify outcomes associated with WSSV infection highlighted the presence of high mortality as the main variable explaining the data. The results obtained from this study are discussed in the context of WSD control and areas requiring further investigation are suggested.KEY WORDS: White spot disease · Aquatic epidemiology · Risk factors · Penaeus monodon · Rice-shrimp farming systemResale or republication not permitted without written consent of the publisher

White spot disease (WSD) is at present the most serious viral disease affecting cultivated shrimp species globally. The causative agent, white spot syndrome virus (WSSV), is extremely virulent, has a wide host range and can also be transmitted from broodstock to their offspring. The shrimp postlarvae (PL) act as asymptomatic, latent carriers of the virus, and stocking of WSSV-infected PL has been reported as a risk factor for WSD outbreaks in culture ponds. However, there is no population-based study on WSSV prevalence in PL of shrimp. The present manuscript documents the approaches and the results in the estimation of prevalence of WSSV in PL populations of Penaeus monodon at the time of stocking. A maximum of 300 PL from each of the 73 batches of PL stocked at various farms in the west coast of India during September 1999 to January 2000 were tested for the presence of WSSV by 2-step nested PCR. Thirty-six (49%) of the 73 batches tested positive for WSSV either by 1-step alone (3 batches) or after 2-step nested PCR (33 batches). Sub-samples of 5 PL each or 1 PL each tested to quantify the proportion of infected PL within batches showed that WSSV prevalence was very high in 1-step PCR-positive batches and low in 2-step PCR-positive batches. The study also showed that appropriate sampling and sample size were major factors in determining the prevalence of WSSV in PL populations, underlining the need for testing large samples of PL to reduce errors from falsely negative results. KEY WORDS: WSSV · PCR · Penaeus monodon · Postlarvae · Prevalence · Aquatic epidemiologyResale or republication not permitted without written consent of the publisher

Whiteleg (Litopenaeus vannamei) and black tiger (Penaeus monodon) shrimp are the most common farm-based shrimp species cultured in Vietnam, but there have been few studies of intensive whiteleg shrimp production. The objectives of this study were to describe production characteristics and to identify factors associated with yield in intensive shrimp production systems in Vietnam. This study included data from shrimp farms in four Vietnam provinces, collected from mid-2014 to early 2016. The data included key production characteristics, such as length of production cycle, number and quality of postlarvae (PL) stocked, feed intake, production yield (kg/m 2 ), harvest size (shrimp/kg), average weight and water quality measurements for some of the crops. We report descriptive statistics for the production parameters and findings from two linear mixed models describing the associations of these parameters with two outcomes: production yield and average weight of shrimp. The average stocking density for the 846 crops included in the dataset was 109 PL/m 2 , average length of production cycle was 92 days, and average production yield was 1.15 kg/m 2 . The shrimp gained 0.2 g per day, on average, and consumed an average of 0.17 kg of feed, daily/1,000 PL stocked. The linear mixed models indicated that stocking density, stocking month and region were the main predictors for production yield, while water temperature 4 weeks prior and PL quality were the main predictors for the average weight of shrimp, after accounting for the length of production cycle. This study describes baseline production parameters for whiteleg shrimp production in Vietnam. K E Y W O R D Sintensive culture, multivariable, production characteristics, Vietnam, whiteleg shrimp, yield 2015). This new species gained popularity due to its better growth performance, tolerance of a wide range of salinities and low

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