Transmission of white spot syndrome virus in improved-extensive and semi-intensive shrimp production systems: A molecular epidemiology study

Experimental evidence suggests that white spot syndrome virus (WSSV) can be transmitted horizontally through water, via carrier organisms and/or by cannibalism of infected shrimp, but also vertically through infected broodstock. However the mode(s) of WSSV transmission in shrimp farming systems and the epidemiological consequences are not well understood. In this study, molecular markers have been used to analyse the spread of WSSV within and between farms in Viet Nam for two different farming systems: improved-extensive and semi-intensive. Samples were periodically obtained from twenty improved-extensive shrimp ponds and twenty-three semi-intensive shrimp ponds applying different sampling schemes. PCR-genotyping was employed using three WSSV molecular marker loci: the variable tandem repeat regions (VNTR) in ORF75, ORF94 and ORF125 (GenBank AF369029 ). An analysis on the combined results of all three VNTR marker loci made it possible to follow transmission pathways of WSSV in the respective shrimp farming systems. The transmission of WSSV infection on improved extensive shrimp farms appeared mainly due to the recycling of WSSV over time in the same pond, whereas in semi-intensive shrimp farms transmission of WSSV was mainly from neighbouring ponds. These findings provide important information for field applications by suggesting pathway-specific control strategies.