Mixed-genotype white spot syndrome virus infections of shrimp are inversely correlated with disease outbreaks in ponds

Outbreaks of white spot syndrome virus (WSSV) in shrimp culture and its relation to virus virulence are not well understood. Here we provide evidence that the presence of WSSV mixed-genotype infections correlate with lower outbreak incidence and that disease outbreaks correlate with single-genotype infections. We tested 573 shrimp samples from 81 shrimp ponds in the Mekong delta with outbreak or non-outbreak status. The variable number tandem repeat (VNTR) loci of WSSV were used as molecular markers for the characterization of single- and mixed-genotype infections. The overall prevalence of mixed-genotype WSSV infections was 25.7%. Non-outbreak ponds had a significantly higher frequency of mixed-genotype infections than outbreak ponds for all VNTR loci, both at the individual-shrimp as well as at the pond level. The genetic composition of WSSV populations appears to correlate with the health status of shrimp ponds. The causal relationship between genotypic diversity and virulence can now be experimentally approached.