Pathotyping of Vibrio isolates by multiplex PCR reveals a risk of virulent strain spreading in New Caledonian shrimp farms.

Two recurring syndromes threaten the viability of the shrimp industry in New Caledonia, which represents the second largest export business. The "Syndrome 93" is a cold season disease due to Vibrio penaeicida affecting all shrimp farms, while the "Summer Syndrome" is a geographically restricted vibriosis caused by a virulent lineage of Vibrio nigripulchritudo. Microbiological procedures for diagnosis of these diseases are time-consuming and do not have the ability to discriminate the range of virulence potentials of V. nigripulchritudo. In this study, we developed a multiplex PCR method to simultaneously detect these two bacterial species and allow for pathotype discrimination. The detection limits of this assay, that includes an internal amplification control to eliminate any false-negative results, were determined at 10 pg purified DNA and 200 cfu/ml. After confirming the effectiveness of our method using experimentally infected animals, its accuracy was compared to standard biochemical methods during a field survey using 94 samples collected over 3 years from shrimp farms encountering mortality events. The multiplex PCR showed very high specificity for the detection of V. penaeicida and V. nigripulchritudo (inclusivity and exclusivity 100%) and allowed us to detect the spreading of highly pathogenic isolates of V. nigripulchritudo to a farm adjoining the "Summer Syndrome area." This assay represents a simple, rapid, and cost-effective diagnostic tool for implementing timely risk management decisions but also understanding the seasonal and geographical distribution of these pathogens.