White spot syndrome virus genetic variants detected in Mexico by a new multiplex PCR method

Abstract

One of the most serious problems confronted by the shrimp farming industry is the disease caused by white spot syndrome virus (WSSV). In 1999, the first cases of this disease in Mexico were not easily diagnosed because some polymerase chain reaction (PCR) primers previously published for WSSV detection failed to detect some variants of the virus. This is the first systematic survey of the gross signs, histopathology, transmission electron microscopy (TEM), and multiplex PCR in WSSV from 123 shrimp farms located in 20 different coastal provinces in Mexico. Using a set of five primer pairs in a multiplex PCR reaction, WSSV-positive shrimp from Thailand (positive control 1) yielded five expected amplicons of 1447, 941, 775, 365, and 306 bp (PCR Profile 1). Using the same multiplex PCR, WSSV-infected shrimp samples from Honduras gave three amplicons of 775, 365, and 306 bp (PCR Profile 2). Based on histopathology, TEM, and PCR, 76 (62%) of 123 Mexican shrimp farms sampled gave WSSV-PCR-positive results. For both Litopenaeus vannamei and L. stylirostris, two PCR profiles were seen, one identical to Profile 2 and another with only one amplicons of 306 bp (PCR Profile 3), which suggests that different outbreaks were caused by two different WSSV genetic variants in Mexico. Wild crabs Callinectes arcuatus also gave the type 3 profile. None of the WSSV-infected Mexican shrimp gave Profile 1. By transmission electron microscopy, samples from Sinaloa State (Mexico) showed ovoid to subcylindrical virions (230–433×81–160 nm), with terminal projections inside the nuclei of gill epithelial cells.