Qualitative and quantitative studies on the relative virus load of tails and heads of shrimp acutely infected with WSSV

There is currently concern and controversy in the shrimp industries of the Americas about the risk posed by the importation and reprocessing of shrimp infected with white spot syndrome virus (WSSV) and yellow head virus (YHV). To further understand the risk, more knowledge concerning the quantitative virus load of infected shrimp is needed. The present study was carried out to better define, using qualitative and quantitative methods, the relative virus load of shrimp heads and tails. For these studies, specific pathogen-free (SPF) Penaeus vannamei were infected with WSSV. Emergency harvest of these shrimp was simulated by collecting the infected shrimp at the onset of postinfection mortalities and determining the relative virus loads of the head and tails by quantitative real-time PCR and histology methods. Routine histology and in situ hybridization assay with a WSSV-specific DNA probe demonstrated qualitatively similar levels of WSSV infection in the heads and tails of experimental infected shrimp. The novel real-time PCR method demonstrated quantitatively that the head had a slightly higher WSSV load than did the tail. However, since the tail represents 58% of the total body weight, the total virus load on a per weight basis turns out to be similar in the head (49%) and tail (51%) of the same shrimp with acute phase WSSV infections. In proportion to the total tail weight, the virus load of the peeled shell represents 55% of the total viral load in tail.