The transition from pre-patent to patent infection of white spot syndrome virus (WSSV) in Penaeus monodon triggered by pereiopod excision

White spot syndrome (WSS) has been found in many species of shrimp and crabs, not just in Asia but globally. The causative agent is known as white spot syndrome virus (WSSV). In the pre-patent stage of infection, there are no gross signs of WSS in the shrimp and they behave as though they were healthy. Although the pre-patent stage might persist for months, the transition can happen within a few hours under stressful conditions. In this study, stress was applied to the shrimp by removing one pereiopod each day. The stage of infection was evaluated by WSSV diagnostic polymerase chain reaction (WSSV PCR) using DNA prepared from the excised pereiopod. From being 2-step WSSV PCR-positive, a latently infected Penaeus monodon could become 1-step WSSV PCR-positive within just 24 to 48 h of being stressed. When the specimen was became 1-step WSSV PCR-positive, the WSSV specific probe and the in situ hybridization reaction revealed that cells of the stomach, epidermis, heart, hemocyte in heart, antennal glands, lymphoid organs, hepatopancreas and muscle gave positive signals. These results demonstrate the quick multiplication and spread of WSSV in P. monodon when triggered by stressor.