Long-term presence of white spot syndrome virus (WSSV) in a cultivated shrimp population without disease outbreaks

PCR and in situ hybridization analysis were used for detection of white spo t syndrome virus (WSSV) in an infected, cultured shrimp population over a l ong period in the absence of disease outbreaks. The shrimp were derived fro m a single WSSV-carrier brooder and cultured first in a tank and then in ou tdoor ponds. Prior to harvest at 13 mo, no 1-step PCR-positive specimens we re found, even though most tested specimens were found to be 2-step PCR-pos itive. At 7 mo, 2-step PCR-positive tissues were found in 5 sampled shrimp. Heart, gill, integument, muscle and stomach tissues best supported viral r eplication. At 13 mo several shrimp died, and 1-step PCR-positive individua ls were found for the first time. Although superficially healthy, 10% of th e surviving adults had tiny white spots on their carapace, and in situ hybr idization analysis revealed WSSV-positive cells in 40% of the specimens exa mined. As before, most were found in the stomach, integument and gills, and only very few in the lymphoid organ and other organs. These observations c ontrasted to those for experimentally infected shrimp with gross signs of t erminal WSSV infection, where strong positive signals were also observed in the lymphoid organ and in other organs of ectodermal or mesodermal origin. Our results showed clearly that whatever the source, WSSV was carried in t he shrimp population at a low intensity (i.e. nested PCR was required for d etection) for a very long time in the absence of massive mortality. We hypo thesize that disease outbreaks do not occur if shrimp defense mechanisms ma nage to contain low-intensity viral infections under low-stress culture con ditions. Conversely, outbreaks may occur under stressful conditions.