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

Abstract

PCR and in situ hybridization analysis were used for detection of white spot syndrome virus (WSSV) in an infected, cultured shrimp population over a long period in the absence of disease outbreaks. The shrimp were derived from a single WSSV-carrier brooder and cultured first in a tank and then in outdoor ponds. Prior to harvest at 13 mo, no l-step PCR-positive specimens were found, even though most tested specimens were found to be 2-step PCR-positive. At 7 mo, 2-step PCR-positive tissues were found in 5 sampled shrimp. Heart, gill, integument, muscle and stomach tissues best supported viral replication At 13 mo several shrimp died, and l-step PCR-positive individuals were found for the first time Although superficially healthy, 10% of the surviving adults had tiny white spots on their carapace, and In sjtu hybridization analysis revealed WSSV-positive cells in 40% of the specimens examined. As before, most were found in the stomach, integument and gills, and only very few in the lymphoid organ and other organs. These observations contrasted to those for experimentally infected shrimp with gross signs of terminal 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 the shrimp population at a low intensity (i.e. nested PCR was required for detection) for a very long time in the absence of massive mortality. We hypothesize that disease outbreaks do not occur if shrimp defense mechanisms manage to contain lowintensity viral infections under low-stress culture conditions. Conversely, outbreaks may occur under stressful conditions.

Studies on effective PCR screening strategies for white spot syndrome virus (WSSV) detection in Penaeus monodon brooders

“…Simllar results were obtained in a continuation study on the offspring of 1 brooder from the earlier study (Tsai et al 1999). We therefore conclude that these samples had borderline concentrations of WSSV DNA and thus yielded different results with different sub-samples.…”

“…Investigation of WSSV infection by 2-step polymerase chain reaction (PCR) revealed that some positive brooders produced larval batches that gave both 2-step WSSV PCR positive and negative test results with different sub-samples (Lo et al 1997). A more recent study of a single WSSV-carrier brooder revealed that 10% of its 13 mo old, outwardly healthy, adult offspring had tiny white spots on their carapace and exhibited WSSV-infected cells in the stomach, integument, gills and other organs by in situ DNA hybridization (Tsai et al 1999). These results lndicate that the quantity of WSSV DNA in tested brooders may sometimes be near the limits of detection methods.…”

Shrimp Viral Diseases, Import Risk Assessment and International Trade

“…It is possible that aquaculture systems provided the environment in which WSSV outbreaks could be triggered. Molecular studies have confirmed that even in aquaculture systems, it is possible to find WSSV positive shrimp growing normally [25,26] and outbreaks are related to environmental stresses [16].…”

Taura syndrome virus loads in Litopenaeus vannamei hemolymph following infection are related to differential mortality

“…Although TSV-induced mortalities cease after the initial acute infection phase and cuticular lesions resolve, shrimp can remain carriers for prolonged periods. In this respect, TSV is similar to white spot syndrome virus, yellow head virus and other viruses which can also be carried in subclinical states at low infection loads (Tsai et al 1999) but are the source of disease outbreaks when shrimp become stressed by culture conditions (Lotz et al 2005).…”