R/DIS-3

Description:

OBJECTIVES By applying tools developed with MDSG development funds to archived frozen crabs from 2008, an outbreak of a fatal blue crab virus (reo-like virus, RLV) was detected in the Rhode River. This virus has previously been shown to be present in a majority of soft shell crab mortalities. To better manage blue crab fisheries, two major questions need to be addressed: 1) How frequent are outbreaks of the RLV? and 2) Do crab shedding systems have the potential to increase RLV prevalence in the waters that receive effluent? We propose to address these questions with two objectives: 1: Assess prevalence of RLV in wild crabs in multiple creeks over a two year period. 2: Assess prevalence of RLV in creeks with / without flow-through soft shell operations. The discovery that a fatal reovirus is associated with dead and dying soft shell crabs throughout the eastern US should motivate crabbers to harvest, transport, and shed their crabs in ways to minimize mortality. METHODOLOGY Experimental design: The study will be conducted for two years, encompassing multiple rivers from the Sassafras to the Crisfield area. Working with collaborators, archived crab samples from several rivers will be analyzed as well. Two sampling areas will be in proximity to large scale commercial soft shell crab production, and will include paired sampling in creeks that are nearby but lack shedding operations. The Rhode River will be intensely monitored. Sampling will be conducted by trawling in early and late season. Through contacts with soft shell crab producers at Deal Island and Crisfield we will gather information on the intensity of shedding at these locations, and whether there is appreciable mortality. Virus will be detected in crab tissue by analysis of double stranded RNA and by Rt-PCR specific for the RLV. This tool was developed with support of MDSG development funds to Dr. Schott. A recent improvement to the Rt-PCR assay has made it quantitative, allowing us to estimate the quantity of virus in samples. Comparisons will be made between virus prevalence at each site and date to detect correlations with proximity to shedding systems, season, year, or river system. RATIONALE Soft shell crab production is plagued by mortalities of 25% or more. We recently discovered that most soft shell crabs that die are infected with a fatal crab-specific virus, called reo-like virus (RLV). Crowding of hundreds of commercially caught crabs in shedding systems may promote transmission of RLV between injured, stressed crabs. We have evidence that sentinel crabs can acquire RLV from water within a shedding system. This suggests that infectious virus is free in the water, including shedding system effluent. Virus prevalence in the wild is generally low (3 to 4%). However, we recently used a PCR-based technique to reveal that that an outbreak (60% prevalence) of RLV occurred in the Rhode River. Discovering whether virus from shedding systems contributes to wild RLV prevalence is a priority, as is finding a way to identify and exclude RLV infected crabs from shedding systems.