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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.
This section describes how this project has advanced scientific knowledge and made a difference in the lives of coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.
SUMMARY: Researchers developed a PCR assay tool to detect a crab-specific virus that is associated with high mortality during the production of soft-shell blue crabs. This tool, developed for use in Chesapeake Bay, is now used by researchers in Florida and Louisiana for studies of the prevalence of reo-like virus (RLV) in coastal waters there.
RELEVANCE: Soft-shell crab production is plagued by mortalities of 25 percent or more. The principal investigators discovered that most of these deaths are associated with a crab-specific virus called reo-like virus (RLV). This virus causes heightened mortality of crabs in laboratory experiments and may increase mortality rates in facilities that produce soft-shell crabs in captivity (referred to as shedding systems). In these facilities, crowding of crabs may promote transmission of RLV among injured, stressed individuals. In addition, some operators discard dead crabs into nearby waterways while their shedding systems discharge water to those same water bodies, offering a potential route for RLV to infect wild crabs.
RESPONSE: This research study, supported by Maryland Sea Grant, was strengthened by repeated sampling at the same locations in Chesapeake Bay. The application of the PCR (polymerase chain reaction) assay enabled confirmation that RLV prevalence in the estuary is highly variable both spatially and temporally. Through a competitive process, Emily Flowers was selected as a Maryland Sea Grant Research Fellow and conducted research on this project..
RESULTS: Researchers shared the PCR assay tool with colleagues who are applying it to better understand RLV. One of the principal investigators made a presentation to the Chesapeake Bay Program’s Sustainable Fisheries Goal Implementation Team. The Florida Fish and Wildlife Research Institute has collaborated with one of the investigators, Eric Schott of the University of Maryland Center for Environmental Science, to use the tool to study the prevalence of the virus in Florida waters; results of this study confirm that RLV is present along Florida’s Gulf coast. At the request of a Louisiana Sea Grant fisheries specialist, the Schott lab confirmed RLV in that state, a finding now published in two manuscripts. As a follow up, Dr. Schott and a colleague submitted a proposal to a national Sea Grant competition for funds to conduct workshops in Maryland and Louisiana and to partner with fishermen to develop improved shedding practices to reduce the effects of RLV.
Flowers, EM; Bachvaroff, TR; Warg, JV; Neill, JD; Killian, ML; Vinagre, AS; Brown, S; Almeida, A; Schott, EJ. 2016. Genome sequence analysis of CsRV1: A pathogenic reovirus that infects the blue crab Callinectes sapidus across its trans-hemispheric range. Frontiers in Microbiology7(126):1 -9. doi:10.3389/fmicb.2016.00126. UM-SG-RS-2016-21.
Flowers, EM; Simmonds, K; Messick, GA; Sullivan, L; Schott, EJ. 2016. PCR-based prevalence of a fatal reovirus of the blue crab, Callinectes sapidus (Rathbun) along the northern Atlantic coast of the USA. Journal of Fish Diseases39(6):705 -714. doi:10.1111/jfd.12403. UM-SG-RS-2016-15.
Rogers, HA; Taylor, SS; Hawke, JP; Schott, EJ; Lively, JAA. 2015. Disease, parasite, and commensal prevalences for blue crab Callinectes sapidus at shedding facilities in Louisiana, USA. Diseases of Aquatic Organisms112(3):207 -217. doi:10.3354/dao02803. UM-SG-RS-2015-02.
Bowers, HA; Messick, GA; Hanif, A; Jagus, R; Carrion, L; Zmora, O; Schott, EJ. 2010. Physicochemical properties of double-stranded RNA used to discover a reo-like virus from blue crab Callinectes sapidus. Diseases of Aquatic Organisms93(1):17 -29. doi:10.3354/dao02280. UM-SG-RS-2010-22.