Document Type



Virginia Institute of Marine Science

Publication Date



Journal Of Shellfish Research





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To determine the nature and abundance of over-wintering P. marinus infections, infected oysters (Crassostrea virginica) collected from the upper James River, VA, were placed in a tray and suspended from a pier in the lower York River, VA in November 1991. Every six weeks through May 1992 oysters (n = 25) were removed from the tray, examined for P. marinus by hemolymph culture in fluid thioglycollate medium (FTM), gradually warmed in individual containers to 25-degrees-C and held for one month. After the incubation period, which permitted the development of very light and/or cryptic parasite stages to detectable levels, the oysters were reanalyzed for P. marinus by both hemolymph and tissue cultures in FTM. A second group of 25 oysters from the tray was sacrificed at the initiation of each incubation, diagnosed using FTM cultures of hemolymph and tissue, and examined for cryptic stages using immunoassays. On the basis of FTM assays, prevalence of P. marinus gradually declined from 100% in November 1991 to 32% in May 1992. Incubation of oysters at 25-degrees-C always resulted in an increase of P. marinus prevalence and intensity, suggesting that the parasite was more abundant than initial FTM cultures indicated. Immunoassay diagnosis revealed infections in many of the oysters diagnosed as negative by FTM cultures. Most infections detected by immunoassay were comprised of individual P. marinus meronts within hemocytes in the midgut epithelial lining. Previously unidentified cryptic stages were not observed. Perkinsus marinus appears to overwinter at very low intensities in a high proportion of oysters. Comparison of P. marinus prevalence and intensity in transplanted oysters maintained in the York River to that in oysters monitored at the original James River collection site suggests that salinity may greatly influence overwintering infections. Infection intensity and prevalence declined earlier and to a greater extent at the James River site (4-12 ppt) than at the York River location (19-23 ppt). It appears that the synergistic effect of low temperature and low salinity may be more important in regulating P. marinus epizootics than either factor acting alone.


Immunoassay; Parasite; Prevalence; Intensity; Oyster; Salinity; Temperature

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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 License.