LABORATORY INVESTIGATIONS OF SUSCEPTIBILITY, INFECTIVITY, AND TRANSMISSION OF PERKINSUS-MARINUS IN OYSTERS

The protozoan parasite, Perkinsus marinus (Dermo), has caused signific ant mortality in the eastern oyster, Crassostrea virginica, along the east coast of the United States and the Gulf of Mexico, since the 1950 s. Because of its current expanded distribution and increased abundanc e, P. marinus is now considered more prevalent in the mid-Atlantic wat ers and the Chesapeake Bay in particular, than another protozoan patho gen, Haplosporidium nelsoni (MSX). The susceptibility, infectivity/pat hogenicity, and transmission of P. marinus in eastern oysters were inv estigated in numerous laboratory studies, The influence of environment al factors such as temperature, salinity, and pollution on the interac tion between the host oyster and the parasite were also examined. Thre e P. marinus life stages, the meront, prezoosporangia, and biflagellat ed zoospore, were found effective in transmitting the disease. The mer ont stage was more effective than the prezoosporangia stage in transmi tting the disease in eastern oysters, suggesting that the meront is th e primary transmission agent in nature. A dose of 10-10(2) freshly iso lated P. marinus cells oyster(-1) was required to cause infection by d irect shell cavity injection. P. marinus susceptibility and disease pr ogression were positively correlated with temperature, salinity, and n umber of infective cells the oyster encountered. Temperature appeared to be the most important factor, followed by the infective cell dose, and then salinity in determining the subsequent disease development in oysters. There was no significant interaction between temperature, sa linity, and infective cell dose on the prevalence of disease in oyster s. However, the interaction between either temperature and salinity or between temperature and P. marinus dose significantly intensified the disease. The Pacific oyster, Crassostrea gigas, was less susceptible, but not completely resistant, to P. marinus compared to the eastern o yster, C. virginica. However, the Pacific oyster was intolerant of hig h temperature (>15 degrees C) and low salinity (<10 ppt), thus vulnera ble to high mortality under high temperature and low salinity environm ental conditions. Pollution has the potential to enhance P. marinus su sceptibility and infection in oysters.