Modeling the Oyster Parasite

    The oyster parasite MSX (Haplosporidium nelsoni), has been an important cause of mortality in Delaware Bay oysters since the late 1950s. MSX and a second oyster parasite, Dermo,(Perkinsus marinus), often cause over 50% mortality yearly in the market-size portion of the oyster population with devastating consequences for the oyster industry. Up to the late 1980s, MSX was by far the more important of the two parasites. Since 1990, Dermo has become the primary source of mortality. Understanding how environmental variations and biologial processes control the relative abundance of these two disease-causing agents and subsequent mortality in the host populations is an important goal in oyster research.

     A collaborative modeling effort between the Haskin Shellfish Research Laboratory (HSRL) at Rutgers University and the Center for Coastal Physical Oceanography (CCPO) at Old Dominion University has resulted in the development of an oyster populations dynamics model and a model for Dermo disease. This model has been used to examine the mechanisms by which epizootics (animal epidemics) are produced by Dermo in oyster populations and the mechanisms that initiate and stop these epizootics. Key environmental variables include temperature, salinity, food supply, and turbidity. Warm winters followed by dry summers, for example, may trigger a Dermo epizootic that results in massive mortalities for several subsequent years. A drop in food supply or an increase in turbidity can also trigger an epizootic.

     Present modeling efforts, being conducted jointly by HSRL, CCPO, and the Cooperative Oxford Laboratory, Maryland Department of Natural Resources, are now directed at the inclusion of MSX in this dual oyster-Dermo model to look at the relationship between these two disease agents and to better predict how each produces mortality under a suite of environmental conditions. This model will contain the effects of temperature, salinity, oyster food supply, and the timing of the spring bloom on MSX development and the effect of cold temperatures and failed MSX sporulation events on regression of the disease. The final model, containing both Dermo and MSX, will be the first dual-disease model developed for any invertebrate. This model will permit examination of the mechanisms by which one of the two diseases becomes the principal source of mortality in oyster populations for a period of years. The model will also permit evaluation of different oyster culture strategies to maintain the commercial harvest during periods of varying intensity of the two diseases.