Modeling the MSX Parasite in Eastern Oyster (Crassostrea virginica) Populations. I. Model Development, Implementation, and Verification

A mathematical model simulating the host-parasite-environmental interactions of eastern oysters (Crassostrea virginica) and the pathogen, Haplosporidium nelsoni, which causes MSX disease, has been developed. The model has 2 components. One replicates the infection process within the oyster and the other simulates transmission. The infection-development component relies on basic physiological processes of both host and parasite, modified by the environment, to reproduce the observed annual prevalence cycle of H. nelsoni. Equations describing these rates were constructed using data from long-term field observations, and field and laboratory experiments. In the model, salinity and temperature have direct effects upon in vivo parasite survival and proliferation as well as on transmission rates. Cold winters depress transmission rates for 1 or 2 years after the event, even if temperatures return to normal. Warm winters have no effect on transmission in subsequent years. Hemocyte activity, parasite density, and the overall environmental quality provided to the parasite by the host also influence the modeled infection process. Hemocytes scavenge and eliminate parasites that die over the winter or that degenerate as a result of failed sporulation. Replication rates of H. nelsoni are slowed at high parasite densities. The environmental quality provided by the host, which is a function of oyster food availability and the oyster\u27s potential growth efficiency, affects doubling times and also determines whether the parasite completes its life cycle by forming spores. Sport production is related to a threshold environmental quality, which occurs only in small oysters because of their high growth efficiency. Simulations that use environmental conditions characteristic of Delaware Bay reproduce the observed seasonal H. nelsoni cycle, consequent oyster mortality, and spore production in juvenile oysters. The oyster-H. nelsoni model provides a quantitative framework for guiding future laboratory and field studies as well as management efforts