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

A mathematical model simulating tile hos1-parasi1e-envi ronmentaJ imeractions of eas1em oys1ers (Cra.tsos1rea ,,irginica) and the pathogen, Haplo,poridi1111111elso11i. which cause~ MSX disease, has been deve loped. The model ha~ 1 components. One replica1es the infec1ion process within the oys1er and 1he other si1nulaies transmis,ion. The infection-development co1nponcnt re lies on basic physiological processes of bolh host and paras ite. modified by the environment. to reproduce the observed annual prevalence cycle of H. 11elso11i. Equa1ions de~cribing 1hese rates were consrructed using data from long-tenn field observation,. and field and laboratory experimenL~. In the model . salini ty and temperature have direct effects upon in ,•i,•o parasite survival and prol iferation as well as on transmiss ion rates. Cold wi111er, depress trans1nis~ion rates for J or 2 years after the even1. even 1f te1npcraturcs retun1 to nonnal. Wann winters have no effect on Lransm,ssion in subsequent year~. Hemocyte activity. parasite density, and the overall environmental quali1y provided 10 the parasi te by the host also innuence the modeled infection process. Hemocy1e, scavenge and eliminate para,,i1es thai die over the winter or that degenerate as a result of failed sporulation. Repl,cation rates of fl. 11e/so11i arc slowed at high parasite densities. The environn1ental quality provided by the host, which is a function of oyster food avai labil ity and the oyster·~ po1cntial growth effi ciency. affects doubling times and also determines whether the parasite completes its life cycle by fonning spores. Spore production is related to a threshold envi ronmental quality. which occurs only i11 small oysters because of their high growth efliciency. 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. 11elso11i model provides a quantitative frainework for guiding future labor3tory and field stud ies as well as management efforts. KE Y lVORDS: Haplo,poridium 11elso11i. numerical modeling, Jv(SX disease. 1narine pathogen. hos1-paras ite environment