- Marine diseases lack a general theory for host-pathogen systems, unlike terrestrial systems
- Disease agents in sedentary animals like oysters are moved between individuals and places by the water currents which complicates understanding of disease transmission processes
- Outbreaks (epizootics) of marine diseases cause extensive mortality in the affected population; the processes controlling the initiation and termination of the disease epizootics are largely unknown for marine systems
- This study combines theoretical, experimental and modeling approaches to examine transmission and biological and environmental controls on marine diseases
The theoretical framework that is the basis for this study has its origins in Susceptible-Infection-Susceptible/Recovery (SIS/SIR) models developed for terrestrial systems. This framework identifies transmission pathways that are critical to the transition between point source and nonpoint source transmission dynamics, and focuses on the relationships between infective dose, infective particle concentration and flux, host population density and within population disease susceptibility.
The experimental and modeling components of this study are designed to identify and quantify the connections and interactions between the host, pathogen and environment and the processes controlling these. The theoretical framework will be tested using dermo disease (caused by Perkinsus marinus) in Eastern oysters (Crassostrea virginica) as a case study.
Physical Modeling Component
The influence of hydrodynamic and biological processes on disease transmission, infectious particle concentrations and local and regional particle availability is being investigated using circulation fields obtained from simulations implemented for idealized, simplified estuaries, and a realistic estuary (Delaware Bay) using the Regional Ocean Modeling System (ROMS) framework. The effects of biological processes such as filtration and growth, release of infectious particles, and mortality will be examined using a circulation model with an embedded benthic organism model. Parameterizations for the models will be based on infective dose and genetic experimental results.
Infective Dose Experimental Component
Many invertebrates can affect local dose of a disease agent through filter-feeding. This interaction between host population density and filtration capacity makes it possible for the host population to affect disease agent transmission rates. Experimental studies will address this interaction by considering:
- What population density is required to reduce particle availability to the average individual to a concentration below infective dose?
These experiments were conducted with Eastern oysters and the dermo disease causative agent at the Aquaculture Innovation Center (AIC). The experiments used infected oysters that released parasites into three replicate exposure tanks. Exposure tanks contained uninfected oysters at typical Delaware Bay densities with either: (1) no recruits, (2) average recruit densities or (3) maximal recruit densities. These experiments provided an estimate of how host density can affect per capita transmission.
Genetics Experimental Component
The genetics component of this study is based around the hypothesis that some individuals in a population are likely to be more vulnerable to infection at low doses of disease particles than others. This variation in individual susceptibility is important in understanding how a disease is transmitted to a native population. To test if the dose required to cause disease varies within the population, 24 groups of native disease-free oysters will be exposed to low doses of dermo particles and the genetic basis for variations in susceptibility will be determined by examining the frequency of genotypes at five dermo-resistant markers that were identified in previous studies.