Microparasitic disease dynamics in benthic suspension feeders: Infective dose, non-focal hosts, and particle diffusion

Benthic suspension-feeders can accumulate substantial numbers of microparasitic pathogens by contacting or filtering particles while feeding, thus making them highly vulnerable to infectious diseases. The study of disease dynamics in these marine organisms requires an innovative approach to modeling. To do so, we developed a single-population deterministic compartmental model adapted from the mathematical theory of epidemics. The model is a continuous-time model, unstructured in spatial or age terms, and configured to simulate the dynamics of diverse dose (body burden)-dependent infectious disease transmission processes in suspension feeders caused by susceptible individuals contacting or absorbing (filtering) infectious waterborne pathogens. Different scenarios were simulated to explore the effect of recruitment, filtration rate, particle loss, diffusion-like processes in the water column and non-focal hosts (i.e. non-susceptible in terms of disease) on disease incidence. An increase in recruitment (i.e. new disease free susceptibles) can reduce the prevalence of infection due to the dilution effect of adding more susceptibles, but the disease can spread faster for the same reason. Lower infective particle accumulation rates or increasing particle loss rates in the environment reduce the prevalence of infection. This effect is trivial when the water is saturated with infective particles released by infected and/or dead animals. Diffusion of particles from the local pool available to suspension feeders to the adjacent remote pool, prompted by a large remote volume and high particle exchange, limits epizootic development. Similarly, the likelihood of an epizootic can be constrained in a large susceptible population when competition for pathogens, more 'active' in active filter feeders than in passive suspension feeders, reduces the per capita infective particle accumulation rate. In passive suspension feeders, decreasing the area of the feeding surface has the same effect in constraining disease development. The effect of competition for infective particles in essence diluting the infective particle concentration in the water column is magnified when the susceptible population is part of a community with non-focal filter feeders, and is particularly effective in limiting disease development in high infective dose systems. At the same time, this active foraging strategy makes filter feeders more vulnerable to epizootics. The model is a suitable framework for studying the disease dynamics and determinants of disease outbreaks in benthic suspension feeders.