Modeling Phocine Distemper Virus Dynamics in the United Kingdom: A Two-Host Application of the SEIR Model

Abstract

Phocine Distemper Virus (PDV) is a morbillivirus that has become a novel target for disease surveillance and control efforts in the North Sea region after large-scale epizootics in 1988 and 2002 led to the mass mortality of common, or harbor, seals (Phoca vitulina) and grey seals (Halichoerus grypus). PDV, as with other marine epizootics, poses a unique modeling challenge: data is reliant on volunteer reporting of strandings, which may not be representative of the true burden of disease, and thus key parameters are difficult to elucidate. Additionally, recent serological research has indicated that, not only can the two relevant species cross-infect, but that the burden of disease differs between them: grey seals typically exist only as carriers of the disease and do not become symptomatic.

First, this thesis aims to articulate a two-host SEIR (Susceptible-Exposed-Infectious-Recovered) model to ascertain key disease parameters, including the reproduction rate and the true burden of disease, and attempt to forecast possible future course(s) of the disease based on existing population data (all data were sourced from the Sea Mammal Research Institute in St. Andrews, Scotland). Second, given the difficult nature of assigned time of death for individual seals during mass mortality events (relevant for understanding the temporal course of the disease), this thesis provides a framework and initial data for an estimate of postmortem interval assessments based on protein degradation.