Publisher
PLoS
Abstract
Variants of the susceptible-infected-removed (SIR) model of Kermack & McKendrick (1927)
enjoy wide application in epidemiology, offering simple yet powerful inferential and predictive
tools in the study of diverse infectious diseases across human, animal and plant populations.
Direct transmission models (DTM) are a subset of these that treat the processes of
disease transmission as comprising a series of discrete instantaneous events. Infections
transmitted indirectly by persistent environmental pathogens, however, are examples where
a DTM description might fail and are perhaps better described by models that comprise
explicit environmental transmission routes, so-called environmental transmission models
(ETM). In this paper we discuss the stochastic susceptible-exposed-infected-removed
(SEIR) DTM and susceptible-exposed-infected-removed-pathogen (SEIR-P) ETM and we
show that the former is the timescale separation limit of the latter, with ETM host-disease
dynamics increasingly resembling those of a DTM when the pathogen's characteristic timescale
is shortened, relative to that of the host population. Using graphical posterior predictive
checks (GPPC), we investigate the validity of the SEIR model when fitted to simulated
SEIR-P host infection and removal times. Such analyses demonstrate how, in many cases,
the SEIR model is robust to departure from direct transmission. Finally, we present a case
study of white spot disease (WSD) in penaeid shrimp with rates of environmental transmission
and pathogen decay (SEIR-P model parameters) estimated using published results of
experiments. Using SEIR and SEIR-P simulations of a hypothetical WSD outbreak management
scenario, we demonstrate how relative shortening of the pathogen timescale comes
about in practice. With atttempts to remove diseased shrimp from the population every 24h,
we see SEIR and SEIR-P model outputs closely conincide. However, when removals are 6-
hourly, the two models' mean outputs diverge, with distinct predictions of outbreak size and
duration.
Year
2021
Category
Refereed journal