When and why direct transmission models can be used for environmentally persistent pathogens
Fig 1
Susceptible-exposed-infectious-removed-pathogen (SEIR-P) environmental transmission model diagram illustrating the four host compartments ((S)usceptible, (E)xposed, (I)nfectious and (R)emoved, or recovered) and single pathogen compartment (P) of the model.
The solid arrows indicate the movement of hosts between host compartments and the loss of viable pathogen from the system. The dotted arrows indicate how the host and pathogen parts of the model influence each other. The parameters α and β are the environmental and direct transmission rates while ϵ and ρ are the rates of pathogen emission and decay. The time that hosts spend in the E and I compartments is determined by the chosen exposed and infectious lifetime distributions, e.g., gamma distributions with shape parameters νδ and νγ and rate parameters λδ and λγ, as shown here. When exponential distributions are assumed, then the rate parameters are denoted respectively by δ and γ (see Table 1 for a summary of all symbols used throughout the paper). When α = ϵ = ρ = 0 we obtain the SEIR direct transmission model as a submodel.