5 research outputs found
Stability of the Endemic Coexistence Equilibrium for One Host and Two Parasites
For an SI type endemic model with one host and two parasite strains, we study the
stability of the endemic coexistence equilibrium, where the host and both parasite strains
are present. Our model, which is a system of three ordinary differential equations,
assumes complete cross-protection between the parasite strains and reduced fertility and
increased mortality of infected hosts. It also assumes that one parasite strain is
exclusively vertically transmitted and cannot persists just by itself. We give several
sufficient conditions for the equilibrium to be locally asymptotically stable. One of them
is that the horizontal transmission is of density-dependent (mass-action) type. If the
horizontal transmission is of frequency-dependent (standard) type, we show that, under
certain conditions, the equilibrium can be unstable and undamped oscillations can occur.
We support and extend our analytical results by numerical simulations and by
two-dimensional plots of stability regions for various pairs of parameters
Global analysis of multi-strains SIS, SIR and MSIR epidemic models
International audienceWe consider SIS, SIR and MSIR models with standard mass action and varying population, with different pathogen strains of an infectious disease. We also consider the same models with vertical transmission. We prove that under generic conditions a competitive exclusion principle holds. To each strain a basic reproduction ratio can be associated. It corresponds to the case where only this strain exists. The basic reproduction ratio of the complete system is the maximum of each individual basic reproduction ratio. Actually we also define an equivalent threshold for each strain. The winner of the competition is the strain with the maximum threshold. It turns out that this strain is the most virulent, i.e., this is the strain for which the endemic equilibrium gives the minimum population for the susceptible host population. This can be interpreted as a pessimization principle.On considère les modèles SIS, SIR et MSIR avec la loi de l'action de masse standard et une population non constante, avec n différentes souches de pathogènes. Nous considérons aussi les même modèles avec transmission verticale. On prouve que sous une condition générique, le principe de compétition exclusive est vérifié. Pour chaque souche, un nombre de reproduction de base est associé. Il correspond au cas où seule cette souche existe. Le nombre de reproduction de base du système complet est le maximum de tous les nombres de reproduction de base pris individuellement. Nous définissons aussi un seuil équivalent pour chaque souche. La souche qui gagne la compétition est celle qui maximise le nombre de reproduction de base. C'est aussi la souche la plus virulente, i.e., c'est la souche pour laquelle l'équilibre endémique donne le minimum des individus susceptibles dans la population hôte. C'est le principe de pessimisation