écologie des virus influenza aviaires en Camargue - IRD

Chapitre IIParameters b m and d m are respectively birth and death rates for month m, N is the number ofbirds and is the rate of infection for each transmission process. We introduced time­forceddemographic parameters that allowed us to reproduce the avian community dynamics asobserved in our study site. For inter­individual transmission with a density­dependentprocess we thus have: =I; for inter­individual transmission with a frequency­dependentprocess: =I/N; and finally, for water­borne transmission: = /( +). This kind oftransmission is assumed to be a saturating function since experimental studies have testedindependently different initial viruses load which all lead to an high infection rate (Lu andCastro 2004;Webster et al. 1978; Webster et al. 1992). In fact, these results suggest athreshold for viral load needed to bring susceptible individuals to an infectious state (θforthe viral load in water). On the other side, since this model incorporates time­forceddemographic parameters (b m and d m ) and consequently dynamic population size (N t ),abundance of viral particles fluctuates in time. If these model does not incorporate thisfeature, abundance of viral particles could reach a persistent value at equilibrium and thesethree transmission routes can be viewed as a continuum. In this case, water­bornetransmission is an intermediate form between density­dependent and frequency­dependenttransmission. Accordingly, we investigated outputs with such an intermediate model,between frequency­dependent and density­dependent. The corresponding transmission66