écologie des virus influenza aviaires en Camargue - IRD

Influenza aviaires en Camarguemechanism is defined by = I/(N ) where is the contact rate between individuals and is the degree of frequency­dependence of transmission ( = 0 for full density­dependenttransmission and = 1 for full frequency­dependent transmission). Estimation of , , , and was performed using the values of σ(recovery rate per individual), θ(minimal viralload to initiate infection), γ(viral shedding per day per infected individual) andπ(viralparticle inactivation rate in water) presented in Table 1 (evaluation procedure is detailed inthe online appendix.). We also investigated transmission patterns which involved twotransmission routes: density dependent + water­borne transmission, and frequency dependent+ water­borne transmission.For each of the disease transmission mechanisms, we optimized the transmissionparameters in our mathematical model to obtain the minimal error against the disease data.More specifically, we maximized the likelihood of all models and applied afterwards alikelihood ratio test in order to select the most parsimonious model which shows the bestdata adequacy. The evaluation procedure is detailed in the online appendix.This mathematical framework starts from the following hypotheses. (i) Each birdspecies has the same duration of virus excretion: a one to two week duration has beenreported for the most common avian species (Webster et al. 1978; Lu and Castro 2004). (ii)Each bird species excretes the same number of viral particles: we assumed that viral particleproduction is equal across different host species. From a mathematical point of view thisassumption is not really important when a threshold of viral particle concentration in theenvironment is reached. We also assumed that viral particles produced by different avianspecies remain infectious for the same duration irrespective of the AIV subtypes. (iii) Watercontacts are equal across avian species: we assumed that there are no differences in terms of67