Seasonal spread and control of Bluetongue in cattle.

Bluetongue is a seasonal midge-borne disease of ruminants with economic consequences on herd productivity and animal trade. Recently, two new modes of transmission have been demonstrated in cattle for Bluetongue virus serotype 8 (BTV8): vertical and pseudo-vertical transmission. Our objective was to model the seasonal spread of BTV8 over several years in a homogeneous population of cattle, and to evaluate the effectiveness of vaccination strategies. We built a deterministic mathematical model accounting for the seasonality in vector abundance and all the modes of transmission. We proposed a counterpart of the basic reproduction number (R(0)) in a seasonal context (R(S)). Set A(t) is the number of secondary cases produced by a primary case introduced at time t. R(S) is the average of A(t). It is a function of midge abundance and vaccination strategy. We also used A*, the maximum of A(t), as an indicator of the risk of an epidemic. Without vaccination, the model predicted a large first epidemic peak followed by smaller annual peaks if R(S)>1. When R(S)<1, small epidemics could occur if A* >1. Vaccination reduced R(S) and A* to less than one, but almost perfect vaccine efficacy and coverage were required to ensure no epidemics occurred. However, a lower coverage resulting in R(S)>1 could decrease infection prevalence. A further step would be to optimize vaccination strategies by targeting an appropriate period of the year to implement the vaccination.