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 (R0) in a seasonal context (RS). Set A(t) is the number of secondary cases produced by a primary case introduced at time t. RS 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 RS>1. When RS<1, small epidemics could occur if A⁎>1. Vaccination reduced RS 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 RS>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.

► We modelled the seasonal spread of BTV8 in a homogeneous population of cattle.
► We proposed a counterpart of the basic reproduction number in a seasonal context: RS.
► We verified for a vector-borne disease model that RS corresponded to R0.
► We proposed to use an additional indicator of disease spread, A⁎, in highly seasonal climate.
► To reduce RS and A⁎, and to control BTV8 spread, a high vaccination coverage was required.