Quantitative investigation of population persistence and recurrent outbreaks of bovine respiratory syncytial virus on dairy farms.

Objective: To determine if transmission of virus among seropositive cattle is a plausible mechanism for the permanent presence of bovine respiratory syncytial virus (BRSV) in dairy herds, and how likely, with the scenario for persistence, there will be only 1 clinical outbreak of BRSV per year.

Design: Build a stochastic model, parameter estimation from serologic data on BRSV, and interpret the estimated parameter values from the model analysis.

Sample Population: Monthly data on the prevalence of antibodies directed against BRSV in sera from all cattle in 6 dairy herds.

Procedure: Parameter estimation applying general linear models, model analysis using calculation of the reproduction ratio for simplified models, and Monte-Carlo simulation for the whole model.

Results: Persistence of BRSV by transmission among seropositive cattle given estimated parameter values would be accompanied by frequent extinctions (once every 10 to 50 years) and long infectious periods in seropositive cattle (100 days). Moreover, in the model, a single clinical outbreak among seronegative cattle only occurred with external forcing.

Conclusions: From these data, transmission among seropositive cattle is not a plausible mechanism for persistence of BRSV in dairy herds.