Modeling the effect of an outbreak of bovine herpesvirus type 1 on herd-level milk production of Dutch dairy farms

One of the impacts of disease is its effect on milk production. In the pres ent study the effect of an outbreak of bovine herpesvirus type 1 on milk pr oduction at the herd level of certified bovine herpesvirus type 1-free dair y farms was modeled. The objective was to study several linear models to qu antify the effects of a bovine herpesvirus type 1 outbreak on milk producti on accounting for the repeated measurements and incorporating our assumptio ns about the most likely duration of effects of this virus. Because milk pr oduction is measured at regular intervals, the data consisted of repeated m easurements at the herd and cow levels. A marginal model, a subject-specific random-effect model, and a transition model were developed. The effect of a bovine herpesvirus type 1 outbreak wa s statistically significant in the random-effect model, and this model fitt ed the investigated farms best. However, a transition model might be a bett er model for generalizing the results to the whole population of Dutch dair y farms. The effect of a bovine herpesvirus type 1 outbreak on milk production deriv ed from the random-effect model amounted to, on average, a loss of 0.92 kg of milk per cow per day during a period of 9 wk. The milk production loss v aried from almost none to 2 kg of milk per cow per day. This reduction resu lted in an average loss of Dfl 372 (Dfl1 = $US $0.50) with lower and upper confidence limits of, respectively, Dfl 12 and Dfl 730 per bovine herpesvir us type 1 outbreak.