Modelling transmission dynamics of Campylobacter in Dutch broiler flocks

Within the multi-disciplinary CARMA (Campylobacter Risk Management and Assessment) project it was decided to focus on evaluation of interventions in the chicken meat chain. To assess the effect of control measures on the farm we developed a model on Campylobacter transmission in Dutch broilers and broiler flocks, resulting in an estimate of the Campylobacter prevalence in broilers for slaughter. The Campylobacter prevalence in broilers depends mainly on 2 parameters, i.e. transmission between broilers within flocks and transmission between flocks. A within-flock model describes Campylobacter transmission between broilers by a logistic growth curve based on experimental data. We found that Campylobacter does spread fast, a flock of 30 000 broilers is completely infected within 3 weeks after introduction. In the between-flock model, infections are introduced via 3 routes: (1) by an infected flock in the previous round in the same house, (2) by other infected flocks present on the farm, (3) by other sources. To quantify the contribution of each of these routes we analysed field data, i.e. observations on the Campylobacter status of all flocks from 10 broiler farms during 13 -14 production rounds. Combination of both transmission models allows us to construct a probability distribution on the Campylobacter prevalence in a random broiler flock at slaughter, while assuming a constant introduction rate of a Campylobacter into a flock during a round. This method is used to evaluate the following control scenarios: improvement of hygienic measures, a ban on different types of livestock on all farms, and a separation of free flocks from infected flocks at slaughter, i.e. logistic slaughtering. For logistic slaughtering and processing it is essential to detect infected flocks with a high certainty. To this end we calculated the probability of detection. Fast test results appeared to be of more importance than the sensitivity of the test.