Modelling infection dynamic of bovine digital dermatitis in pasture-based system in New Zealand

Introduction
Understanding the infection dynamics of bovine digital dermatitis (BDD) at the population level is crucial for the development of BDD control. Previously, two mathematical simulation models have been built to explore the lesion transition dynamics in housed cow setting (Döpfer et al., 2012; Biemans et al., 2018). However, neither the transmission process nor the parameters involved in the process may be applicable to dairy herds in NZ where cows are normally kept entirely at pasture and routine treatments for BDD lesions are extremely rare. Therefore, the aim of this study was to develop a unique mathematical model to simulate the infection dynamics of BDD infection in dairy cattle in New Zealand. 

Material and methods
A simple deterministic compartmental model was developed to capture the transmission dynamics of BDD in a typical spring-calving New Zealand dairy herd. This included a demographic component and disease component. The demographic component follows the dairy calendar of a typical spring calving herd in NZ. As the presentation of BDD in the pasture-based system was different from that reported in housed cows (almost all the lesions were M4, with very few ulcerative (M1/M2) lesions and no post-treatment M3 lesions, the lesions were classified into two types based on appearance and chronicity: Type 1 (small grey or black lesions with moderate skin thickening which did not persist over a prolonged period) and type 2 (larger more obviously proliferative lesions which persist). Detailed description and photos of type I and type II lesions will be presented at the conference. […]