How to avoid degradation of paratuberculosis prevalence in dairy cattle herd? An individual-based modelling approach
Résumé
Paratuberculosis is an infectious gastrointestinal disease of cattle worldwide [1, 2]. Caused by Mycobacterium avium subsp. paratuberculosis (Map) [3], it can lead to emaciation, profuse diarrhoea, decreased milk production, and early culling of animals [4]. Several measures are available to control Map spread: to improve calf rearing, to separate as soon as possible calves from their dam, and to cull quickly high shedders [5, 6]. However, measure efficiency is not very well established and might depend on the prevalence of infection in the herd at the start of measure implementation.
Our objective was to determine which control measures and which minimal level of implementation were needed to avoid degrading prevalence of infected adults within dairy cattle herds according to the starting adult prevalence of infection. We evaluated three situations with contrasted levels of prevalence representing realistic ranges observed in dairy cattle herds in Brittany (France). We developed an original individual-based model of Map spread within a dairy cattle herd, and use this model to assess the effect of control measures on adult prevalence variations. We considered 388 strategies based mostly on reducing exposure of young animals to adult environment and on test-and-cull measures. The model includes 6 age groups (from new-borns to adults), 6 health states (susceptible, no more susceptible, transiently infectious, latently infected, moderately infectious, and highly infectious), and all of the described transmission routes (in utero, faecal-oral, milk and colostrum ingestion). The individual-based paradigm allows following the evolution of animal characteristics over time, and thus to evaluate precise test-and-cull measures with the model.
We demonstrated that, irrespective of the initial adult prevalence, both control measures should be combined to avoid degrading adult prevalence in dairy cattle herds. Reducing calf exposure to adult environment was the most important lever to reach a higher probability of prevalence maintenance or decrease. For intermediate prevalence (from 7% to 21%), we showed that it was difficult to avoid degrading prevalence even with a high level implementation of control measures, this probability ranging from 25% to 50% after 5 years of control implementation.
Our results showed that combining control measures is required to avoid degrading the prevalence of infected adults in dairy cattle herds, but that similar options are relevant irrespective of the prevalence at the start of control implementation. This result will simplify advices to be provided by Animal Health Services to farmers to help controlling Map spread at local scale.