Abstract
Simulation models can predict the outcome of different strategies for the control and eradication of paratuberculosis (PTB) in dairy herds. Two main transmission modes have previously been used to simulate the spread of PTB: direct (contact between animals) and indirect (through the environment). In addition, previous models were calibrated to either low or high within-herd prevalence levels, which we refer to as normal and low hygiene levels, respectively. We simulated both direct and indirect transmission with the same model in both normal and low hygiene level scenarios. The effectiveness of a test-and-cull strategy was dependent on the calibration level of the simulation model, and eradication occurred less frequently with the more biologically plausible indirect transmission mode. The results were compared to within-herd prevalence records from 314 dairy herds. The prevalence in 50% of the herds varied less than 0.9% per year on average, and less than 4% in 90% of the herds. We therefore conclude that the normal-hygiene scenario best describes most dairy herds in Denmark. Finally, we simulated different levels of farmer compliance with a test-and-cull strategy and found that a 60% compliance level was not sufficient to reach eradication within 10 years.
Highlights
Paratuberculosis (PTB) is a chronic disease in cattle that is caused by Mycobacterium avium ssp. paratuberculosis (MAP) and shed predominantly in the feces, and in milk from infected animals
We found the same patterns in the simulated scenarios with sensitivity reduced to 50% and 20% of the original, but the efficiency of the test-and-cull strategy was reduced (Figs 2 and 3) and eradication could not be achieved within 10 simulated years in the median result for the indirect transmission mode (ITM) model with 50% sensitivity
The results indicate that the difference between simulation models resulting in very high true prevalence and those with more moderate true prevalence predictions lies in the calibration
Summary
Paratuberculosis (PTB) is a chronic disease in cattle that is caused by Mycobacterium avium ssp. paratuberculosis (MAP) and shed predominantly in the feces, and in milk from infected animals. It is unclear whether results from the DTM and ITM models differ due to the transmission mode or other reasons such as parameterization, population dynamics, etc Another feature that separates the currently published PTB simulation models into two groups is the resulting true within-herd prevalence after several years in a no-control situation. Massaro et al.[16] simulated 10 years and predicted a prevalence of 37%, and the model by Al-Mamun et al.[12] resulted in a true prevalence of 49% after 25 simulated years These models all resulted in a high true prevalence in a no-control scenario, and we can consider that they all reflect low-hygiene scenarios because the true prevalence (and the level of MAP transmission) is closely linked to the hygiene in the herd. This was the case for the study by Al-Mamun et al.[19], where calibration resulted in 10%, 23% or 35% true prevalence
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