MiCull2 -- simulating mastitis transmission through milking order

Contagious mastitis pathogens can be transmitted through milking. However, previously published simulation models, such as MiCull, have not directly taken this into account. We have reimplemented the MiCull model to model transmission of contagious mastitis pathogens through milking in a milking parlor. This additional complexity requires a substantial increase in computations and a need to structure the program code to make it more flexible for future use. The aim of this paper was threefold: First, to implement the new model in a faster programming language; secondly, to describe the new model, in particular transmission of a contagious mastitis pathogen through milking; and thirdly, to compare three different milking order strategies in regards to prevalence and incidence of intramammary infections. For each scenario, 500 herds with 200 cows each were simulated over 10 years. The model was calibrated using available mastitis parameters from the literature. We hypothesized that milking order should have a considerable effect on disease transmission, especially if the infected cows with clinical enter the milking parlor first and thereby have a high risk of infecting the following cows. The milking order scenarios examined were random milking order and milking clinical cases first, or last. Unexpectedly, there were no large differences between these scenarios for reasonably sized infection rates corresponding to a herd with a moderate level of clinical mastitis in the herd. Larger differences are expected to be found in herds with very high infection rates. We have developed a transmission simulation model of mastitis pathogens using a new mode of transmission by milking order. We expect that this new version of MiCull will be useful for both researchers and advisors since it is flexible, can be fitted to various in-herd situations and the computations are fast.