Modeling mastitis reveals relative impact of (a)symptomatic infectivity

Despite widespread compliance with control recommendations, chronic infectious disease remains problematic in animal agriculture. Efforts to disrupt transmission target symptomatic individuals, but disease persists at the population level. Staphylococcus aureus mastitis in dairy cows is a painful inflammation of the udder that reduces milk volume and quality. The popularly targeted transmission mechanism is contact between the milk of symptomatic cows and the udders of susceptible cows via milking equipment. A common control practice is to milk symptomatic cows last, known as the Milk Last Intervention (MLI). However, asymptomatic cows are also infectious. We suspect that they play an important role in maintaining the disease in a herd. In this study, we develop a mathematical model parameterized from the literature, as well as from USDA industry surveys, to address the following question: Given that asymptomatic cows can still transmit, how does MLI impact transmission dynamics? We classify cows according to infection stage and investigate how varying the infectivity between infectious classes influences our model predictions. Even when accounting for asymptomatic transmission, our study predicts that for certain probabilities of infection MLI delays reaching the maximum prevalence of sick animals in the herd. However, reducing asymptomatic transmission could produce a more biologically significant delay, even across wider infectivity ranges. Our results highlight the importance of stochasticity and asymptomatic individuals in this small population system. This contributes to a broader understanding of S. aureus transmission, which is urgently needed to inspire new non-pharmaceutical interventions as antibiotic resistant strains already confound traditional attempts at control.