Abstract
Before an efficient control strategy for livestock-associated methicillin resistant Staphylococcus aureus (LA-MRSA) in pigs can be decided upon, it is necessary to obtain a better understanding of how LA-MRSA spreads and persists within a pig herd, once it is introduced. We here present a mechanistic stochastic discrete-event simulation model for spread of LA-MRSA within a farrow-to-finish sow herd to aid in this. The model was individual-based and included three different disease compartments: susceptible, intermittent or persistent shedder of MRSA. The model was used for studying transmission dynamics and within-farm prevalence after different introductions of LA-MRSA into a farm. The spread of LA-MRSA throughout the farm mainly followed the movement of pigs. After spread of LA-MRSA had reached equilibrium, the prevalence of LA-MRSA shedders was predicted to be highest in the farrowing unit, independent of how LA-MRSA was introduced. LA-MRSA took longer to spread to the whole herd if introduced in the finisher stable, rather than by gilts in the mating stable. The more LA-MRSA positive animals introduced, the shorter time before the prevalence in the herd stabilised. Introduction of a low number of intermittently shedding pigs was predicted to frequently result in LA-MRSA fading out. The model is a potential decision support tool for assessments of short and long term consequences of proposed intervention strategies or surveillance options for LA-MRSA within pig herds.
Highlights
Methicillin-resistant Staphylococcus aureus (MRSA) are a group of S. aureus that have acquired the MecA or MecC gene, which make them resistant to most β-lactam antibiotics [1]
If MRSA was introduced in the weaner unit or finisher unit, the simulations indicated that spread to the sow units was still likely to occur, despite animals not being moved backwards (S3 and S4 Figs)
We modelled the spread of MRSA between animals within a pig farm mechanistically
Summary
Methicillin-resistant Staphylococcus aureus (MRSA) are a group of S. aureus that have acquired the MecA or MecC gene, which make them resistant to most β-lactam antibiotics [1]. Despite the assumption of no use of risk antimicrobials (tetracyclines and β-lactam antibiotics) in the herd (and use of the low transmission rates associated with this), introduction of MRSA shedders in a few cases (0.6–13.0%) still led MRSA to spread throughout the herd and become established. The observation of MRSA being able to fade out following introduction of a few IS, does not seem unrealistic given that during an investigation on Norwegian pig farms, 32 of 51 farms did not become MRSA positive, despite having positive suppliers [36] For twelve farms, this was explained by the farms only being sporadically supplied from the infected farms, which is comparable to the scenarios modelled. The true risk of transmission will dependent on multiple local factors i.e. internal biosecurity and design and location
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