Mathematical modelling of the transmission of salmonella between pigs

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2007
Thesis identifier
  • T13167
Qualification Level
Qualification Name
Department, School or Faculty
  • Salmonella spp. are the second most common cause of foodborne illness in the United Kingdom (UK) and the European Union (EU). Pigs are relatively more likely to be infected with Salmonella spp. compared to other species of livestock. A recent EFSA survey isolated Salmonella from 21.2% of lymph node samples. Given the high prevalence of Salmonella infection in pigs, with a serotype of human health signi cance, pig meat is thought to be the third most important contributor to Salmonella infection in humans, behind poultry meat and eggs (EFSA, 2006). This has prompted the EU to set targets for reduction of Salmonella spp. in slaughter-age pigs and breeding pigs, which are due to come into force in the next 1-2 years. For reductions in the prevalence of Salmonella infection in live pigs, intervention at the farm level (be it at the sow or pig level) will be required. Current evidence for the e ect of farm level interventions such as the use of organic acids in feed/water or vaccination is scarce, as experimental or observational studies are expensive and so studies thus far have been small. Hence, the relevance of these limited and small studies is limited when interpreting the results for the development of a National Control Plan for Salmonella in pigs, therefore mathematical modelling studies are useful to assess the e ectiveness of on-farm interventions in reducing the burden of human salmonellosis. This thesis presents a number of determinsitic and stochastic models on the subject of Salmonella introduction/transmission between pigs, progressing from a simple determinstic SIR model of grower- nisher production to a detailed stochastic model incoproating all stages of production and considering the source of infection. The dynamics of infection in the deterministic models was similar. With current parameter estimation, infection was self-sustaining in pen populations across the models, whether there were 1 or 300 pens. Stability analysis of each of the models suggested that the homogeneous infected steady state would be the result of at least one infected pig entering the herd. Travelling wave analysis of the multipen models suggested that the speed of transmission between pens, via faecal-oral transmission, was relatively slow, such that infection would probably be limited to a few pens by the time pigs were sent to slaughter. Very different dynamics were observed for the stochastic models, where stochastic fade-out was the most common result from infection entering a herd.;The models developed in this thesis allow an insight into the complex dynamics of transmission and intervention on pig farms, which is currently not possible through observational study due to the large number of variables that must be controlled. The nal model incorporates several advancements in the eld of Salmonella in pig transmission modelling that have not been considered before (e.g. the explicit inclusion of the magnitude of (intermittent and variable) shedding, farm management systems and sources of infection). These advances highlight new and interesting dynamics, suggesting that the sow is by far the most important source of infection of pigs. In particular, the level of Salmonella shedding of individual pigs/sows appears to be crucial to the dynamics of infection, but this has not been captured before. This seems a fairly intuitive conclusion, given that Salmonella is mainly transmitted via the faecal-oral route and is dose-dependent. However, it is not normally captured in models because of the complexity of doing so, and the lack of data to parameterise such a model. In the case of Salmonella in pigs, when dealing with various sources, complex management systems and highly variable shedding rates, then the inclusion of shedding dynamics at a more detailed level appears warranted, as the dynamics change markedly according to whether it is included or not. In conclusion, this thesis has established a set of models for the investigation of the introduction, transmission and intervention of Salmonella in pigs. The final model suggests that the sow is a major source of infection, and hence intervention should rst and foremost be introduced to the breeding herd. However, decreasing the resistance of the weaner/ nisher pig to infection, and conducting All-In-All-Out production, would lessen the transmission of infection between pigs during later stages of production. The final model has already been used to inform the development of the UK National Control Plan (to investigate the accuracy of several sampling schemes and as an input for Cost-Benefit Analysis), and research will continue to improve the assumptions and parameter estimation of the model.
Resource Type
  • Strathclyde theses - ask staff. Thesis no. : T13167
Date Created
  • 2007
Former identifier
  • 947481