Studies on verotoxigenic Escherichia coli O157 in Swedish cattle From sampling to disease spread modelling

Verotoxigenic Escherichia coli O157:H7 (VTEC O157) is an important zoonotic pathogen capable of causing infections in humans, sometimes with severe symptoms such as hemorrhagic colitis and hemolytic uremic syndrome (HUS). Cattle are considered to be the main reservoir of the bacterium. In this thesis, sampling strategies to detect VTEC O157 in a cattle herd, risk factors for the introduction and the spread of VTEC O157 in Swedish cattle herds, as well as options for control, are studied. A spatial data-driven stochastic model was developed to explore the spread of VTEC O157 by livestock movements and local transmission among proximal holdings in the complete Swedish cattle population. Overshoe sampling alone or in combination with dust and/or pooled pat sampling were established to be reliable for identifying cattle herds with animals shedding VTEC O157. Results from field studies and computer simulations show that animal movements and local spread are important for the transmission of VTEC O157 in the Swedish cattle population. However, simulated control measures based on reducing the between-herd VTEC O157 transmission by animal movements and local spread, had marginal effect in decreasing the prevalence. On the other hand, simulated control measures based on reducing the shedding and susceptibility, efficiently decreased the prevalence of VTEC O157 in the Swedish cattle population.