Verocytotoxin producing E. Coli O157 on farms : prevalences, risk factors and transmission

Infection with verocytotoxin producing Escherichia coli (VTEC) O157 in humans can lead to mild or bloody diarrhoea, with e.g. the haemolytic uraemic syndrome (HUS) as possible complication. Cattle appear to be important reservoirs of O157 VTEC. The main objectives of research described in this thesis are investigating prevalences, risk factors, and transmission of O157 VTEC to understand the dynamics of O157 VTEC in Dutch cattle. Data from a monitoring program in Dutch animal herds indicated that O157 VTEC is endemic in The Netherlands, with higher prevalences during summer and early fall. Risk factors for infection were identified. Within-herd prevalence, potential environmental reservoirs, intermediate hosts and DNA types of O157 VTEC isolates were determined in a longitudinal study of a positive dairy farm. DNA clusters indicated persistence on the farm during winter and spring. Quantification of transmission using data from this dairy herd and from an experiment with calves showed that transmission was higher in calves. Transmission of O157 VTEC during summer might differ from transmission during winter. Two other experiments indicated that both previously infected heifers and previously contaminated pastures did not function as reservoir of O157 VTEC between shedding seasons. A retrospective cohort study established that positive farms from the Dutch monitoring program were only slightly more likely to be found positive in a next shedding season than previously negative farms, so between-herd transmission seems to occur. Factors associated with a positive test at second sampling implied (re-)introduction rather than long-term persistence of infection. Results of this thesis suggest that several types of O157 VTEC might persist on a farm for some time, with possibly different strains as the most prevailing types in subsequent shedding seasons. Ultimately, the within-herd infection might become extinct due to limiting numbers of susceptible cattle. Between-herd transmission of O157 VTEC can lead to persistence in a larger region, e.g. The Netherlands, maintaining the endemic status in Dutch cattle populations. Because of this, exposure of humans to O157 VTEC cannot be ruled out in The Netherlands. Humans might become infected through food- and waterborne transmission and by transmission directly from animals to humans. When aiming at reducing risks for humans by interventions at farm-level, it is of importance to reduce the number of positive animals and farms. For this, more specific research for the effect of intervention measures on introduction, transmission and survival of O157 VTEC on farms, and economic (cost-benefit) analysis, should be performed.