Verotoxinogenic Escherichia coli O157:H7 in Swedish Cattle and Pigs

Verotoxinogenic E. coli O157:H7 (VTEC O157:H7) can cause severe disease in humans, with bloody diarrhea and complications such as haemolytic uraemic syndrome (HUS), thrombotic thrombocytopenic purpura (TTP) and even death. Animals carry VTEC O157:H7 asymptomatically. Ruminants, especially cattle, are considered to be the main reservoir, although the bacterium can occasionally be isolated from other species, such as pigs. The main aim of this thesis was to increase our knowledge of VTEC O157:H7 in Swedish cattle and pigs and to assess the extent to which they could be a potential source of human infections. The studies have included prevalence investigations of VTEC O157:H7 in slaughtered Swedish cattle and pigs, with estimated prevalences of 1.2% and 0.1%, respectively. Moreover, a study was performed on 371 dairy herds, where VTEC O157:H7 was detected in 8.9% of the herds. Indentified risk factors for herds to prove positive were: median age of sampled animals, herd size, farms located in Halland and presence of pigs on a dairy farm. Studies were also performed on farms where pigs shed VTEC O157:H7. Direct or indirect contact with ruminants seemed to be of major importance for presence of the bacterium in pigs. Young pigs were monitored during rearing for slaughter and were found to rid themselves of the bacteria prior to slaughter. When VTEC O157:H7 isolates from the cattle prevalence studies (n=181) and farms linked to human cases (n=19) were subtyped, a specific variant, VTEC O157:H7 (PT4:vtx2,vtx2c), predominated among the strains isolated from farms associated with disease in humans. By extended subtyping it was established that strains of this specific variant belonged to a group of putative hyper-virulent strains, clade 8, suspected of causing more severe disease in humans. Furthermore, different molecular subtyping techniques were evaluated regarding their ability to distinguish between VTEC O157:H7 strains isolated from Swedish cattle and pigs.