A particularly virulent O149:H10 enterotoxigenic Escherichia coli clone harbours a newly characterized plasmid pTENT2 carrying the tetracycline-resistance tetA and the virulence genes estA, paa, and sepA that were not present in less virulent clones. The objectives of this study were to assess whether the additional genes on pTENT2 played a role in the increased severity of post-weaning diarrhoea and if they provided any potential advantage for the emergence of the highly virulent clone. Groups of pigs were dosed orally with isogenic pTENT2-positive and pTENT2-negative ETEC strains, and the clinical and pathological changes were compared between the groups. Two additional groups were given the pTENT2-positive strains and maintained on feed with or without chlortetracycline to assess the effect of subtherapeutic levels of tetracycline on the short-term persistence of the ETEC O149:H10 clone. The severity of diarrhoea within the first few hours post-inoculation was significantly increased ( p = 0.0408) in animals receiving pTENT2-positive strains as compared to animals receiving pTENT2-negative strains. There were no consistent or significant histopathological differences between any of the groups and no significant difference in the persistence of ETEC between groups.