I also forgot to mention that E. coli can undergo virulent strain formation while in the intestines. So it is possible that the bacteria formed a virulent, disease causing form within your pet as a natural strain evolution of the bacteria. Typically in situ virulent strain formation will show up as just mild to moderate diarrhea; it is also possible, on occasion, to form more deadly strains. Young, geriatric or immune compromised pets are at highest risk for this. Just another thought. Often when we see our pets suddenly struck ill we naturally look for a cause. Often there is not necessarily a preventable reason - especially with the pets of this forum who have some of the most attentive, affectionate, caring and proactive owners that a pet could wish for.