Prevalence and molecular typing of attaching and effacing Escherichia coli among calf populations in Belgium

Attaching and effacing Escherichia coli are involved in diarrhea in 2 to 8-week old calves. The virulence factors of these bacteria include: (i) the secretion of proteins (i.e. EspB) involved in microvilli effacement, (ii) the production of the intimin, a 94 kDa outer membrane protein encoded by the eaeA gene and involved in the intimate attachment of bacteria to epithelial cell and (iii) the production of verotoxins: VT1 and/or VT2. We investigated the presence and the pathotype of these strains in several calf populations by colony hybridization or by genetic amplification. Using the colony hybridization method we showed first that only 5% of calves who died from diarrhea presented EaeA+ E. coli strains and secondly that 19% of healthy calves showed an asymptomatic carriage. However, using colony hybridization and genetic amplification, we identified EaeA+ strains in 91% of calves living in farms with recurrent diarrhea problems. In 66% of the calves, there was a correlation between the presence of AEEC and diarrhea. At the pathotype level, most of the EaeA+ isolates were negative for VT probes. In VT+ bacteria, the majority were VT1+. The number of VT positive bacteria was significantly higher in calves who died from diarrhea than in healthy or sick calves. This underlined the aggravating role of verotoxins in the disease. Moreover, only 25% of the bovine AEEC were positive with the EaeB probe. Surprisingly, the proportion of EaeB+ strains was significantly higher in healthy calves than in other populations.