Escherichia coli O157 is a shigatoxin producing pathogen with major public health impacts. The primary reservoir of E.coli O157 is considered to be cattle. Most human disease caused by E.coli O157 is via the foodborne route, and often seen as outbreaks. In most humans, disease is characterised by diarrhea and is self-limiting. However, in some individuals (especially infants and the elderly) it can progress to haemolytic uraemic syndrome (HUS) and kidney failure. In Australia in 2014 there were 0.4 cases/100,000 population of shigatoxin E.coli (STEC) infection, compared to 2.32 cases/100,000 population in the US in 2013. The health costs of STEC infections in Australia and the US are estimated to be $2.6 million and $300 million per year, respectively. To understand the dynamics of E.coli O157 shedding and supershedding, a cohort study was undertaken in a herd of replacement dairy heifers at Camden NSW between 2012 and 2013.

This cohort was sampled (fecal and rectoanal mucosal swabs) 18 times at approximately weekly intervals. The overall prevalence of E.coli O157 shedding was 44% (412/930 samples), and ranged from 10 to 94% at individual sampling points. All of the 52 heifers in this cohort yielded at least one isolate of E. coli O157 during the study period. Super-shedding (> 104 CFU / gm faeces) was detected at a sample level of 3.6% (32/893) and ranged between nil and 10%. Of the 52 heifers, 24 (46%) were detected to be super-shedding at some point during the study, 19 of which were detected as super-shedding at only one point.

The evidence from this study did not support the theory of a small subset of a herd being persistently infected and supershedding E.coli O157, thus spreading and maintaining infection in a herd. Rather, all herd members of this cohort shed E.coli O157 at some point in time, and heifers experienced super-shedding events regularly and in a non-clustered pattern. Further studies on risk factors are discussed.