Super-shedding cattle and the transmission dynamics of Escherichia coli O157.
The prevalence of Escherichia coli O157 displays striking variability across the Scottish cattle population. On 78% of farms, in a cross-sectional survey of 952, no shedding of E. coli O157 was detected, but on a small proportion, approximately 2%, very high prevalences of infection were found (with 90-100% of pats sampled being positive). We ask whether this variation arises from the inherent stochasticity in transmission dynamics or whether it is a signature of underlying heterogeneities in the cattle population. A novel approach is taken whereby the cross-sectional data are viewed as providing independent snapshots of a dynamic process. Using maximum-likelihood methods to fit time-dependent epidemiological models to the data we obtain estimates for the rates of immigration and transmission of E. coli O157 infection - parameters which have not been previously quantified in the literature. A comparison of alternative model fits reveals that the variation in the prevalence data is best explained when a proportion of the cattle are assumed to transmit infection at much higher levels than the rest - the so-called super-shedders. Analysis of a second dataset, comprising samples taken from 32 farms at monthly intervals over a period of 1 year, additionally yields an estimate for the rate of recovery from infection. The pattern of prevalence displayed in the second dataset also strongly supports the super-shedder hypothesis.
Matthews
L
L
Centre for Tropical Veterinary Medicine, University of Edinburgh, Easter Bush, Roslin, Midlothian, UK. louise.matthews@ed.ac.uk
McKendrick
I J
IJ
Ternent
H
H
Gunn
G J
GJ
Synge
B
B
Woolhouse
M E J
ME
D000822:Animal Husbandry; D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000382:microbiology / Q000635:transmission*; D003430:Cross-Sectional Studies; D003625:Data Collection; D004927:Escherichia coli Infections / Q000382:microbiology / Q000635:transmission*; D019453:Escherichia coli O157 / Q000472:pathogenicity*; D008962:Models, Theoretical; D015995:Prevalence; D012606:Scotland / Q000453:epidemiology