Risk factors for the presence of high-level shedders of Escherichia coli O157 on Scottish farms

Abstract

Escherichia coli O157 infections are the cause of sporadic or epidemic cases of often bloody diarrhea that can progress to hemolytic uremic syndrome (HUS), a systematic microvascular syndrome with predominantly renal and neurological complications. HUS is responsible for most deaths associated with E. coli O157 infection. From March 2002 to February 2004, approximately 13,000 fecal pat samples from 481 farms with finishing/store cattle throughout Scotland were examined for the presence of E. coli O157. A total of 441 fecal pats from 91 farms tested positive for E. coli O157. From the positive samples, a point estimate for high-level shedders was identified using mixture distribution analysis on counts of E. coli O157. Models were developed based on the confidence interval surrounding this point estimate (high-level shedder, greater than 10(3) or greater than 10(4) CFU g(-1) feces). The mean prevalence on high-level-shedding farms was higher than that on low-level-shedding farms. The presence of a high-level shedder on a farm was found to be associated with a high proportion of low-level shedding, consistent with the possibility of a higher level of transmission. Analysis of risk factors associated with the presence of a high-level shedder on a farm suggested the importance of the pathogen and individual host rather than the farm environment. The proportion of high-level shedders of phage 21/28 was higher than expected by chance. Management-related risk factors that were identified included the type of cattle (female breeding cattle) and cattle stress (movement and weaning), as opposed to environmental factors, such as water supply and feed.

FIG. 1.

Results of mixture distribution analysis. Normal-normal distribution. Point estimate of threshold for high-level shedding, 3,135; 95% CI for threshold estimate, 1,658 and 10,395.

FIG. 2.

Geographical distribution of farms. (A) Distribution of all 481 farms sampled in Scotland between March 2002 and February 2004. Red dots indicate positive farms (n = 91). Prevalence estimated at 18.9% (exact binomial 95% CI = 15.5 to 22.7). (B) Distribution of positive farms used in all analyses (n = 77) showing the designation of shedding. The maps were created on the basis of border data from http://www.edina.ac.uk.

FIG. 3.

Comparison of proportions of the three major phage types (and others) for low-level and high-level shedders using the point estimate of the high-level threshold (≥3,135). Chi square was significant (chi square = 9.135; P = 0.003; exact odds ratio = 2.97).

FIG. 4.

Comparison of distribution of prevalences of E. coli O157 on low-level- and high-level-shedding farms using the point estimate of the high-level threshold (≥3,135). The resulting GLMM model was significant (F = 5.27; df = 1,71; P = 0.025; odds ratio = 2.28 [1.11 to 4.65]).

FIG. 5.

Distribution of regression coefficients for “number of positives” variable from GLM analysis. Graphs summarize 10,000 runs. The arrow represents the location of the coefficient from the original GLM. (A) Model using ≥10³ CFU g⁻¹ feces to define high-level shedder. The original parameter (−4.26) was not extreme relative to the pseudodistribution (two-tailed test, P = 0.34). (B) Model using ≥10⁴ CFU g⁻¹ feces to define high-level shedder. The original parameter (−4.21) was not extreme relative to the pseudodistribution (two-tailed test, P = 0.15).