University of Nebraska Cooperative Extension MP 76-A

Summary

This diagnostic strategy can be used in food-safety research or as a monitoring tool in animal production food-safety programs to classify feedlot pens by the percentage of cattle shedding Escherichia coli O157:H7.

Introduction

The principles of hazard-analysiscritical- control-points (HACCP) were developed to minimize the likelihood that food will be contaminated with potentially dangerous pathogens. Ideally food-safety would be maximized if HACCP principles were applied at all levels of food production and processing. Unfortunately, there is insufficient knowledge of the epidemiology and ecology of E. coli O157:H7 to design and implement HACCP-based food safety programs in cattle feedyards.

Research or development of on-farm HACCP programs to control E. coli O157:H7 in feedlot production systems have been hampered by difficulty in determining the infection status of cattle at any point in time. The difficulty in diagnosis results because infection with E. coli O157:H7 in cattle occurs without clinical signs, except in calves, and because there is a lack of field-validated methods to monitor livestock for food safety pathogens.

Determining if individual live cattle are shedding E. coli O157:H7 is expensive and impractical. For example, culture of the feces from most, if not all, animals in a feedlot pen requires considerable labor and supplies. Handling finished cattle prior to shipping is not desirable because of the loss in value to cattle due to shrink, dark cutters and bruising. It may be possible to control E. coli O157:H7 in feedlots without knowing the infection status of individual cattle because control points or interventions for reducing human foodborne pathogens in feedlot cattle would most likely be directed towards pens of cattle. Therefore, the E. coli O157:H7 status of pens of feedlot cattle is an important outcome for feedlot production food safety research and HACCP monitoring.

Research and development of HACCP-based feedlot food safety programs could advance if pens of cattle, rather than individuals, could be accurately and economically classified by the level of fecal shedding of E. coli O157:H7. Such a pen-test could serve as a monitoring tool in feedlot production food-safety programs, and would allow researchers to test potential interventions, or look for feedlot production methods related to the presence or absence of food-borne pathogens. The objective of this study was to evaluate diagnostic strategies to efficiently identify pens of feedlot cattle with a high prevalence of cattle shedding E. coli O157:H7.

Procedure

Twenty-nine feedlot pens from five Midwestern feedlots, ranging in size from 36 to 231 (median 107) cattle, were each studied once between June and September, 1999 (Smith et al. 2001 Beef Report). Seven pen-test devices that cattle could rub, lick or chew were placed in the pens the evening prior to sample collection. The morning of sampling, feces were collected from the rectums of all cattle in each pen and concurrent samples were collected of pen-test devices and a single composite sample of 20 fresh fecal pats from the pen surface. Culture methods were specific to the type of sample but included selective enrichment and immunomagnetic separation. Isolates were confirmed by standard methods including PCR. Non-parametric statistical methods were used to test either rank differences or rank correlations between pen-level classifications and the results of individual animal testing.

Results

Escherichia coli O157:H7 was isolated from at least one animal in each of the 29 pens. The percentage of cattle shedding detectable levels of the organism within a pen ranged from 0.7% to 79.8% (median 17.1%). E. coli O157:H7 was recovered from at least one pen-test device from 15 pens and from the composite fecal samples of eight pens ( Figure 1 ). Recovery of E. coli O157:H7 from at least one pen test device or from the composite fecal sample was most likely to occur from the pens with higher prevalence (Wilcoxon rank sums P=0.001).

The pen-test devices and composite feces were evaluated singly as diagnostic tools to differentiate high prevalence pens from low prevalence pens. The new pen-test device most accurately detected as positive (greatest percentage of pens classified correctly) pens with 16% or greater prevalence (pen-level sensitivity = 82%, pen-level specificity = 92%). Culture of composite feces most accurately detected as positive pens with a 37% percent or higher prevalence (penlevel sensitivity = 86%, pen-level specificity = 91%).

Information from culture of the pentest devices and the composite feces was combined to classify pens by three levels of fecal shedding prevalence. Pens were classified as high prevalence if E. coli O157:H7 was recovered from the composite fecal sample; pens were classified as medium prevalence if the organism was recovered from the device, but not from the composite feces; pens were classified as low prevalence if the organism was not recovered from the device or the composite feces. This classification scheme correlated well (Spearman s r = 0.76, P<0.0001) with the pen-prevalence determined by culturing the feces from individual cattle. Pens classified as high prevalence had significantly higher rankings in pen-prevalence than pens classified as medium prevalence (P=0.05) or low prevalence (P=0.0006), and pens classified as medium prevalence had significantly higher rankings in pen-prevalence than pens classified as low prevalence (P=0.005).

The premise of the pen-test was to culture a few devices from which many cattle in a pen could have contributed organisms. Culture of the pen-test devices alone or in parallel with culture of a composite fecal sample may be a diagnostically efficient strategy to characterize E. coli O157:H7 fecal shedding in feedlot pens. This diagnostic strategy may be useful as a research tool or as a monitoring tool in the development of animal production food safety programs.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Elbert C. Dickey, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.

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