Pre-harvest interventions to reduce carriage of E. coli O157 by harvest-ready feedlot cattle

Transcription

1 Meat Science 71 (2005) Review Pre-harvest interventions to reduce carriage of E. coli O157 by harvest-ready feedlot cattle G.H. Loneragan a, *, M.M. Brashears b MEAT SCIENCE a Division of Agriculture, Killgore Research Building, West Texas A&M University, WTAMU Box 60998, Canyon, TX , USA b Department of Animal and Food Sciences, Animal Science Building, Texas Tech University, Lubbock, TX , USA Abstract Escherichia coli O157 is an important cause of food-borne illness. The primary reservoir for this organism is cattle and at present the major site of control is within abattoirs. Recent data have highlighted the importance of the pathogen load entering abattoirs on harvest-ready feedlot cattle. The likelihood for in-plant intervention failure increases as the proportion of cattle carrying E. coli O157 within a pen increases. Pre-harvest reduction of E. coli O157 colonization will require targeted intervention strategies and should reduce contamination of carcasses thereby enhancing public health. Several pre-harvest interventions show substantial promise, such as specific strains of direct-fed microbials, vaccine technology, sodium chlorate, and neomycin sulfate, whereas others such as Brown Seaweed or chlorination of water have little or no detectable benefit. Selection of validated interventions strategies will be important as efforts to control pre-harvest carriage of E. coli O157 increase. Ó 2005 Published by Elsevier Ltd. Keywords: Food safety; E. coli O157; Interventions; Feedlot; Cattle Contents 1. Introduction Avenues for pre-harvest control Pre-harvest interventions Direct-fed microbials Vaccine technology Sodium chlorate Neomycin sulfate Conclusions References Introduction * Corresponding author. Tel.: ; fax: address: gloneragan@mail.wtmau.edu (G.H. Loneragan). Escherichia coli serotype O157 has been well characterized as a food-borne pathogen (Wells et al., 1983). This organism possesses several important virulence fac /$ - see front matter Ó 2005 Published by Elsevier Ltd. doi: /j.meatsci

2 G.H. Loneragan, M.M. Brashears / Meat Science 71 (2005) tors that contribute to its importance in public health such as intimin, shiga toxins, and enterohemolysins (Le- Blanc, 2003). Children and the elderly appear at greatest risk for developing disease and subsequent complications (e.g., hemolytic uremic syndrome) after exposure to E. coli O157 (Taylor, White, Winterborn, & Rowe, 1986). Each year there are an estimated 70,000 cases of E. coli O157-induced disease, which in turn results in several thousand hospitalizations and approximately 60 deaths (Mead et al., 1999). However, the number of illnesses reported by Mead and colleagues relies heavily on assumptions and as such they have calculated their estimates using a 20-fold under-reporting of bloody diarrhea. Moreover, these estimates do not reflect recent evidence of a substantial decline in the reported number of cases (Anonymous, 2004). Disease attributed to this pathogen can occur in outbreaks but the majority of cases are sporadic or not associated with an outbreak situation. Exposure to livestock, particularly cattle, livestock facilities, or consumption of beef products are frequently identified as risk factors for disease (Chapman, 2000; Kassenborg et al., 2004; Smith et al., 2004; Varma et al., 2003; Wells et al., 1983). As a consequence, much effort has focused on better understanding the epidemiology of this organism in cattle, particularly cattle housed in feedlots. Based on the current information, it appears that the initial infection with this organism occurs early in life and does not induce protective immunity from subsequent colonization (Gannon et al., 2002; Laegreid, Elder, & Keen, 1999). At the feedlot, prevalence varies seasonally with greatest recovery of E. coli O157 occurring in warmer months (Barkocy-Gallagher et al., 2003; Anonymous, 2001). Based on various large-scale studies, it can be determined that E. coli O157 is ubiquitous among cattle operations and more variation in prevalence occurs pen-to-pen than from feedlot-to-feedlot (Hancock, Rice, Thomas, Dargatz, & Besser, 1997; Sargeant, Sanderson, Griffin, & Smith, 2004a; Sargeant, Sanderson, Smith, & Griffin, 2004b; Smith et al., 2001). As a consequence of this pen-to-pen variation, it has been possible to evaluate pen-level variation in prevalence at the time of harvest and subsequent likelihood of carcass contamination. In a study performed by Elder et al. (2000), the authors recovered E. coli O157 from 26.2%, 13.0%, 43.4%, 18.3%, and 1.9% of fecal samples, hides, pre-evisceration carcasses, post-evisceration carcasses, and carcasses post-intervention, respectively. Importantly, they reported that the pre-harvest prevalence (prevalence in feces or on hides) was associated (r = 0.58, 95% CI , P < 0.01) with prevalence of carcasses positive at any site (pre- or post-evisceration, or post-intervention). In another study, Ransom et al. (2003) visited feedlots immediately prior to shipment to an abattoir and then sampled hides and carcasses from these pens of cattle during harvest. In pens of cattle with a fecal prevalence greater than 20%, they recovered E. coli O157 from 22.5%, 46.3%, 12.5%, 2.5%, and 0.6%, of hides, colons, pre-evisceration carcasses, post-evisceration carcasses and carcasses in the cooler, respectively. The risk of pathogen recovery was less in pens of cattle in which 20% or less of the fecal pats were positive. In these pens, E. coli O157 was recovered from 5.7%, 7.1%, 7.1%, 0.0%, and 0.0%, of hides, colons, pre-evisceration carcasses, post-evisceration carcasses and carcasses in the cooler, respectively. Visits to packing plants and recovered E. coli O157 from 75.7% of hides, 14.7% of pre-evisceration carcasses, 3.8% of post-evisceration carcasses, 0.3% of post-intervention samples and 0.0% of chilled carcasses (Arthur et al., 2004). In addition, they reported that E. coli O157 prevalence on hides was associated (R 2 = 0.68, P < 0.05) with the prevalence on pre-evisceration carcasses. There are two salient points that arise from the three studies briefly discussed above. The first is that in-plant interventions are highly effective at reducing E. coli O157 contamination on carcasses. The second is that despite the efficacious in-plant interventions, if the pathogen load on hides (as estimated by prevalence) entering the plant is large then the likelihood for carcass contamination (even in the cooler) increases. These data stress the importance of pre-harvest manipulation of E. coli O157 carriage, as greater carriage is more likely to result in an in-plant intervention failure. If it is possible to reduce prevalence of E. coli O157 pre-harvest, then presumably carcass contamination and human exposure would also be decreased and consequently, public health may benefit. 2. Avenues for pre-harvest control It is possible to broadly categorize options for preharvest control of E. coli O157 into two avenues. The first is modification of an existing management strategy such as manipulation of a feed ingredient or frequency with which water troughs are washed. The second avenue for control is development and implementation of novel targeted intervention technologies. If the former method of control were possible, then it would be preferable, as modifying an existing management practice could be implemented expeditiously with minimal impact on feedlot operations. The second avenue would require significant investment in development and implementation. The challenges of the latter should not be underestimated as many feedlots may be reluctant to invest the time and resources required to implement a technological change whose immediate benefit does not directly related to day-to-day feeding of cattle. Unfortunately, studies to date have failed to identify a single management practice that is consistently associated with E. coli O157 prevalence and importantly, can

3 74 G.H. Loneragan, M.M. Brashears / Meat Science 71 (2005) readily be modified. One study identified an association with muddy pen surfaces and E. coli O157 prevalence but this variable was not shown to be statistically associated with prevalence in another study (Sargeant et al., 2004b; Smith et al., 2001). Effectively, the only factor consistently associated with carriage is a temporal (seasonal) one (Barkocy-Gallagher et al., 2003; Anonymous, 2001), which is not necessarily modifiable. Even in studies where water troughs have been repeatedly cleaned or pens sanitized, no significant impact on prevalence was detected (Elder & Keen, 1999). Given our present understanding of the epidemiology of E. coli O157 in feedlot cattle, pre-harvest control will require development and implementation of pre-harvest interventions. This paper is not meant to serve as a clearinghouse for information on all potential interventions but rather, we will provide information on specific interventions that are currently available for use or are pending regulatory approval and substantive studies have been performed in settings to facilitate inferential judgment on their potential efficacy in commercial feedlots. Focus has been placed on interventions that appear to exert a positive impact. Interventions with little or no benefit such as chlorination of water (LeJeune et al., 2004) or Ascophyllum nodosum (Brown Seaweed or TASCO14) are not discussed but it is important to note that not all proposed interventions are efficacious. Some of the studies reported herein have estimated the prevalence of E. coli O157 in feces while others have estimated prevalence on hides or both. Hides appear to be the major source for carcass contamination. Presumably, feces are the major source of E. coli O157 for hide contamination. If this is so, then benefits could be achieved with an intervention directed at reducing hide contamination if applied just prior to harvest. Additionally, long-term control could be achieve with an intervention directed at reducing colonization at the rectoanal junction (Naylor et al., 2003) and other sites within the gastrointestinal tract. Such interventions are reliant on an association between fecal prevalence and hide prevalence. To evaluate and describe the relationship between fecal and hide E. coli O157 prevalence, we used logistic regression techniques on a series of published (Loneragan & Brashears, 2005) and unpublished data. These data were derived from six commercial feedlot studies and included cattle housed in 122 pens. Representative fecal and hide samples were collected from 2846 animals. The outcome variable of interest was pen-level binomial response variable (i.e., positive samples/possible events) and the independent variable was fecal prevalence as a continuous variable from 0 to 1. This analysis was performed using the GLIMMIX procedure for SAS (release 9.1.2, Cary, NC). Overall, 17.7% and 22.3% of fecal and hide samples were positive, respectively. E. coli O157 was detected in feces or on hides Fig. 1. Model of hide prevalence as predicted by fecal prevalence (solid line). The dashed lines represent the 95% confidence limits of the prediction. The Y-axis represents the predicted pen-level E. coli O157 prevalence on hides (dependent variable) and the X-axis represents pen-level fecal prevalence (independent variable). in 90.2% of pens. In those pens with at least one positive animal (either feces or hides), the prevalence was 19.9% and 25.3%. In the final logistic model, hide prevalence was associated with fecal prevalence (P < 0.01); the model with parameter estimates was: log½p=ð1 pþš ¼ þ fecal prevalence; where p is the expected hide prevalence at a given fecal prevalence. This model and the 95% confidence limits of the p are depicted in Fig. 1. The importance of this model is that we do not necessarily have to place less importance on studies in which an intervention was associated with a significant reduction in prevalence where only feces were collected. Based on the model above, one can infer that prevalence on hides should also be reduced. 3. Pre-harvest interventions 3.1. Direct-fed microbials This broad category includes probiotics and competitive exclusion. Most of the applied research in this area has focused on a specific Lactobacillus-based direct-fed microbial. The specific strain that appears to be most efficacious based on the current body of literature is NP51 (otherwise known as NPC747). In a study by Brashears, Galyean, Loneragan, Mann, and Killinger- Mann (2003a), this and another strain, NPC750, were evaluated in feedlot cattle housed in five head pens. Averaged over the duration of the study, the likelihood of recovery of E. coli O157 from feces was 49% lower in animals receiving NP51 compared to controls

4 G.H. Loneragan, M.M. Brashears / Meat Science 71 (2005) (OR = 0.51%, 95% CI , P < 0.01). The effect of NPC750 was less so with only a 30% reduction in the likelihood of recovery relative to the controls (OR = 0.7%, 95% CI 0.5B1.1, P = 0.12). A significant benefit of both Lactobacillus strains was detected for hide contamination at harvest (P < 0.05). In a subsequent study, Younts, Galyean, Loneragan, Elam, and Brashears (2004) observed a similar benefit with a 58% reduction in fecal prevalence in animals administered NP51. Interestingly, when fed with another strain of Lactobacillus, the benefit lessened particularly when NP51 was included at a lower dose (10 6 vs NP51 cells per head per day). As with the previous study, benefits were also observed for hide contamination. Salient points from these studies indicate that dose and selection of bacterial strains to include in a directfed microbial product appear to be important. Selection of strains was highlighted by research performed by Brashears, Jaroni, and Trimble (2003b) in which they evaluated approximately 650 candidate strains. Through a systematic series of studies, including the strainõs ability to inhibit E. coli O157, five promising strains were identified, one of which was NP51. The practical importance of strain variation in observed response is that if the wrong direct-fed microbial strain is selected then conceivably, E. coli O157 shedding and resultant hide contamination could inadvertently be increased rather than the desired decrease. To evaluate the effect of NP51 dose, Younts-Dahl et al. (2005) evaluated three daily doses of 10 7,10 8, and 10 9 NP51 cells per head. Averaged over time, the highest dose provided the greatest reduction, which amounted to a 77% lower likelihood of E. coli O157 recovery compared to controls (OR = 0.23%, 95% CI , P < 0.01). While the lower doses also resulted in reduce shedding averaged over time compared to the controls, their benefit was not as great. A similar benefit was observed in hide samples collected at slaughter. This study also highlighted the importance of appropriate strain selection as inclusion of another Lactobacillus strain in addition to NP51 actually resulted in increased fecal shedding and hide contamination at slaughter compared to controls (OR = 1.53%, 95% CI , P = 0.37). This product, NP51, is commercially available and at the time of manuscript submission, and approximately 10% of the fed cattle in the US are fed this product (personal communication, Douglas Ware, Nutrition Physiology Corporation). Another potentially promising direct-fed microbial is an Enterococcus-based product. As yet, no peerreviewed publications in which the benefits of this product have been reported are available. However, company data are encouraging; even though prevalence was quite low, animals fed this candidate direct-fed microbial were 56% less likely to have E. coli O157 recovered from their feces. Competitive exclusion using non-pathogenic E. coli appears to be a practical option in confined animal feeding operations (Tkalcic et al., 2003; Zhao et al., 2003). This application has largely been restricted to artificially infected animals; however, results appear promising. More research is needed for Enterococcus- and E. coli-based direct-fed microbials before wholesale recommendations can be given. However, preliminary work is encouraging and warrants consideration as promising future pre-harvest interventions Vaccine technology Development of vaccine technology would hold great practical application within the beef industry because (1) cattle producers are familiar with administration of vaccines; (2) incorporation into existing management of cattle would be fairly simple; (3) vaccines could be used in all sectors of the industry, not just confined animal feeding operations. Eliciting an immune response within the gastrointestinal tract and other challenges associated with developing an efficacious vaccine whose outcome would be decreased E. coli O157 carriage should not be underestimated. For example, natural exposure does not induce protective immunity. Despite the many challenges, it appears vaccines may be a viable pre-harvest intervention of the future. Investigators administered a vaccine containing type III secreted proteins to feedlot cattle on three occasions 21 days apart (Potter et al., 2004). Averaged over the feeding period, 8.8% and 21.3% of vaccinates and controls, respectively, were positive for E. coli O157 in their feces. These data indicate an average vaccine efficacy of 58.7%, i.e., 58.7% lower risk of recovery of E. coli O157 in vaccinates vs. controls. Hide contamination was not estimated in this study. As described above, however, hide contamination is directly correlated with fecal shedding and one would expect such a vaccine to demonstrate comparable reduction in the proportion of hides positive for E. coli O157 at harvest. Another bacterin vaccine has also been evaluated and has demonstrated the potential to reduce prevalence of E. coli O157 in feces and on hides at harvest (personal communication, Keith Belk, Colorado State University) Sodium chlorate Sodium chlorate is metabolized within facultative anaerobic bacteria to highly toxic chlorite (Anderson et al., 2001b). Ultimately, sodium chlorate acts as a suicide substrate for certain bacteria within the gastrointestinal tract. In preliminary studies of cattle and sheep challenged with E. coli O157, sodium chlorate effectively reduced pathogen carriage (Callaway et al., 2002, 2003). These studies used cattle and sheep artificially inoculated with specific strains of E. coli O157 and the

5 76 G.H. Loneragan, M.M. Brashears / Meat Science 71 (2005) animals were fed potassium nitrate to up-regulate the nitrate reductase enzyme. Because of these factors, extrapolation to an expected benefit in naturally infected feedlot cattle not consuming supplemental potassium nitrate is tenuous. However, there is no reason to suspect that sodium chlorate would not be an efficacious pre-harvest intervention. A potential benefit beyond E. coli O157 control is that sodium chlorate could be used for a variety of food-borne pathogens that are facultative anaerobic such as Salmonella and Shield. In fact, an in vitro evaluation of sodium chlorate demonstrated bactericidal effects among Salmonella Typhimurium DT104; a similar benefit was observed in pigs artificially inoculated with Salmonella (Anderson et al., 2000, 2001a) Neomycin sulfate Intuitively, E. coli O157 colonization within the gastrointestinal tract might be modified by the use of antimicrobial drugs. Of E. coli O157 isolates recovered from naturally infected animals, all were susceptible to neomycin sulfate (personal communication, Robert Elder, Seaboard Farms, Inc.). This antimicrobial drug is approved for in-feed or in-water administration to cattle for the control or treatment of colibacillosis and because of its route of administration, can be applied to entire groups of animals. In a commercial feedlot study in which neomycin sulfate was included in the water for 2 days, fecal shedding and hide contamination were markedly reduced in harvest-ready cattle (P < 0.01 for both models; unpublished data). In the control and treated cattle, E. coli O157 was recovered from 22.1% and 0.4% of feces and 50.0% and 2.5% of hides, respectively. This represents a 98.2% and 95.0% reduction in fecal and hide recovery, respectively. As hide contamination is indicative of cumulative fecal contamination over time and because samples were collected approximately 24 h after the second dayõs administration of neomycin sulfate, it is unclear why such an effect was seen on hides. It is possible that (1) the presence of neomycin in feces prevented successful recovery of viable E. coli O157 cells on hides; or (2) hide carriage is more dynamic than previously thought. It seems highly improbable that animal sampling inadvertently led to selection of negative animals in the treated group and positive animals in the controls simply due to chance. Moreover, in another commercial feedlot study (personal communication, Keith Belk, Colorado State University), a similar effect in feces and on hides was seen in response to neomycin sulfate administration to harvest-ready cattle. The use of antimicrobial drugs, particularly in-feed products, in animal agriculture is becoming increasingly contentious. As it currently stands, a label change will be required before neomycin sulfate can be used to control E. coli O157 in cattle. In the event of such a label change, adoption of neomycin sulfate as a pre-harvest intervention to control E. coli O157 will likely generate much debate. The conventional applications of antimicrobial drugs in animal agriculture are to improve animal health or the efficiency of production. The use of neomycin sulfate would be to reduce the load of E. coli O157 entering packing plants, i.e., a potential human health benefit. The argument concerning neomycin sulfate use, therefore, will have to weigh the potential benefits for public health against the potential costs in terms of favoring development of resistance. 4. Conclusions Based on the studies outlined above, it is clear that several interventions show considerable promise. Other candidate interventions that may be beneficial but are at present in the early developmental phase and are beyond the scope of this manuscript. Currently, the only available intervention that has been broadly evaluated is the NP51-containing direct-fed microbial. Other interventions that are presently available, such as the Enterococcus-based direct-fed microbials, require additional validation before one should recommend their use. Several challenges exist for those interventions that require regulatory approval. E. coli competitive exclusion products, sodium chlorate, and neomycin sulfate all require approval by the Food and Drug Administration (FDA). Such approvals are time consuming and costly. Animal vaccines are traditionally licensed by the Center for Veterinary Biologics (CVB) within the US Department of Agriculture and are intended to improve animal health. A vaccine to reduce intestinal carriage of E. coli O157 would have little if any benefit to animal health and be administered to cattle solely to improve public health. As such, it is unclear if the CVB will license E. coli O157 vaccines or if they will fall under the regulatory authority of the FDA. The FDAapproval process for vaccines is far more convoluted, time consuming, and costly than that of the CVB. Ulti- Table 1 Food Safety and Inspection Service (FSIS) reported ground beef samples positive for E. coli O157 and Centers for Disease Control and Prevention (CDC) estimate for human incidence of E. coli O157 induced illnesses based on FoodNet data Year Source of data FSIS CDC Not available

6 G.H. Loneragan, M.M. Brashears / Meat Science 71 (2005) mately, the costs associated with FDA licensing may make the vaccines prohibitively expensive for routine use. It is the opinion of the authors that vaccines to reduce the carriage of E. coli O157 in cattle should be licensed by the CVB. If so, the products could be available sooner and the associated licensing costs would be less. Regardless of the challenges associated with approval and implementation of pre-harvest interventions, the potential for efficacious, on-farm control of E. coli O157 exists. The Food Safety and Inspection Service has reported a reduction in the percentage of positive trim samples, while the Centers for Disease Control and Prevention have reported a reduction in the incidence of E. coli O157-induced human illnesses (Table 1). It is likely that much of the former (and potentially some of the latter) reduction is attributable to in-plant interventions and management. Some of the reductions may also be due to the adoption of the NP51-containing direct-fed microbial. Another possibility analogous to the emergence of E. coli O157 is that newly emerging serotypes may be replacing the O157. Implementation of efficacious pre-harvest interventions should reduce the failure rate of in-plant interventions. In other words, reducing the prevalence of E. coli O157 carried by cattle entering abattoirs will improve the efficiency of in-plant interventions. This should improve the safety of beef, reduce the public exposure to this pathogen, and improve public health. References Anderson, R. C., Buckley, S. A., Callaway, T. R., Genovese, K. J., Kubena, L. F., Harvey, R. B., et al. (2001a). Effect of sodium chlorate on Salmonella Typhimurium concentrations in the weaned pig gut. Journal of Food Protection, 64, Anderson, R. C., Buckley, S. A., Kubena, L. F., Stanker, L. H., Harvey, R. B., & Nisbet, D. J. (2000). Bactericidal effect of sodium chlorate on Escherichia coli O157:H7 and Salmonella typhimurium dt104 in rumen contents in vitro. Journal of Food Protection, 63, Anderson, R. C., Callaway, T. R., Buckley, S. A., Anderson, T. J., Genovese, K. J., Sheffield, C. L., et al. (2001b). Effect of oral sodium chlorate administration on Escherichia coli O157:H7 in the gut of experimentally infected pigs. International Journal of Food Microbiology, 71, Anonymous. (2001). Escherichia coli O157 in United States feedlots. United State Department of Agriculture, Animal and Plant Health Inspection Service, Veterinary Services: Centers for Epidemiology and Animal Health. Fort Collins: CO Info Sheet Publication #N Anonymous. (2004). Preliminary Foodnet data on the incidence of infection with pathogens transmitted commonly through foodselected sites, United States, Morbidity and Mortality Weekly Report, 53, Arthur, T. M., Bosilevac, J. M., Nou, X., Shackelford, S. D., Wheeler, T. L., Kent, M. P., et al. (2004). Escherichia coli O157 prevalence and enumeration of aerobic bacteria, enterobacteriaceae, and Escherichia coli O157 at various steps in commercial beef processing plants. Journal of Food Protection, 67, Barkocy-Gallagher, G. A., Arthur, T. M., Rivera-Betancourt, M., Nou, X., Shackelford, S. D., Wheeler, T. L., et al. (2003). Seasonal prevalence of shiga toxin-producing Escherichia coli, including O157:H7 and non-o157 serotypes, and Salmonella in commercial beef processing plants. Journal of Food Protection, 66, Brashears, M. M., Galyean, M. L., Loneragan, G. H., Mann, J. E., & Killinger-Mann, K. (2003a). Prevalence of Escherichia coli O157:H7 and performance by beef feedlot cattle given Lactobacillus direct-fed microbials. Journal of Food Protection, 66, Brashears, M. M., Jaroni, D., & Trimble, J. (2003b). Isolation, selection, and characterization of lactic acid bacteria for a competitive exclusion product to reduce shedding of Escherichia coli O157:H7 in cattle. Journal of Food Protection, 66, Callaway, T. R., Anderson, R. C., Genovese, K. J., Poole, T. L., Anderson, T. J., Byrd, J. A., et al. (2002). Sodium chlorate supplementation reduces E. coli O157:H7 populations in cattle. Journal of Animal Science, 80, Callaway, T. R., Edrington, T. S., Anderson, R. C., Genovese, K. J., Poole, T. L., Elder, R. O., et al. (2003). Escherichia coli O157:H7 populations in sheep can be reduced by chlorate supplementation. Journal of Food Protection, 66, Chapman, P. A. (2000). Sources of Escherichia coli O157 and experiences over the past 15 years in Sheffield, UK. Journal of Applied Bacteriology, 51 S60. Elder, R.O., & Keen, J.E., (1999). Effects of pen cleaning and group vs. individual penning on fecal shedding of naturallyacquired enterohemorrhagic E. coli (EHEC) O157 in beef feedlot cattle. In Conference of research workers in animal diseases, Chicago, IL. Elder, R. O., Keen, J. E., Siragusa, G. R., Barkocy-Gallagher, G. A., Koohmaraie, M., & Laegreid, W. W. (2000). Correlation of enterohemorrhagic Escherichia coli O157 prevalence in feces, hides, and carcasses of beef cattle during processing. Proceedings of the National Academy of Science USA, 97, Gannon, V. P., Graham, T. A., King, R., Michel, P., Read, S., Ziebell, K., et al. (2002). Escherichia coli O157:H7 infection in cows and calves in a beef cattle herd in Alberta, Canada. Epidemiology and Infection, 129, Hancock, D. D., Rice, D. H., Thomas, L. A., Dargatz, D. A., & Besser, T. E. (1997). Epidemiology of Escherichia coli O157 in feedlot cattle. Journal of Food Protection, 60, Kassenborg, H. D., Hedberg, C. W., Hoekstra, M., Evans, M. C., Chin, A. E., Marcus, R., et al. (2004). Farm visits and undercooked hamburgers as major risk factors for sporadic Escherichia coli O157:H7 infection: data from a case-control study in 5 Foodnet sites. Clinical Infectious Diseases, 38(Suppl. 3), S271 S278. Laegreid, W. W., Elder, R. O., & Keen, J. E. (1999). Prevalence of Escherichia coli O157:H7 in range beef calves at weaning. Epidemiology and Infection, 123, LeBlanc, J. J. (2003). Implication of virulence factors in Escherichia coli O57:H7 pathogenesis. Critical Reviews in Microbiology, 29, LeJeune, J. T., Besser, T. E., Rice, D. H., Berg, J. L., Stilborn, R. P., & Hancock, D. D. (2004). Longitudinal study of fecal shedding of Escherichia coli O157:H7 in feedlot cattle: predominance and persistence of specific clonal types despite massive cattle population turnover. Applied and Environmental Microbiology, 70, Loneragan, G. H., & Brashears, M. M. (2005). Effects of using retention-pond water for dust abatement on performance parameters of feedlot steers and carriage of E. coli O157 and Salmonella. Journal of the American Veterinary Medical Association, 226(8),

7 78 G.H. Loneragan, M.M. Brashears / Meat Science 71 (2005) Mead, P. S., Slutsker, L., Dietz, V., McCaig, L. F., Bresee, J. S., Shapiro, C., et al. (1999). Food-related illness and death in the United States. Emerging Infectious Diseases, 5, Naylor, S. W., Low, J. C., Besser, T. E., Mahajan, A., Gunn, G. J., Pearce, M. C., et al. (2003). Lymphoid follicle-dense mucosa at the terminal rectum is the principal site of colonization of enterohemorrhagic Escherichia coli O157:H7 in the bovine host. Infections and Immunity, 71, Potter, A. A., Klashinsky, S., Li, Y., Frey, E., Townsend, H., Rogan, D., et al. (2004). Decreased shedding of Escherichia coli O157:H7 by cattle following vaccination with type III secreted proteins. Vaccine, 22, Ransom, J.R., Sofos, J.N., Belk, K.E., Dewell, G.A., McCurdy, K.S., Smith, et al. (2003). Prevalence of Escherichia coli O157 in feedlot cattle feces, on their hides and on carcasses from those cattle. In 10th Meeting of the international symposium of veterinary epidemiology and economics, Vina Del Mar, Chile. Sargeant, J. M., Sanderson, M. W., Griffin, D. D., & Smith, R. A. (2004a). Factors associated with the presence of Escherichia coli O157 in feedlot-cattle water and feed in the Midwestern USA. Preventive Veterinary Medicine, 66, Sargeant, J. M., Sanderson, M. W., Smith, R. A., & Griffin, D. D. (2004b). Associations between management, climate, and Escherichia coli O157 in the faeces of feedlot cattle in the Midwestern USA. Preventive Veterinary Medicine, 66, Smith, D., Blackford, M., Younts, S., Moxley, R., Gray, J., Hungerford, L., et al. (2001). Ecological relationships between the prevalence of cattle shedding Escherichia coli O157:H7 and characteristics of the cattle or conditions of the feedlot pen. Journal of Food Protection, 64, Smith, K. E., Stenzel, S. A., Bender, J. B., Wagstrom, E., Soderlund, D., Leano, F. T., et al. (2004). Outbreaks of enteric infections caused by multiple pathogens associated with calves at a farm day camp. Pediatric Infectious Diseases Journal, 23, Taylor, C. M., White, R. H., Winterborn, M. H., & Rowe, B. (1986). Haemolytic-uraemic syndrome: clinical experience of an outbreak in the west midlands. British Medical Journal, 292, Tkalcic, S., Zhao, T., Harmon, B. G., Doyle, M. P., Brown, C. A., & Zhao, P. (2003). Fecal shedding of enterohemorrhagic Escherichia coli in weaned calves following treatment with probiotic Escherichia coli. Journal of Food Protection, 66, Varma, J. K., Greene, K. D., Reller, M. E., DeLong, S. M., Trottier, J., Nowicki, S. F., et al. (2003). An outbreak of Escherichia coli O157 infection following exposure to a contaminated building. Journal of American Medical Association, 290, Wells, J. G., Davis, B. R., Wachsmuth, I. K., Riley, L. W., Remis, R. S., Sokolow, R., et al. (1983). Laboratory investigation of hemorrhagic colitis outbreaks associated with a rare Escherichia coli serotype. Journal of Clinical Microbiology, 18, Younts, S. M., Galyean, M. L., Loneragan, G. H., Elam, N. A., & Brashears, M. M. (2004). Dietary supplementation with lactobacillus- and Propionibacterium-based direct-fed microbials and prevalence of Escherichia coli O157 in beef feedlot cattle and on hides at harvest. Journal of Food Protection, 67, Younts-Dahl, S. M., Osborn, G. D., Galyean, M. L., Rivera, J. D., Loneragan, G. H., & Brashears, M. M. (2005). Reduction of Escherichia coli O157 in finishing beef cattle by various doses of Lactobacillus acidophilus in direct-fed microbials. Journal of Food Protection, 68, Zhao, T., Tkalcic, S., Doyle, M. P., Harmon, B. G., Brown, C. A., & Zhao, P. (2003). Pathogenicity of enterohemorrhagic Escherichia coli in neonatal calves and evaluation of fecal shedding by treatment with probiotic Echerichia coli. Journal of Food Protection, 66,