| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
FOOD SAFETY SYMPOSIUM |
Food Animal Health Research Program, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691
 |
Abstract |
|---|
 Top Abstract INTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
Key Words: cattle • Escherichia coli O157 • food safety • preharvest
|
INTRODUCTION |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
; USDA, 2005). Despite adoption of in-plant intervention strategies focused specifically to reduce this pathogen and other foodborne pathogens in the finished meat product, illnesses are continuously being reported. Furthermore, in-plant intervention strategies do not affect the frequency of human exposure to this pathogen from direct contact with cattle nor via indirect exposure to the organism from manure-contaminated foods such as fruits and vegetables, water, or the environment. Although testing and segregation of E. coli O157:H7-positive cattle near or immediately before slaughter until the animals test negative has been considered as a method to limit the number of cattle contaminated with E. coli O157:H7 entering the food chain, this approach is problematic for several reasons, including 1) the sporadic pattern of fecal excretion of E. coli O157:H7 is such that an animal might test negative one time but be reexposed before the next test period or slaughter; 2) the hide may be contaminated in the absence of fecal excretion, and 3) the expense and logistics of handling large numbers of test-positive animals is prohibitive. Thus, considering the diverse sources and routes through which humans may contact contaminated bovine manure, intervention strategies that target the pathogen in live animals on the farm before slaughter, termed preharvest intervention or control, may have the largest impact on improving beef safety.
|
PREHARVEST INTERVENTION CONCEPTS |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
; Pyburn et al., 2005). Analytical and theoretical models of preharvest control of E. coli O157:H7 have likewise predicted significant impacts of on-farm intervention strategies on human cases of this foodborne bacterium (Jordan et al., 1999a,b). More recently, empirical data collected by Elder et al. (2000) confirm the importance of lowering the preharvest prevalence of E. coli O157 on the carcass contamination rate. In recent years, the frequency of E. coli O157 contamination in ground beef has declined, and this decrease is concurrent with a decrease in the incidence of infections caused by E. coli O157; however, it has not been possible to demonstrate that these 2 parameters are causally associated with one another.
Unfortunately, several epidemiological and ecological characteristics of E. coli O157:H7 represent constraints to the effectiveness of some strategies used for preharvest control of E. coli O157:H7. The widespread distribution of E. coli O157:H7, its persistence in environmental sources, its apparent ability to reinfect cattle, as well as its wide host range (including wildlife), make complete eradication of E. coli O157:H7 an unrealistic goal (Hancock et al., 2001). An achievable objective for preharvest intervention for E. coli O157:H7 in cattle is to reduce the magnitude or the prevalence of fecal excretion, or both. These goals may be reached by limiting farm-to-farm dissemination of the organism, cattle-to-cattle dissemination within a farm, or proliferation within an individual animal. These preharvest intervention methods can be grouped into 3 general categories: 1) exposure reduction strategies; 2) exclusion strategies, and 3) direct antipathogen strategies. Some of these preharvest intervention strategies are available for application, some will be available in the near future, and other strategies still require more research into their effectiveness at controlling E. coli O157:H7 before they can be available to livestock producers.
|
EXPOSURE REDUCTION STRATEGIES |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
). It is logical that reducing the frequency of exposure of cattle to these contaminated sources will reduce E. coli O157:H7 prevalence in live animals. However, at present, it is not known how much each of these potential sources of exposure contributes to the overall herd prevalence of this organism.
Water Quality
Several researchers have demonstrated a positive association between drinking water contamination with E. coli O157:H7 and the presence of this organism in cattle feces (Faith et al., 1996; LeJeune et al., 2004; Davis et al., 2005). When water sources are positive for E. coli O157, inevitably cattle are also positive for this organism, and vice versa. Clearly, water sources for cattle are frequently contaminated with relatively high numbers of generic E. coli, and water troughs can be reservoirs and vehicles for dissemination of E. coli O157:H7 (LeJeune et al., 2001a,b). Whether the presence of E. coli O157 in these troughs is simply a result of bovine fecal contamination or a principle source of exposure to this foodborne pathogen remains undetermined. Attempts to enhance livestock drinking water quality by various treatment methods including chlorination, electrolyzed water, and ozonation have yielded marginal impacts on water quality variables (LeJeune et al., 2004), poor palatability (Zhao et al., 2006), or negligible effectiveness on E. coli O157:H7 prevalence in cattle when tested in the field (LeJeune et al., 2004; Besser et al., 2005; Zhao et al., 2006). Furthermore, coliform or E. coli contamination levels in water are not predictive of E. coli O157:H7 presence (LeJeune et al., 2004; Sanderson et al., 2005). Although eliminating E. coli O157:H7 from cattle drinking water may be a meritorious goal, until such time that practical, economical, and effective measures that reduce livestock drinking water contamination have been demonstrated to significantly affect the epidemiology of E. coli O157 or other pathogens in cattle are demonstrated, investment in extensive water trough management programs may not be warranted from a food safety standpoint.
Feed Hygiene
Similar to water contamination, the hygiene of livestock feeds are thought to play key role in the exposure of cattle to E. coli O157:H7 (Crump et al., 2002). Feed previously exposed to cattle, as well as feed components intended for cattle feeds, have been reported to be contaminated with E. coli O157:H7 with prevalence values approaching 1 and 15%, respectively (Davis et al., 2003; Dodd et al., 2003). To date, however, there have been no published reports describing a correlation among the magnitude or frequency of feed contamination with coliforms, generic E. coli, or E. coli O157:H7 and bovine prevalence of the organism. Two recent studies failed to find any correlation between the magnitude of coliform contamination of feeds and E. coli O157:H7 prevalence in cattle (Dodd et al., 2003; LeJeune et al., 2006). Nevertheless, in several European and Scandinavian countries strict animal feed hygiene controls are in place, and the incidence of Salmonella (also thought to be frequently feedborne) is low in cattle and swine (Boqvist et al., 2003). It remains to be seen if increased feed hygiene results in measurable decreases in E. coli O157 and other foodborne pathogen in cattle.
Environmental Exposure
Given the natural grooming behavior of cattle and the potential for bacteria to become dispersed on dust particles it would seem logical that the hygienic conditions of the livestock’s immediate environment, such as the floor of feedlot cattle pens or bedding provided for dairy cattle, would have significant impacts on bovine exposure to E. coli O157:H7. In fact, Smith et al. (2001) reported that the prevalence of E. coli O157 in cattle was greatest in pens that were muddy or wet. Dairy cattle housed in barns with flush-type manure removal systems were more likely to have E. coli O157:H7 than animals housed in barns where manure was removed by scraping (Garber et al., 1999).
Although dusty feedlot conditions are not desirable for many environmental, animal health, and human occupational health concerns, decreasing dust formation through sprinkling had no significant impact on fecal shedding of E. coli O157:H7 in feedlot cattle (Loneragan and Brashears, 2005; Morrow et al., 2005). Other studies have demonstrated that gram-negative bacteria are infrequently recovered from dust at feedlots, possibly because of their rapid inactivation by UV irradiation (Wilson et al., 2002). In contrast, animals housed indoors may be exposed to gram-negative bacteria on dust particles (Zucker et al., 2000). A greater prevalence of E. coli O157:H7 was reported (2005) in dairy cows housed on sawdust bedding compared with herds bedded with sand (P = 0.05). Clearly, given the diverse management difference between beef and dairy cattle, preharvest control measures in these 2 production systems will probably differ significantly.
Animal Density
Several studies have identified grouping of calves before weaning as a risk factor for increased fecal shedding of E. coli O157:H7 excretion by calves. Calves typically excrete E. coli O157 more frequently and in greater numbers than adult animals (Cray and Moon, 1995). Thus, calves may serve a reservoir of E. coli O157:H7 from which the organisms are maintained and periodically disseminated into adult animals housed nearby. Housing calves separately, away from other stock, may provide a mechanism to prevent O157 on farms; however, this hypothesis has not been validated. Housing preweaned calves in a physically separate location from older animals may be possible for dairy calves but not for cow-calf pairs on pasture. Off-site heifer raising is another option, but biosecurity risks associated with bringing bred heifers back onto the farm from outside sources must carefully be considered (Hegde et al., 2005).
Most studies that have explored the relationship between herd size and E. coli O157:H7 prevalence in dairy herds have not yielded significant correlations between these 2 factors (Dodson and LeJeune, 2005). However, Vidovic and Korber (2006) recently reported a significantly greater E. coli O157:H7 prevalence in feedlot cattle that were housed at high density compared with cattle in pens housed with more area per cow.
Wildlife Exclusion
Escherichia coli O157:H7 has been isolated from a large number of nonbovine sources on farms. This includes other domestic animals (i.e., dogs, other ruminants, horses, and pigs; Hancock et al., 1998). In addition, others have documented E. coli O157 in wildlife including raccoons, opossums (Renter et al., 2004), rats (Cizek et al., 1999), wild birds (Nielsen et al., 2004), and deer (Rice et al., 1995). Although cattle are considered the primary reservoir of this organism, it is possible that novel strains of E. coli O157:H7 are introduced into cattle populations through feed or water contaminated with the feces from these animals (Daniels et al., 2003; Wetzel and LeJeune, 2006). Once introduced, passage through even a single cow could effectively amplify the organism to levels that could result in a large number of cattle becoming colonized with E. coli O157:H7. Thus, exclusion of wildlife from livestock feeds and water would appear to be a beneficial practice. Similar to the situation with feed and water hygiene, the effects of wildlife exclusion, to the extent currently possible, on E. coli O157:H7 prevalence in cattle have yet to be documented.
|
EXCLUSION STRATEGIES |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
). The most common of these methods includes dietary manipulation based on the use of specific feed components, probiotics, prebiotics, and competitive exclusion agents.
Feed Component Management
Feed management has been suggested to influence environmental conditions within the gastrointestinal tract that modify the competitiveness or survival of E. coli O157:H7. Early epidemiological studies identified cottonseed and clover feeding as reducing fecal excretion of E. coli O157:H7 in dairy cattle (Garber et al., 1995; Dargatz et al., 1997). However, other studies reported a positive association between cottonseed in the diet of feedlot cattle and E. coli O157:H7 prevalence (Sargeant et al., 2004). Likewise, corn silage, barley, and beet pulp appear to increase the carriage rates of E. coli O157:H7 in cattle (Dargatz et al., 1997; Herriott et al., 1998). In a commercial-scale field trial, Berg et al. (2004) demonstrated that feedlot cattle fed barley had a greater prevalence and magnitude of E. coli O157:H7 than cornfed cattle. A controversial study on the subject of E. coli and feed components, specifically high concentrate diets and forage-rich diets, in cattle appeared in Science in 1998 (Diez-Gonzalez et al.). The authors’ claim that briefly switching cattle from a grain-fed diet to hay shortly before slaughter would reduce the number of cattle positive for E. coli O157:H7 entering the food chain prompted numerous investigations on the subject, none of which provided convincing evidence of predictable or repeatable results (Hovde et al., 1999; Buchko et al., 2000; Callaway et al., 2003).
There are many biologically plausible explanations for how a particular feed might influence the gastrointestinal microbiota (e.g., altering volatile fatty acid concentrations, changing the pH conditions, and altering the composition of the resident bacteria). Our current understanding of the complex mechanisms involving maintenance of the microbial ecosystem in the digestive tract of cattle and E. coli O157H7’s response to this change in environment, however, are inadequate to predict with reasonable certainty the impact of specific feed components on E. coli O157:H7. Until such time that these interactions are better understood, selection of particular feeds to control E. coli O157:H7 in live animals will be a hit-and-miss process, which will require multiple large, expensive trials to validate.
Probiotics
Probiotics are defined as "a preparation of or a product containing viable, defined microorganisms in sufficient numbers, which alter the microflora in a compartment of the host and that exert beneficial health effects in this host" (Schrezenmeir and de Vrese, 2001). These preparations are also called direct fed microbials. Direct fed microbial preparations generally consist of individual bacterial species or mixtures of bacterial species and yeast, often including lactic acid bacteria. Direct fed microbials have been used in the cattle industry for over 20 yr to enhance animal health and production (Nocek and Kautz, 2006). More recently, interest has focused on the potential of these products to affect foodborne pathogen carriage in animals (Tournut, 1989; Brashears et al., 2003; Younts-Dahl et al., 2005). Researchers have isolated several E. coli strains from cattle capable of producing colicins (proteins that specifically inhibit E. coli) that show potential to displace E. coli O157:H7 in live cattle (Zhao et al., 2003) and reduce fecal excretion of enterohemorrhagic Escherichia coli in neonatal calves (Zhao et al., 2003) and in weaned calves also (Tkalcic et al., 2003). This culture’s effectiveness in preventing E. coli O157:H7 colonization under commercial conditions has yet to be examined. Another product, a Lactobacillus acidophilus culture, has repeatedly demonstrated effectiveness at reducing E. coli O157:H7 in feedlot cattle by up to 50% (Brashears et al., 2003; Elam et al., 2003; Younts-Dahl et al., 2005). This product is currently available commercially in the United States and is being used in many, if not most, large US feedlots (Callaway et al., 2004).
Prebiotics
Prebiotics are organic compounds such as fructo-oli-gosaccharides that are unavailable to, or indigestible by, the host animal, but are digestible by a specific bacterial species (Schrezenmeir and de Vrese, 2001). The use of prebiotics has been used in humans to promote intestinal health, and in pigs to improve nutrition, but little information is available on the use of these products in cattle (de Vaux et al., 2002). A preliminary report by Braden et al. (2004) described an inhibitory effect of brown seaweed (Ascophyllum nodosum) on E. coli O157:H7 in feedlot cattle. However, comparing prevalence values among multiple pens of animals is required to further elucidate the effect of this product, if any, on bovine E. coli O157:H7 prevalence.
|
DIRECT ANTIPATHOGEN STRATEGIES |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
Hide Washing
This approach involves physical removal of contaminants from the hide and hooves from the cattle immediately before, or just after, slaughter. The initial research indicates that such interventions have significant impacts on carcass contamination (Bosilevac et al., 2005a). Alternatives that have been evaluated include wash water or treated water (ozonated or electrolyzed; Bosilevac et al., 2005b). However, such practices should be weighed against humane handling concerns, food quality issues (e.g., increased dark cutters), and even a potential increase in contamination from slaughtering animals with wet hides.
Antimicrobial Compounds
For unknown reasons, E. coli O157:H7 does not typically exhibit resistance to the large number of antibiotics frequently observed in other enteropathogenic E. coli and other foodborne pathogens, such as Salmonella enterica. Most isolates are susceptible to the antibiotic, neomycin sulfate (Galland et al., 2001; Stephan and Schumacher, 2001; Mora et al., 2005). Neomycin sulfate has been demonstrated to decrease fecal populations and excretion of E. coli O157:H7 in cattle (Elder et al., 2002; Woerner et al., 2006). Neomycin is a good candidate for use in the cattle industry because of its approved use and 24-h withdrawal; however, it is closely related to other antibiotics from the same amino-glycoside family (e.g., streptomycin, kanamycin, and gentamycin) that are used to treat some human infections. Because the risk associated with antimicrobial resistance in human health, its use in cattle remains controversial. The use of ionophores does not appear to significantly influence the prevalence of E. coli O157:H7 in cattle (Edrington et al., 2003; Lefebvre et al., 2006).
A nonantibiotic alternative for the selective killing of E. coli O157:H7 and other Enterobacteriaceae, including Salmonella, is the use of sodium chlorate. When administered in feed and drinking water, it reduces E. coli O157:H7 populations in the feces and in the intestinal content of cattle (Callaway et al., 2002) and pigs (Anderson et al., 2001) under experimental conditions. The beneficial effects of sodium chlorate are ascribed to the anaerobic reduction of this chemical by nitrate reductase to chlorite, a bactericidal metabolite. Treatment with sodium chlorate is suggested as a peri-harvest intervention to be applied shortly before slaughter; nevertheless, the impacts on beneficial rumen microflora appear to be negligible (Anderson et al., 2000). The use of sodium chlorate in cattle and other food animals has not yet been approved for use in the United States.
Bacteriophage Therapy
Bacteria are subject to lysis and killing by a large number of viruses called bacteriophages, many of which are commonly found in the intestinal microbial flora of food-producing animals and also in the environment (Klieve and Bauchop, 1988; Klieve and Swain, 1993). Some bacteriophages have fairly narrow host ranges (Barrow and Soothill, 1997; Barrow et al., 1998) and therefore have been studied as potential therapeutic agents for the selective elimination of specific pathogens from a mixed microbial populations, such as those present in the gastrointestinal tract of live animals. Several in vitro studies show remarkable effects of bacteriophages on E. coli O157:H7. The principle of phage therapy to combat E. coli O157:H7 in cattle is further supported by in vivo work in murine models (Tanji et al., 2005; Sheng et al., 2006). Unfortunately, the effectiveness of bacteriophage treatment to decrease E. coli O157:H7 in cattle or other ruminants has not been frequently reported in the peer-reviewed literature. The few available reports demonstrate the potential for the use of this technology to reduce E. coli O157:H7 carriage in cattle, but commercial application of bacteriophage therapy is likely still several years away (Sheng et al., 2006). Widespread use of bacteriophages for treatment of foodborne pathogens in animals will require both regulatory approval and consumer acceptance.
Vaccination
The use of vaccination to prevent pathogen colonization and fecal excretion in agricultural animals is based on the priming of the animal’s immune system against antigens expressed by E. coli O157:H7 to prevent the colonization of this organism in the gastrointestinal tract. Achieving these goals is not without challenges; specifically, priming the mucosal immune system to mount a protective response against an otherwise commensal organism has been difficult. Nonetheless, researchers have developed experimental vaccines against the proteins and other cellular components thought to play critical roles in bacterial adherence to mammalian cells and the intestinal mucosa of calves and piglets. These include the Type III E. coli-secreted proteins, Tir, intimin, and the O157 lipopolysaccharide (Konadu et al., 1999; Dean-Nystrom et al., 2002; Potter et al., 2004). Preliminary results (Potter et al., 2004) were deemed to be encouraging enough to warrant further investigation; however, the reasons why similar decreases in E. coli O157:H7 were not observed among vaccinated cattle in a larger, commercial-scale study involving 218 pens of feedlot cattle in 9 feedlots in Canada remain unanswered (Van Donkersgoed et al., 2005). In the future, it may be possible to engineer effective vaccines, including ones that can be delivered in the feed (Judge et al., 2004; Wen et al., 2006); however, at present, there is no vaccine that is available to reduce E. coli O157 in cattle.
In summary, because of the ecology of E. coli O157, notably its ability to colonize multiple animal hosts and to survive extended periods in the environment, complete eradication of the organism from cattle populations is unlikely. Nevertheless, control of this food-borne pathogen on the farm may be achieved by restricting the dissemination of pathogens between farms and by limiting proliferation, survival, and dissemination of pathogens within a farm. Considerable efforts have been made in identifying plausible herd management practices and environmental factors that may reduce the prevalence and magnitude of E. coli O157 in cattle. Preharvest interventions should be economical, practical, and suitable from an animal welfare perspective.
The reasons that most of the interventions attempted to date have not been successful are unknown. It is possible that the desired goal was not adequately achieved to test the hypothesis because of scientific or practical limitations. For example, it is possible that we have been unable to consistently deliver sterile feed or water to cattle or induce a strong mucosal immune response to a vaccine that could affect E. coli O157 carriage. An alternative explanation is that the mechanisms we are trying to interrupt are insignificant in the larger picture of total E. coli O157 exposure and colonization mechanisms. Although good agricultural practices or good management practices may have an impact on animal health, productivity, and environmental sustainability, unless they are specifically validated with respect to the biological contribution (attributable risk) that each of these factors has on pathogen reduction, they should not be viewed as critical control points for E. coli O157:H7 prevention.
|
SUMMARY AND CONCLUSIONS |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
|
Footnotes |
|---|
2 Corresponding author: lejeune.3{at}osu.edu
Received for publication September 7, 2006. Accepted for publication November 28, 2006.
|
LITERATURE CITED |
|---|
|
TopAbstractINTRODUCTIONPREHARVEST INTERVENTION CONCEPTSEXPOSURE REDUCTION STRATEGIESEXCLUSION STRATEGIESDIRECT ANTIPATHOGEN STRATEGIESSUMMARY AND CONCLUSIONSLITERATURE CITED |
|---|
This article has been cited by other articles:
|
|
 |
|
  T. Standley, J. Paterson, K. Skinner, B. Rainey, A. Roberts, T. Geary, G. Smith, and R. White The Use of an Experimental Vaccine in Gestating Beef Cows to Reduce the Shedding of Escherichia coli O157:H7 in the Newborn Calf Professional Animal Scientist, June 1, 2008; 24(3): 260 - 263. [Abstract] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
