Biocontainment is achieved by implementing strategies to reduce risk associated with the transmission of pathogenic agents among cattle within a feedyard. The large number of animals and relatively high population density in modern feedyards make biocontainment an important issue. Disease of cattle within feedyards is inevitable, but it can be managed with strategies such as segregation of sick animals from healthy animals or cleaning and disinfection of equipment and facilities to decrease exposure of susceptible cattle. These principles apply to many diseases endemic to cattle in US feedyards.
Biosecurity is an important aspect of disease prevention in any agricultural production system. Traditional biosecurity at the herd level has played a role in animal husbandry since the late 19th century and eradication of contagious bovine pleuropneumonia from the United States.1 Diseases such as bovine spongiform encephalopathy and the 2001 outbreak of FMD in the United Kingdom have made biosecurity more important than ever before. The global marketplace in agricultural trade coupled with increasing amounts of international travel has created a demand to consider interventions to prevent unintentional introduction of foreign animal diseases. Furthermore, the increase of international terrorist groups with intent to harm the United States coupled with domestic terrorist groups intent on harming US agriculture has increased the potential for an intentional disease introduction to US agriculture.2,3 Beef feedyards are particularly vulnerable to disease introduction because of the large numbers of cattle arriving from multiple sources, which makes it challenging to provide biosecurity. For the purposes of the information reported here, biosecurity refers to reducing risk associated with the entry of pathogenic agents to a particular feedyard by the implementation of mitigation strategies.
The large concentration of animals makes a feedyard an inviting bioterrorism target for domestic or international terrorist groups, which highlights the need for good feedyard security. Security practices in feedyards are aimed at controlling access to the facility in an effort to protect everything within it from theft, damage, or contamination. Although we are not aware of any reports of intentional disease introductions in the United States, toxins have been intentionally introduced into feedstuffs fed to food-producing animals in the United States. These intentional introductions have resulted in cattle deaths as well as losses attributable to extended withdrawal times.4 Contamination of animal feeds with chemical or biological agents is a current threat from extremist groups and activists.5,6 Although objective data describing real versus perceived risks of intentional disease introduction are difficult to obtain, data describing biocontainment, biosecurity, and security in feedyards can be feasibly obtained. Economic losses resulting from introduction of a contagious foreign animal disease into a feedyard would be substantial, but biosecurity and security plans for each feedyard must be economically justifiable.7 The study reported here surveyed personnel at feedyards in the Central Plains with regard to biocontainment, biosecurity, and security to characterize standard practices currently used in the feedyard industry.
Materials and Methods
Sample population—Survey data were collected from feedyard managers during interviews. Questions were tested for clarity by initially visiting 6 feedyard managers in Kansas. After suggested revisions were made, the survey was administered in person to feedyard management personnel from feedyards in 5 Central Plains states (Colorado, Kansas, Nebraska, Oklahoma, and Texas). Approval for this survey was obtained from the Kansas State University Institutional Review Board Committee for Research Involving Human Subjects.
Survey—The surveya consisted of 189 questions in 4 basic areas, which were background information on the feedyard (41 questions), biocontainment (29 questions), biosecurity (68 questions), and security (51 questions). Some questions were relevant for more than 1 area.
Feedyards were identified from a list of active, concentrated animal feeding operation permit holders obtained from Colorado, Kansas, Nebraska, Oklahoma, and Texas. Only feedyards with the capacity to feed > 1,000 cattle for finish before slaughter were included in the study. Feedyards were stratified on the basis of categories for 1-time capacity as defined by the NASS. Strata were 1,000 to 3,999 cattle, 4,000 to 15,999 cattle, 16,000 to 31,999 cattle, and ≥ 32,000 cattle. Sample size was calculated to allow estimation of the prevalence of management practices within categories of 1time capacity with a 90% confidence interval and confidence width of ± 15% for management practices with a prevalence of approximately 25%. In Kansas, contact was attempted for all feedyards that held a concentrated animal feeding operation permit for > 1,000 cattle. Feedyards in other states were selected for contact on the basis of their proximity to Kansas; feedyards located in the panhandles of Oklahoma and Texas, northeastern Colorado, and throughout Nebraska were selected to facilitate travel and collection of data. Up to 4 telephone calls to each feedyard were made in an effort to contact management personnel to solicit participation in the survey.
For managers of feedyards who agreed to participate, visits were scheduled during a follow-up telephone call, and a meeting time was arranged with a member of the feedyard management. The consulting veterinarian for the feedyard was notified of the survey in advance. Personnel at feedyards were called and visits were scheduled for each size category until the required number of feedyards for each category was filled. Questions regarding facility design, security, employees, disease preparedness, feedstuffs, hospital or treatment systems, sanitation, cattle sources, handling of sick cattle, and disposal of carcasses were included in the survey.
Statistical analysis—All data were analyzed by use of a commercial statistical software packageb to provide the frequencies of responses to each question. To assess relationships among feedyard strata and between crossclassified responses and to account for expected cell frequencies of < 5 in some tables, a Fisher exact test was used. Values of P < 0.05 were regarded as significant.
Results
Results of efforts to contact feedyard management for completion of the survey were summarized (Table 1). The survey was completed for 106 feedyards. For feedyard personnel that were contacted and whose feedyards met the inclusion criteria, 159 of 221 (72%) agreed to participate, and 106 of 159 (67%) of these were surveyed. Distribution of feedyards in each of the 4 NASS categories was 29 feedyards with 1,000 to 3,999 cattle, 32 feedyards with 4,000 to 15,999 cattle, 25 feedyards with 16,000 to 31,999 cattle, and 20 feedyards with ≥ 32,000 cattle. The 1-time capacity in each feedyard in the survey ranged from 1,300 to 125,000 cattle. Surveys were completed by 19 owners, 85 managers, and 2 assistant managers.
Results of efforts to contact feedyard personnel, number that consented and refused, and number of feedyards visited to conduct a survey of feedyard biosecurity, biocontainment, and security practices in 5 Central Plains states.
The number and percentage of feedyards at which selected management practices relevant to biocontainment were implemented (stratified on the basis of NASS category) were determined (Table 2). Variation was evident among the feedyards of various size strata for the proportion of feedyards that used common equipment for handling feed, manure, and dead cattle; cleaning and disinfection practices of equipment used for oral administration of treatments; use of a common facility for receiving new cattle and treating sick cattle; and cleaning the processing and treatment facilities. Feedyards that unloaded arriving cattle into a multiple-purpose facility used for treatment and unloading did not clean the facility significantly (P = 0.35) more often than those that used a separate unloading facility.
The number and percentage of feedyards at which selected management practices relevant to biosecurity were implemented (stratified on the basis of NASS category) were determined (Table 3). Only the proportion of feedyards that required trailers to be cleaned before loading incoming cattle and that collected a history of international travel from visitors varied among feedyard strata. In this survey, it was reported for 76 of 106 (71.7%) feedyards that the holding area for dead cattle was located outside of the typical feedyard traffic pattern, but 75 of 92 (81.5%) feedyards that used rendering services reported that rendering trucks drove across the feedyard traffic pattern to collect dead cattle.
The number and percentage of feedyards at which selected management practices relevant to security were implemented (stratified on the basis of NASS category) were determined (Table 4). Security practices varied the most among various feedyard strata. The proportion of feedyards that implemented security practices decreased as feedyard size decreased. Variation was evident among the feedyard strata for the proportion of feedyards that had a perimeter fence, kept cattle behind locked gates, secured micronutrients from unauthorized access, employed a night watchman, used signs to direct visitors to the office, maintained a visitor log, checked employee references, and performed a criminal background check on employees.
Proportion of 106 feedyards in 5 Central Plains states at which practices relevant to biocontainment were implemented, on the basis of NASS strata for feedyard capacity.
Proportion of 106 feedyards in 5 Central Plains states at which practices relevant to biosecurity were implemented, on the basis of NASS strata for feedyard capacity.
Proportion of 106 feedyards in 5 Central Plains states at which practices relevant to feedyard security were implemented, on the basis of NASS strata for feedyard capacity.
The purpose of the survey reported here was to assess the current practices at feedyards with regard to biocontainment, biosecurity, and security. The states included in the survey represented 5 of the top 7 beefproducing states in the United States. These states were feeding 9,230,000 (77%) of the total cattle on feed in US feedyards with a 1-time capacity of ≥ 1,000 cattle as of January 1, 2007.8
A feedyard is a population-dense environment with numerous opportunities for disease transmission. Segregation of sick cattle to prevent direct exposure of healthy cattle to pathogens is the basis of transmission control for endemic agents.9,10 Despite this, a substantial number of the feedyards in the survey (35/95 [36.8%]) allowed fence-line contact between sick cattle in the hospital system and healthy cattle.
At a fourth of the feedyards, indirect contact was allowed between healthy cattle and dead cattle through use of the same equipment to handle manure, dead cattle, and feedstuffs. The prevalence of this practice increased as feedyard size decreased (Table 2). Among those feedyards that used the same equipment to handle manure, dead cattle, and feed, only 4 of 27 (14.8%) cleaned and disinfected that equipment prior to handling feed. Pathogens associated with feces or dead cattle can continue to threaten healthy cattle at a feedyard when equipment used for handling manure or carcasses is subsequently used for handling feed. This risk is highest when feedyards do not thoroughly clean and disinfect equipment before it is used for moving feedstuffs. Transmission and persistence of agents such as BVDV, Escherichia coli O157, and Salmonella spp may be facilitated by this practice. Indirect environmental transmission of BVDV has been reported.11 Escherichia coli O157 can survive for days in manure, depending on the storage temperature.12 Salmonella dublin has been cultured from dried manure after 5 years and from an empty slurry pit after 4 years.13 Ideally, feed equipment should only be used to handle feed. Proper cleaning and disinfection or the use of changeable loader buckets is the best protocol for feedyards where the feed equipment must be used for handling carcasses or manure.
Another potential means for transmission within feedyards is equipment used for oral administration of treatments. Enteric agents such as Salmonella spp and BVDV are shed in saliva and may be transmitted iatrogenically among calves when proper sanitation is not practiced. Less than half of the feedyards in this survey cleaned equipment used for oral administration of treatments after each animal, and only approximately a fourth disinfected equipment after oral administration to each animal. A feedyard that cleans and disinfects such equipment daily or less often is unlikely to reduce the risk of disease transmission among calves in the feedyard via this route.
Effective cleaning and disinfection strategies may also decrease risk of indirect transmission within treatment and unloading facilities. Less than half of the feedyards cleaned the treatment facility or unloading facility daily. This may be especially important when a single multiple-purpose facility is used for unloading healthy incoming cattle and treating sick cattle. The proportion of feedyards that used a multiple-purpose facility for treatment of sick cattle and unloading newly received cattle varied among feedyard strata, with smaller feedyards more likely to use a multiple-purpose facility. This may have been attributable to the capital investment of building and maintaining multiple facilities, with fewer cattle over which to spread the cost. Feedyards were not significantly (P = 0.35) more likely to clean facilities used for both treatment and unloading, compared with feedyards that used separate facilities for each. Studies12–14 have suggested that transport increases the fecal shedding of agents such as E coli O157 and Salmonella spp and that, because of environmental persistence, agents may accumulate in an unloading facility that is not cleaned on a regularly scheduled basis. Data regarding the risk of disease transmission in unloading or treatment facilities, or the necessary frequency, effectiveness, and economic value of cleaning and disinfecting strategies in mitigating that risk, are not available. However, disease incidence is accepted to be the result of interactions among the agent, host, and environment. An environment that allows persistence of the disease agent in an amount sufficient to form an infective dose increases disease risk. Even when cleaning and disinfection do not completely eliminate an infectious agent, it may decrease the concentration below the infective dose. For these reasons, it seems likely that dirty facilities would increase the risk for incoming cattle and those resident cattle that are being treated in the same multiple-purpose facility, but research is needed on the optimal amount of intervention.
Traditional biosecurity in the sense of preventing introduction of disease into a feedyard is difficult at best and impractical in many cases. By the nature of the industry, feedyards accept a certain degree of risk by purchasing a large number of cattle from multiple sources. In many cases, these cattle may have been purchased at an auction market where they have been commingled with cattle from multiple origins prior to transport to the feedyard. In addition, feedyards may import cattle from multiple auction markets on a continuous basis. Many of the diseases of importance to the feedyard industry are also relatively ubiquitous within the cattle population. For example, some of the agents associated with bovine respiratory disease complex are found in all cattle populations, and efforts to exclude them are generally ineffective. Shedding of enteric agents such as Salmonella spp13 and the food-safety agent E coli O157: H714 may increase during transport. This large and continuous influx of cattle and the endemic nature of many diseases of importance to feedyards make it challenging and often impractical to prevent disease introduction.
Feedyards cannot control exposure to pathogens at the auction market, but they may be able to control transportation practices. Although a relatively uncommon practice, requiring that trailers be cleaned and, when possible, disinfected before incoming cattle are loaded for shipment to a feedyard may help control exposure to pathogens such as E coli O157:H7, Salmonella spp, or BVDV.15 Other food animal industries have accepted standards of cleanliness for prevention of disease introduction into a facility. Cleaning and disinfecting livestock trailers can prevent indirect infection with porcine reproductive and respiratory syndrome in swine.16 Foot-and-mouth disease virus can survive for several days in bovine feces,17 and should FMD virus be introduced into the United States, feces-contaminated trucks could increase the risk of indirect transmission. The amount of reduction in disease risk and the economic value of requiring trucking companies to clean their trucks between loads of cattle are unknown, and additional research is needed before precise recommendations can be made.
Fomites such as shoes and clothing worn by humans may serve to transmit Salmonella organisms, FMD virus, and other disease-causing agents and may pose a threat to feedyard cattle. Feedyards may consider screening visitors to determine whether they had previous animal contact, especially when returning from a foreign country with FMD or in the event FMD virus should be introduced into the United States. In another survey,18 feedyard managers and consulting veterinarians indicated that they believe FMD virus is the most likely pathogenic agent to be introduced by bioterrorists.
Wearing clean boots or shoes when around feedstuffs may be an important practice to decrease the risk of fecal-oral contamination, yet feedyards in the survey reported here rarely required visitors to wear clean boots or shoes. A study19 of large dairies found that Salmonella spp could be isolated from the surface of rubber boots in 12 of 27 dairies after people walked through housing areas. In another study20 in which ampicillin-resistant E coli were applied to rubber boots by immersion in a slurry, bacteria could be isolated from boot tracks on a concrete surface for up to 48 m (150 feet). Wearing clothing and shoes that have not been contaminated by exposure to other animals or animal waste is a standard of practice in swine confinement systems. Wearing clean outerwear may even prevent transmission of FMD virus in swine.21 The authors are unaware of specific investigations of transmission of disease in feedyards via footwear, but analysis of a study22 in swine suggests that plastic boots worn over the top of personal footwear may decrease transmission of some viral agents. The economic value of this practice in feedyards is not clear, and additional research would be needed to identify the optimal amount of control; however, the potential for disease transmission via shoes and boots clearly exists.
Most feedyards in the survey reported here piled dead cattle outside the regular feedyard traffic pattern but also reported that rendering trucks drove across the feedyard traffic pattern to collect dead cattle. Dead cattle may have been piled at the back of the feedyard, where they were out of sight and removed from the regular traffic flow. Feedyard management may have perceived that the carcasses were a potential source for pathogen transmission to other healthy cattle, but the feedyard environment and healthy cattle may have been exposed to contamination by the rendering trucks as they traversed the feedyard.
The current risk for infection with FMD virus is extremely low. Should FMD virus enter the United States, the risk for each feedyard will be higher and justify more careful implementation of biosecurity measures. The amount of time needed for a feedyard to implement an increased level of biosecurity would be decreased if plans were ready for implementation should there be an outbreak of FMD. Most feedyards in the study did not have a written response plan for introduction of FMD virus, and the proportion of feedyards that did have such a plan varied among feedyard categories, with larger feedyards being more likely to have a written plan.
Security plays an important role in feedyards, but it is challenging because in most operations the cattle and some feedstuffs are kept outdoors. Several incidents involving cattle in the United States and other countries have been characterized by feed contaminants and toxicants.5,23,24 History suggests that cattle producers should consider the risk of feed contamination and methods of risk reduction. A perimeter fence with restricted-access entrance gates allows feedyard management to limit access to only authorized personnel. Most surveyed feedyards had a light continuously on at the main entrance, but only a third of the feedyards kept the main gate locked at night, and two thirds did not employ a night watchman. In some cases, managers or other employees live on or near the operation, which may serve to deter unauthorized entry. These controls help limit access to cattle and to on-site feed mills where micronutrients and protein supplements could be contaminated and then unknowingly fed to the cattle. A secured perimeter also limits access to feed bunks should a perpetrator desire to contaminate feed that has already been fed to the cattle. Less than half of the feedyards reported that they had a fence that would prevent human access. Deterrence of unauthorized entry may reduce the risk for intentional introduction of diseases or toxicants by increasing the difficulty of gaining access to critical areas. Although determined terrorists could still gain access through an appropriately designed and constructed fence, they may be inclined to target a facility with fewer security measures and easier access. Objective data on real versus perceived risk are difficult to obtain for intentional introduction of diseases. Domestic terrorist groups have targeted animal-related facilities, but the authors are unaware of any known attack on a feedyard. People for the Ethical Treatment of Animals have made statements indicating that they would welcome the introduction of FMD virus into the United States.6
Less than half of the feedyards in our survey secured all protein and micronutrients from unauthorized access, and the proportion that did varied among feedyard sizes. In the USDA feedlot survey,25 86.3% of feedyards reported storing protein supplements and 37.3% reported storing mineral supplements in sealed containers. That feedlot survey did not report whether supplements in sealed containers were secured from unauthorized access. Locking the sources of the supplements within a container or behind a fence may deter malicious intentional contamination of feed.
Signs directing traffic and visitors to the office communicate the importance of biosecurity and security to visitors and provide feedyard staff with an opportunity to discuss the purpose of the visit. Asking visitors to sign out as they leave the premises also conveys the perception of surveillance by feedyard employees of visitors within the facility. Additionally, identifying visitors that have checked in through the office and training feedyard personnel to challenge all unidentified visitors may be useful in deterring malicious acts. Terrorist organizations, both domestic and international, commonly probe defenses of prospective targets to identify security weaknesses. Disgruntled employees or neighbors may also be aware of weaknesses. An appropriately implemented security policy, although not invulnerable, may make a feedyard a less inviting target.
The USDA feedlot survey25 revealed that 25.6% of feedyards with capacity of > 8,000 cattle restricted the movement of people (eg, deny access or require clean clothing), compared with only 15.5% of feedyards with a capacity of 1,000 to 7,999 cattle. We detected a similar pattern in our survey, with larger feedyards more likely to have implemented strategies to reduce unauthorized visitors from accessing cattle and feed. Larger feedyards represent a greater investment in the livestock operation, and management may believe security measures are justified to decrease the risk to their assets. Larger feedyards also have more staff to support the monitoring of feedyard traffic during the day and at night. By monitoring the activity of visitors, feedyard staff may limit exposure to cattle and feedstuffs to deter intentional introduction of diseases or contamination of feedstuffs.
Most feedyards in the survey reported here actively examined a potential employee's history prior to hiring. Checking references and visiting with a prior employer may provide information regarding experience and beliefs about production agriculture. A criminal background check may reveal illegal actions taken against other animal facilities. Feedyard personnel in this survey may not have pursued information about potential future employees through reference or criminal background checks because in some cases, prospective employees may be known, long-time members of the local agricultural community. In instances in which an applicant is not known by the feedyard management, a more thorough check of references and background may be prudent to assess whether potential employees can be trusted to work in vulnerable areas of the feedyard.
The survey reported here characterized practices for biocontainment, biosecurity, and security in feedyards in Central Plains states. Information gained from the survey results can be used by consulting veterinarians and feedyard managers as a basis for discussion and to target training efforts. Although the feedyards included were not a random sample of all feedyards in the 5 states, they represented operations in a large portion of the Central Plains states where most cattle in the United States are fed as well as a wide range of feedyard sizes. For the feedyards for which we were able to contact personnel and that met the inclusion criteria, there was a high acceptance rate. Subsequently, we were able to obtain completed surveys for most of these feedyards. Although this may have resulted in bias in the estimates of the proportion of feedyards at which particular management strategies were practiced, we believe that the data provided valuable insights into the general practices of feedyards in the Central Plains and surrounding areas.
Production animal agriculture has advanced in the development of biosecurity and biocontainment practices that decrease risks of disease. The swine and poultry26 industries have tailored production systems around these practices. However, biosecurity and biocontainment in the feedyard industry are more challenging in many ways. Careful assessment and implementation of key principles may be effective in controlling risk from accidental or intentional introduction of diseases or toxins. In general, feedyards in the survey reported here did not practice high levels of biocontainment, biosecurity, or security. Small feedyards were generally less likely to use good practices than were large feedyards. This difference was most consistent in the areas of biocontainment and security.
Feedyards included in this survey may not have implemented more biocontainment, biosecurity, and security practices for several reasons. They may have been unaware of the risks or the appropriate mitigation strategies to decrease risks. Veterinarians should help managers to better understand the routes of transmission for diseases that are most threatening to their operations and to develop optimal plans aimed at preventing disease transmission. In contrast, managers may have understood the risks but perceived that the mitigation strategies were ineffective or uneconomical. Information about disease risks and mitigation strategies should be combined with cost-benefit analyses by veterinarians and managers to establish best management practices for each feedyard. Action plans developed in consultation with veterinarians to address disease outbreaks would be valuable to managers for use in educating feedyard employees about preventive actions and steps for an effective response. Additional studies will be needed to enable veterinarians and farm managers to better understand the risks and to determine those mitigation strategies that provide the most economic benefits. Veterinarians are pivotal in educating feedyard staff about the dynamic risk of disease introduction and transmission within the feedyard, which characterizes the industry, and identifying best management practices for biocontainment, biosecurity, and security.
ABBREVIATIONS
| FMD | Foot-and-mouth disease |
| NASS | National Agricultural Statistics Service |
| BVDV | Bovine viral diarrhea virus |
A copy of the survey is available from the corresponding author.
PROC FREQ, SAS for Windows, version 9.1, SAS Institute Inc, Cary, NC.
References
- 1.↑
Martin S, Meek A, Willeberg P. Epidemiologic concepts. In: Veterinary epidemiology. Ames, Iowa: Iowa State University Press, 1987;3–21.
- 2.
Abt CC. Current and improved biodefense cost-benefit assessment. In: Richardson HW, Gordon P, Moore JE, eds. The economic impacts of terrorist attacks. Northampton, Mass: Edward Elgar Publishing, 2005;119–132.
- 3.
Leitenberg M. Biological weapons and bioterrorism in the first years of the twenty-first century. Politics Life Sciences 2002;21:3–27.
- 4.↑
Kosal ME, Anderson DE. An unaddressed issue of agricultural terrorism: a case study on feed security. J Anim Sci 2004;82:3394–3400.
- 5.
Neher NJ. The need for a coordinated response to food terrorism: the Wisconsin experience. Ann N Y Acad Sci 1999;894:181–183.
- 6.↑
Knowles T, Lane J, Bayens G, et al. Defining law enforcement's role in protecting American agriculture from agroterrorism. US Department of Justice, 2005. Available at: www.ncjrs.gov/pdffiles1/nij/grants/212280.pdf. Accessed Sep 10, 2007.
- 7.↑
Elbakidze L. The economics of agricultural bio-security: an interpretive literature review. College Station, Tex: Department of Agricultural Economics, Texas A&M University. Available at: agecon2.tamu.edu/people/faculty/mccarl-bruce/papers/1123.pdf. Accessed Nov 30, 2007.
- 8.↑
USDA, Agriculture Statistics Board, National Agricultural Statistics Service. Cattle on feed, February 23, 2007. Available at: www.nass.usda.gov/Publications/Todays_Reports/reports/cofd0207.pdf. Accessed Sep 10, 2007.
- 9.
Smith RA, Stokka GL, Radostits OM, et al. Health and production management in beef feedlots. In: Radostits OM, ed. Herd health: food animal production medicine. 3rd ed. Philadelphia: WB Saunders Co, 2001;581–633.
- 10.
Marshall B, Petrowski D, Levy SB. Inter-and intraspecies spread of Escherichia coli in a farm environment in the absence of antibiotic usage. Proc Natl Acad Sci U S A 1990;87:6609–6613.
- 11.↑
Niskanen R, Lindberg A. Transmission of bovine viral diarrhoea virus by unhygienic vaccination procedures, ambient air, and from contaminated pens. Vet J 2003;165:125–130.
- 12.↑
Himathongkham S, Bahari S, Riemann H, et al. Survival of Escherichia coli O157:H7 and Salmonella typhimurium in cow manure and cow manure slurry. FEMS Microbiol Lett 1999;178:251–257.
- 13.↑
Plym-Forshell L, Ekesbo I. Survival of salmonellas in urine and dry faeces from cattle—an experimental study. Acta Vet Scand 1996;37:127–131.
- 14.↑
Bach SJ, McAllister TA, Mears GJ, et al. Long-haul transport and lack of preconditioning increases fecal shedding of Escherichia coli and Escherichia coli O157:H7 by calves. J Food Prot 2004;67:672–678.
- 15.↑
Barham AR, Barham BL, Johnson AK, et al. Effects of the transportation of beef cattle from the feedyard to the packing plant on prevalence levels of Escherichia coli O157 and Salmonella spp. J Food Prot 2002;65:280–283.
- 16.↑
Dee S, Deen J, Burns D, et al. An assessment of sanitation protocols for commercial transport vehicles contaminated with porcine reproductive and respiratory syndrome virus. Can J Vet Res 2004;68:208–214.
- 17.↑
Bartley LM, Donnelly CA, Anderson RM. Review of foot-andmouth-disease-virus survival in animal excretions and on fomites. Vet Rec 2002;151:667–669.
- 18.↑
Brandt AW, Sanderson MW, DeGroot BD, et al. Feedyard manager and veterinarian responses to a Delphi-like feedyard biosecurity survey. Bovine Pract 2007;41:77–83.
- 19.↑
Kirk JH, Sischo WM, Barnett SC, et al. Salmonella contamination of rubber boots worn on dairies. Bovine Pract 2002;36:11–14.
- 20.↑
Kirk JH, Boggs C, Jeffery J, et al. Efficacy of disinfectants for sanitizing boots under dairy farm conditions. Bovine Pract 2003;37:50–53.
- 21.↑
Amass SF, Mason PW, Pacheco JM, et al. Procedures for preventing transmission of foot-and-mouth disease virus (O/TAW/97) by people. Vet Microbiol 2004;103:143–149.
- 22.↑
Dee S, Deen J, Pijoan C. Evaluation of 4 intervention strategies to prevent the mechanical transmission of porcine reproductive and respiratory syndrome virus. Can J Vet Res 2004;68:19–26.
- 23.
Cameron G, Pate J. Covert biological weapons attacks against agricultural targets: assessing the impact against US agriculture. Terrorism Polit Violence 2001;13:61–82.
- 24.
Carus WS. Bioterrorism and biocrimes: the illicit use of biological agents since 1900. Washington, DC: National Defense University Center for Counterproliferation Research, 2001.
- 25.↑
USDA. Part III: health management and biosecurity in US feedlots, 1999. No. N336.1200. Fort Collins, Colo: USDA, APHIS, Veterinary Services, Centers for Epidemiology and Animal Health, 2000.
- 26.↑
Shane SM, Halvorson D, Hill D, et al. Biosecurity in the poultry industry. In: American Association of Avian Pathologists. Kennett Square, Pa: University of Pennsylvania, 1995;26–28.
