Biosecurity for Cow-Calf Enterprises

The term biosecurity generally includes two components—preventing the introduction of new pathogens or toxins onto a farm and efforts to control spread of disease and/or intoxication within a herd. To be more exact, biosecurity is concerned with preventing the introduction of pathogens or toxins that have the potential to damage the health or productivity of a herd of cattle or the safety and quality of a food product. Biocontainment is a closely related concept and refers to efforts to control the spread of disease or intoxication within a herd. Biosecurity plans need to be based on the particular herd’s current disease status, the particular disease to be controlled, cost of prevention, likelihood of an outbreak, impact of an outbreak, and risk aversion of the producer.

Multiple tools are available to decrease the risk of importing disease into a herd and include the following:

• 1.Only importing cattle from herds with known health status. That is, only import from disease-free herds or herds with an appropriate vaccination program and actual records of excellent health and performance.
• 2.Testing strategies for incoming cattle to identify those carrying infectious disease.
• 3.Quarantine strategies for incoming cattle to prevent contact with the resident herd until any incubating disease is manifest or acute disease is resolved.
• 4.Vaccination requirements for both incoming cattle before entry and the resident herd.
• 5.Management practices to control exposure of cattle to pathogens from other risk groups, neighboring herds, and wildlife reservoirs.

SOURCES OF EXPOSURE

Cattle are ruminant animals designed to consume forage and convert it to meat and milk. This is generally done in an extensive range environment, and in the course of doing so they are exposed to neighboring herds and a wide variety of wildlife that can expose the herd to disease. Biosecurity for cow-calf producers is challenging because of the extensive nature of cow-calf production and subsequent exposure to a complex and diverse environment (Box 113-1 ).

Box 113-1. Sources of Exposure to the Cow-Calf Herd.

• 1.New additions—bulls, replacement heifers, stockers, semen, embryos
• 2.Neighbors' herds—common grazing, fence-line contact, fence breaches, and commingling
• 3.Wildlife—deer, elk, rodents, birds, insects
• 4.Feed and water—imported feed, water drainage
• 5.Suppliers and visitors—rendering trucks, contaminated boots, contaminated equipment

Most cow-calf herds allow entry of new animals. At a minimum, new bulls are imported and somewhat commonly other classes of cattle such as cows, heifers, and calves are imported as well. Semen and embryos are also potential sources of disease introduction if not handled correctly. Additionally, herds commonly have contact with other cattle over the fence between neighboring herds and through occasional breaches in the fence and intermingling of herds. Disease can also be introduced through wildlife contacts and contaminated feed or water sources. Finally, herd suppliers may introduce disease to the herd as well. Rendering trucks that pick up dead animals are a particular risk because they are carrying dead animals that are at high risk for carrying infectious disease. Visitors and perhaps especially veterinarians may introduce disease by coming onto the farm with dirty clothes and boots and by treating animals with inadequately cleaned and disinfected equipment.

The differences between herds lie in whether medical or production records from the source herd are known and whether the imported animals are tested or quarantined for a period after arrival.

ASSESSING THE LEVEL OF RISK

Numerous risks for disease introduction exist, and the level of risk from particular sources varies in every herd because of their location and management practices. The risk from importing new cattle varies with the disease status of the source herd. Generally veterinarians do not have much, if any, documentation of the disease status of the source herd. If whole herd testing for particular pathogens is available from the source herd, it serves as the best evidence of disease freedom. A second source of information on disease status is documented herd health and performance records. If, for example, the herd has documented records of high reproductive performance, the likelihood of a reproductive pathogen in the herd is lessened. Similarly, if a herd has a documented biosecurity program, in addition to good performance and low morbidity and mortality levels, the risk of a significant pathogen is decreased. Source herds for semen and embryos are subject to the same concerns. Semen obtained from reputable sources that operate according to Certified Semen Services guidelines are a low risk for disease introduction, as are properly washed embryos for embryo transfer.

Imported feed is a risk for bovine spongiform encephalopathy and Salmonella, as well as potential toxic substances. Using reputable suppliers in compliance with federal guidelines for feed handling and ruminant protein and with on-site quality assurance programs will minimize risk. Risk of exposure by way of water depends on the amount of shared water source and the disease status of herds that share the water.

Risk level from suppliers and visitors depends on their practices and origin. Visitors or neighbors may introduce disease accidentally with dirty boots or clothes. Likewise, veterinarians can be a significant source of exposure be-cause of their exposure to multiple herds and particularly herds with sick cattle. Veterinarians who arrive with dirty clothes, boots, and equipment increase the herd risk for disease introduction.

BIOSECURITY TOOLS

Several tools are available for the implementation of biosecurity programs. The implementation and importance of each depends on the individual farm and the importance of individual disease agents for that farm. They include the following:

• •Quarantine
• •Testing
• •Vaccination
• •Traffic control
• •Environmental control

Quarantine

Quarantine is the separation of one group of cattle from another to prevent disease transmission. Complete quarantine of the resident herd from all outside herds is generally not practical for beef herds. We can, however, use short-term quarantine to control disease exposure.

Quarantine provides a period where incubating clinical disease, if present, may be expressed or detected before animals are introduced to the resident herd. As such, for acute diseases with short incubation times, and no inapparent carrier state, such as bovine respiratory syncytial virus (BRSV) and bovine viral diarrhea (BVD) in non–persistently infected animals, quarantine may be effective even when used alone. In contrast, for many diseases such as Johne’s, brucellosis, leptospirosis, neosporosis, salmonellosis, BVD in persistently infected individuals, and leukosis, quarantine is not an effective biosecurity measure because of the inapparent carrier state. These animals can appear clinically normal but still carry and potentially shed the organism. In these cases the animals will need to be tested to detect the inapparent carrier state.

Proper quarantine involves preventing more than nose-to-nose contact between imports and the resident herd; drainage, distance, and duration need to be considered. For diseases that are shed in feces or urine such as Salmonella, Leptospira, and BVD, drainage from the quarantine site must not contact the resident herd. For diseases such as bovine leukemia virus or Anaplasma, which may have a significant insect vector, quarantine must be far enough away to prevent travel of vectors from quarantined cattle to the resident herd.

Duration of quarantine is again dictated by the disease of concern and its epidemiologic characteristics. For diseases where testing is undertaken, the quarantine period must last at least long enough for test results to return and establish the disease status of the cattle. If testing is not done, the quarantine period should at least exceed the incubation and shedding period of the diseases of concern. In general, the duration of the quarantine period should be 21 to 30 days.

Wildlife may be a source of exposure to Leptospira, Salmonella, and Neospora, but quarantine of cattle from wildlife is not likely possible or economically viable given the extensive nature of the cow-calf environment. Some practical steps, however, can be taken to minimize effective contact between the herd and local wildlife. Feedstuffs should be stored in a manner to prevent contamination by wildlife feces or urine. Control of standing water in corrals and pens and water sources will limit the environmental reservoir and decrease exposure to Leptospira spp. Population control of wild animals that are in contact with the herd may be prudent as well.

Testing

Testing of imported cattle can be useful in decreasing risk of introducing disease into the herd. Testing in addition to a quarantine period is required to prevent introduction of diseases with a clinically normal, inapparent carrier state such as Johne’s, brucellosis, leptospirosis, neosporosis, salmonellosis, and leucosis. The quarantine period serves to prevent transmission to the resident herd while waiting for the test to establish disease status. Testing of imported animals is no panacea, however. Tests must be carefully evaluated for use to ensure they achieve the desired goal of decreasing risk of disease entry, and at an acceptable cost. Testing can be a valuable tool in a biosecurity program when appropriately applied. Test performance and utility will depend on the prevalence of the specific disease in the source herd and the sensitivity and specificity of the test.

Tests with high sensitivity will accurately identify a high proportion of positive animals. When applied to animals from a source herd with a low prevalence of disease, however, the positive predictive value will be poor, indicating a large proportion of the test-positive animals are false positives. This may be unacceptable if the cost of a false positive is high, resulting, for example, in the unnecessary exclusion of valuable genetics. Alternatively, if the cost of false negatives is high, then a significant level of false positives may be acceptable to exclude true positives. The negative predictive value is high when the test specificity is high and prevalence is low (i.e., negative test results are likely correct, but the test results provide little additional assurance that the animal is negative because the initial prevalence was low).

Tests with low sensitivity by definition fail to accurately identify a high proportion of true positive animals. When such tests are applied to individuals in populations with moderately low prevalence, the positive predictive value is low and the negative predictive value is approximately equal to 1-prevalence, so a negative test provides little information. The Johne’s enzyme-linked immunosorbent assay (ELISA) test is just such a test. Prevalence of Johne’s disease in beef cows is approximately 1%. The test is only about 25% sensitive in young preclinical cows, although sensitivity increases as the disease progresses. It is a specific test, correctly identifying about 97% to 99% of negative animals. When applied to a group of animals with a 1% prevalence of disease, the positive predictive value is 8% to 20%, so 80% to 92% of the positive test results are false positives. The negative predictive value is 99.2%, an excellent value until the veterinarian remembers that he or she was 99% sure any given animal was negative before the test was run (if 1% of cows were positive, then 99% were negative). Under these circumstances the test provides little useful information for the money that was spent and potentially provides false security that Johne’s disease has been excluded from the herd. Further discussion of testing issues and test application are available (see Additional Resources on p. 599; Smith, Chapter 3, and Chenoweth, Chapter 3).

Vaccination

The use of vaccination for maintenance of immunity in the resident herd is another way to manage risk from imported cattle. It has probably been the most common way veterinarians and producers have attempted to mitigate biosecurity and biocontainment risks. Vaccination, however, should not be looked on as the only, or even primary, means of decreasing risk of disease. Clinical trial data on the effectiveness of most vaccines are limited. Even under optimal conditions, not all cattle will respond to vaccination, nor will all that respond to vaccination be protected from infection. In most cases vaccines do not prevent infection but work to decrease clinical disease and potentially shedding of pathogens. Vaccination programs, however, can be useful adjuncts to other management practices. Producers may vaccinate the resident herd to increase immunity to pathogens that may be introduced by imported cattle or may require vaccination of imported cattle before entry into the herd to decrease introduction of disease agents.

Traffic Control

Traffic control has probably received little attention in the past but is important for both prevention of disease introduction to the herd (biosecurity) and control of disease transmission within the herd (biocontainment). Biosecurity traffic control involves controlling contact between visitors and the resident herd. Visitors and service providers can track manure and other biologic substances onto the farm, resulting in the introduction of disease. Agents such as Salmonella can survive for extended periods of time in dried manure, soil, etc. and in many cases have a relatively small infectious dose. Feed deliveries, vaccine deliveries, cattle deliveries, etc. should occur at the periphery of the farm and not contact cattle. Trucks delivering cattle may introduce disease because the truck was not cleaned between loads and the delivered cattle were exposed on the truck ride. Rendering truck pick-ups should especially be at the periphery of the farm.

Because of the nature of the profession in treating sick cows, veterinarians may also be a risk for disease introduction and should be especially careful to arrive with clean clothes, boots, and properly disinfected equipment. Equipment that is difficult to disinfect properly such as rope halters would best be provided by the producer rather than the veterinarian.

Environmental Control

Environmental control includes practices to decrease pathogen survival or accumulation in the environment and thus decrease disease exposure. For example, decreasing accumulation of standing water in corrals and pens will limit exposure of cows to Leptospira or other water-borne pathogens. Maintaining a well-drained and dry calving area will decrease pathogen survival and disease exposure in neonatal calves.

SPECIFIC BIOSECURITY APPLICATIONS

Sexually Transmitted Agents—Campylobacter and Trichomonas

Sexually transmitted diseases are perhaps the simplest to establish biosecurity for. Only one portal of entry is possible, so veterinarians can concentrate efforts on it. Artificial insemination using semen from reputable bull studs is an excellent way to decrease risk from sexually transmitted disease; however, each organism can contaminate semen collections and can survive freezing if proper precautions are not taken.

The primary reservoir for both Trichomonas and Campylobacter is the persistently infected bull. Importation of infected bulls or infected cows is the main source of introduction to the herd. Exposure to neighbor’s bulls through communal grazing or fence breaks may also serve as a source of exposure. Ideally, only truly virgin bulls and females should be imported. If any breeding animal imports are not virgin, a quarantine and testing program should be established. Because both Campylobacter and Trichomonas are persistent infections that show no clinical signs in the infected bull or cow, quarantine alone is not effective in preventing introduction into a herd. Quarantine and testing of all nonvirgin bulls before contact with the herd can effectively prevent introduction of colonized bulls. Serial cultures should be taken for Campylobacter and Trichomonas at 1-week intervals for 3 weeks to increase the likelihood that infected bulls will be identified. The sensitivity of Trichomonas culture in commercial media has been estimated at 70% to 97%.

Vaccines for Campylobacter and Trichomonas are available either alone or in combination with Leptospira. Field trial data suggest that Campylobacter vaccination is effective in decreasing the effect of disease on the herd. Campylobacter vaccine efficacy has been estimated at 38% to 67% in cows. In bulls vaccination provides protection from persistent colonization. Trichomonas vaccine trials suggest the vaccine is effective in decreasing the duration of shedding and increasing pregnancy rates following exposure (vaccine efficacy 45%) in cows, but it does not appear to be effective in bulls.

Agents with Environmental Reservoirs—Leptospira and Neospora

Leptospira

Leptospirosis is an important cause of abortion and infertility in North American cattle. Cattle are the reservoir for the host adapted serovar Leptospira borgpetersenii serovar hardjo. All other serovars result in incidental infection in cattle and are adapted to and maintained in other species. Transmission of infections is through contact with a contaminated environment or by venereal transmission for the bovine host-adapted serovar. Survival of and exposure to leptospires in the environment is assisted in moderate temperatures and standing water.

Cattle infected with the serovar hardjo are typically normal clinically and continue to shed for months to years, so quarantine alone is not effective in preventing introduction of this form of leptospirosis. Quarantine could be effective in segregating imported animals until testing could determine their infection status. Additionally, runoff from the quarantine area must be controlled and not allowed to contact the resident herd. Utilization of polymerase chain reaction (PCR) to detect leptospires in urine and serology to identify the most likely serovar is the most practical testing strategy. Alternatively, one dose of long-acting oxytetracycline at 20 mg/kg eliminated renal shedding of L. borgpetersenii serovar hardjo in experimentally infected cattle. It may be helpful to treat all imported cattle with antibiotics to clear any infection during a quarantine period.

Vaccination of the resident herd and imported animals may be a useful way to provide some level of protection for the herd. Required frequency of vaccination varies with the level of exposure from one time per year in semiarid regions to two to three times per year in wetter environments with higher exposure.

Particularly in wetter climates, control of the environment may be important in reducing the exposure level of cows by decreasing the environmental survival of leptospires. Corrals should be graded to prevent the accumulation of standing water as a suitable environment for leptospires. A mild slope to the corral and a relatively impervious surface will aid water drainage. A program to limit contact between wildlife and cattle, pens, feed, and water may also be helpful.

Neospora

Neospora caninum has become a commonly recognized cause of bovine abortion in cattle. Evidence of Neospora infection is common in U.S. beef and dairy herds. Vertical transmission is common; transmission occurs between dam and fetus in approximately 90% of cases. Positive cattle are at increased risk for an abortion. Available evidence indicates horizontal transmission also occurs but less commonly. Dogs and coyotes are definitive hosts for N. caninum and deer appear to be able to serve as intermediate hosts. Dogs and wild canids should be prevented from consuming placenta and fetal tissues from abortions. Placentas and fetuses should be collected and disposed of as promptly as possible. Cattle feed sources should be protected from dogs and wild canids to prevent them from defecating in feed.

N. caninum causes no disease in the mature animal, so quarantine alone will not identify positive animals. An ELISA test kit is commercially available to identify infected animals for exclusion.

Recently, a vaccine for N. caninum received full approval for commercial availability in the United States (Neogaurd, Intervet Inc., Millsboro, Del.). Limited efficacy data from experimental studies or controlled clinical trials are available, so the efficacy of the vaccine is uncertain. A vaccination program could complicate differentiation of naturally infected animals from vaccinates and limit culling options.

Ubiquitous Agents—Bovine Viral Diarrhea and Infectious Bovine Rhinotracheitis

Bovine Viral Diarrhea

Seroprevalence studies indicate that BVD infection is widely distributed in cattle herds. The primary source of infection in an endemic herd is the persistently infected animal, which sheds high numbers of viral particles. Importation of a new animal, persistently or acutely infected, to the herd frequently precedes an outbreak of BVD. BVD can be introduced to the herd by any class of cattle. Exposure of pregnant cows or heifers to BVD virus (BVDV) from this imported animal from approximately 45 to 125 days of gestation may result in persistently infected calves that expose the rest of the herd well after the initial outbreak. Contact with neighboring cattle herds also provides some risk to the resident herd.

Preventing BVDV from entering a herd revolves around identification of acute and persistent infections in imported animals before introduction into the herd. Quarantine alone is not adequate to exclude BVDV from a herd because persistently infected animals may appear normal and not manifest disease during a quarantine period. Diagnostic testing is necessary in conjunction with quarantine during the time that tests are pending. Several good options exist for testing animals for BVD. If a persistently infected animal is identified during quarantine, it should be immediately culled. Acutely infected animals do not remain viremic, and when the acute infection has passed, the animal will be safe to add to the herd. Once a herd has a BVDV infection, biocontainment depends on identification and removal of PI animals. Specific testing strategies for identification and removal depend on the specific circumstances of the herd.

Vaccination can be a useful adjunct to proper management in the control of reproductive disease from BVDV. The vaccination must provide fetal protection to be useful in a control program focused toward reproductive disease. Available evidence suggests that vaccination may provide some level of fetal protection. Published reports of vaccine efficacy in preventing PI calves range from 57% to 82%. Clearly, vaccination alone will not prevent the birth of persistently infected calves if biosecurity methods allow introduction of infected cattle. It will, however, limit the effect of an inadvertent introduction of BVDV whether from imported cattle, contact with neighboring cattle, or an outside reservoir.

Infectious Bovine Rhinotracheitis

Seroprevalence studies suggest that bovine herpes virus (BHV)-1 infection is widely distributed in cattle herds. In susceptible herds 25% to 60% of cows may abort following exposure during pregnancy. BHV causes latent infection in cattle, and these latent infections may be reactivated by stressful conditions. Because of this latent reactivation, cattle previously exposed to BHV-1 can serve as a source of infection for susceptible animals. Persistence of infection in the herd is a result of both acute infections in susceptible animals and reactivation of shedding in latently infected animals.

A period of quarantine to allow possible acute or reactivated infections to manifest and resolve may be useful to avoid introducing active infection. It also allows time for vaccination and development of immunity in incoming cattle. Any seropositive animal may serve as a source of infection to a negative herd when reactivation of shedding occurs. Testing and exclusion of seropositive animals to establish a negative herd is not likely to be practical in a cow-calf herd.

Maintenance of immunity in individuals and the herd through vaccination is a crucial factor in an infectious bovine rhinotracheitis biosecurity program. Both modified live virus and killed products are available. Vaccine efficacy of a modified live virus vaccine in an experimental study was 90% 7 months after vaccination. Vaccination will not prevent establishment of latency, and modified live virus vaccination may induce latency.

Enteric Disease Biosecurity

In beef cattle operations the primary risk from enteric disease is in calves. Calf diarrhea is a multifactorial disease resulting from the interaction of host, agent, and environmental factors. Numerous agents are associated with diarrhea in calves including Rotavirus, Coronavirus, Escherichia coli, Salmonella, Cryptosporidia, Coccidia, and potentially BVDV. They share a common fecal-oral transmission route, so the methods of controlling disease introduction and spread are also substantially common between them. Most agents of calf enteric disease are ubiquitous organisms, present in the gastrointestinal tract of a large proportion of animals within a herd. In many cases the mature animals are the reservoir and calves are exposed as they enter the herd. The difference in disease rates between herds is due to differences in management practices and generally not the presence or absence of disease agents. In beef herds Salmonella and some specific strains of E. coli are not ubiquitous but may be introduced to herds from outside making biosecurity an important issue. Introduction of calves for grafting onto cows that have lost their calves is probably the greatest risk. This is particularly true for calves purchased through auction markets.

The two main enteric diseases of biosecurity and biocontainment interest in adult cattle are Salmonella and Mycobacterium avium ssp. paratuberculosis (MAP, or Johne’s). Numerous Salmonella serotypes can infect cattle and may originate from multiple different reservoirs. Generally it is introduced to a herd by importation of a subclinical carrier animal. Calves or adult cows purchased through salebarn channels may be of particular risk due to the extensive exposure they have had to other cull cattle. Dairy source animals may present an increased risk as compared with beef animals. Wildlife exposure may also be a source of exposure for beef operations. Anecdotal evidence in the United States suggests turkeys and geese may carry strains that can subsequently infect cattle. Salmonella are hardy in the environment and may survive for years to produce infections. Salmonella are also a zoonotic concern and clients should be warned of the potential for infection, especially if immunocompromised persons are in the household.

Mycobacterium avium ssp. paratuberculosis has been considered more of a problem in dairy cattle but is receiving increasing attention in beef herds. Sero-prevalence estimates based on ELISA serology in beef cattle have ranged from 0.4% to 3% at the individual cow level and 8% to 44% at the herd level. Prevalence is higher in dairy herds. Adult cattle imported to the herd may introduce MAP and subsequently spread it to calves, resulting in establishment in the herd. Once MAP is in the herd, biocontainment practices to control contact of calves with adult manure is critical to minimize transmission. In a beef operation efforts should be focused on keeping a clean environment so that calves are not exposed to excess adult manure on udders, around feeding areas, or in feed or water. Use of dairy animals for nurse cows, embryo recipients, or as a source of colostrum may increase risk of importing MAP to the herd. Testing individual animals before entry into the herd is not a useful technique to prevent introduction of MAP as discussed earlier under testing. Alternately, importing cattle only from herds with a whole herd testing program in place for MAP provides some level of certainty that they are negative or low-prevalence herds and can substantially decrease risk.

SUMMARY

Rational biosecurity programs are a function of effectiveness and economics. Biosecurity is ideally implemented in a risk-analysis approach that assesses the risk of introducing disease, consequences of introduction (e.g., economic, reputation, labor), cost of a mitigation program, and effectiveness of the mitigation program (amount of risk is decreased). Adequate understanding of the epidemiology and ecology of the particular disease agent is necessary to strategically identify effective control points.