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Review Article |
Correspondence: 1 Corresponding Author: H Houe, The Royal Veterinary and Agricultural University, Department of Large Animal Sciences, Section for Veterinary Epidemiology, Grønnegårdsvej 8, DK-1870 Frederiksberg C, Denmark
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Abstract |
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 TOPSources and manufacturers Abstract IntroductionOverview of Current Diagnostic...Objectives of Diagnostic Tests...ConclusionReferences |
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Key Words: Bovine viral diarrhea virus • cattle • control • eradication • test strategies
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Introduction |
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TOPSources and manufacturersAbstractIntroductionOverview of Current Diagnostic...Objectives of Diagnostic Tests...ConclusionReferences |
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The terms "control" and "eradication" are used in literature to refer to different degrees of disease reduction. Useful definitions of these terms were formulated by Andrews and Langmuir in 1963:2 "Control is the purposeful reduction of specific disease prevalence to relatively low level of occurrence, though transmission occurs frequently enough to prevent its permanent disappearance; eradication is the purposeful reduction of specific disease prevalence to the point of continued absence of transmission within a specified area." This distinction has been sharpened by Yekutiel,55 who defines eradication as: "The purposeful reduction of specific disease prevalence to the point of continued absence of transmission within a specified area by means of a time limited campaign." In effect, this latter definition was an early attempt to put a cost-benefit argument for eradication: in stating that eradication involves time-limited investment, it implicitly points to the continuous high costs of a control program.
In the case of BVDV infection, the distinction between control and eradication is broadly reflected in the 2 main strategies used to combat the disease BVD: vaccination for control,6 or avoidance of vaccination coupled with biosecurity measures aiming at eradication.5,28 However, recently this distinction has been rendered inadequate by the development of a 2-phase program comprising a systematic test-and-removal scheme followed by vaccination and, later, eradication.31 It has, therefore, been suggested that the key distinction should be between nonsystematic and systematic control approaches: nonsystematic control refers to any measures implemented on a herd basis, and systematic control refers to a goal-oriented reduction in the incidence and prevalence of infections. In systematic control, progress is monitored so that it can be evaluated.29,32 This new distinction also implies that, whether vaccination is included in the program, equal consideration must be given to the various diagnostic techniques.
Before systematic control programs were introduced, it was usually clinical disease in individual animals that triggered diagnostic follow-up evaluation. However, in systematic control programs, the logical sequence is different. The diagnostic questions that arise are first addressed at herd level: which herds are infected and which herds are uninfected, and how can absence of infection versus presence of infection be confirmed? Secondarily, questions of the following kind arise about infected herds: what specific clinical manifestations of infection can be found, and how can all PIs be found and eliminated? After this, a third question needs to be addressed: how can the absence of infection be reconfirmed before proceeding to surveillance at herd level? Address of these questions requires the test strategy to have the following problem-oriented order of the overall diagnostic objectives or phases: 1) initial tests to classify herd status, 2) follow-up tests to identify individual infected animals in infected herds, 3) continuous monitoring to confirm BVDV-free status, and 4) testing as part of the additional biosecurity measures that need to be implemented. Before the testing strategies are described, a brief overview of the possible laboratory techniques is given. After this, examples of systematic control schemes in European countries are presented. The progress and effectiveness of these programs on the ground have been reviewed recently32 and are, therefore, not covered here.
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Overview of Current Diagnostic Tests |
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TOPSources and manufacturersAbstractIntroductionOverview of Current Diagnostic...Objectives of Diagnostic Tests...ConclusionReferences |
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Virus Detection
Live BVDV can be detected using a virus isolation technique in which cultured cells are inoculated with test specimens, and then subjected to immunofluorescence or immunoperoxidase staining 3–5 days later. Virus isolation is usually considered the most reliable virus detection technique.13 However, if the samples are suboptimal (e.g., attributable to autolysis), it is not the most sensitive test. Still, it is recommended as the reference test.49 It is possible to quantify the amount of virus in a specimen by titration. Virus isolation requires cell culturing facilities and careful quality control to ensure that cultured cells and the fetal serum used as medium supplement are free of BVDV and its antibodies. Enyme-linked immunosorbent assays (ELISAs) can be used for BVDV antigen detection. The system produces a color reaction measured in corrected or relative optical density (OD) values. Antigen ELISAs have the advantage of being fast and inexpensive, of obviating the need for cell culture facilities, and of usually offering high sensitivity and specificity.49 This technique can also be used on eluate from fresh ear notch samples.
BVDV antigen detection on skin biopsy specimens using immunohistochemical (IHC) analysis or antigen ELISAs is increasingly common. Skin biopsy testing has proved to be valid for detecting high viremia/antigenemia levels in PI animals; it also has the advantage of not being adversely affected by the presence of colostral antibodies.7,13,15,48 IHC analysis of ear notch tissue specimens is particularly attractive because ear tagging and tissue sampling can be undertaken in one step.7,26 The presence of BVDV RNA can be detected by RT-PCR analysis. Moreover, this technique offers high sensitivity, making it suitable for testing specimens with potentially low quantities of virus, such as bulk milk, pooled samples of serum or plasma from transiently infected, as well as PI animals, or other biological materials.34,43,49
Antibody Detection
The most commonly used antibody detection techniques are the virus neutralization test (VNT) and ELISAs. The VNT is based on the inhibitory effect of antibodies on virus replication in cultured cells. Therefore, it requires cell culturing facilities and contamination control. The VNT is suitable for quantification of antibodies by titration. It is a labor-intensive and, therefore, expensive test, and in Europe, it is mostly used as the reference test.49 As an alternative to the VNT, indirect and blocking ELISAs are commonly used.49 The ELISA systems use a color reaction measured as optical density (OD). In the blocking ELISA, the reaction is measured as percentage of inhibition relative to that of a negative-control sample. ELISAs have the advantage of being fast and inexpensive, and they do not depend on cell culturing facilities. The OD values and percentage inhibition values may also be used as semiquantitative measures.14
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Objectives of Diagnostic Tests in Control Programs |
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TOPSources and manufacturersAbstractIntroductionOverview of Current Diagnostic...Objectives of Diagnostic Tests...ConclusionReferences |
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The main diagnostic components of BVDV control programs (objectives or phases) can be categorized as follows: 1) initial tests to classify herd status; 2) follow-up tests to identify individual infected animals in infected herds; 3) continued monitoring to confirm infection-free status.
Initial Tests to Classify Herd Status—performance of Diagnostic Tests at the Herd Level
Herd diagnosis is defined as diagnosis that is based on testing samples from multiple representative individuals in a herd. The goal is usually to detect the presence or absence of a given disease within the herd without testing every animal separately. Diagnosis can involve testing several animals individually, or the pooling of samples (serum/plasma or milk) from several animals before testing. For BVDV, the following tests have been used in herd level diagnosis: 1) detection of antibodies in bulk milk; 2) detection of antibodies in individual or pooled serum/plasma samples from young stock, or in pooled samples (of milk or serum/plasma) from primiparous cows; and 3) virus detection in bulk milk.
Measurement of Antibodies in Bulk Milk
Once an appropriate test has been selected, the level of BVDV antibody in bulk milk correlates well with the prevalence of seropositive animals. A study of 123 dairy herds in Sweden indicated that herds with bulk milk indirect ELISA OD values 
0.20 had low or zero prevalence of seropositive cows (prevalence range 0–26.5%), whereas herds with OD values > 0.8 had high prevalence of seropositive animals (range 87–100%).35 Similarly, a study of 117 dairy herds in France indicated that herds with bulk milk OD percentage (OD%) < 75% had mean prevalence of 8.9% seropositive animals, compared with 60.6% in herds with OD% 
0.75.4 A Dutch study indicated that, among 25 herds with PI animals, 24 had detectable antibodies in bulk milk, whereas 16 of 24 herds without PI animals also had a positive bulk milk result.56 This corresponds to herd sensitivity (HSe) of 0.96 and herd specificity (HSp) of only 0.33 for identifying a herd with PI animals. Applying an antibody-blocking ELISA on bulk milk from 352 Danish dairy herds, HSe of 0.82 and HSp of 0.88 for identifying herds with PI animals were obtained at a cut-off value of 80% (using the young stock test as gold standard for presence of PI animals). When the cut-off value of the blocking ELISA was reduced to 50%, the HSe increased to 1 and the HSp was reduced to 0.62.5,19 In a Norwegian study, the HSe and HSp of bulk milk, compared with those in a young stock test, were estimated at around 0.9 and 0.8, respectively, but varied according to the cut-off value.50
Thus, the sensitivity of tests using antibody levels in bulk milk to predict the presence of PI animals is close to 1 in herds not vaccinated for BVDV, whereas the specificity is lower. Obviously, false-negative results (low sensitivity) will occasionally be obtained for recently infected herds in which only a few animals have seroconverted. A repeated bulk milk test a few months later will solve this problem.
Low specificity (many false-positive results) in bulk milk predictions of the presence of PI animals typically is attributable to the high prevalence of seropositive animals in herds from which PI animals recently have been removed. The time it takes for the herd to become totally immunologically naïve is a function of the replacement rate, given that replacements are born in the herd or are antibody negative at purchase. It usually will take a few years for the antibody levels in bulk milk to decrease significantly,19 and bulk milk will not be suitable for identifying herds without active infection until the herd is almost totally immune naïve. In conclusion, because of the high sensitivity and low specificity of bulk milk antibody testing, this method is useful in identifying almost all true-positive herds (i.e., herds with PI animals), but it also detects a number of false-positive herds. Logically, the next step, therefore, is to do further tests in all test-positive herds.
Measurement of Antibodies in Individual or Pooled Serum/Plasma Samples from Young Stock, or Detection of Antibodies in Pooled Serum/Plasma or Milk Samples from Primiparous Cows
Although it may take a few years for antibody levels in bulk milk to decrease significantly, it takes only a few months after the last PI animal is removed from the herd for young stock to become antibody negative after colostral antibody levels have waned. Therefore, young stock older than 6–8 months is the key to identifying herds that may have received false-positive results in the bulk milk test. The testing of a few young animals for the presence of antibodies to indirectly indicate presence or nonpresence of PI animals is often referred to as "spot testing." Among 26 Danish dairy herds in which serum antibody testing of all animals in the herd was used as the reference test, the HSe and HSp, when stock samples from young animals were used, was 0.93 and 1, respectively.16,18,19 Similar to that of bulk milk antibody testing, the sensitivity of antibody testing among young stock is high, but in contrast to bulk milk testing, the specificity also is high. Pillars and Grooms reported HSe and HSp of 0.66 and 1, respectively.42
The low HSe here was attributable to unfortunate sampling of PI animals for the antibody tests. Similar to that of bulk milk antibody testing, the HSe of antibody testing of young stock may be affected by PI animals which, because they have only recently been introduced or born, are unlikely to have infected their herd mates to a substantial extent. Problems with the specificity of this test also have been reported in areas where there is wide variation in antibody prevalence among young stock.56 Furthermore, in a North American study involving 38 beef herds, serologic testing of young stock was (in contrast to the European experience) found to be not useful.53 There are at least 2 possible reasons for this. First, in spot testing, where a subsample of animals is used, it is important that the animals selected for testing are representative of the group. Thus, for example, it is necessary to exclude recently purchased animals or animals that were not part of the herd when they were young. Second, if the animals have been separated into groups for management purposes, it is important for the spot tests to be performed on each group.
The principle of the young stock test can be applied to primiparous cows, where milk testing is also a possibility. However, as indicated earlier, it will take longer for seropositive adult animals to be replaced by antibody-negative individuals after PI animals have been removed, relative to younger stock.
Spot Tests in Herds Using Vaccination
The study by Pillars and Grooms42 focused on herds in which vaccination might have been used, except in young stock. As has been mentioned, the application of the antibody spot test on 5 unvaccinated young animals was useful in identifying herds with PI animals.
In herds in which killed-virus vaccines are used, antibody titers will be significantly higher in animals from herds with PI animals than they will be in animals from herds without PI animals.21 Thus, the significance of finding at least 3 animals with high titer among a screening sample of 5 young stock animals was P < 0.01 in herds in which killed-virus vaccines were used when no PI animals were present, whereas it was P > 0.99 in similar herds with PI animals. Antibody titers among weaned calves have also been found to be useful in distinguishing between herds that contain PI animals and uninfected herds that received multiple modified-live virus (MLV) vaccines.51 The proportion of animals with low antibody titer has been used to calculate sample sizes for predicting the presence of PI animals.25
Virus Detection in Bulk Milk
Occasionally, PI animals reach reproductive age (heifers and cows). These animals will secrete virus in the milk. The RT-PCR technique has been used to detect whether BVDV-infected (PI and transiently infected) animals are present among lactating cows. In one report, use of this method detected 1 PI animal among 162 lactating cows.9 However, in most instances, PI animals are only found among the young stock. PCR analysis of bulk milk is, therefore, not suitable as a herd test for the presence of PI animals, but can be useful as an initial test to indicate whether there are any PI or transiently infected animals among the lactating cows.
Follow-up Tests to Identify Infected Individual Animals in Infected Herds
If the antibody test using bulk milk and the spot tests of young stock (and perhaps also RT-PCR analysis of bulk milk) indicate the presence of PI animals in the herd, the next step is to identify individual PI animals.
Presence of Clinical Bvdv Manifestations
Any information about BVD-like clinical signs in the herd should be recorded (an overview of diagnostic techniques for individual clinical cases is shown in Table 1). Because of the characteristic pathogenesis of BVDV infections, there is a typical sequence of events that can help predict the time when PI animals are likely to be born. Clinical signs arising from transient infection (or acute BVD) will appear about 6–9 days after infection. If these signs are severe, acute BVD may be followed by abortion, immediately or shortly after seroconversion, because of the general condition of the cow. If the animal is infected around the time of breeding, repeat breeding may be observed a few weeks later. Congenital defects in newborn calves may develop 4–6 months after transient infection, as these are induced in mid-gestation, whereas PI animals are born 5–9 months after transient infection. This characteristic timeline is valuable in the diagnostic follow-up evaluation within the herd and for identifying periods when PI animals may be born.3,17
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An alternative strategy is to first screen all animals for antibodies and, thereafter, only test the antibody-negative animals for the virus. However, in populations in which PI animals have a high likelihood of encountering PI animals from other herds, or in which vaccination is practiced, this procedure may be hampered as PI animals may seroconvert to heterologous strains. The benefit of the initial screening for antibodies is that the predictive value of the virologic test is improved, and that information on individual antibody status is obtained which gives the investigator an opportunity to predict how long the virus clearance procedure will last.
Other tests are available for virus detection in which interference by maternal immunity is not a factor (e.g., tests in which neonates are assessed by ear notch IHC analysis or by PCR analysis of ear notch or serum samples). However, these tests do not offer an opportunity to obtain serologic information. Especially in countries not using vaccination, the information obtained from testing blood samples for antibodies has been important at all stages of the program. In those countries, ear notch IHC analysis has, therefore, not been the preferred method.
Continued Monitoring to Confirm Infection-free Status
The methods of continuous monitoring used to confirm infection-free status are essentially the same as those used to establish initial herd status. However, the accuracy of the relevant tests depends to a large extent on 3 situational factors: time immediately after removal of PI animals (clearance); time and mode of herd reinfection; and reemergence of PI animals.
Time Immediately after Removal of PI Animals
When all PI animals, including those that had been included as fetuses, have been removed from a herd, and when the herd is closed and all incoming cattle are pretested before entry, the transmission of BVDV will essentially stop.29 Because of the high infection pressure that exists when PI animals are present, a large proportion of the herd—typically > 90% of the animals—will be antibody positive during the period immediately after removal of the last PI animal; because of the usually lifelong duration of antibodies after natural infection, the first animals to become seronegative in these herds will be young calves losing their colostral antibodies. The herd will gradually become seronegative, starting with calves 6–8 months old, as it is restocked with offspring. Therefore, spot tests should be used in the period immediately after the removal of PI animals.
Herd Reinfection
If a PI animal, or a dam carrying a PI fetus is the source of infection, the infectious pressure immediately rises to a high level. However, when animal movements are controlled, it is more common for the infection to be introduced by other means.28 In such instances, the incidence of infection within the herd tends to be low and of short duration.17,20,36,37 Continuous seroconversion in the absence of PI animals also has been described,30,33 but here, the literature displays a lack of consensus, as other studies suggest that acutely infected animals are unlikely to maintain the infection for longer periods.36,37 In any case, before the first PI animal (if any) is born, the incidence of infection will be low. Herds at this introductory stage of the infection cycle display considerable variation in prevalence of seropositive cattle.
If a new infection develops in a herd in which most of the cows are still antibody positive, it may be difficult to detect an increase in bulk milk antibody level. When the prevalence of seropositive animals decreases, the antibody levels in bulk milk also decrease, making it easier to detect a new infection by examining the bulk milk antibodies. In immunologically naïve herds, the presence of a single antibody-positive animal (especially an animal carrying a PI fetus) can make the bulk milk test result positive, even in herds with up to 100–200 cows.28
Reemergence of PI Animals
When PI animals are born 6–9 months after the dam's transient infection, the infectious pressure soon becomes high. This situation is the same as when the infection is introduced by a dam carrying a PI fetus, or when there is direct purchase of a PI animal. However, the impact of the latter will depend on the age of the latter and, hence, on the group to which it is introduced. The incidence risk of infection will be high for animals housed in the same building, or even on the same farm, if the farm buildings are in close proximity. Thus, for a 6-month period, an incidence risk of 0.97 for seroconversion was seen among animals in farms with PI animals.20,22 However, the speed of transmission depends on the distance between PI animals and susceptible animals.54 At that stage, a herd infection is usually easily identified by either antibody detection in bulk milk or use of the spot test.
The antibody profile of vaccinated herds containing PI animals resembles profiles in unvaccinated herds with PI animals.21 If a few animals older than 6 months are kept unvaccinated, they can serve as sentinels indicating whether BVDV infection is present.32,42
Examples of European Schemes with Systematic Control
The first systematic programs designed to eradicate BVDV were introduced in 1993–1994 in Denmark, Finland, Norway, and Sweden. In general, these started out as voluntary programs and were later institutionalized by legislation. In 1997, Austria followed with a regional program that was extended to the entire country in 2004. Northern Germany introduced a compensation scheme for culled PI animals in 1988. This was followed by certification of individual herds that were free of PI animals. However, in 1992, because of reinfections, systematic vaccination was recommended in herds that had been cleared. Because many countries have used vaccination without any noticeable reduction in prevalence, a program was developed that combines a systematic reduction in prevalence with the precautionary use of vaccines, with the final aim of eradication.31 Denmark, Sweden, Norway, and Finland have never used BVDV vaccines. In Austria, only a small number of cattle have been vaccinated.
Initially, prevalence varied substantially among these countries. In Denmark, Sweden, and Germany, approximately 40–50% of dairy herds were estimated to have PI animals or recent infection.1,5,12,22,35 In Austria, results of a study that was based on bulk milk antibody testing indicated a prevalence of approximately 10% for herds with PI animals. However, PI animals were only found in 10% of those herds (i.e., in 1% of all herds).45 In Norway, 7.1% of dairy herds had high antibody levels indicative of current or recent infection.52 Finland had low starting prevalence, with only 1% of the dairy herds having any antibodies.38
Principles of Bvdv Eradication Programs That Do Not Involve Use of Vaccines
The programs have been similar in that they consist of the following 4 testing phases (discussed previously)29: 1) initial tests to classify herd status; 2) follow-up tests to identify individual infected animals in infected herds; 3) continuous monitoring to confirm infection-free status; and 4) individual biosecurity measures (e.g., test certificates for individual animals before movement). These certificates are only introduced for animals from herds that are not infection free, including infected herds, if they are allowed to sell at all.
However, the specific tests used at each phase, as well as the combinations of tests, have varied among countries28 and over time. The following describe some typical testing regimens. Some of the rules have only been in place for a limited period, but many of them still apply. In general, the rules have become more rigorous over time, as prevalence has decreased. Control protocols and legislation contain detailed information on the number of animals that should be tested and the testing intervals, as well as specifying exceptions to the general rules (e.g., in connection with farms only producing calves that go directly to slaughter).
In general, the diagnostic tests used in implementing biosecurity measures (phase 4) have aimed at preventing transmission of infection between infected and noninfected herds. Individual testing should ensure that no PI animals are moved to other herds or pastures, or to auctions, exhibitions, and the like. Antibody-positive pregnant animals should be isolated. Their offspring should be tested for PI status, and the sale of such animals should be avoided. To prevent the introduction of acutely infected animals, it has been recommended that all incoming animals be quarantined for 3 weeks before being introduced into herds. Although some suggest that acutely infected animals could transmit the virus for longer periods,8 this has not raised any real issues for those engaged in the eradication of BVDV from European countries.
With regard to phases 1–3, there have been some notable differences between the countries. A schematic presentation of the programs has been given in an earlier review.28 In Denmark, stage 1 began with bulk milk antibody testing at regular intervals using a blocking ELISA.5 A herd was categorized as free of infection if 2 sequential tests of bulk milk had blocking ELISA result < 50%. If the bulk milk was positive, at least 3 young stock animals were tested individually for the presence of antibodies. If these animals also tested positive, the herd was considered infected and an individual herd plan (phase 2) was prepared with the aim of identifying and removing PI animals. The sequential application of phases 1 and 2 testing ensured that labor-intensive testing (phase 2) was required for as few herds as possible (Fig. 1). The intensity of testing varied from herd to herd, from the testing of all animals and calves born in the following months (either before colostral intake or when they were 3 months old) to the testing of a selected age group in which PI animals were expected to be found. After removal of all PI animals and a negative result of young stock testing, the herd could be declared free of infection. Thereafter, negative status was confirmed annually (phase 3) by testing at least 3 young stock animals; after a few years, this monitoring was undertaken by testing bulk milk for antibodies. Beef herds were tested in a similar manner, except that the testing regimen in phase 1 started with testing young stock, and the monitoring regimen (phase 3) focused on young stock.
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In Norway, the testing regimen (phase 1) in dairy cattle started with testing for antibodies in bulk milk using an indirect ELISA.52 In test-positive herds, this was followed by testing pooled milk from a few (3–5) first–parity cows. If results of these tests were positive, pooled sera from 3–5 young calves between 8 and 12 months old were tested for antibodies. Herds classified as positive were given movement restrictions, and follow-up testing was undertaken to identify and remove PI animals (phase 2). The herd could be declared infection free after having at least 2 consecutive antibody-negative samples (bulk milk, pooled milk from primiparous cows, or pooled serum from young stock). Thereafter, monitoring (phase 3) involved use of the 3 aforementioned antibody testing methods. Beef herds were monitored by testing of young stock as described previously.
Because Finland had a low prevalence of infection, the testing of individual animals was performed in herds with antibody-positive bulk milk ( 0.25 indirect ELISA OD units).38,44 In herds with > 50% antibody-positive animals, the seronegative animals were tested for virus. Finland has few beef herds. Sera from beef animals were obtained at slaughter and were tested for antibodies. In herds with > 50% antibody-positive animals, the seronegative animals were tested for virus.
The eradication program in Austria was established by following the Scandinavian model.41,46,47 Bulk milk samples were tested regularly for antibodies. Among young stock, at least 5 animals were tested for antibodies. If the young stock test had evidence of active infection, follow-up testing was used to identify PI animals. An RT-PCR assay was used on pooled blood samples followed by testing of individual animals to identify PI animals.47
Principles of Bvdv Eradication Programs That Include Vaccination
Under circumstances in which, for one reason or another, the Scandinavian approach to eradication of BVDV is not applicable, systematic vaccination after removal of PI cattle is an option for minimizing the risk of reintroduction of the virus into cleared herds. In Germany, the first regional voluntary control schemes combining testing, removal of PI cattle, and vaccination were initiated in 1988 and 1992. In late 2004, BVDV became a notifiable disease in Germany. Regulatory statutes covering the disease are currently being drafted and will go into effect in 2006; at that point, BVDV control will change from a voluntary to a systematic approach. Several BVDV vaccines, including MLV preparations, are commercially available in Germany, where vaccination has been used in nonsystematic BVDV control for several years. This situation requires an approach to testing of herds that differs from that in countries in which vaccination is banned, since the results for bulk milk samples are not necessarily indicative of a herd's BVDV infection status.
The following elements comprise the German program: 1) initial tests to classify herd status; 2) follow-up testing to identify and remove PI animals from infected herds; 3) systematic vaccination using a vaccine with a protocol documented to protect pregnant animals against fetal infection; 4) continued monitoring to confirm infection-free status; and 5) individual biosecurity measures (e.g., test certificates for individual animals before movement).
Since vaccination might interfere with bulk milk testing, the initial screening of herds depends on the epidemiologic situation. In vaccinated herds, a few animals older than 6 months should be kept unvaccinated so that they can serve as sentinels to indicate the infection pressure, if any, in the herd.32,42 In nonvaccinated herds, an initial bulk milk sample can be used to test for BVDV-specific antibodies.
If results for any of the samples are positive, the herd is tested for PI animals. There are several options for identifying PI animals. In dairy herds, bulk milk samples can be tested by RT-PCR analysis. When results of a bulk sample are positive, PI animals need to be identified by additional testing. The offspring of PI cows are, by definition, PI and should also be removed. The rest of the herd can be tested using either individual blood or ear notch samples that will be analyzed using an antigen-capture ELISA. Pooled samples of serum or plasma can be analyzed by use of an RT-PCR assay. After the testing and removal of PI animals from a herd, follow-up testing of all calves born in the ensuing 12 months is essential.
After all of the PI animals identified in the course of the first follow-up evaluation have been removed, vaccination can be mandatory or banned, depending on the epidemiologic situation. In areas with a high density of cattle, intensive trade, and high prevalence of BVDV, systematic vaccination should be used until the incidence of PI cattle has fallen to a low level. In areas with a low prevalence of BVDV, vaccination should be banned, because the risk of reintroduction of BVDV into infection-free herds is likely to be low. If vaccination is to be used, a 2-step vaccination protocol is recommended, provided that a licensed, MLV vaccine is available.10,31 Ideally, an inactivated-virus vaccine will be administered, followed by a booster vaccination 4 weeks later using an MLV vaccine. Animals are vaccinated no later than 8 weeks before pregnancy. This protocol ensures a long-lasting immune response and comprehensive fetal protection.11,39 Moreover, where this protocol—in which MLV vaccine is only used on animals primed with an inactivated vaccine—has been used, vaccine virus has never been observed to be shed after vaccination. In any case, the principal goal of vaccination is to obtain the broadest and most-enduring fetal protection possible. A group of young animals older than 6 months should be kept unvaccinated and monitored for BVDV antibodies to confirm that there is no new introduction of infection.
Continuous monitoring is an essential element of the program, and herds are tested serologically using either bulk milk samples from nonvaccinated herds or spot tests of unvaccinated young stock animals in herds that apply systematic vaccination.
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TOPSources and manufacturersAbstractIntroductionOverview of Current Diagnostic...Objectives of Diagnostic Tests...ConclusionReferences |
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TOPSources and manufacturersAbstractIntroductionOverview of Current Diagnostic...Objectives of Diagnostic Tests...ConclusionReferences |
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