首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
OBJECTIVES: To assess the sensitivity of the current surveillance program used in Denmark for detecting outbreaks of infectious bovine rhinotracheitis (IBR) at the herd level and to evaluate the impact of alternative sample collection strategies on the sensitivity of the system in an acceptable time frame. SAMPLE POPULATION: Data from the Danish Central Husbandry Register on cattle of 24,355 and 25,233 beef herds and on 13,034 and 12,003 dairy herds in the years 2000 and 2001, respectively. PROCEDURES: Surveillance programs were evaluated under current sample collection conditions and under 3 alternative scenarios by use of simulation modeling. Data from the current detection component of the surveillance system were used as input, taking into consideration the sensitivity and specificity of bulktank milk and serologic testing. RESULTS:The current system identifies infected dairy herds within a 3-month period with desired accuracy largely because of the test characteristics and number of bulk-tank milk samples. The system is less likely to detect infected beef herds in a timely manner because surveillance in beef herds depends solely on serologic testing at the time of slaughter. The efficiency of surveillance in dairy cattle herds was not decreased substantially when the slaughter-surveillance component was omitted. CONCLUSIONS AND CLINICAL RELEVANCE:Geographically targeted sample collection during the high-risk season (winter) was predicted to increase the probability of rapid detection of IBR infection in cattle. This approach can be used for assessing other surveillance systems to determine the best strategies for detection of infected herds.  相似文献   

2.
AIM: To use disease modelling to inform a response team about the number of animals per herd/flock to be examined, and the start date and duration of clinical surveillance required to be confident that foot-and-mouth disease (FMD) was not present on an island in New Zealand with a population of approximately 1,600 cattle, 10,000 sheep and a small number of pigs, goats and alpacas. METHODS: Because the probability of detecting clinical disease in (the) primary case(s) in larger herds and flocks was extremely low, deterministic and stochastic mathematical SLIR (susceptible, latent, infectious, recovered) models for the transmission of infection were constructed to estimate the date when clinical lesions in herds and flocks would be detected with 95% confidence. Surveillance targeted the first wave of infections following a suspect index case. RESULTS: If 70 cattle in herds of about 400 cattle were examined it was estimated it would take approximately 13 (90% stochastic range 9-19) days from first exposure before it would be possible to achieve 95% confidence for detecting clinical signs for a low-virulence virus, and 9 (7-14) days for a high-virulence virus. The duration of sufficiently accurate clinical detection was 17 (15-19) days and 13 (12-14) days for low- and high-virulence viruses, respectively. A sample of 70 sheep from flocks of >1,000 would be required to achieve clinical detection at about the same time but with a shorter period of detection than for cattle. The duration of effective detection could be increased by examining a larger sample in most sheep flocks, however the small size of many cattle herds in the study population limited the confidence of detecting group-level disease in cattle, therefore necessitating repeated herd inspections. The model suggested that group-level detection was not feasible if it was based on elevated body temperature alone because of short durations of fever in infected animals. CONCLUSION AND CLINICAL RELEVANCE: Simulation modelling is a useful and powerful tool for informing ongoing surveillance activities in the face of an exotic disease incursion. Results of modelling suggested to start clinical inspection activities at 4 days and to continue regular inspection twice a week for about 35 days after the date of first exposure, to satisfy the required 95% confidence threshold of clinical detection of FMD in cattle herds and sheep flocks.  相似文献   

3.
Within the European Union (EU), detailed legislation has been developed for cattle, but not deer, to minimise disease risks associated with trade in animals and animal products. This legislation is expressed as input-based standards, providing a detailed outline of the activity required (for example, testing of animals and application of defined control measures), on the expectation that an adequate output (for example, confidence in freedom) will be achieved. Input-based standards are at odds with the increasing shift towards output-based standards, particularly in OIE rules governing international trade. In this paper, we define output-based standards to achieve and maintain freedom from tuberculosis (TB) in farmed deer, with reference to EU member states. After considering the probability of freedom achieved for cattle under existing EU legislation, we defined a ‘free farmed deer holding’ as one with a probability of freedom from infection of at least 99%. We then developed an epidemiological model of TB surveillance systems for deer holdings, incorporating different surveillance strategies, including combinations of diagnostic tests, and a variety of different scenarios relating to the potential for introduction of infection. A range of surveillance strategies were identified to achieve and maintain a free farmed deer holding, and worked examples are presented. The surveillance system sensitivity for varying combinations of screening and confirmatory tests in live animals, animals at slaughter and on-farm deaths is also presented. Using a single test at a single point in time, none of the TB tests routinely used in farmed deer is able to achieve an acceptable probability of TB freedom. If repeat testing were undertaken, an acceptable probability of TB freedom could be achieved, with differing combinations of the surveillance system sensitivity, frequency of testing and risk of introduction. The probability of introduction of infection through the importation of infected deer was influenced by the use of a pre-movement test (assumed 90% test sensitivity and negative test results), the TB prevalence in the source herd and the number of animals imported. A surveillance system sensitivity of at least 81% was achieved with different combinations of annual live animal surveillance and surveillance of animals at slaughter or on-farm deaths. This methodology has broad applicability and could also be extended to other diseases in both deer and other species with relevance to trade in animals and animal products.  相似文献   

4.
‘Confidence’ in freedom from disease is generally derived from multiple sources of varied surveillance information, and typically this surveillance evidence has been accumulated over time. In the state of Western Australia (WA) the main surveillance evidence supporting Free Zone status in the national bovine Johne's disease (BJD) program comprises periodic surveys and the ongoing clinical diagnostic system. This paper illustrates a simple approach to current valuation of historical surveillance information, based on the calculated sensitivity of the surveillance processes, the time elapsed since the data were accumulated, and the probability of new introduction of disease into the population during that elapsed time. Surveillance system components (SSCs) contributing to the overall sensitivity of the surveillance system were the clinical diagnostic system and periodic targeted surveys. Sensitivity of each component was estimated using a stochastic scenario tree model of the surveillance process as implemented. Probability of introduction of BJD into WA during each time period was estimated retrospectively from a stochastic import risk analysis model applied to actual cattle importation data. The probability that the WA cattle population was free from infection (at design prevalences of 0.2% of herds and 2% of animals within an infected herd) was estimated following each of 11 years, giving a median probability that the State was free of BJD (at these design prevalences) at the end of 2005 of 0.89. The meaning of this result is discussed.  相似文献   

5.
In the first survey for contagious bovine pleuropneumonia (CBPP) in the southern region of the Sudan (Bahr el Ghazal Province), 8.1% of Dinka-owned, and 9.2% of Fellata-owned, cattle, and 48 and 20% of their respective herds were serologically positive for CBPP. Dinka cattle between 3 months and 3 years of age had significantly higher test reaction rates than those younger or older. There was a positive association between reaction rates and herd size. A similar association was found between crude mortality rates and herd size, though virtually none of the variability among herds in crude mortality was accounted for by presence of CBPP infection. Overall, the study indicated a highly endemic situation for CBPP infection with rare overt disease under conditions of little prior intervention. Under such conditions, it is suggested that a “surveillance and selective actions” rather than a “mass actions” approach might be the most suitable, with vaccination limited by surveillance to those areas at highest risk of outbreaks of clinically important CBPP, namely areas of low herd immunity or those where actual outbreaks of clinical disease have already occurred. Such an approach to CBPP could serve as a “model” for establishing a system of improved disease intelligence overall in the southern Sudan.  相似文献   

6.
A stochastic simulation model was developed to assess the risk of introduction of Mycobacterium avium subsp. paratuberculosis infection into a dairy herd through purchase of female replacement cattle. The effects of infection prevalence in the source herd(s), number of females purchased, and testing by enzyme-linked immunosorbent assay (ELISA) alone or ELISA and fecal culture as risk mitigation strategies were evaluated. Decisions about negative test results were made on a lot and individual basis. A hypothetical dairy herd, free from M. a. paratuberculosis, which replaced 1 lot (10, 30, or 100) of cows per year, was considered. Probability distributions were specified for the sensitivities and specificities of ELISA and fecal culture, the proportion of infected herds and within-herd prevalence for randomly selected replacement source herds (high prevalence) and herds in level 2 (medium prevalence) and level 3 (low prevalence) of the Voluntary Johne's Disease Herd Status Program (VJDHSP). Simulation results predicted that 1-56% of the lots had at least 1 M. a. paratuberculosis-infected cow. Assuming that ELISA sensitivity was 25%, simulation results showed on a lot basis that between 0.4% and 18% and between 0.1% and 9% were predicted to have at least 1 infected cow not detected by ELISA and by a combination of ELISA and fecal culture, respectively. On an individual cow basis, between 0.1% and 8.3% of ELISA-negative cattle in ELISA-positive lots were estimated to be infected. In both the lot and individual analyses, the probability of nondetection increased with larger lot sizes and greater prevalence. Sensitivity analysis indicated that the effect of a lower ELISA sensitivity (10%) was a variable decrease in mean detection probabilities for all combinations of prevalence and lot size. The benefit of testing introduced cattle with ELISA alone or in combination with fecal culture was found to be minimal if cows were purchased from known, low-prevalence (level 3) herds. The value of testing by ELISA alone or in combination with fecal culture was greatest in high-prevalence herds for all lot sizes. Testing of random-source cattle, bought as herd replacements, can partially mitigate the risk of introduction of M. a. paratuberculosis but not as well as by using low-prevalence source herds (level-3 VJDHSP), with or without testing.  相似文献   

7.
For the eradication of an infectious agent, like bovine herpesvirus 1 (BHV-1), surveillance and certification can be used to reduce the transmission between herds. The goal of surveillance is that a certified herd that becomes infected is detected timely so that infection of several other certified herds is prevented. What counts is whether the reproduction ratio R, i.e. the average number of certified herds infected by one infected certified herd can be kept below 1. To support policy makers in making decisions about the minimal demands for a surveillance programme in an eradication campaign of BHV-1 in cattle, two mathematical models were investigated. With these models, the basic reproduction ratio between herds was calculated. The surveillance programmes were characterised with sample size, sampling frequency, test sensitivity, herd size, vaccination status, and contacts between herds. When R between herds is below 1, then the surveillance programme is sufficiently good to prevent spread of infection, provided that R is estimated well. In the model based on bulk milk testing sample size was replaced by a threshold at which bulk milk can be found positive. The R between herds was mainly influenced by the vaccination status, sampling frequency, and contacts between herds. Herd size moderately affected the outcome. Test sensitivity and sample size, however, were of minor importance. If herds of 50 cows became free of BHV-1 without vaccination, then spread of infection between herds might be prevented when animals within herds are sampled once a year (milk or blood samples). This frequency needs to be intensified, being twice a year, for larger herds and/or herds with extensive contacts with other herds. When bulk milk is sampled instead, sampling should be done at least every 5 months and more intensively, being each month, with larger herd sizes and more contacts between herds.  相似文献   

8.
OBJECTIVE: To describe the distribution and prevalence of cattle herds with detectable antibody to bovine pestivirus in Queensland in 1994/95. MATERIALS AND METHODS: The study used 7,838 serum samples collected from 250 herds in Queensland, as part of a structured animal health surveillance program conducted in 1994 and 1995. Samples were collected from female cattle bred on the property. In each herd, 10 to 20 heifers less than two years of age and 10 to 15 older cows were sampled giving a 95% probability of detecting one or more seropositive animals if the seroprevalence was approximately 10% or greater. Sera were analysed for antibodies to bovine pestivirus using a virus neutralisation test. RESULTS: Total cattle numbers in sampled herds varied from 62 to 24,600 head, while total area of properties sampled varied from 50 to 395,400 hectares. Eleven percent of herds contained no seropositive animals among those sampled, and in 38% of herds, all sampled cattle aged one to two years of age were seronegative. There was a trend for larger herds to have one or more animals seropositive for bovine pestivirus (chi-squared for Linear trend = 3.656, p = 0.056). Herds with more than 500 head of cattle were significantly more likely than herds with less than 500 head to contain one or more seropositive animals in any age group (prevalence ratio = 1.12; 95% confidence interval 1.01 - 1.23; p = 0.026). Age specific seroprevalence increased from around 10% in heifers, to between 75% and 85% in cows aged 10 years. The average annual incidence risk for bovine pestivirus infection varied from 0.12 to 0.24 seroconversions per cattle year at risk, and did not vary with age. The overall crude seroprevalence adjusted for herd size was 45%. There was a wide range of seroprevalence recorded for each level of stocking intensity. CONCLUSIONS: This survey provides valuable baseline data on bovine pestivirus infection in Queensland cattle herds.  相似文献   

9.
Pre-movement testing for bovine tuberculosis (BTB) was compulsory in Ireland until 1996. We determined the proportion of herd restrictions (losing BTB-free status) attributable to the recent introduction of an infected bovid; described events between restoration of BTB-free status (de-restriction) and the next herd-level test for BTB; estimated the proportion of undetected infected cattle present at de-restriction; identified high-risk movements between herds (movements most likely to involve infected cattle); and determined the potential yield of infected cattle discovered (or herds that would not lose their BTB-free status) by pre-movement testing, relative to the numbers of cattle and herds tested. We used national data for all 6252 herds with a new BTB restriction in the 12 months from 1 April 2003 and 3947 herds declared BTB-free in the 12 months from 1 October 2001. We identified higher-risk animals from our logistic generalized estimating-equation models. We attributed 6-7% of current herd restrictions to the recent introduction of an infected animal. There were considerable changes to herd structure between de-restriction and the next full-herd test, and infection was detected in 10% of herds at the first assessment (full-herd test or abattoir surveillance) following de-restriction. Following movement from a de-restricted herd, the odds of an animal being positive at the next test increased with increasing time in the source herd prior to movement, increasing time between de-restriction and the next full-herd test and increasing severity of the source herd restriction. The odds decreased with increasing size of the source herd. We estimated that 15.9 destination-herd restrictions per year could be prevented for every 10,000 cattle tested pre-movement and that 3.3 destination-herd restrictions per year could be prevented for every 100 source herds tested pre-movement. The yield per pre-movement test can be increased by focusing on high-risk movements; however, this would result in a substantial decrease in the total number of potential restrictions identified.  相似文献   

10.
Financial resources may limit the number of samples that can be collected and analysed in disease surveillance programmes. When the aim of surveillance is disease detection and identification of case herds, a risk-based approach can increase the sensitivity of the surveillance system. In this paper, the association between two network analysis measures, i.e. 'in-degree' and 'ingoing infection chain', and signs of infection is investigated. It is shown that based on regression analysis of combined data from a recent cross-sectional study for endemic viral infections and network analysis of animal movements, a positive serological result for bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) is significantly associated with the purchase of animals. For BCV, this association was significant also when accounting for herd size and regional cattle density, but not for BRSV. Examples are given for different approaches to include cattle movement data in risk-based surveillance by selecting herds based on network analysis measures. Results show that compared to completely random sampling these approaches increase the number of detected positives, both for BCV and BRSV in our study population. It is concluded that network measures for the relevant time period based on updated databases of animal movements can provide a simple and straight forward tool for risk-based sampling.  相似文献   

11.
Making valid inferences about herd prevalence from data collected at slaughter is difficult because the observed sample is dependent on the number of animals sampled from each herd, which varies with herd size and culling practices, and the probability of a positive test result, which depends on variable within-herd prevalence levels as well as test sensitivity and specificity. In this study, brucellosis herd prevalence among beef cow-calf operations is estimated from slaughter surveillance data using a method that combines process modeling with Bayesian inference. Inferences are made for two populations; the first population comprises cow-calf beef herds in a typical U.S. state. The second population represents all beef herds in a collection of 46 low-risk states. The Bayesian Monte Carlo method used in this study links process model inputs to observed surveillance results via Bayes Theorem. The surveillance evidence across multiple years is accumulated at a discounted rate based on the probability of introducing new infection into an area. The process model's inputs include herd size, culling rate per herd, within-herd prevalence, serologic test performance, and the probability of successfully investigating positive results. The surveillance results comprise the number of cows and bulls tested at slaughter and the number of affected herds detected each year. The results find at least 95% confidence that brucellosis herd prevalence among beef cow-calf herds is less than 0.014% (3 per 21,500 herds) and 0.00081% (5 per 6,15,770) after 5 years of slaughter surveillance (with no detections of affected herds) in a typical U.S. state and across 46 low-risk U.S. states, respectively. These results were based on conservative modeling assumptions, but sensitivity analysis suggests only slight changes in the results from changing the assumed process model input values. The most influential analytic input was the probability of introducing new infection into a putatively brucellosis-free state or group of states.  相似文献   

12.
Vector-borne diseases pose a special challenge to veterinary authorities due to complex and time-consuming surveillance programs taking into account vector habitat. Using stochastic scenario tree modelling, each possible surveillance activity of a future surveillance system can be evaluated with regard to its sensitivity and the expected cost. The overall sensitivity of various potential surveillance systems, composed of different combinations of surveillance activities, is calculated and the proposed surveillance system is optimized with respect to the considered surveillance activities, the sensitivity and the cost. The objective of this project was to use stochastic scenario tree modelling in combination with a simple cost analysis in order to develop the national surveillance system for Bluetongue in Switzerland. This surveillance system was established due to the emerging outbreak of Bluetongue virus serotype 8 (BTV-8) in Northern Europe in 2006. Based on the modelling results, it was decided to implement an improved passive clinical surveillance in cattle and sheep through campaigns in order to increase disease awareness alongside a targeted bulk milk testing strategy in 200 dairy cattle herds located in high-risk areas. The estimated median probability of detection of cases (i.e. sensitivity) of the surveillance system in this combined approach was 96.4%. The evaluation of the prospective national surveillance system predicted that passive clinical surveillance in cattle would provide the highest probability to detect BTV-8 infected animals, followed by passive clinical surveillance in sheep and bulk milk testing of 200 dairy cattle farms in high-risk areas. This approach is also applicable in other countries and to other epidemic diseases.  相似文献   

13.
We constructed a stochastic bio-economic model to determine the optimal cost-efficient surveillance program for bovine tuberculosis. The surveillance programs differed in combinations of one or more detection methods and/or sampling frequency. Stochastic input variables in the epidemiological module described the dynamics of infection and the probability of detection. By means of an efficiency frontier, the trade-off between the expected cost and the epidemiological risk parameter relating to the outbreak size was evaluated. The surveillance scheme based on visual inspection of lesions on carcasses at slaughter was optimal given the current prevalence of the disease in the Netherlands if the objective was to minimise the expected costs. However, the efficient set also included two other schemes: slaughterhouse inspection in combination with GAMMA-interferon testing of blood samples and slaughterhouse inspection in combination with two-stage tuberculin testing. The choice ultimately will depend on the risk attitude of the decision-maker; a more-stringent surveillance scheme will be enforced if the expected outbreak size is to be constrained. In future scenarios, ELISA testing of bulk-tank milk in combination with the current slaughterhouse inspection procedure would outperform the surveillance scheme of solely slaughterhouse inspection if ELISA testing of bulk-tank milk becomes feasible.  相似文献   

14.
Bovine tuberculosis surveillance alternatives in Belgium   总被引：1，自引：0，他引：1  
Belgium obtained the bovine tuberculosis (bTB) officially free status in 2003 (EC Decision 2003/467/EC). This study was carried out to evaluate the components of the current bTB surveillance program in Belgium and to determine the sensitivity of this program. Secondly, alternatives to optimize the bTB surveillance in accordance with European legislation (Council Directive 64/432/EEC) were evaluated. Separate scenario trees were designed for each active surveillance component of the bTB surveillance program. Data from 2005 to 2009 regarding cattle population, movement and surveillance were collected to feed the stochastic scenario tree simulation model. A total of 7,403,826 cattle movement history records were obtained for the 2,678,020 cattle from 36,059 cattle herds still active in 2009. The current surveillance program sensitivity as well as the impact of alternative surveillance protocols was simulated in a stochastic model using 10,000 iterations per simulation. The median (50% percentile) of the component sensitivities across 10,000 iterations was 0.83, 0.85, 0.99, 0.99, respectively, for (i) testing the cattle only during the winter screening, (ii) testing only imported cattle, (iii) testing only purchased cattle and (iv) testing only all slaughtered cattle. The sensitivity analysis showed that the most influential input parameter explaining the variability around the output came from the uncertainty distribution around the sensitivity of the diagnostic tests used within the bTB surveillance. Providing all animals are inspected and post mortem inspection is highly sensitive, slaughterhouse surveillance was the most effective surveillance component. If these conditions were not met, the uncertainty around the mean sensitivity of this component was important. Using an antibody ELISA at purchase and an interferon gamma test during winter screening and at import would increase greatly the sensitivity and the confidence level of Belgium's freedom from bTB infection status.  相似文献   

15.
Bovine tuberculosis (bTB) is a persistent problem in cattle herds in Great Britain and Ireland. Farm management and cattle husbandry practices can influence the risk of transmission of bTB and hence the likelihood of bTB breakdown (>or=1 reactor to the tuberculin skin test). Biological differences are expected in the transmission dynamics, and hence risk factors for bTB breakdown, on farms where infection persists in the herd compared to farms where infection is more sporadic or short-lived. Comparative case-control studies were performed to test farm management practices as potential risk factors for transient (under breakdown restrictions for 6 months) bTB breakdown over 5 years (1995-1999) on 179 and 171 UK cattle farms, respectively. Farms were characterised for badger sett density and farm habitat composition by ground survey, farmers were questioned retrospectively on management practices, and cases and controls were identified from national tuberculin test records. Controlling for routine tuberculin testing interval, log-transformed herd size, regional location, badger sett density and farm habitat complexity, multivariable logistic regression identified increased odds of both transient and persistent breakdown on farms that bought-in cows (odds ratio (OR)>or=4.9; 95% confidence interval (CI)>or=1.1;22.8). In addition, the purchase of >50 head of cattle (OR=4.0, 95% CI=1.0;16.0) and the storage of manure for >or/=6 months (OR=4.4; 95% CI=1.3;15.4) were risk factors for transient breakdown, whereas the use of silage clamps (OR=9.1; 95% CI=2.0;40.8) increased the risk of persistent breakdown. Decreased odds of both transient and persistent breakdown were associated with higher stocking densities (>3cattle/ha) (OR相似文献   

16.
Aggregate testing for the evaluation of Johne's disease herd status   总被引：4，自引：0，他引：4  
This paper examines methods for evaluating herd Johne's disease status that could be used in a survey of the cattle industry. Emphasis is placed on aggregate testing, a process whereby a random sample of cattle from a herd is assessed using an imperfect test, such as an ELISA for detecting antibody in serum. Important aggregate test parameters discussed include: sample size, herd-level sensitivity, herd-level specificity, the number of reactors used for declaring a positive herd result, and the expected within-herd prevalence of disease. Aggregate testing may be useful for several livestock diseases. However, problems arise when it is applied to Johne's disease because of the poor sensitivity of the available diagnostic tests, the low within herd prevalence of infection, and clustering of false positives within a herd.  相似文献   

17.
OBJECTIVE: To investigate the epidemiologic and financial impacts of targeted sampling of subpopulations of cows, compared with random sampling of all cows, for classification of dairy herd infection status for paratuberculosis. ANIMALS: All cows from 4 infected herds with a low-to-moderate prevalence of paratuberculosis and from 1 noninfected herd in California. PROCEDURE: The infection status of each cow was classified on the basis of results of an ELISA or combined ELISA and fecal culture results. Thirteen sampling schemes designed to randomly sample cows on the basis of lactation number, stage of lactation, and milk production were evaluated. Sampling without replacement was used to obtain a probability of herd detection of paratuberculosis for each evaluated sampling method and for simulated sample sizes between 30 and 150 cows. Marginal cost-effectiveness analysis was used to determine the cost increase relative to the increase in detection probability. RESULTS: Sampling cows in the third or higher lactation and > or = 200 days into lactation yielded the highest detection probability in most instances, resulting in a detection probability that was 1.4 to 2.5 times that obtained by sampling 30 cows in the second or higher lactation. Costs of testing via the alternative method with a 95% detection probability were approximately dollar 300 lower in a high-prevalence herd (31%) and dollar 800 lower in a low-prevalence herd (9%), compared with use of the reference method. CONCLUSIONS AND CLINICAL RELEVANCE: Detection of herds with paratuberculosis could be improved, and costs of testing substantially reduced by sampling targeted groups of cows.  相似文献   

18.
Abortion, premature calving, hemolytic anemia and fatal hematuria were associated with high levels (titer > 10(-4)) of antibody to Leptospira interrogans serovar hardjo and with isolation of hardjo in a herd of 265 beef cattle in the Great Clay Belt of northern Ontario. This herd was bred by artificial insemination, after heat detection by vasectomized bulls. The antibody prevalence rate in the herd was 54 to 60% over a five year period. The rate tended to reach 100% by age three years and to be below 5% in yearlings, which were raised in isolation from older cattle. Hardjo was isolated from the urine of a cow that aborted in the eighth month of pregnancy, and from kidneys of yearling steers which had been exposed to an older cow. Maternal antibody levels in calves paralleled those in their dams, protecting calves while they were being naturally exposed to infection, thus contributing to the achievement of balance between host and parasite. A controlled vaccination trial was conducted in 50 initially seronegative yearling steers and heifers. Serological response to vaccine was limited to a maximum agglutinin titer of 10(-2) in 8% of vaccinated cattle. Vaccination reduced the infection rate from 86% in the controls to 46% in the treated group, indirectly reducing the number of calves for which colostral antibody against hardjo would be available. A vaccination program was not implemented in the herd. Hardjo infection appeared to die out over a period of six years following the initial five year study period, with antibody prevalence falling from 60% to 0.7% and reactors persisting only in two eight year old cows. Decline in infection was coincident with changes in management which protected heifers from exposure to infection until their third pregnancy, and which probably lowered the reservoir of infection by increased culling from older age classes.  相似文献   

19.
A dairy herd (102 cattle) which had been enrolled under a paratuberculosis control program for two years utilizing a complement fixation test (carbohydrate antigen) and intradermal skin test (johnin PPD) was subjected to two further herd tests and followed to slaughter to determine infection status by culture and histology. Mycobacterium paratuberculosis infection was demonstrated in 37 of the animals of which only five were considered reactors on the basis of the last two herd tests applied. Cultural and histopathological evaluation indicated the testing procedures had eliminated heavily infected animals. The limitations of these testing procedures under free stall housing conditions are discussed.  相似文献   

20.
As a result of a BVDV infection in a herd consisting of 95 adult cattle 17 cows aborted their calves within a period of 3.5 months, one third got severe diarrhoea, 3 cows died and an increased percentage of the cattle got lochiometra after calving or abortion. The disease was diagnosed by paired serological testing of cattle with diarrhoea or abortion and post mortem examination of several aborted calves. From one foetus BVDV virus was isolated and subsequently subtyped by sequencing. Of aborting cattle, the testing results were influenced by the interval between infection and abortion. These results indicate that a primary infection with BVDV subtype 1b can cause severe clinical symptoms in a dairy herd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号