Modelling the effect of surveillance programmes on spread of bovine herpesvirus 1 between certified cattle herds

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.     

Simulation of alternatives for the Dutch Johne's disease certification-and-monitoring program

To identify optimal method(s) for certification and subsequent monitoring of Mycobacterium avium subsp. paratuberculosis (Map)-unsuspected herds, certification-and-monitoring schemes were studied using a stochastic simulation model ("JohneSSim"). JohneSSim simulated the within-herd transmission and economic aspects of Map in closed Dutch dairy herds. The model was validated with field observations on Map-unsuspected herds. The current Dutch certification-and-monitoring schemes were compared with 11 alternative schemes in which individual and pooled fecal culture, ELISA, Johnin-intradermal test and gamma-IFN ELISA were used, varying the test frequency, tested age group and number of tested animals. On reaching the 'Map-free' status with the standard certification scheme, 11% of the simulated herds were not truly Map-free. Therefore, the designation 'Map-free' should be changed into, for instance, 'low-risk Map'. In the most-attractive alternative certification scheme, the 'Map-free' status was reached after four herd examinations (at 2-year intervals) consisting of serial testing of all cattle > or = 2 years of age with a pooled fecal culture and individual fecal culture of positive pools. This scheme resulted in lower total and annual discounted costs and a lower animal-level prevalence at reaching the 'Map-free' status compared to the standard scheme, assuming that there was no new introduction of the infection. Schemes to monitor the 'Map-free' status were compared, assuming that this status was reached with the standard certification scheme. In comparison to the standard monitoring scheme, none of the alternative monitoring schemes resulted in both a lower animal-level prevalence of undetected pre-existing Map infections in closed herds, and lower median annual discounted costs. Results of the model were very sensitive to the assumed sensitivity of the fecal culture test and to management measures that prevent within-herd transmission of Map. If these preventive measures were taken, the probability of undetected Map infections in closed 'Map-free' herds was decreased substantially.     

Experiences in the control of brucellosis in cattle herds in the government district of Hannover

Caused by imported beef cattle new outbreaks of Brucellosis (subtype III) were observed in the government district of Hannover, which was "Brucellosis free" over a long period. With the aim to interrupt the series of infections from herd to herd it seemed to be necessary to introduce a herd screening system including frequent tests. It was not possible to screen the herds by the usual way of blood serum testing because of reasons of practicability and economy. A practicable alternative was the ELISA supported tank investigation. Four of the last six outbreaks were detected by this milk sampling. The other two infected herds were detected by only clinical symptoms because the lactating cows were not infected and had no contacts to the infected separately held heifers. The required test frequency in the dairy cattle herds could only be realized applying an ELISA supported highly sensitive tank milk test method. This method offered the chance of discovering infected herds as soon as possible and prevented greater economical losses and animal health risks. The tank milk screening of the dairy cattle herds to detect antibodies against Brucellosis (and additional EBL and IBR) has now become a standard method to continue the official status "free from..." This is a safer and a more economic alternative to the previous blood sampling of cattle older than two years with a three years interval because this method guarantees a safe information about the serological status of each cow which is now tested at least once a year, depending on the time of dry standing status.(ABSTRACT TRUNCATED AT 250 WORDS)     

Simulation model estimates of test accuracy and predictive values for the Danish Salmonella surveillance program in dairy herds

The Danish government and cattle industry instituted a Salmonella surveillance program in October 2002 to help reduce Salmonella enterica subsp. enterica serotype Dublin (S. Dublin) infections. All dairy herds are tested by measuring antibodies in bulk tank milk at 3-month intervals. The program is based on a well-established ELISA, but the overall test program accuracy and misclassification was not previously investigated. We developed a model to simulate repeated bulk tank milk antibody measurements for dairy herds conditional on true infection status. The distributions of bulk tank milk antibody measurements for infected and noninfected herds were determined from field study data. Herd infection was defined as having either >or=1 Salmonella culture-positive fecal sample or >or=5% within-herd prevalence based on antibody measurements in serum or milk from individual animals. No distinction was made between Dublin and other Salmonella serotypes which cross-react in the ELISA. The simulation model was used to estimate the accuracy of herd classification for true herd-level prevalence values ranging from 0.02 to 0.5. Test program sensitivity was 0.95 across the range of prevalence values evaluated. Specificity was inversely related to prevalence and ranged from 0.83 to 0.98. For a true herd-level infection prevalence of 15%, the estimate for specificity (Sp) was 0.96. Also at the 15% herd-level prevalence, approximately 99% of herds classified as negative in the program would be truly noninfected and 80% of herds classified as positive would be infected. The predictive values were consistent with the primary goal of the surveillance program which was to have confidence that herds classified negative would be free of Salmonella infection.     

Ante mortem diagnosis of tuberculosis in cattle: a review of the tuberculin tests, gamma-interferon assay and other ancillary diagnostic techniques

The early, preclinical stages of bovine TB can be detected in live animals by the use of tests of cellular immunity (the skin, gamma-interferon and lymphocyte transformation tests). Tests of humoral (antibody) immunity, Mycobacterium bovis PCR probes on early tissue cultures or live cattle specimens, and tests based on "electronic nose" technology have been developed more recently. The key measure of diagnostic test accuracy is the relationship between sensitivity and specificity, which determines the false-positive and false-negative proportions. None of the tests currently available for the diagnosis of bovine TB allow a perfectly accurate determination of the M. bovis infection status of cattle. Although various factors can reduce the sensitivity and specificity of the skin tests, these remain the primary ante mortem diagnostic tools for TB in cattle, providing a cost-effective and reliable means of screening entire cattle populations. Despite the inescapable limitations of existing diagnostic tests, bovine TB has been effectively eradicated from many developed countries and regions with the implementation of sound programmes of regular tuberculin skin testing and removal of reactors, coupled with slaughterhouse surveillance for undetected infections, repeat testing and culling of infected herds, cattle movement restrictions to prevent introduction of infected animals and occasional slaughter of entire herds with intractable breakdowns. This is likely to remain the mainstay of bovine TB control programmes for the foreseeable future. Additionally, newer ancillary in vitro diagnostic assays are now available to TB control programme managers to supplement the skin tests in defined circumstances according to the specific disease situation in each country or region. The strategic deployment of ancillary in vitro tests alongside the primary skin tests has enhanced the detection of M. bovis-infected cattle and reduced the number of animals slaughtered as false positives.     

Control of BVDV-infection on common grassland--the key for successful BVDV-eradication in Lower Austria

A bovine viral diarrhea/mucosal disease (BVD/MD) control and eradication program was introduced in Lower Austria in 1996, according to the Swedish model. At present 9800 out of 17,000 herds are part of this program. An important risk factor for BVDV-transmission under local conditions is communal grazing. Approximately 3-4% of livestock share common pastures, in which susceptible pregnant cattle may be mixed with unrecognised persistently infected (PI) animals. Rules and regulations were defined to allow only herds free from BVDV-infection on to common grassland. At the moment, 5067 herds are certified free from BVDV. The percentage of BVDV-free herds in regions with intensive pasture utilisation is higher (57.3%) than in the other regions (43.0%) of Lower Austria. With a reliable system for identification of PI-animals and a high certainty of prevention of PI-animals on common grassland, the main transmission of BVDV infection can be stopped, even if the animals are derived from infected herds and transiently infected animals cannot be excluded.     

Potential infection-control benefit for Ireland from pre-movement testing of cattle for tuberculosis

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.     

Review of pathogenesis and diagnostic methods of immediate relevance for epidemiology and control of Salmonella Dublin in cattle

Salmonella enterica subsp. enterica serovar Dublin (S. Dublin) receives increasing attention in cattle production. It is host-adapted to cattle, and leads to unacceptable levels of morbidity, mortality and production losses in both newly and persistently infected herds. Cattle health promoting institutions in several countries are currently constructing active surveillance programmes or voluntary certification programmes, and encourage control and eradication of S. Dublin infected cattle herds. There is a need to understand the underlying pathogenesis of the infection at both animal and herd level to design successful programmes. Furthermore, knowledge about and access to diagnostic tests for use in practice including information about test accuracy and interpretation of available diagnostic test methods are requested.     

Comparison of the complement-fixation and agar gel immunodiffusion tests for diagnosis of subclinical bovine paratuberculosis

The performance of the serum complement fixation (CF) test was compared with that of a serum agar gel immunodiffusion (AGID) test on 74 subclinically infected and 154 uninfected cattle in 6 commercial midwestern dairy herds with Mycobacterium paratuberculosis infection and on 30 cattle in a herd that was free of infection. Infection status of cattle within herds was established by performance of a series of 3 or more fecal cultures and of ileocecal lymph node cultures of culled cattle. In cattle with subclinical infection detected by culturing, the sensitivity estimates of the CF and AGID tests were 10.8% (3.6% SE) and 18.9% (4.5% SE), respectively. In the cattle classified as disease free, the specificity estimates of the CF and AGID tests were 97.4% (1.3% SE) and 99.4% (0.6% SE), respectively. Neither set of estimates was significantly different. Negative test results obtained with the use of either test in apparently normal cattle from suspect herds should be interpreted with caution because both tests suffer from low sensitivities in subclinically infected animals. However, the AGID test may be more useful in regulatory situations in which the CF test is currently used because the AGID test is easier to perform and to interpret.     

Estimating the number of undetected multi-resistant Salmonella Typhimurium DT104 infected pig herds in Denmark

In Denmark, the detection of multi-resistant Salmonella Typhimurium DT104 (MRDT104)-infected pig herds relies on the national Salmonella surveillance programme at the farm and slaughterhouse levels of production. With the surveillance sampling protocol and the diagnostic methods currently used, some herds might remain undetected. The number of undetected Danish pig herds infected with MRDT104 in the period 1 August 2001–31 July 2002 was estimated and compared with the number of culture-confirmed detected herds. A flow chart was constructed to illustrate where infected herds will go undetected in the surveillance system and Monte Carlo simulation was used to model the actual number of pig herds infected with MRDT1104. We estimated that 52 (90% CI [28, 178]) finisher herds were infected with MRDT104 compared to 23 (44%) detected. Among sow herds with production of weaners or growers, we estimated that 38 (90% CI [23, 74]) were infected with MRDT104 compared to 7 (18%) actually detected. Among breeder and multiplier herds, we estimated that five (90% CI [3, 8]) herds were infected with MRDT104 compared to three (60%) detected. In total, we estimated that 102 pig herds were infected with MRDT104 from 1 August 2001 till 31 July 2002 (90% CI [63, 228]). In comparison, 33 (32%) infected herds were detected in this period. The predicted proportion of undetected herds varied considerably with herd type. We infer that the proportion of detected MRDT104 infected herds depended on the intensity of the combined serological and bacteriological testing.     

Estimating the Prevalence of Salmonella spp. in Swine Herds – Influence of Sensitivity and Specificity of Salmonella Detection

Information about the proportion of truly Salmonella‐free herds is required for an evaluation of the epidemiological situation, the development of control strategies and their implementation. Findings regarding the presence of salmonellas in faeces and intestinal lymph nodes as well as the presence of Salmonella antibodies in meat juice from slaughtered pigs were obtained in the context of a study conducted by a number of institutes. These data were used for an analysis of the validity of data on the prevalence of infected animals within herds and on the prevalence of infected herds. The proportion of batches or herds with exclusively negative individual findings was found to depend not only on the true proportion of truly Salmonella‐free animals within herds but quite essentially also on the distribution of the proportion of infected animals within herds, the sensitivity of the methods of examination and sample sizes. When taking into account the existing dependencies, it was found that among the swine, the real numbers of Salmonella carriers were much higher than shown by bacteriological and serological examination. Regarding salmonellosis in swine, also a number of contaminated herds must be expected which is far higher than that shown by the number of herds with positive findings in at least one animal. Even a low contamination of all or almost all herds would result in the numbers of ‘negative’ batches observed, i.e. batches with exclusively negative individual findings. A rating of the salmonella exposure of herds as high, low, or very low is possible and may, and should be, used for measures of consumer protection, irrespective of the proportion of truly Salmonella‐free herds.     

Brucellosis in Ontario: a case control study.

Data from cattle herds infected with brucellosis and from control (noninfected) herds were collected and analyzed using case control techniques. It appeared that herds located close to other infected herds and those herds whose owners made frequent purchases of cattle had an increased risk of acquiring brucellosis, particularly those who made purchases from other herds or from cattle dealers. Infected herds had a lower level of vaccination than noninfected herds. However, the percentage vaccinated was highly variable in each group. Vaccination per se did not appear to adversely influence the interpretation of serological test results nor did it appear to protect the individual animal. Once infected, the time required to become free of brucellosis was increased by large herd size and/or loose housing. Closed herds also took longer to become brucellosis free than more open herds. The percentage of animals removed from the herd was increased by active abortion. Those herds with multiple serological reactors (positives and questionables) at the first herd test after the imposition of quarantine had the highest percentage of cattle removed.     

ELISA and fecal culture for paratuberculosis (Johne's disease): sensitivity and specificity of each method

The sensitivity and specificity of the ELISA and fecal culture tests for paratuberculosis in dairy cattle are examined. ELISA and fecal culture data from seven dairy herds where both fecal cultures and ELISA testing was done concurrently are included. A cohort of 954 cattle including 697 parturient adults, cultured every 6 months from 10 herds followed over 4 years served as the basis to determine fecal culture sensitivity. The fecal culture technique utilized a 2g sample with centrifugation and double incubation. Of the 954 cattle cohort of all ages (calf to adult) that were fecal sampled on the first herd visit, 79 were culture positive. An additional 131 animals were detected as culture positive over the next seven tests at 6-month intervals. The sensitivity of fecal culture to detect infected cattle on the first sampling was 38%. Of the 697 parturient cattle cohort, 67 were positive on the first fecal culture, while an additional 91 adult cattle were culture positive over the next seven tests, resulting in a sensitivity of 42% on the first culture of the total animals identified as culture positive. Animals culled from the herds prior to being detected as infected and animals always fecal culture negative with culture positive tissues at slaughter are not included in the calculations. Both groups of infected cattle will lower the apparent sensitivity of fecal culture. Infected dairy herds tested concurrently with both fecal culture and ELISA usually resulted in more than twofold positive animals by culture compared to ELISA.The classification of infected cattle by the extent of shedding of Mycobacterium paratuberculosis in the feces helps define the relative proportion of cattle in each group and therefore the likelihood of detection by the ELISA test. ELISA has a higher sensitivity in animals with a heavier bacterial load, i.e. high shedders (75%) compared to low shedders (15%). Repeated testing of infected herds identifies a higher proportion of low shedders which are more likely to be ELISA negative. Thus, the sensitivity of the ELISA test decreases with repeated herd testing over time, since heavy shedders will be culled first from the herds.     

The Animal and Plant Health Inspection Service (APHIS) brucellosis eradication program in the United States

Efforts to eradicate brucellosis caused by Brucella abortus in the United States began in 1934 as part of an economic recovery program to reduce the cattle population because of the Great Depression and concurrent severe drought conditions. A number of states saw this as an opportunity to reduce the level of brucellosis, which was the most significant livestock disease problem in the US at the time. In 1934 and 1935, the reactor rate in adult cattle tested was 11.5%. In 1954, the magnitude of the brucellosis problem in the United States in terms of economics to the cattle industry and human health prompted Congress to appropriate funds for a comprehensive national effort to eradicate brucellosis. The brucellosis eradication program was designed as a cooperative effort between the federal government, the states, and livestock producers. As the science and technology of brucellosis has developed over the years through research and experience, the eradication program has been modified many times.

As of 31 December 2000, there were no affected cattle herds in the United States. This was the first time in the history of the brucellosis program that the United States had no known brucellosis affected herds. However, brucellosis has a variable, sometimes quite lengthy incubation period, so it is expected that additional affected herds will be disclosed. It is likely that additional affected herds will be disclosed before brucellosis is finally eradicated from cattle. Animal health officials remain prepared to aggressively pursue any newly disclosed affected herds to eliminate the disease as quickly as possible.

The State-Federal Brucellosis Eradication Program has made tremendous progress since its inception. In an eradication program, it is critically important to recognize that, despite all the tools that are available to eliminate the disease, an effective surveillance system is the critical first step that must be in place in order to be successful. It is imperative, not only to be able to find the disease and eliminate it, but to find it before it spreads to susceptible herds. When brucellosis can be identified, contained, and eliminated before spread occurs, eradication can be achieved.     

Use of serologic evaluation for antibodies against bovine viral diarrhea virus for detection of persistently infected calves in beef herds

OBJECTIVE: To determine whether use of serologic evaluation of a sentinel sample of calves or cows for antibodies against bovine viral diarrhea virus (BVDV) would accurately predict whether an animal persistently infected with BVDV could be detected in beef herds. SAMPLE POPULATION: 27 cow-calf herds in which the status of persistently infected calves was not known and 11 herds known to have persistently infected calves. Procedure-Detection of persistently infected calves was determined through immunohistochemical testing of tissue obtained at necropsy of all calves that died during calving season and skin (ear notch) specimens obtained from all young stock in the fall of 2002. Serum samples were collected from 30 spring-born calves and 10 mature cows. RESULTS: Optimum serologic test performance at time of weaning was detected when 10 calves were evaluated. At least 3 of 10 randomly selected calves were likely to have a titer > 1:1000 against BVDV type I or II in 53% of herds in which a persistently infected calf was detected during that year (sensitivity, 53%). However, at least 3 of 10 randomly selected calves were also likely to have a titer > 1:1000 in 20% of herds that did not have a persistently infected calf detected during that year (specificity, 80%). CONCLUSIONS AND CLINICAL RELEVANCE: Despite the use of a number of various cutoff values and sample sizes, serologic evaluation of a small number of calves or cows could not be used to accurately predict the presence of persistently infected cattle in a herd.     

Evaluation of conventional and radiometric fecal culture and a commercial DNA probe for diagnosis of Mycobacterium paratuberculosis infections in cattle.

Radiometric (RCM) and conventional fecal culture (HEY) and a commercial polymerase chain reaction/DNA probe were evaluated as diagnostic tests for subclinical paratuberculosis in dairy cattle using fecal specimens from a repository of paratuberculosis specimens. The case definition of subclinical bovine paratuberculosis was isolation of Mycobacterium paratuberculosis, by conventional or radiometric culture, from fecal samples or internal organs of dairy cattle without diarrhea or chronic weight loss. Animals designated as free of the disease originated exclusively from certified paratuberculosis-free herds in Wisconsin. Among 182 infected cattle, RCM and HEY fecal culture and the DNA probe had test sensitivities of 54.4%, 45.1% and 33.5%, respectively. Fecal samples from only 111 of the M. paratuberculosis-infected cows tested positive by at least one of the three tests and these cows were designated as fecal shedders; the remaining 71 were considered to have prepatent infections. Among the 111 M. paratuberculosis fecal shedders, RCM, HEY and the probe detected the organism in 89.2%, 73.8% and 55.0% of the fecal specimens, respectively. Herd prevalence significantly affected the sensitivity of all three diagnostic tests (p less than 0.05) but only affected the fecal shedder detection efficiency of the DNA probe (p less than 0.01). No positive DNA probe results were found on 100 randomly selected fecal samples from cows in four certified paratuberculosis-free herds, thus the DNA probe was 100% specific. Probe analyses could be performed in 24 h or less. Time to complete the culture-based tests was 12 wk for HEY and 7 wk for RCM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification of Mycobacterium avium ssp. paratuberculosis infected dairy herds by environmental sampling

In herds with known prevalence (P) use of environmental sampling (ES) to detect Mycobacterium avium ssp. paratuberculosis (MAP) infected cattle herds was proofed in relation to P. In 31 MAP-infected free stall dairy herds and 15 non-infected herds P was defined by annually repeated whole herd testing by fecal culture (34 877 individual samples). Eight infected herds had a very low (> 0-2%), 14 a low (> 2-5%), four a medium (> 5-10%), and five a high P (> 10%). A mean number of nine environmental samples per herd were collected from the floor of lactating cows, milking, calving and sick cow areas and the crossover to the calf area. After twelve weeks cultivation on HEYM-medium with and without mycobactin positive samples were further characterized by PCR. All non-infected herds (100%) showed negative and 22 (71%) of the infected herds positive results in ES. Nine infected herds with negative ES results had a low P (0.04-4,04%). Proportion of positive ES depended on P and on sampling areas with 53.3% positive results in lactating cow areas and 45.2% in milking areas. For P > 5%, ES in these two areas caused a positive herd status; herds with P < 5% required sampling in the other areas too. The ES method has a herd sensitivity of 87% for dairy herds with P > 2% and provides an efficient tool to determine MAP infection status or herd prevalence.     

The Dutch Brucella abortus monitoring programme for cattle: the impact of false-positive serological reactions and comparison of serological tests

The Dutch national Brucella abortus eradication programme for cattle started in 1959. Sporadic cases occurred yearly until 1995; the last infected herd was culled in 1996. In August 1999 the Netherlands was declared officially free of bovine brucellosis by the European Union. Before 1999, the programme to monitor the official Brucella-free status of bovine herds was primarily based on periodical testing of dairy herds with the milk ring test (MRT) and serological testing of all animals older than 1 year of age from non-dairy herds, using the micro-agglutination test (MAT) as screening test. In addition, serum samples of cattle that aborted were tested with the MAT. The high number of false positive reactions in both tests and the serum agglutination test (SAT) and complement fixation test (CFT) used for confirmation seemed to result in unnecessary blockade of herds, subsequent testing and slaughter of animals. For this reason, a validation study was performed in which three indirect enzyme-linked immunosorbent assays (ELISAs), the CFT and the SAT were compared using a panel of sera from brucellosis-free cattle, sera from experimentally infected cattle, and sera from cattle experimentally infected with bacteria which are known to induce cross-reactive antibodies (Pasteurella, Salmonella, Yersinia, and Escherichia). Moreover, four ELISAs and the MRT were compared using a panel of 1000 bulk milk samples from Brucella-free herds and 12 milk samples from Brucella abortus- infected cattle. It is concluded that the ELISA obtained from ID-Lelystad is the most suitable test to monitor the brucelosis free status of herds because it gives rise to fewer false-positive reactions than the SAT.     

Udder health: the influence of BVD-free status

The effect of BVD v-free certification of BVD v-infected dairy herds on udder health was determined, by comparing parameters of udder health in 319 cases and 629 controls. Cases were dairy herds that originally had at least one BVD V antigen-positive animal but which subsequently became BVD v-free and maintained this certified status for at least 2 years. Controls had an unknown BVD V status. Each case was matched with two controls by region and herd size. The three udder health parameters were bulk-milk somatic cell count (scc), the proportion of cows with a high scc, and the proportion of cows with a high scc after previously having a normal scc (these cows were considered to have a new intramammary infection). Udder health was better in the cases in the 2 years preceding the BVD V-free period than in the controls. BVD V-free status was not associated with bulk-milk scc or the proportion of cows with a high scc but was associated with occurrence of a new intramammary infection:fewer case cows had a new intramammary infection than control cows (difference 0.6%; P < 0.05).     

Evaluation of the specificity of the gamma-interferon test in Italian bovine tuberculosis-free herds

We investigated the specificity of the gamma-interferon test for bovine tuberculosis (TB) in 1,557 cattle in 30 paratuberculosis-free and officially certified TB-free dairy herds, located in three provinces of the Lombardy Region in Northern Italy. The TB-free status of the herds under examination was further confirmed by the tuberculin skin test, by an antibody assay and by post mortem examination of animals culled from the herds during the study period. The specificity of the gamma-interferon tests after a single test and a double sampling scheme were 88.8% and 95.4%, respectively. After a single test, 11.7% of dubious reactors were also detected, while most cattle (47.4%) were shown to be avian reactors, probably due to contamination from infected birds and/or forage. There was strong evidence that the specificity of the test could be related to the animals' interaction with environmental mycobacteria and/or ageing. To reduce the percentage of nonspecific bovine reactors under alleged TB-free conditions, test procedures might involve the use of more specific antigens and/or different reaction thresholds.