Probability of freedom from disease after the first detection and eradication of PRRS in Sweden: Scenario-tree modelling of the surveillance system

In July 2007, PRRS was detected for the first time in Sweden. A total of eight positive herds were identified and various measures were taken to eradicate the disease, including restrictions and slaughter of infected herds. Subsequently, both active and passive surveillance activities were undertaken. This study describes stochastic scenario-tree modelling of all the various surveillance system components, to estimate the current probability that Sweden is free from PRRS. The model includes all actions taken after the first positive herd was detected. The surveillance system components included in the model were as follows: investigations undertaken in association with the outbreak, a serological study based on samples collected at slaughter, samples collected in the national PRRS surveillance programme and passive clinical surveillance. The probability of freedom was estimated in time steps of 1 month, from July to December 2007. After each time step, the calculated posterior probability of freedom from the previous month, combined with the probability of introduction, was used as a prior probability for the next month.The result from the model showed a 99.8% probability that Sweden was free from PRRS at the end of December 2007. The estimated total sensitivity of the surveillance system varied between 81.2% and 94.3% and was highest during the first months after the outbreak. For sensitivity analysis purposes, the model was also applied using higher risks of introduction. However, this did not make considerable difference to the final estimates.     

Incidence of classical swine fever (CSF) in wild boar in a densely populated area indicating CSF virus persistence as a mechanism for virus perpetuation

A virological survey was carried out to establish the distribution of classical swine fever (CSF) virus among wild boar in the Federal State of Brandenburg, Germany. Organ materials and blood samples were collected from 11,670 wild boar shot or found dead during the period March 1995 to December 1997. In total 211 (1.8%) wild boar were positive for CSF virus or antigen. The incidence of CSF-positive animals decreased continuously from 4.6% at the beginning of the epidemic in 1995 to 0.7% in 1997. The highest incidence of positive animals (22%) was found in wild-boar piglets younger than 3 months of age in 1995. The findings were indicative for the decisive role which young wild boar play in the epidemiology of CSF. Following intrauterine transfer some of the wild-boar piglets were probably persistently infected with CSF virus as experienced experimentally. Such piglets can be held responsible for CSF virus perpetuation within the wild-boar population. No CSF virus was isolated from adult wild boar weighing more than 75 kg. During 3 years of monitoring a sufficient number of susceptible wild boar, in particular young animals, was available to maintain the infection chain in that area. It was concluded that persistently infected piglets and the high population density of wild boar in the Brandenburg region offered optimal conditions for the establishment of an CSF epidemic.     

Incidence of Classical Swine Fever (CSF) in Wild Boar in a Densely Populated Area Indicating CSF. Virus Persistence as a Mechanism for Virus Perpetuation

A virological survey was carried out to establish the distribution of classical swine fever (CSF) virus among wild boar in the Federal State of Brandenburg, Germany. Organ materials and blood samples were collected from 11 670 wild boar shot or found dead during the period March 1995 to December 1997. In total 211 (1.8%) wild boar were positive for CSF virus or antigen. The incidence of CSF-positive animals decreased continuously from 4.6% at the beginning of the epidemic in 1995 to 0.7% in 1997. The highest incidence of positive animals (22%) was found in wild-boar piglets younger than 3 months of age in 1995. The findings were indicative for the decisive role which young wild boar play in the epidemiology of CSF. Following intrauterine transfer some of the wild-boar piglets were probably persistently infected with CSF virus as experienced experimentally. Such piglets can be held responsible for CSF virus perpetuation within the wild-boar population. No CSF virus was isolated from adult wild boar weighing more than 75 kg. During 3 years of monitoring a sufficient number of susceptible wild boar, in particular young animals, was available to maintain the infection chain in that area. It was concluded that persistently infected piglets and the high population density of wild boar in the Brandenburg region offered optimal conditions for the establishment of an CSF epidemic.     

Evaluation of diagnostic tests for the detection of classical swine fever in the field without a gold standard.

Knowledge of the sensitivity of diagnostic tests for infectious diseases under field conditions can be used to design a surveillance program that increases the effectiveness of the control policy. In this study, the sensitivity of tests for the detection of classical swine fever (CSF) virus (CSFV) under field conditions was estimated without knowledge of the true disease status of the animals tested. During the CSF epidemic of 1997-1998 in The Netherlands, tonsil samples from pigs of CSF suspect farms were collected for laboratory diagnosis of CSE These specimens were tested in a fluorescence antibody test (FAT1) for the presence of CSFV antigen. When at least 1 specimen in a particular sample series from a farm was positive, this farm was declared CSFV infected. Specimens of that series, either FAT1 negative (98) or FAT1 positive (127), were subsequently tested again (FAT2). After that, a suspension was made of the remaining tissue, and this suspension was evaluated with a virus isolation test. In total, 225 tonsil specimens were examined. A statistical model was formulated, and the sensitivity of the 3 tests and the prevalence of positive specimens in the sample were estimated by the method of maximum likelihood. The sensitivity of the FAT1, the test that was used for confirmation of CSFV infection in a pig herd, was approximately 78% (95% confidence interval [CI] = 62-92%). The effectiveness of the selection process of animals on the farm by the veterinarian was estimated to be 77% (64-87%). The sensitivity of the combination of FAT1 and FAT2 (60%) indicates that at least 5 animals should be selected on a CSF-suspect farm to gain a detection probability for CSFV of 99%.     

Implementing a probabilistic definition of freedom from infection to facilitate trade of livestock: putting theory into praxis for the example of bovine herpes virus-1

International trade of livestock and livestock products poses a significant potential threat for spread of diseases, and importing countries therefore often require that imported animals and products are free from certain pathogens. However, absolute freedom from infection cannot be documented, since all test protocols are imperfect and can lead to false-negative results. It is possible instead to estimate the "probability of freedom from infection" and its opposite, the probability of infection despite having a negative test result. These probabilities can be estimated based on a pre-defined target prevalence, known surveillance efforts in the target population and known test characteristics of any pre-export test. Here, calculations are demonstrated using the example of bovine herpes virus-1 (BoHV-1). In a population that recently became free of BoHV-1 without using vaccination, the probability of being infected of an animal randomly selected for trade is 800 per 1 million and this probability is reduced to 64 (95% probability interval PI 6-161) per 1 million when this animal is tested negatively prior to export with a gB-ELISA. In a population that recently became free of BoHV-1 using vaccination, the probability of being infected of an animal randomly selected for trade is 200 per 1 million, and this probability can be reduced to 63 (95% PI 42-87) when this animal is tested negatively prior to export with a gE-ELISA. Similar estimations can be made on a herd level when assumptions are made about the herd size and the intensity of the surveillance efforts. Subsequently, the overall probability for an importing country of importing at least 1 infected animal can be assessed by taking into account the trade volume. Definition of the acceptable level of risk, including the probability of false-negative results to occur, is part of risk management. Internationally harmonized target prevalence levels for the declaration of freedom from infection from selected pathogens provide a significant contribution to the facilitation of international trade of livestock and livestock products by allowing exporting countries to design tailor-made output-based surveillance programs, while providing equivalent guarantees regarding the probability of freedom from infection of the population. Combining this with an approach to assess the overall probability of introducing at least 1 infected animal into an importing country during a defined time interval will help importing countries to achieve their desired level of acceptable risk and will help to assess the equivalence of animal health and food safety standards between trading partners.     

The effectiveness of classical swine fever surveillance programmes in The Netherlands

Consequences of classical swine fever (CSF) epidemics depend on the control measures, but also on the number of infected herds at the end of the high-risk period (HRP). Surveillance programmes aim to keep this number as low as possible, so the effectiveness of surveillance programmes can be measured by the number of infected herds at the end of the HRP. In this paper, an evaluation of the effectiveness of the following five Dutch CSF surveillance programmes is presented: (1) routine gross pathology of severely diseased pigs; (2) routine virological tests of tonsils of all pigs, submitted under 1; (3) daily clinical observation by the farmer; (4) periodic clinical inspection by a veterinarian; (5) leucocyte counts in blood samples from diseased animals on a herd where antimicrobial 'group therapy' is started. The evaluation was done by a modelling study, in which virus transmission, disease development, and actions and diagnostic tests in surveillance programmes were simulated. Also, the yearly costs of the programmes were calculated, and direct costs of CSF epidemics were related to the number of infected herds at the end of the HRP. It appeared that the current Dutch surveillance programmes, without the leucocyte counts, keep the number of infected herds at the end of the HRP below 20 with 95% probability. Leaving out the most-expensive programme of periodic inspection (12.5M per year) does not change this result - indicating that (for CSF surveillance) the programme could well be stopped. If the leucocyte programme, which is currently not effective due to the low sample submission rate, optimally were applied, the 95th percentile could be reduced to 10 infected herds. However, whether application is beneficial is unclear, because of uncertainty of the economic benefits due to the many expected false-positive herds each year.     

Current value of historical and ongoing surveillance for disease freedom: surveillance for bovine Johne's disease in Western Australia

‘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.     

Simulating the spread of classical swine fever virus between a hypothetical wild-boar population and domestic pig herds in Denmark

Denmark has no free-range wild-boar population. However, Danish wildlife organizations have suggested that wild boar should be reintroduced into the wild to broaden national biodiversity. Danish pig farmers fear that this would lead to a higher risk of introduction of classical swine fever virus (CSFV), which could have enormous consequences in terms of loss of pork exports. We conducted a risk assessment to address the additional risk of introducing and spreading CSFV due to the reintroduction of wild boar. In this paper, we present the part of the risk assessment that deals with the spread of CSFV between the hypothetical wild-boar population and the domestic population. Furthermore, the economic impact is assessed taking the perspective of the Danish national budget and the Danish pig industry. We used InterSpreadPlus to model the differential classical swine fever (CSF) risk due to wild boar. Nine scenarios were run to elucidate the effect of: (a) presence of wild boar (yes/no), (b) locations for the index case (domestic pig herd/wild-boar group), (c) type of control strategy for wild boar (hunting/vaccination) and (d) presence of free-range domestic pigs. The presence of free-range wild boar was simulated in two large forests using data from wildlife studies and Danish habitat data. For each scenario, we estimated (1) the control costs borne by the veterinary authorities, (2) the control-related costs to farmers and (3) the loss of exports associated with an epidemic. Our simulations predict that CSFV will be transmitted from the domestic pig population to wild boar if the infected domestic pig herd is located close to an area with wild boar (<5 km). If an outbreak begins in the wild-boar population, the epidemic will last longer and will occasionally lead to several epidemics because of periodic transfer of virus from groups of infected wild boar to domestic pig herds. The size and duration of the epidemic will be reduced if there are no free-range domestic pig herds in the area with CSF-infected wild boar. The economic calculations showed that the total national costs for Denmark (i.e. the direct costs to the national budget and the costs to the pig industry) related to an outbreak of CSF in Denmark will be highly driven by the reactions of the export markets and in particular of the non-EU markets. Unfortunately, there is a substantial amount of uncertainty surrounding this issue. If hunting is used as a control measure, the average expenses related to a CSF outbreak will be 40% higher if wild boar are present compared with not present. However, a vaccination strategy for wild boar will double the total costs compared with a hunting strategy.     

An economic model to evaluate the mitigation programme for bovine viral diarrhoea in Switzerland

Economic analyses are indispensable as sources of information to help policy makers make decisions about mitigation resource use. The aim of this study was to conduct an economic evaluation of the Swiss national mitigation programme for bovine viral diarrhoea virus (BVDV), which was implemented in 2008 and concludes in 2017. The eradication phase of the mitigation programme comprised testing and slaughtering of all persistently infected (PI) animals found. First, the whole population was antigen tested and all PI cattle removed. Since October 2008, all newborn calves have been subject to antigen testing to identify and slaughter PI calves. All mothers of PI calves were retested and slaughtered if the test was positive. Antigen testing in calves and elimination of virus-carriers was envisaged to be conducted until the end of 2011. Subsequently, a surveillance programme will document disease freedom or detect disease if it recurs. Four alternative surveillance strategies based on antibody testing in blood from newborn calves and/or milk from primiparous cows were proposed by Federal Veterinary Office servants in charge of the BVDV mitigation programme. A simple economic spreadsheet model was developed to estimate and compare the costs and benefits of the BVDV mitigation programme. In an independent project, the impact of the mitigation programme on the disease dynamics in the population was simulated using a stochastic compartment model. Mitigation costs accrued from materials, labour, and processes such as handling and testing samples, and recording results. Benefits were disease costs avoided by having the mitigation programme in place compared to a baseline of endemic disease equilibrium. Cumulative eradication costs and benefits were estimated to determine the break-even point for the eradication component of the programme. The margin over eradication cost therefore equalled the maximum expenditure potentially available for surveillance without the net benefit from the mitigation programme overall becoming zero. Costs of the four surveillance strategies and the net benefit of the mitigation programme were estimated. Simulations were run for the years 2008-2017 with 20,000 iterations in @Risk for Excel. The mean baseline disease costs were estimated to be 16.04m CHF (1 Swiss Franc, CHF=0.73 € at the time of analysis) (90% central range, CR: 14.71-17.39m CHF) in 2008 and 14.89m CHF (90% CR: 13.72-16.08m CHF) in 2009. The break-even point was estimated to be reached in 2012 and the margin over eradication cost 63.15m CHF (90% CR: 53.72-72.82m CHF). The discounted cost for each surveillance strategy was found to be smaller than the margin, so the mitigation programme overall is expected to have a positive net economic benefit irrespective of the strategy adopted. For economic efficiency, the least cost surveillance alternative must be selected.     

Demonstrating freedom from disease using multiple complex data sources 2: case study--classical swine fever in Denmark

A method for quantitative evaluation of surveillance for disease freedom has been presented in the accompanying paper (Martin et al., 2007). This paper presents an application of the methods, using as an example surveillance for classical swine fever (CSF) in Denmark in 2005. A scenario tree model is presented for the abattoir-based serology component of the Danish CSF surveillance system, in which blood samples are collected in an ad hoc abattoir sampling process, from adult pigs originating in breeding herds in Denmark. The model incorporates effects of targeting (differential risk of seropositivity) associated with age and location (county), and disease clustering within herds. A surveillance time period of one month was used in the analysis. Records for the year 2005 were analysed, representing 25,332 samples from 3528 herds; all were negative for CSF-specific antibodies. Design prevalences of 0.1-1% of herds and 5% of animals within an infected herd were used. The estimated mean surveillance system component (SSC) sensitivities (probability that the SSC would give a positive outcome given the animals processed and that the country is infected at the design prevalences) per month were 0.18, 0.63 and 0.86, for among-herd design prevalences of 0.001, 0.005 and 0.01. The probabilities that the population was free from CSF at each of these design prevalences, after a year of accumulated negative surveillance data, were 0.91, 1.00 and 1.00. Targeting adults and herds from South Jutland was estimated to give approximately 1.9, 1.6 and 1.4 times the surveillance sensitivity of a proportionally representative sampling program for these three among-herd design prevalences.     

Application of modelling to determine the absence of foot-and-mouth disease in the face of a suspected incursion

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.     

Optimizing early detection of avian influenza H5N1 in backyard and free-range poultry production systems in Thailand

For infectious diseases such as highly pathogenic avian influenza caused by the H5N1 virus (A/H5N1 HP), early warning system is essential. Evaluating the sensitivity of surveillance is a necessary step in ensuring an efficient and sustainable system. Stochastic scenario tree modeling was used here to assess the sensitivity of the A/H5N1 HP surveillance system in backyard and free-grazing duck farms in Thailand. The whole surveillance system for disease detection was modeled with all components and the sensitivity of each component and of the overall system was estimated. Scenarios were tested according to selection of high-risk areas, inclusion of components and sampling procedure, were tested. Nationwide passive surveillance (SSC1) and risk-based clinical X-ray (SSC2) showed a similar sensitivity level, with a median sensitivity ratio of 0.96 (95% CI 0.40-15.00). They both provide higher sensitivity than the X-ray laboratory component (SSC3). With the current surveillance design, the sensitivity of detection of the overall surveillance system when the three components are implemented, was equal to 100% for a farm level prevalence of 0.05% and 82% (95% CI 71-89%) for a level of infection of 3 farms. Findings from this study illustrate the usefulness of scenario-tree modeling to document freedom from diseases in developing countries.     

Policy-driven development of cost-effective, risk-based surveillance strategies

Animal health and residue surveillance verifies the good health status of the animal population, thereby supporting international free trade of animals and animal products. However, active surveillance is costly and time-consuming. The development of cost-effective tools for animal health and food hazard surveillance is therefore a priority for decision-makers in the field of veterinary public health. The assumption of this paper is that outcome-based formulation of standards, legislation leaving room for risk-based approaches and close collaboration and a mutual understanding and exchange between scientists and policy makers are essential for cost-effective surveillance. We illustrate this using the following examples: (i) a risk-based sample size calculation for surveys to substantiate freedom from diseases/infection, (ii) a cost-effective national surveillance system for Bluetongue using scenario tree modelling and (iii) a framework for risk-based residue monitoring. Surveys to substantiate freedom from infectious bovine rhinotracheitis and enzootic bovine leucosis between 2002 and 2009 saved over 6 million € by applying a risk-based sample size calculation approach, and by taking into account prior information from repeated surveys. An open, progressive policy making process stimulates research and science to develop risk-based and cost-efficient survey methodologies. Early involvement of policy makers in scientific developments facilitates implementation of new findings and full exploitation of benefits for producers and consumers.     

Bovine viral diarrhea (BVD) eradication in Switzerland--experiences of the first two years

A national eradication programme was designed with the aim of achieving total freedom from bovine viral diarrhea virus (BVDV) infection in the Swiss cattle population. The eradication programme consisted of testing every Swiss bovine for antigen, culling virus-positive animals and applying movement restrictions. Starting in 2008, the campaign achieved the goal of reducing the proportion of newborn calves that were virus-positive from 1.8% to under 0.2% within two years (situation in September 2010). Both good data flow between the parties involved as well as speed and efficiency (e.g. concerning the application of tests, movement restrictions and slaughter) are central to the success of the programme. Since the beginning of the programme 2.85 million cattle have been tested for bovine viral diarrhea virus (BVDV). The BVD-prevalence in cattle at the individual and herd levels following the implementation of the eradication programme was assessed. Using data collected during this campaign a risk factor analysis was conducted in order to identify factors associated with the appearance of virus positive newborn calves in herds where BVD had not previously been detected; these risk factors would allow targeting of future surveillance. Herd size, early death rate (i.e. the number of animals that either die before 15 days of age or are stillborn per number of newborns per year), buying in stock, using communal summer grazing, production type, age structure and management strategy were factors associated with the appearance of new cases of infection. Testing of newborn calves for antigen will continue to be conducted until the end of 2011, this is combined with outbreak investigation of newly infected herds (consisting of re-testing dams of virus-positive calves and if necessary all cattle on or that recently left the farm). This process is done to identify infected animals that may have been missed during prior testing (false negatives), it also serves to identify other factors that may be responsible for the introduction of BVDV onto the farm. Since October 2009, testing of calves for antigen combined with outbreak investigation has led to the detection of 55 infected animals that had tested negative (presumably false negative) during previous rounds of testing.     

Theoretical basis and empirical implications of the bovine brucellosis market slaughter testing program in the United States

With reductions in federal funding, the Market Slaughter Testing (MST) surveillance system might once again become the primary bovine brucellosis surveillance system for the beef-cow population in the United States. Thus, understanding the weaknesses and/or strengths of the MST surveillance system will be crucial in determining the most effective program in a limited funding control/eradication program.An estimation of a hypergeometric distribution used in previous studies to describe the effectiveness of the MST system is employed and parameters of this distribution are estimated from secondary data. Results analyzed by herd size, and within-herd infection level yield a distinct herd size bias in the MST surveillance system. For example, the probability of detection in a 9-cow herd is 24% compared to 85.4% for a 645 cow herd after 1 year of infection. This herd size bias implies that secondary testing may be efficiently u used by concentrating testing in the smaller herds when funds for secondary testing are limited.MST detection probabilities can vary 30% between the build-up and liquidation phases of the cattle cycle, i.e., secular upswings and downswings in cattle inventory numbers. First Point of Concentration testing (FPC) of purchased replacements can partially offset the effects of the cattle cycle. A 95% herd vaccination level can reduce the probability of detection as much as 40% when compared to a similar herd that is not vaccinated.     

When can a veterinarian be expected to detect classical swine fever virus among breeding sows in a herd during an outbreak?

The herd sensitivity (HSe) and herd specificity (Hsp) of clinical diagnosis of an infection with classical swine fever (CSF) virus during veterinary inspection of breeding sows in a herd was evaluated. Data gathered from visits to herds during the CSF outbreak in 1997-1998 in The Netherlands were used for the analysis. Herds were visited one or more times by the same or by different veterinarians. On the basis of the veterinarians' reports, each visit was coded as 0 (negative clinical diagnosis) or 1 (positive clinical diagnosis). The HSe for clinical diagnosis of CSF was modelled as a function of days elapsed since introduction of the virus. The moment of introduction of the CSF virus in the CSF-positive herds was unknown, so for each herd, a probability distribution for the unknown number of days since introduction was derived from serum samples collected at depopulation. The information from the reports of the veterinarians and from the test results of the serum samples at depopulation was combined in a Bayesian analysis. Data from CSF-negative herds were analysed to estimate HSp of clinical diagnosis of CSF. The HSe of clinical diagnosis was 0.5 at 37 days after virus introduction (95% CI: 31, 45) and reached 0.9 at 47 days after virus introduction (95% CI: 41, 54). The estimated herd specificity was 0.72 (95% CI: 0.64, 0.79). Dependence of HSe and HSp on characteristics of the veterinarians and the herds also was studied. Specialisation of the veterinarian significantly, although not markedly, affected the HSe.     

Evaluation of antibody-ELISA and real-time RT-PCR for the diagnosis and profiling of bluetongue virus serotype 8 during the epidemic in Belgium in 2006

In 2006 bluetongue (BT) emerged for the first time in North-Western Europe. Reliable diagnostic tools are essential in controlling BT but data on the diagnostic sensitivity (Se) and specificity (Sp) are often missing. This paper aims to describe and analyse the results obtained with the diagnostics used in Belgium during the 2006 BT crisis. The diagnosis was based on a combination of antibody detection (competitive ELISA, cELISA) and viral RNA detection by real-time RT-PCR (RT-qPCR). The performance of the cELISA as a diagnostic tool was assessed on field results obtained during the epidemic and previous surveillance campaigns. As the infectious status of the animals is unknown during an epidemic, a Bayesian analysis was performed. Both assays were found to be equally specific (RT-qPCR: 98.5%; cELISA: 98.2%) while the diagnostic sensitivity of the RT-qPCR (99.5%) was superior to that of the cELISA (87.8%). The assumption of RT-qPCR as standard of comparison during the bluetongue virus (BTV) epidemic proved valid based on the results of the Bayesian analysis. A ROC analysis of the cELISA, using RT-qPCR as standard of comparison, showed that the cut-off point with the highest accuracy occurred at a percentage negativity of 66, which is markedly higher than the cut-off proposed by the manufacturer. The analysis of the results was further extended to serological and molecular profiling and the possible use of profiling as a rapid epidemiological marker of the BTV in-field situation was assessed. A comparison of the serological profiles obtained before, during and at the end of the Belgian epidemic clearly showed the existence of an intermediate zone which appears soon after BTV (re)enters the population. The appearance or disappearance of this intermediate zone is correlated with virus circulation and provides valuable information, which would be entirely overlooked if only positive and negative results were considered.     

Laboratory decision-making during the classical swine fever epidemic of 1997-1998 in The Netherlands

The National Reference Laboratory for classical swine fever (CSF) virus in the Netherlands examined more than two million samples for CSF virus or serum antibody during the CSF epizootic of 1997–1998. The immense amount of samples and the prevalence of border disease (BD) virus and bovine viral diarrhoea (BVD) virus infections in Dutch pig herds necessitated the diagnostic efforts of the laboratory to be focused on generating CSF specific test results throughout the eradication campaign.

Detection of 82% of the 429 outbreaks was achieved through the combined use of a direct immunofluorescence and peroxidase assay (FAT/IPA) with samples (tonsils) collected from clinically-suspected pigs. This suggests that in the majority of the outbreaks, the pigs had clinical signs that were recognised by the farmer and/or veterinarians, indicating the presence of CSF virus in a pig herd. A positive diagnosis of 74% of all the tissue samples (tonsils) collected at infected pig holdings was established by FAT. More than 140,000 heparinised blood samples were examined by virus isolation, resulting in the detection of 4.5% of the infected herds. CSF virus was isolated in approximately 29% of all the blood samples collected from pigs at infected or suspected farms.

Several serological surveys — each done within a different framework — led to the detection of 13.5% of the total number of outbreaks. The detection of CSF virus antibody in serum was carried out by semi-automated blocking ELISA. Approximately 28.5% of the sera which reacted in the ELISA were classified as CSF virus-neutralising antibody positive and 26.5% as positive for other pestiviruses following the virus neutralisation test (VNT).

We concluded that two of the CSF laboratory diagnostic methods described were determinative in the eradication campaign: first, the FAT for the screening of diseased pigs; and second, the ELISA and VNT when millions of predominantly healthy pigs needed to be screened for the presence of CSF serum antibody. Decision-making on the basis of results generated by either method can, however, be seriously hindered when samples are examined from pig herds with a high prevalence of non-CSF pestiviruses.     

Defining output-based standards to achieve and maintain tuberculosis freedom in farmed deer, with reference to member states of the European Union

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.