[BVD situation on farms with high serological values in the tank milk - individual animals have a greater influence than previously known]

INTRODUCTION

Bovine virus diarrhea (BVD) is an economically important disease in cattle. Switzerland started an eradication program in 2008. After the initial virological examination of all cattle followed by the examination of all newborn calves, the BVD prevalence in 2012 was low enough to start serological monitoring. An unusually high number of increased tank milk values were observed during this serological monitoring in autumn 2019. No seropositive animals were found on most farms in the follow-up cattle group testing. The present study was designed in the form of a multiple case report to better assess the BVD situation in a herd after a serological tank milk result above the cut-off value. The tank milk of 13 farms with serological values above the cut-off value from the analyses in autumn 2019 was examined again with two different ELISA tests in spring 2020. In addition, at the same time blood samples were taken to obtain serological values of all adult cattle on the farm. The results of the two tests that were used to examine the tank milk samples correlated well with each another. The results of the tank milk serology showed a low correlation with the seroprevalence in the lactating cows, but no correlation with the seroprevalence of all adult animals in the herd. A single seropositive animal had a major influence on the results of the tank milk serology in some herds. In addition, correct tank milk sampling must be ensured because the representativeness of the tank milk sample is decisive for a meaningful result for the investigated farm. If the result of the tank milk test is above the cut-off value, the examination of a group of cattle is still the best way to identify a current infection in a herd.

Eradication of Bovine Viral Diarrhoea (BVD) in Cattle in Switzerland: Lessons Taught by the Complex Biology of the Virus

Bovine viral diarrhoea virus (BVDV) and related ruminant pestiviruses occur worldwide and cause considerable economic losses in livestock and severely impair animal welfare. Switzerland started a national mandatory control programme in 2008 aiming to eradicate BVD from the Swiss cattle population. The peculiar biology of pestiviruses with the birth of persistently infected (PI) animals upon in utero infection in addition to transient infection of naïve animals requires vertical and horizontal transmission to be taken into account. Initially, every animal was tested for PI within the first year, followed by testing for the presence of virus in all newborn calves for the next four years. Prevalence of calves being born PI thus diminished substantially from around 1.4% to <0.02%, which enabled broad testing for the virus to be abandoned and switching to economically more favourable serological surveillance with vaccination being prohibited. By the end of 2020, more than 99.5% of all cattle farms in Switzerland were free of BVDV but eliminating the last remaining PI animals turned out to be a tougher nut to crack. In this review, we describe the Swiss BVD eradication scheme and the hurdles that were encountered and still remain during the implementation of the programme. The main challenge is to rapidly identify the source of infection in case of a positive result during antibody surveillance, and to efficiently protect the cattle population from re-infection, particularly in light of the endemic presence of the related pestivirus border disease virus (BDV) in sheep. As a consequence of these measures, complete eradication will (hopefully) soon be achieved, and the final step will then be the continuous documentation of freedom of disease.

Development of Reporting Guidelines for Animal Health Surveillance-AHSURED

With the current trend in animal health surveillance toward risk-based designs and a gradual transition to output-based standards, greater flexibility in surveillance design is both required and allowed. However, the increase in flexibility requires more transparency regarding surveillance, its activities, design and implementation. Such transparency allows stakeholders, trade partners, decision-makers and risk assessors to accurately interpret the validity of the surveillance outcomes. This paper presents the first version of the Animal Health Surveillance Reporting Guidelines (AHSURED) and the process by which they have been developed. The goal of AHSURED was to produce a set of reporting guidelines that supports communication of surveillance activities in the form of narrative descriptions. Reporting guidelines come from the field of evidence-based medicine and their aim is to improve consistency and quality of information reported in scientific journals. They usually consist of a checklist of items to be reported, a description/definition of each item, and an explanation and elaboration document. Examples of well-reported items are frequently provided. Additionally, it is common to make available a website where the guidelines are documented and maintained. This first version of the AHSURED guidelines consists of a checklist of 40 items organized in 11 sections (i.e., surveillance system building blocks), which is available as a wiki at https://github.com/SVA-SE/AHSURED/wiki. The choice of a wiki format will allow for further inputs from surveillance experts who were not involved in the earlier stages of development. This will promote an up-to-date refined guideline document.

Cost distribution of bluetongue surveillance and vaccination programmes in Austria and Switzerland (2007-2016)

Bluetongue virus (BTV) is an emerging transboundary disease in Europe, which can cause significant production losses among ruminants. The analysis presented here assessed the costs of BTV surveillance and vaccination programmes in Austria and Switzerland between 2007 and 2016. Costs were compared with respect to time, type of programme, geographical area and who was responsible for payment. The total costs of the BTV vaccination and surveillance programmes in Austria amounted to €23.6 million, whereas total costs in Switzerland were €18.3 million. Our analysis demonstrates that the costs differed between years and geographical areas, both within and between the two countries. Average surveillance costs per animal amounted to approximately €3.20 in Austria compared with €1.30 in Switzerland, whereas the average vaccination costs per animal were €6.20 in Austria and €7.40 in Switzerland. The comparability of the surveillance costs is somewhat limited, however, due to differences in each nation’s surveillance (and sampling) strategy. Given the importance of the export market for cattle production, investments in such programmes are more justified for Austria than for Switzerland. The aim of the retrospective assessment presented here is to assist veterinary authorities in planning and implementing cost-effective and efficient control strategies for emerging livestock diseases.

Evidence for Emergency Vaccination Having Played a Crucial Role to Control the 1965/66 Foot-and-Mouth Disease Outbreak in Switzerland

Foot-and-mouth disease (FMD) is a highly contagious disease that caused several large outbreaks in Europe in the last century. The last important outbreak in Switzerland took place in 1965/66 and affected more than 900 premises and more than 50,000 animals were slaughtered. Large-scale emergency vaccination of the cattle and pig population has been applied to control the epidemic. In recent years, many studies have used infectious disease models to assess the impact of different disease control measures, including models developed for diseases exotic for the specific region of interest. Often, the absence of real outbreak data makes a validation of such models impossible. This study aimed to evaluate whether a spatial, stochastic simulation model (the Davis Animal Disease Simulation model) can predict the course of a Swiss FMD epidemic based on the available historic input data on population structure, contact rates, epidemiology of the virus, and quality of the vaccine. In addition, the potential outcome of the 1965/66 FMD epidemic without application of vaccination was investigated. Comparing the model outcomes to reality, only the largest 10% of the simulated outbreaks approximated the number of animals being culled. However, the simulation model highly overestimated the number of culled premises. While the outbreak duration could not be well reproduced by the model compared to the 1965/66 epidemic, it was able to accurately estimate the size of the area infected. Without application of vaccination, the model predicted a much higher mean number of culled animals than with vaccination, demonstrating that vaccination was likely crucial in disease control for the Swiss FMD outbreak in 1965/66. The study demonstrated the feasibility to analyze historical outbreak data with modern analytical tools. However, it also confirmed that predicted epidemics from a most carefully parameterized model cannot integrate all eventualities of a real epidemic. Therefore, decision makers need to be aware that infectious disease models are useful tools to support the decision-making process but their results are not equal valuable as real observations and should always be interpreted with caution.

Evaluation of farm-level parameters derived from animal movements for use in risk-based surveillance programmes of cattle in Switzerland

Background

This study focused on the descriptive analysis of cattle movements and farm-level parameters derived from cattle movements, which are considered to be generically suitable for risk-based surveillance systems in Switzerland for diseases where animal movements constitute an important risk pathway.

Methods

A framework was developed to select farms for surveillance based on a risk score summarizing 5 parameters. The proposed framework was validated using data from the bovine viral diarrhoea (BVD) surveillance programme in 2013.

Results

A cumulative score was calculated per farm, including the following parameters; the maximum monthly ingoing contact chain (in 2012), the average number of animals per incoming movement, use of mixed alpine pastures and the number of weeks in 2012 a farm had movements registered. The final score for the farm depended on the distribution of the parameters. Different cut offs; 50, 90, 95 and 99 %, were explored. The final scores ranged between 0 and 5. Validation of the scores against results from the BVD surveillance programme 2013 gave promising results for setting the cut off for each of the five selected farm level criteria at the 50th percentile. Restricting testing to farms with a score ≥ 2 would have resulted in the same number of detected BVD positive farms as testing all farms, i.e., the outcome of the 2013 surveillance programme could have been reached with a smaller survey.

Conclusions

The seasonality and time dependency of the activity of single farms in the networks requires a careful assessment of the actual time period included to determine farm level criteria. However, selecting farms in the sample for risk-based surveillance can be optimized with the proposed scoring system. The system was validated using data from the BVD eradication program. The proposed method is a promising framework for the selection of farms according to the risk of infection based on animal movements.

Economic comparison of the monitoring programmes for bluetongue vectors in Austria and Switzerland

With the bluetongue virus serotype 8 (BTV-8) outbreak in 2006, vector monitoring programmes (according to EU regulation 1266/2007) were implemented by European countries to obtain information on the spatial distribution of vectors and the vector-free period. This study investigates the vector monitoring programmes in Austria and Switzerland by performing a retrospective cost analysis for the period 2006–2010. Two types of costs were distinguished: costs financed directly via the national bluetongue programmes and costs contributed in-kind by the responsible institutions and agricultural holdings. The total net costs of the monitoring programme in Austria amounted to €1,415,000, whereby in Switzerland the costs were valued at €94,000. Both countries followed the legislation complying with requirements, but differed in regard to sampling frequency, number of trap sites and sampling strategy. Furthermore, the surface area of Austria is twice the area of Switzerland although the number of ruminants is almost the same in both countries. Thus, for comparison, the costs were normalised with regard to the sampling frequency and the number of trap sites. Resulting costs per trap sample comprised €164 for Austria and €48 for Switzerland. In both countries, around 50 per cent of the total costs can be attributed to payments in-kind. The benefit of this study is twofold: first, veterinary authorities may use the results to improve the economic efficiency of future vector monitoring programmes. Second, the analysis of the payment in-kind contribution is of great importance to public authorities as it makes the available resources visible and demonstrates how they have been used.

Cost and sensitivity of on-farm versus slaughterhouse surveys for prevalence estimation and substantiating freedom from disease

Within the framework of Swiss surveillance for epizootic diseases, dairy cattle are sampled using bulk tank milk while non-dairy cattle are sampled on the farm. The latter method is costly, time-demanding and dangerous for the personnel. However, slaughterhouses could be an alternative sampling point for this population. To assess the cost-effectiveness and sensitivity of such an approach, surveillance using slaughterhouse sampling was modelled with data from the 2012 Swiss animal movement database (AMD). We simulated a cross-sectional study for bluetongue (BT), and surveillance programmes to substantiate freedom from infectious bovine rhinotracheitis (IBR) and enzootic bovine leucosis (EBL) (combined) to compare the outcome of random on-farm sampling versus slaughterhouse sampling. We found that, under Swiss conditions, slaughterhouse sampling results in low herd-level sensitivities because animals are sent by owners to slaughter individually and not in large groups, restricting the number of samples per herd. This makes slaughterhouse sampling inappropriate for prevalence surveys at the herd-level. However, for prevalence surveys at the animal-level and for substantiation of freedom from disease, slaughterhouse surveillance is equally or more cost-efficient than on-farm sampling.

Data management systems for the bovine viral diarrhoea eradication programme in Switzerland

A programme to eradicate bovine viral diarrhoea was launched in Switzerland in 2008 with the aim of eradicating the causal virus. During the first year of the programme, the entire population of 1.6 million cattle were tested for the presence of the virus; in the following three years an additional 1.8 million calves were tested. The complexity of information generated during the eradication programme, together with a tight schedule, made computerised data management a necessity. To organise, coordinate and supervise the programme, extensions were made to the computerised information system ISVet, of the Swiss Veterinary Service, which provides automated documents for both the Veterinary Service and private veterinarians. Specific data are accessible by user groups via the BVD-Web platform, ISVet and the Swiss animal movement database. The functionalities of the structure and the reports needed to control the progress of the programme are described in detail. The authors also discuss the major advantages, disadvantages and pitfalls when planning an eradication programme using a national centralised database over a distributed computer network.

Serosurveillance of Schmallenberg virus in Switzerland using bulk tank milk samples

Infections with Schmallenberg virus (SBV), a novel Orthobunyavirus transmitted by biting midges, can cause abortions and malformations of newborns and severe symptoms in adults of domestic and wild ruminants. Understanding the temporal and spatial distribution of the virus in a certain territory is important for the control and prevention of the disease. In this study, seroprevalence of antibodies against SBV and the spatial spread of the virus was investigated in Swiss dairy cattle applying a milk serology technique on bulk milk samples. The seroprevalence in cattle herds was significantly higher in December 2012 (99.5%) compared to July 2012 (19.7%). This high between-herd seroprevalence in cattle herds was observed shortly after the first detection of viral infections. Milk samples originating from farms with seropositive animals taken in December 2012 (n=209; mean 160%) revealed significantly higher S/P% ratios than samples collected in July 2012 (n=48; mean 103.6%). This finding suggests a high within-herd seroprevalence in infected herds which makes testing of bulk tank milk samples for the identification farms with past exposures to SBV a sensitive method. It suggests also that within-herd transmission followed by seroconversion still occurred between July and December. In July 2012, positive bulk tank milk samples were mainly restricted to the western part of Switzerland whereas in December 2012, all samples except one were positive. A spatial analysis revealed a separation of regions with and without positive farms in July 2012 and no spatial clustering within the regions with positive farms. In contrast to the spatial dispersion of bluetongue virus, a virus that is also transmitted by Culicoides midges, in 2008 in Switzerland, the spread of SBV occurred from the western to the eastern part of the country. The dispersed incursion of SBV took place in the western part of Switzerland and the virus spread rapidly to the remaining territory. This spatial pattern is consistent with the hypothesis that transmission by Culicoides midges was the main way of spreading.

Prevalence of antibodies against bluetongue virus serotype 8 in bulk-tank milk samples from dairy cattle herds located in risk areas for bluetongue virus transmission after a vaccination programme in Switzerland

Switzerland had been affected by the bluetongue virus serotype 8 (BTV-8) epidemic in Europe in the years 2007 to 2009. After three years of mandatory vaccination and comprehensive surveillance, Switzerland showed to be free of BTV-8 in 2012. In the future Elisa testing of bulk-tank milk (BTM) samples as a very sensitive and cost-effective method should be used for the surveillance of all serotypes of BTV. To determine the prevalence of seropositive herds, BTM from 240 cattle herds was sampled in July 2012. The results showed an apparent seroprevalence of 98.7% in the investigated dairy herds. Most plausible, the high prevalence was caused by the vaccination campaigns rather than by infections with BTV-8. In the outbreak the cumulative number of BTV-8 cases in Switzerland had been 75.Thus it is very likely that the used inactivated vaccines induced long-term antibody titres. Due to the high seroprevalence, investigating for BT-antibodies cannot be used for early recognition of a new introduction of BTV at the moment. Nonetheless, testing of BTM samples is appropriate for an annual evaluation of the seroprevalence and especially as an instrument for early recognition for incursions as soon as the antibody prevalence declines.To determine this decline the BTM testing scheme should be conducted each year as described in this work.

Seroprevalence of hepatitis E virus in domestic pigs and wild boars in Switzerland

Hepatitis E is considered an emerging human viral disease in industrialized countries. Studies from Switzerland report a human seroprevalence of hepatitis E virus (HEV) of 2.6-21%, a range lower than in adjacent European countries. The aim of this study was to determine whether HEV seroprevalence in domestic pigs and wild boars is also lower in Switzerland and whether it is increasing and thus indicating that this zoonotic viral infection is emerging. Serum samples collected from 2,001 pigs in 2006 and 2011 and from 303 wild boars from 2008 to 2012 were analysed by ELISA for the presence of HEV-specific antibodies. Overall HEV seroprevalence was 58.1% in domestic pigs and 12.5% in wild boars. Prevalence in domestic pigs was significantly higher in 2006 than in 2011. In conclusion, HEV seroprevalence in domestic pigs and wild boars in Switzerland is comparable with the seroprevalence in other countries and not increasing. Therefore, prevalence of HEV in humans must be related to other factors than prevalence in pigs or wild boars.

Evaluation of the benefit of emergency vaccination in a foot-and-mouth disease free country with low livestock density

Foot-and-mouth disease (FMD) is highly contagious and one of the most economically devastating diseases of cloven-hoofed animals. Scientific-based preparedness about how to best control the disease in a previously FMD-free country is therefore essential for veterinary services. The present study used a spatial, stochastic epidemic simulation model to compare the effectiveness of emergency vaccination with conventional (non-vaccination) control measures in Switzerland, a low-livestock density country. Model results revealed that emergency vaccination with a radius of 3 km or 10 km around infected premises (IP) did not significantly reduce either the cumulative herd incidence or epidemic duration if started in a small epidemic situation where the number of IPs is still low. However, in a situation where the epidemic has become extensive, both the cumulative herd incidence and epidemic duration are reduced significantly if vaccination were implemented with a radius of 10 km around IPs. The effect of different levels of conventional strategy measures was also explored for the non-vaccination strategy. It was found that a lower compliance level of farmers for movement restrictions and delayed culling of IPs significantly increased both the cumulative IP incidence and epidemic duration. Contingency management should therefore focus mainly on improving conventional strategies, by increasing disease awareness and communication with stakeholders and preparedness of culling teams in countries with a livestock structure similar to Switzerland; however, emergency vaccination should be considered if there are reasons to believe that the epidemic may become extensive, such as when disease detection has been delayed and many IPs are discovered at the beginning of the epidemic.

Putative risk factors for infections with Toggenburg Orbivirus in goat herds in Southern Switzerland (Canton of Ticino)

Toggenburg Orbivirus (TOV), only detected in goats, has been described as a member of the Bluetongue virus (BTV) serogroup. The transmission pathway of the virus seems different from other Bluetongue viruses (BTVs). The objective of this study was to explore risk factors, especially the influence of alpine pasture and the presence of other livestock species, for the presence of TOV infected goats on farms. Between February 2008 and September 2009, blood samples were collected and analyzed for TOV and hereupon a total of 60 goat farm owners (37 TOV-positive and 23 TOV-negative holdings) were interviewed. Additionally, goatlings were tested for TOV by rRT-PCR prior and after alpine pasture in 2009. These goatlings were positive for TOV only after the alpine pasture. The final logistic regression model included: "exposure to goats from other farms" (OR=10.12, p=0.007), "exposure of the goats to red deer" (OR=4.79, p=0.04) and "exposure to sheep from other farms" (OR=0.05, p=0.002). These variables do not implicitly include direct contact, and the findings are only vaguely indicative for a contact-driven transmission. Furthermore, it is likely that they are only associated with, and thus indicative for, an unknown risk factor associated with alpine pasture not measured in the study. The results of this screening study do not indicate iatrogenic transmission pathways as a main transmission mode and stimulate the formulation of hypotheses on the origin, the transmission pathway and other host species for TOV. Especially, the involvement of an insect vector in transmission on alpine pasture and the relevance of vertical transmission are to be clarified.

Using scenario tree modelling for targeted herd sampling to substantiate freedom from disease

Background

In order to optimise the cost-effectiveness of active surveillance to substantiate freedom from disease, a new approach using targeted sampling of farms was developed and applied on the example of infectious bovine rhinotracheitis (IBR) and enzootic bovine leucosis (EBL) in Switzerland. Relevant risk factors (RF) for the introduction of IBR and EBL into Swiss cattle farms were identified and their relative risks defined based on literature review and expert opinions. A quantitative model based on the scenario tree method was subsequently used to calculate the required sample size of a targeted sampling approach (TS) for a given sensitivity. We compared the sample size with that of a stratified random sample (sRS) with regard to efficiency.

Results

The required sample sizes to substantiate disease freedom were 1,241 farms for IBR and 1,750 farms for EBL to detect 0.2% herd prevalence with 99% sensitivity. Using conventional sRS, the required sample sizes were 2,259 farms for IBR and 2,243 for EBL. Considering the additional administrative expenses required for the planning of TS, the risk-based approach was still more cost-effective than a sRS (40% reduction on the full survey costs for IBR and 8% for EBL) due to the considerable reduction in sample size.

Conclusions

As the model depends on RF selected through literature review and was parameterised with values estimated by experts, it is subject to some degree of uncertainty. Nevertheless, this approach provides the veterinary authorities with a promising tool for future cost-effective sampling designs.

Spatial analysis of bluetongue cases and vaccination of Swiss cattle in 2008 and 2009

Bluetongue (BT) is a vector-borne viral disease of ruminants. The infection is widespread globally with major implications for international animal trade and production. In 2006, BT virus serotype 8 (BTV-8) was encountered in Europe for the first time, causing extensive production losses and death in susceptible livestock. Following the appearance of BTV-8 in Switzerland in 2007, a compulsory vaccination programme was launched in the subsequent year. Due to social factors and difficulties to reach animals on high pasture, the regional vaccination coverage varied across the country in both 2008 and 2009. In this study, the effect of vaccination on the spatial occurrence of BTV-8 and the associated relative disease risk in Switzerland in 2008 and 2009 were investigated by a spatial Bayesian hierarchical approach. Bayesian posterior distributions were obtained by integrated nested Laplace approximations, a promising alternative to commonly used Markov chain Monte Carlo methods. The number of observed BTV-8 outbreaks in Switzerland decreased notably from 2008 to 2009. However, only a non-significant association between vaccination coverage and the probability of a spatial unit being infected with BTV-8 was identified using the model developed for this study. The relative disease risk varied significantly across the country, with a higher relative risk of BTV-8 infection in western and north-western Switzerland where environmental conditions are more suitable for vector presence and viral transmission. Examination of the spatial correlation between disease occurrence, control measures and associated ecological factors can be valuable in the evaluation and development of disease control programmes, allowing prioritisation of areas with a high relative risk of disease.

[Infectious bovine rhinotracheitis (IBR) in the canton of Jura: an epidemiological outbreak investigation]

Following an abortion in a beef herd in the summer of 2009, three outbreaks of infectious bovine rhinotracheitis (IBR) were diagnosed in the cantons of Jura and Neuchatel. An epidemiological outbreak investigation was conducted with the aims to identify the source of introduction of the bovine herpes virus 1 (BoHV-1) into the affected herds and to prevent further spread of the disease. The attack rates in the three outbreak farms were 0.89, 0.28 and 0, respectively. BoHV-1 could be isolated from nasal swabs of two animals originating from one of the affected farms. Comparative restriction enzyme analysis revealed slight differences between the isolates of the two animals, but a high similarity to previous BoHV-1 isolates from the canton of Jura, as well as to a French BoHV-1 isolate. This IBR outbreak has shown the importance of reporting and analyzing abortions. The current disease outbreaks recall the main risk factors for the spread of IBR in Switzerland: purchase and movement of bovines and semen of often unknown IBR status.

Milk concentration improves Bluetongue antibody detection by use of an indirect ELISA

A national Bluetongue antibody surveillance in cattle through bulk milk was conducted in Switzerland between July 2007 and June 2008. Using ID Screen Bluetongue Milk ELISA (ID VET, Montpellier, France), samples from 15 out of 210 dairy farms at least once gave a positive result. In only three of these herds bluetongue positive animals were found. Therefore, specificity for bulk milk was not as good as expected and when individual milk samples were tested, it was even lower. As further investigations of positive results were time-consuming and no other ELISA was available at that time, we aimed at discriminating false from true positive samples with a confirmatory test using a protein precipitation method followed by retesting with the same ELISA. Additionally, we examined whether testing of single milk samples can reliably be used to assess status of cows, and whether sampling at the beginning or at the end of milking, as well as freezing and thawing of the milk could influence the performance of the test. Screening with ID VET milk ELISA and confirmatory testing after protein precipitation yielded a clear increase of specificity without any loss of sensitivity in both bulk and single milk samples. This testing scheme allowed minimizing follow-up investigations by blood testing. Antibody levels in plasma and milk showed a good correlation. Tested by logistic regression, none of the possible influencing factors (time point of sample collection, freezing, or milk content of the samples) had a significant influence on the test performance.

Surveillance and simulation of bovine spongiform encephalopathy and scrapie in small ruminants in Switzerland

BACKGROUND

After bovine spongiform encephalopathy (BSE) emerged in European cattle livestock in 1986 a fundamental question was whether the agent established also in the small ruminants' population. In Switzerland transmissible spongiform encephalopathies (TSEs) in small ruminants have been monitored since 1990. While in the most recent TSE cases a BSE infection could be excluded, for historical cases techniques to discriminate scrapie from BSE had not been available at the time of diagnosis and thus their status remained unclear. We herein applied state-of-the-art techniques to retrospectively classify these animals and to re-analyze the affected flocks for secondary cases. These results were the basis for models, simulating the course of TSEs over a period of 70 years. The aim was to come to a statistically based overall assessment of the TSE situation in the domestic small ruminant population in Switzerland.

RESULTS

In sum 16 TSE cases were identified in small ruminants in Switzerland since 1981, of which eight were atypical and six were classical scrapie. In two animals retrospective analysis did not allow any further classification due to the lack of appropriate tissue samples. We found no evidence for an infection with the BSE agent in the cases under investigation. In none of the affected flocks, secondary cases were identified. A Bayesian prevalence calculation resulted in most likely estimates of one case of BSE, five cases of classical scrapie and 21 cases of atypical scrapie per 100'000 small ruminants. According to our models none of the TSEs is considered to cause a broader epidemic in Switzerland. In a closed population, they are rather expected to fade out in the next decades or, in case of a sporadic origin, may remain at a very low level.

CONCLUSIONS

In summary, these data indicate that despite a significant epidemic of BSE in cattle, there is no evidence that BSE established in the small ruminant population in Switzerland. Classical and atypical scrapie both occur at a very low level and are not expected to escalate into an epidemic. In this situation the extent of TSE surveillance in small ruminants requires reevaluation based on cost-benefit analysis.

Establishing a cost-effective national surveillance system for Bluetongue using scenario tree modelling

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.

Field performance of two rapid screening tests in active surveillance of transmissible spongiform encephalopathies in small ruminants

Recently, screening tests for monitoring the prevalence of transmissible spongiform encephalopathies specifically in sheep and goats became available. Although most countries require comprehensive test validation prior to approval, little is known about their performance under normal operating conditions. Switzerland was one of the first countries to implement 2 of these tests, an enzyme-linked immunosorbent assay (ELISA) and a Western blot, in a 1-year active surveillance program. Slaughtered animals (n = 32,777) were analyzed in either of the 2 tests with immunohistochemistry for confirmation of initial reactive results, and fallen stock samples (n = 3,193) were subjected to both screening tests and immunohistochemistry in parallel. Initial reactive and false-positive rates were recorded over time. Both tests revealed an excellent diagnostic specificity (>99.5%). However, initial reactive rates were elevated at the beginning of the program but dropped to levels below 1% with routine and enhanced staff training. Only those in the ELISA increased again in the second half of the program and correlated with the degree of tissue autolysis in the fallen stock samples. It is noteworthy that the Western blot missed 1 of the 3 atypical scrapie cases in the fallen stock, indicating potential differences in the diagnostic sensitivities between the 2 screening tests. However, an estimation of the diagnostic sensitivity for both tests on field samples remained difficult due to the low disease prevalence. Taken together, these results highlight the importance of staff training, sample quality, and interlaboratory comparison trials when such screening tests are implemented in the field.

Atypical scrapie in a Swiss goat and implications for transmissible spongiform encephalopathy surveillance

Different types of transmissible spongiform encephalopathies (TSEs) affect sheep and goats. In addition to the classical form of scrapie, both species are susceptible to experimental infections with the bovine spongiform encephalopathy (BSE) agent, and in recent years atypical scrapie cases have been reported in sheep from different European countries. Atypical scrapie in sheep is characterized by distinct histopathologic lesions and molecular characteristics of the abnormal scrapie prion protein (PrP(sc)). Characteristics of atypical scrapie have not yet been described in detail in goats. A goat presenting features of atypical scrapie was identified in Switzerland. Although there was no difference between the molecular characteristics of PrP(sc) in this animal and those of atypical scrapie in sheep, differences in the distribution of histopathologic lesions and PrP(sc) deposition were observed. In particular the cerebellar cortex, a major site of PrP(sc) deposition in atypical scrapie in sheep, was found to be virtually unaffected in this goat. In contrast, severe lesions and PrP(sc) deposition were detected in more rostral brain structures, such as thalamus and midbrain. Two TSE screening tests and PrP(sc) immunohistochemistry were either negative or barely positive when applied to cerebellum and obex tissues, the target samples for TSE surveillance in sheep and goats. These findings suggest that such cases may have been missed in the past and could be overlooked in the future if sampling and testing procedures are not adapted. The epidemiological and veterinary public health implications of these atypical cases, however, are not yet known.

Comparison of the historic recycling risk for BSE in three European countries by calculating the basic reproduction ratio R0

A deterministic model of BSE transmission is used to calculate the R(0) values for specific years of the BSE epidemics in the United Kingdom (UK), the Netherlands (NL), and Switzerland (CH). In all three countries, the R(0) values decreased below 1 after the introduction of a ban on feeding meat and bone meal (MBM) to ruminants around the 1990s. A variety of additional measures against BSE led to further decrease of R(0) to about 0.06 in the years around 1998. The calculated R(0) values were consistent with the observations made on the surveillance results for UK, but were partially conflicting with the surveillance results for NL and CH. There was evidence for a dependency of the BSE epidemic in NL and CH from an infection source not considered in the deterministic transmission model. Imports of MBM and feed components can be an explanation for this discrepancy, and the importance of imports for these observations is discussed.

Use of mapping and statistical modelling for the prediction of bluetongue occurrence in Switzerland based on vector biology

Due to the spread of bluetongue (BT) in Europe in the last decade, a sentinel surveillance programme was initiated for Switzerland in 2003, consisting of serological sampling of sentinel cattle tested for BT virus antibodies, as well as entomological trapping of Culicoides midges from June until October. The aim of this study was to create a 'suitability map' of Switzerland, indicating areas of potential disease occurrence based on the biological parameters of Obsoletus Complex habitat. Data on Culicoides catches from insect traps together with various environmental parameters were recorded and analysed. A multiple regression analysis was performed to determine correlation between the environmental conditions and vector abundance. Meteorological data were collected from 50 geo-referenced weather stations across Switzerland and maps of temperature, precipitation and altitude were created. A range of values of temperature, precipitation and altitude influencing vector biology were obtained from the literature. The final combined map highlighted areas in Switzerland which are most suitable for vector presence, hence implying a higher probability of disease occurrence given the presence of susceptible animals. The results confirmed the need for an early warning system for the surveillance of BT disease and its vectors in Switzerland.

A stochastic simulation model to determine the sample size of repeated national surveys to document freedom from bovine herpesvirus 1 (BoHV-1) infection

Background

International trade regulations require that countries document their livestock's sanitary status in general and freedom from specific infective agents in detail provided that import restrictions should be applied. The latter is generally achieved by large national serological surveys and risk assessments. The paper describes the basic structure and application of a generic stochastic model for risk-based sample size calculation of consecutive national surveys to document freedom from contagious disease agents in livestock.

Methods

In the model, disease spread during the time period between two consecutive surveys was considered, either from undetected infections within the domestic population or from imported infected animals. The @Risk model consists of the domestic spread in-between two national surveys; the infection of domestic herds from animals imported from countries with a sanitary status comparable to Switzerland or lower sanitary status and the summary sheet which summed up the numbers of resulting infected herds of all infection pathways to derive the pre-survey prevalence in the domestic population. Thereof the pre-survey probability of freedom from infection and required survey sample sizes were calculated. A scenario for detection of infected herds by general surveillance was included optionally.

Results

The model highlights the importance of residual domestic infection spread and characteristics of different import pathways. The sensitivity analysis revealed that number of infected, but undetected domestic herds and the multiplicative between-survey-spread factor were most correlated with the pre-survey probability of freedom from infection and the resulting sample size, respectively. Compared to the deterministic pre-cursor model, the stochastic model was therefore more sensitive to the previous survey's results. Undetected spread of infection in the domestic population between two surveys gained more importance than infection through animals of either import pathway.

Conclusion

The model estimated the pre-survey probability of freedom from infection accurately as was shown in the case of infectious bovine rhinotracheitis (IBR). With this model, a generic tool becomes available which can be adapted to changing conditions related to either importing or exporting countries.

Evaluation of the effectiveness of selected measures against bovine spongiform encephalopathy (BSE) in Switzerland by use of the basic reproduction ratio R0

The effectiveness of two measures against Bovine Spongiform Encephalopathy (BSE), the compulsory processing of animal by products to meat and bone meal (MBM) at 133 degrees C under 3 bars of pressure for 20 minutes in February 1993 and the exclusion of fallen stock, heads with eyes and spinal cord of cattle older than 30 month from MBM production in April 1996, was evaluated in a process model. The transmission of BSE by calculation of the basic reproduction ratio R0 was modelled. The results were verified by use of a cohort model, based on observed surveillance data. Prior to 1990, before the ban of feeding MBM to ruminants, R0, as calculated in the process model, was above 1, coherent with a slowly progressing BSE epidemic. Since 1991, values of R0 were low at 0.06. The corresponding R0 values derived from the cohort model were higher, the lowest value 0.13 calculated for 1996. Given such low R0 values, the epidemic should have died out. Additionally, no influence of the two measures was obvious at that time given the low level of R0. The discrepancy between the results of the two models is evidence for a dependency of the BSE epidemic from an infection source not considered in the process model. This infection source is most likely importation of feed ingredients and MBM.

BSE, feed and cattle in Switzerland: Is there a spatial relation?

Cross-contamination of cattle feed with meat and bone meal (MBM) allowed in feed for other species is regarded as the current hypothesis for the infection pathway of Bovine Spongiform Encephalopathy (BSE) cases occurring after the implementation of a ban on feeding MBM to cattle. This study was aimed at establishing a spatial relation between BSE cases in Switzerland and the findings of MBM in cattle feed. A cluster analysis and a cohort study were performed. Two hundred sixteen BSE cases born after December 1990 and detected until August 1st 2005, screening data of 504 feed producers between 1996 and 2001 and population data from the Swiss 2001 cattle census were included. The cluster analysis showed feed producer, positive for MBM contaminations in cattle feed, as possible cluster centres for BSE cases. In the cohort study, farms within a radius of 2 and 10 km around positive feed producers showed significantly higher odds to have a BSE case than the control group. The odds ratio and its 95% confidence interval were 2.23 (1.26-3.93) for the 2 km radius and 1.38 (1-1.9) for the 10 km radius. The results provide evidence for a spatial relation between cross-contamination and BSE occurrence. These findings support the hypothesis of cross-contamination to be an important route for BSE transmission after a feed ban.

Cases of bovine spongiform encephalopathy born in Switzerland before and after the ban on the use of bovine specified risk material in feed

In Switzerland there was a reduction in the number of cases of bovine spongiform encephalopathy, observed in the birth cohorts from 1995 to 1996, but no further reduction in the following birth cohorts up to 1998. From the records of 34 cases born after April 30, 1996 (BAB96) and 174 cases born before April 30, 1996 and after December 1990 (BAB90), observed up to April 30, 2004, the risk factors at the farm level, possible routes of exposure and the geographical distribution of the cases were analysed to try to explain the observations. No evidence was found for a rate of exposure other than feed. There was some evidence that the risk factors at farm level were different between the BAB90 and BAB96 cases. A large proportion of the BAB96 cases was born in cantons that had reported only a few BAB90 cases, but a small cluster of the BAB96 cases was found in a region where there had been a cluster of BAB90 cases. The spatial distribution of these cases indicated that the risk of exposure to infection had been more randomly distributed than during the period up to April 1996. Farms with mixed livestock had a higher risk of having a case born after the ban on the feeding of specified risk material in 1996. In a regression model, a trend towards an association between cases of BSE and presence of small ruminants on the farm was observed for the BAB96 cases, and the presence of pigs and the pig:cattle ratio were significant for the BAB90 cases.

Analysis of national serological surveys for the documentation of freedom from porcine reproductive and respiratory syndrome in Switzerland

Results of national serological surveys for porcine reproductive and respiratory syndrome (PRRS) conducted in Switzerland in 2001 and 2004 were analyzed. In 2001, 41,124 breeding sows from 2,540 herds out of 6,406 were sampled, and in 2004, 7,498 animals were sampled from 1,074 herds out of 5,320. All serum samples were tested for PRRS using an ELISA developed at the Institute of Virology and Immunoprophylaxis (IVI), Switzerland with a sensitivity (Se) and specificity (Sp) of 94 and 97%, respectively. Positive samples were re-tested with a commercial ELISA (IDEXX) with Se of 100% and Sp of 99%. Samples positive in the second test were confirmed with the fluorescent antibody test (FAT). A stochastic model using data from the main survey conducted in 2001 was done to verify whether the sampling scheme used could detect at least one infected herd with 99% confidence level if the herd designed prevalence was at 0.1 or 0.2%. Additionally, a Bayesian approach was conducted to calculate the post-survey probability of freedom from PRRS using data from the 2001 and 2004 surveys. A Monte Carlo simulation with 5000 iteration was run for each model. Eleven samples in 2001 and six in 2004, all from different farms, could not be conclusively confirmed as negative by the FAT. All other samples were negative. Truly infected animals and herds were not predicted by a stochastic model at the 99% confidence level and 0.1% herd prevalence using data from the 2001 survey. However, it was demonstrated that the prior probability of freedom from PRRS increased from 89.3 to 99.2% after the 2001 survey. Upon completion of the 2004 survey, the probability of freedom from PRRS reached a value of 99.7%. Based on our results, we could conclude that the pig industry in Switzerland is free of PRRS virus with this level of confidence. Restricted import activities over the last decades are a possible explanation for the continuing absence of PRRS-infection in the Swiss swine population. Import requirements defined by the pig industry minimize the risk of introduction of PRRS-infected animals in the future.

Use of an area index to retrospectively analyze the elimination of fox rabies in European countries

Oral vaccination of foxes (OVF) is a powerful tool to combat rabies in wildlife, and large parts of western Europe have been freed from rabies using this tool. Nevertheless, the success of OVF, given with the number of campaigns needed to eliminate the disease, depends on many factors. This article for the first time focuses on and assesses difference in OVF with respect to the spatial setting of vaccinated areas with time. The size of the areas vaccinated with time and the size of the overlapping area of consecutively vaccinated areas are particularly considered. In order to integrate these two aspects into one single figure, an Area Index is proposed ranging between 0 and 1. A statistical analysis indicates that the number of campaigns needed for rabies elimination significantly decreases on condition that the total rabies endemic area is consecutively treated right from the beginning of oral vaccination. Hence, from an economical and environmental point of view, vaccination areas should be selected the way that guarantees an Area Index close to 1. The concept of an Area Index, as described here, is a useful tool not only in the context of OVF, but it could also be used for other control schemes against infectious diseases in wildlife.

[Twelve years experience in Switzerland: Quo vadis BSE?]

When the first BSE case in Switzerland became public in 1990, it precipitated a chain of events with many diverse consequences. During this time, we continued to gain knowledge, implement new measures, and make practical recommendations in order to attempt to control the disease. This was made possible by the establishment of a huge data base containing information on all cases of BSE that had been diagnosed in Switzerland, as well as by the use of information from experiences in other countries. Analysis of these data allowed the sequence of events to be described temporally and spatially and an epidemiological evaluation to be made. These data also allow predictions to be made regarding the future of BSE in Switzerland, although these predictions must be interpreted cautiously.

[Geographic distribution of BSE in Switzerland]

Visual evaluation of the occurrence of BAB (born after ban) cases pointed towards spatial clustering. Therefore a statistical analysis of spatial case clustering was conducted using GIS technology. In the initial analysis, all 376 cases (135 BAB, 241 BBB/born before ban) diagnosed through mid of March 2001 were investigated using the spatial scan statistic. Two clusters of BBB cases were identified in the western part of Switzerland, and two clusters of BAB cases in the eastern part. Epidemiological investigations performed within the BAB clusters showed an increased pig density in these cluster regions. Pig density is considered an indicator for the probability of contamination of cattle feed with feed containing meat-and-bone meal that is intended for other species (cross-contamination). In an updated cluster analysis including all cases reported until end of June 2002 (data set B), clusters were identified in the same regions as previously. It was shown that the BAB clusters occurred in different time periods. The small scale differences in cluster size and location are discussed, with an emphasis on the implications for following epidemiological investigations.