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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.

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.

The outbreak of foot and mouth disease (FMD) in Great Britain in 2001 let to discussions and especially emergency vaccination was deemed as an alternative to the culling of vast numbers of healthy animals. The project emergency vaccination for FMD in Switzerland was conducted to compare the effectiveness of conventional control strategies during a FMD outbreak alone and with ring vaccination of 3 km and 10 km, respectively. The results of this project showed that emergency vaccination conducted at the beginning of an epidemic was not favorable compared to conventional disease control strategy in Switzerland. In case of an advanced FMD epidemic, a 10 km ring vaccination could support the disease control in a positive way. However, the goal of emergency vaccination to save animal live can hardly be achieved due to actual legal basis and the consequent restriction measures within vaccination zones which will lead to welfare culling.

The neutralization test is used commonly for quantifying neutralizing antibodies and for distinguishing among different virus serotypes (serotyping). Due to the co-circulation of multiple serotypes of Bluetongue virus (BTV), the neutralization test has become an important surveillance method in Europe. However, the existence of different protocols makes test standardization and interpretation of results difficult. The current paper describes the development of a neutralization test using plasma and addresses the factors critical for detection of neutralizing antibodies against BTV serotype 8 (BTV-8), such as virus propagation, stability of virus infectivity and origin of the BTV-8 strain. The results indicated that animals exposed to the Northern European BTV-8 strain developed low neutralizing antibody titers, particularly after vaccination and experimental infection. Although clearly ELISA-positive, these samples often yielded false negative results when tested by the neutralization test using the OIE recommended virus concentration of 100 TCID₅₀/50 μl. The sensitivity of the neutralization test could be improved significantly with retained specificity by using a reduced TCID₅₀ and the homologous European BTV-8 strain instead of the South African reference strain.

Blood samples and cloacal swabs from poultry were collected in 107 small chicken flocks and 62 pure-bred poultry flocks to determine their status regarding Newcastle disease virus (NDV) infection. A questionnaire emphasizing potential contacts of poultry with wild birds and management practices associated with NDV infection was completed for each flock. Additionally, 1576 wild bird carcasses of 115 different bird species were collected from hunters and taxidermists. Poultry sera and tissue fluids of wild birds were tested for NDV antibodies using a blocking ELISA. Cloacal swabs were subjected to reverse transcription polymerase chain reaction (RT-PCR) for NDV genome detection. In-herd NDV seroprevalences between 5 and 29% were found in one small chicken flock, as well as in four pure-bred poultry flocks. NDV antibody positive wild birds were found in 10.2% of all wild birds examined. Highest proportions (i.e. 15%) of positive birds per species were found among sparrowhawks, kites, tawny owls, eagle owls, barn owls, cuckoos, swifts, cormorants and grebes. No NDV genome was detected in cloacal swabs. This study suggests that buying eggs or poultry abroad and exchanging poultry within the country were factors, more important than wild birds, to explain the higher NDV seropositivity in pure-bred poultry flocks.

Im Sommer und Herbst 2011 wurden bei Milchkuhen aus dem Nordwesten der Niederlande und aus Nordrhein-Westfalen in Deutschland unspezifi sche Krankheitssymptome wie wassriger Durchfall, Fieber und Milchleistungsruckgang beobachtet, welche nach wenigen Wochen wieder abklangen. Bei den Abklarungen auf bekannte Viren konnte keine Krankheitsursache ermittelt werden. (Garigliany et al., 2012, Muskens et al., 2012). Beginnend im Dezember des folgenden Winters traten, uber ganz Deutschland und Holland verteilt, leichte bis schwerwiegende Missbildungen bei neuund totgeborenen Lammern und Kalbern auf. Die Muttertiere zeigten weder bei der Trachtigkeit noch bei der Geburt die oben beschriebenen Symptome. Unter den festgestellten Missbildungen waren Arthrogrypose, Torticollis, Skoliose, Kyphose, Brachygnathia inferior und eine unterschiedlich ausgepragte Hypoplasie des Grossund Kleinhirns und des Ruckenmarks haufi g (van den Brom et al., 2012). Im November 2011 wurde am Friedrich-Loffl er-Institut (FLI) in Deutschland eine Metagenom-Analyse der aus Plasmaproben isolierten DNA und RNA von im Oktober 2011 erkrankten Kuhen durchgefuhrt und ein fur Europa neuartiges Orthobunyavirus detektiert (Hoffmann et al., 2011). Der Erreger wurde nach der Herkunft der Proben Schmallenberg-Virus genannt. Das Virus wurde in der Folge auch in den missgebildeten Kalbern und Lammern nachgewiesen (Bilk et al., 2012). Beim neu entdeckten Virus handelt es sich um ein RNAVirus aus der Simbu-Serogruppe, welcher viele verschiedene sogenannte Arboviren (arthropod-borne viruses) angehoren. Phylogenetisch nahverwandte Viren («Shamonda-like»-Viren, z. B. Akabane), welche ebenfalls kongenitale Missbildungen und neurologische Veranderungen bei Rindern und kleinen Wiederkauern verursachen, sind in Afrika, Asien und Ozeanien schon seit Jahrzehnten bekannt (Al-Busaidy et al., 1987, Sellers et al., 1981). Die Ubertragung der Schmallenberg-Viren erfolgt durch den Stich von Gnitzen (Culicoides) und eventuell von Stechmucken sowie transplazentar (Rasmussen et al., 2012; van den Brom et al., 2012). Das Virus lasst sich nicht durch direkten Kontakt von Tier zu Tier zu ubertragen. Nach aktuellem Wissensstand sind Rinder, kleine Wiederkauer, aber auch Hirsche, Rehe, Alpakas und Muffl ons furs das Virus empfanglich (EFSA, 2012). Nach einer Infektion mit dem Schmallenberg-Virus konnen Rinder unspezifi sche Symptome wie Fieber, Durchfall, Appetitlosigkeit und einen massiven Milchleistungsruckgang bis auf die Halfte der Ausgangsleistung zeigen. Die sehr kurze Viramie dauert nur 1 bis 6 Tage, und die Symptome klingen innerhalb weniger Tage wieder ab. Bei adulten Ziegen und Schafen sind bisher nur in seltenen Fallen Symptome bekannt (Lievaart-Peterson et al., 2012). Ungefahr 10 Tage spater wurden Antikorper im Serum nachgewiesen (Hoffmann et al., 2012). Kommt es wahrend der Trachtigkeit von Rindern, Schafen und Ziegen zu einer Infektion des Muttertieres, so kann das Virus auf den Fotus ubertragen werden. Je nach Trachtigkeitsstadium kann die Infektion des Feten zu Abort, Fruhoder Totgeburt fuhren sowie schwerwiegende Missbildungen und Storungen des Zentralnervensystems verursachen. Diese Missbildungen werden als «Arthrogrypose-Hydranencephalie-Syndrom» (AHS) zusammengefasst und sind typisch fur Infektionen mit Viren aus der Simbu-Serogruppe. (OIE technical factsheet und Kurogi et al., 1977).

Recently, a new member of the Bluetongue virus (BTV) serogroup named Toggenburg Orbivirus (TOV) in goats from Switzerland has been described. The epidemiology and host range of TOV are currently unknown. Since TOV causes cross-reactions in laboratory tests used for BTV diagnosis, this study was carried out in order to determine the spatial and temporal spread of TOV. Therefore, serum samples from a national survey in goats, collected during winter and spring 2008 in Switzerland, were serologically examined. Additionally, cattle and sheep from holdings with seropositive goats were tested for the presence of viral RNA and antibodies against BTV and TOV. All goat samples analysed within routine diagnostics at the Institute of Virology and Immunoprophylaxis from 2008 to 2009 were also tested for the presence of TOV. Finally, goat sera collected 1998 in the Canton of Ticino (TI) were analysed. Although the TOV index cases had been identified in flocks north of the Alps, no additional TOV-positive herds were found by serological testing in this region. In contrast, south of the Alps, i.e. in the Canton of Ticino (TI), an apparent seroprevalence of 49% in goats was found at animal and 60% at herd level. In the eastern and western part of the Swiss Alps 15.2% and 10% of tested goats were serologically positive, respectively. A within-herd prevalence of up to 100% was found in some of the positive flocks. The positive flocks in TI were mainly found in three of the five districts, but seropositive animals were identified in each district. Certain selected seropositive flocks were investigated virologically. By RT-qPCR and genome sequencing, the presence of TOV could be confirmed in all investigated seropositive flocks. By testing the goats within routine diagnostics, TOV genome was detected in one goat showing BT-like clinical symptoms from the central Alps and in three healthy animals imported from Germany. Although 3.8% of the sheep from flocks with TOV-positive goats or in contact with these animals showed a positive antibody reaction, TOV-specific RNA was not found in any of the tested sheep and also not in cattle from flocks with TOV-positive goats. Serological and virological test results from archived Swiss goat samples collected in 1998 indicated the presence of TOV already at that time, prior to any Bluetongue disease outbreak in this part of Europe. The results of this study demonstrate that TOV is widespread in certain parts of Switzerland and suggests that this virus has been present in the goat population for at least a decade, albeit without causing any disease signs.

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.

Clinical disease of bluetongue (BT) in sheep may differ depending on breed, age and immunity of infected sheep and may also vary between serotype and strain of BT virus (BTV). Since there are no data available on the susceptibility of Swiss sheep breeds for BT, we performed experimental infection of the 4 most common Swiss sheep breeds and the highly susceptible Poll Dorset sheep with the BTV serotype 8 (BTV-8) circulating in Northern Europe since 2006. Clinical signs were assessed regarding severity, localisation, progression and time point of their appearance. The results clearly show that the Swiss sheep breeds investigated were susceptible to BTV-8 infection. They developed moderate, BT-characteristic symptoms, which were similar to those observed in Poll Dorset sheep. Regardless of breed, the majority of infected animals showed fever, swelling of the head as well as erosions of the mouth and subcutaneous haemorrhages.

Since 2006 bluetongue disease is rapidly spreading across Europe and reached Switzerland in October 2007. In the present article a short overview about the disease and the virus is given, and the first three clinical bluetongue disease cases in cattle, and the respective laboratory findings are presented.

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.

Bluetongue (BT) is a vector-borne animal disease of economical importance due to the international trade restrictions likely to be put into place in a country once the infection is discovered. The presence of BT and its vectors in countries adjacent to Switzerland stresses the need of implementing a surveillance system and to raise disease awareness among potential stakeholders. A national survey in Switzerland 2003 indicated freedom of Bluetongue virus (BTV), although a single individual of the main BT vector Culicoides imicola was caught in the canton of Ticino. The survey also demonstrated that potential BT vectors, C. obsoletus and C. pulicaris are locally abundant in Switzerland. Therefore, a new surveillance method based on sentinel herds in high risk areas was implemented in 2004 for the early detection of both an incursion of BT vectors into Switzerland, and potential virus circulation among cattle.

We report the results of the first survey for antibody against bluetongue virus (BTV) that was conducted in Switzerland in the year 2003. In a nationwide cross-sectional study with partial verification, 2437 cattle sera collected from 507 herds were analysed using competitive enzyme-linked immunosorbent assays (c-ELISA). To adjust for misclassification, 158 sera, including 86 that were recorded equivocal in Switzerland, were sent to the Office Internationale des Epizooties designated regional reference laboratory in the UK for confirmation. No BTV antibody was detected in any of these samples, confirming the absence of BTV from Switzerland in 2003. The specificity of the c-ELISA used in Switzerland for individual Swiss cattle was calculated to be 96.5%. The mean herd sensitivity achieved in our survey ranged from 78.9% to 98.8% depending on the with-in herd prevalence and test sensitivity used for the calculations. The cumulated confidence level achieved with the survey based on a minimal expected prevalence of 2%, was 99.99% and therefore it was concluded that there was no evidence of BTV circulation in Switzerland in 2003.

Since two different types of porcine reproductive and respiratory syndrome virus (PRRSV), the European (EU) and the North American (US) strain, occur or coexist in European swine herds, their rapid and reliable detection and differentiation is essential for disease surveillance. A quantitative TaqMan® reverse transcription-polymerase chain reaction (RT-PCR) is described for PRRSV detection and strain differentiation. Sensitivity and specificity were compared with a conventional PRRSV RT-PCR and to the detection of both PRRSV types in cell cultures and both were found to be equal or superior to the reference methods. Reproducibility was tested and proved that the assay was very reliable. Standard dilutions included in each test allowed absolute quantitation of the amount of viral RNA. The TaqMan® assay described below is time-saving, easy to handle, exhibits a decreased risk of cross-contamination and is highly sensitive and specific. It is, therefore, considered to be a powerful tool for the rapid detection and differentiation of PRRSV.

A method for storing samples containing classical swine fever virus (CSFV) or foot-and-mouth disease virus (FMDV), respectively, was developed, which abolishes the infectivity of both plus strand RNA viruses, and allows storage of samples above 0 degrees C for an extended time, yet preserves the viral RNA in a state which allows its detection by reverse transcription-polymerase chain reaction (RT-PCR), and even rescue of infectious virus after transfection of the extracted RNA into susceptible cells. Supernatants from infected cell cultures as well as organs from diseased animals were stored in Trizol(R) for 1-4 weeks at -20 degrees C, 4 degrees C, room temperature, or 37 degrees C. RNA was then extracted and used subsequently for RT-PCR, as well as transfection into susceptible cells to initiate the replication of progeny virus. Formaldehyde-fixed samples were also included in this study. Storage up to 4 weeks at 37 degrees C in Trizol(R) still yielded positive RT-PCR results and rescue of infectious virus upon RNA transfection. In contrast, formaldehyde fixation reduced drastically the detectability of viral RNA. This method represents a safe and inexpensive alternative to -70 degrees C (dry ice) storage or transport of samples, and abolishes the biosafety risks involved in shipping deep-frozen infectious materials.

Toggenburg Orbivirus (TOV) is the prototype of bluetongue virus serotype 25 (BTV-25). It was first detected in goats in Switzerland in 2008. The virus does not induce clinical signs in infected goats. In field samples viral RNA could be detected only in goats and never in other ruminants. BTV-25 RNA was repeatedly detected for more than one year in the blood of goats from a single flock in Principality of Liechtenstein. Since viral persistence over such a long period has never been reported for bluetongue, blood samples from 110 goats and 2 sheep of that flock were collected during a period of up to two years and analyzed for the presence of BTV-25 RNA and antibodies. Most of the animals which tested positive for BTV-25 RNA, remained positive during the whole investigation period. Moreover, five of these goats were BTV-25 RNA positive over a period of 19-25 months. A weak antibody response against BTV VP7 was commonly observed. As BTV-25 cannot be propagated in any culture system, the presence of virus could only be demonstrated in samples by viral RNA detection using RT-qPCR. To address the question of infectivity of the virus in blood from long-term positive animals, goats were experimentally infected with this blood. Viral replication was demonstrated by increasing RNA amounts. Thus, our findings provide evidence that BTV-25 can persist much longer in an infected host than known so far for other BTV serotypes. Hence, persistence of infectious BTV represents an additional important factor in BTV epidemiology.

The Toggenburg orbivirus (TOV), a recently discovered virus related to bluetongue virus (BTV), has been identified in goats in Switzerland, Italy and Germany. Isolation of TOV in vitro has not yet been achieved and the transmission mechanisms are still unknown. In the experimental infection of pregnant goats described here, TOV could not be detected in secretion/excretion samples or fetal blood. Material from the goat experiment was used as inoculum for propagating the virus in vitro. To enhance the infectivity of TOV several modified protocols, e.g. pretreatment of the virus with trypsin, polyethylene glycol-mediated infection and lipofection were applied. Isolation of TOV, attempts to infect Culicoides nubeculosus by feeding TOV-positive blood and intracerebral inoculation of newborn mice were unsuccessful. The results of these studies suggest that TOV requires specific but different factors than other BTVs for infection and replication outside of its natural caprine host.

After the spread of H5N1 highly pathogenic avian influenza virus (AIV) from Asia into Russia, the Middle East, Europe, and Africa in 2005–06, the Swiss national AIV surveillance program was extended. One of the new focal points was Lake Constance, where sentinel duck stations and swim-in traps were established within the project Constanze in collaboration with Germany and Austria. More than 2000 samples from 41 species were collected in Switzerland between September 2006 and December 2008. Approximately 4% were AIV-positive by quantitative reverse transcriptase–PCR. Subsequent typing revealed 13 different AIV subtypes, of which H5N2 and H9N2 were the most prevalent. All H7 isolates and all but one H5 isolate were characterized as low pathogenic; however, a highly pathogenic H5N1 AIV was detected in a healthy pochard. This study demonstrates that a wide selection of different AIV subtypes can cocirculate among the waterfowl population in wild bird habitats.

Twenty-seven sheep of the four most common Swiss breeds and the English breed Poll Dorset were experimentally infected with a northern European field strain of bluetongue virus serotype 8 (BTV-8). Animals of all breeds developed clinical signs, viremia and pathological lesions, demonstrating that BTV-8 is fully capable of replicating and inducing bluetongue disease (BT) in the investigated sheep. Necropsy performed between 10 and 16 days post-infectionem (d.p.i.) revealed BT-typical hemorrhages, effusions, edema, erosions and activation of lymphatic tissues. Hemorrhages on the base of the Arteria pulmonalis and the left Musculus papillaris subauricularis were frequently present. Histology confirmed the macroscopical findings. Using a score system, clinical manifestation and pathology were found to be significantly related. Furthermore, clinical signs and fever were shown to be indicative for the concurrent presence of high amounts of viral ribonucleic acid (RNA) in blood. Spleen, lung, lymph nodes and tonsils from all animals were analyzed regarding viral RNA loads and infectivity using real-time reverse transcriptase PCR (rRT-PCR) and virus isolation in cell culture, respectively. The highest amount of viral RNA was detected in spleen and lung and rRT-PCR revealed to be a more sensitive method for virus detection compared to virus isolation. A long-term follow-up was performed with three sheep showing that BTV-8 viral RNA in blood was present up to 133 d.p.i. and in certain tissues even on 151 d.p.i. No significant breed-related differences were observed concerning clinicopathological picture and viremia, and the Swiss sheep were as susceptible to BTV-8 infection as Poll Dorset sheep, demonstrating a remarkably high virulence of BTV-8 for indigenous sheep breeds.

Newcastle disease (ND) is a highly contagious avian disease. Rapid diagnosis of ND plays an important role in controlling outbreaks. Until now, time-consuming isolation of ND virus (NDV) in embryonated chicken eggs was used for NDV detection. For rapid diagnosis, reverse transcription-polymerase chain reaction (RT-PCR) with RNA extracted from tissue samples and faeces originating from experimentally and contact-infected chickens was established. Conjunctiva, lung, caecal tonsil and kidney proved to be the most suitable organs. In infected animals, NDV was detected most frequently between day 4 and 6 post-infection. Contact-infected animals gave most positive results between day 6 and 13 after exposure. RT-PCR was also able to reproducibly detect NDV in faecal samples. The RT-PCR did not show any cross-reactivity with other avian paramyxovirus serotypes, and additionally offers the possibility of subsequent sequencing of the amplified DNA allowing pathotyping of the isolate.

At the Swiss Federal Veterinary Office risk analyses are conducted according to international standards. A risk analysis contains the elements risk management, risk assessment and risk communication. A risk assessment is based on risk profile, hazard identification and a pathway model. All available information is gathered, documented and assessed and the risk estimated. The question. "What is the probability that unprocessed wild boar meat imported to Switzerland from the federal state Mecklenburg Western Pommerania is contaminated with classical swine fever virus?" was answered by a release assessment. The hazard identification recognized classical swine fever virus and attenuated live virus vaccine used for oral immunization as hazards. The probability of contamination was estimated to be small. The question: "What is the likelihood to introduce Aujeszky's disease to Switzerland and infect the indigenous pig population with the disease, by means of importing pork and meat products?" was answered by assessing the release, exposure and resulting consequences. The risk of an infection of the indigenous pig population was estimated to be very small, as 80% of the imported products derive from countries or zones free from Aujeszky's disease. Furthermore the majority of the imported products are processed. The strict implementation of the regulations governing feeding of food wastes to pigs reduces the probability of exposure. In all assessments the risk management decides on a strategy to deal with the risk, taking into consideration the results and recommendations derived from the risk assessment as well as other relevant factors.

Two types of porcine reproductive and respiratory syndrome virus (PRRSV) have been reported, the European type (EU PRRSV) and the North American type (US PRRSV). We developed a dual enzyme-linked immunosorbent assay (ELISA) for the simultaneous detection and differentiation of serum antibodies directed against either of the two PRRSV types. This tandem PRRS ELISA is based on affinity-purified recombinant nucleocapsid protein expressed in Escherichia coli . Sensitivity and specificity were assessed by using the IDEXX HerdChek PRRS ELISA and the indirect immunofluorescence assay as reference tests. A total of 1,571 sera originating from the United States, Europe, and two PRRS-free countries, i.e., Switzerland and New Zealand, were used for validation of the tandem PRRS ELISA. The new test performed at least as well as the reference tests in regard to sensitivity (0.94 for the US PRRS ELISA and 0.93 for the EU PRRS ELISA) and specificity (0.96 for the US PRRS ELISA and 0.99 for the EU PRRS ELISA). Positive sera were correctly differentiated in 582 of 591 cases, indicating a high differentiation capability of this dual ELISA. The robustness and repeatability of the test were assessed and found to be appropriate for diagnostic applications. Taken together, the data indicate that the tandem PRRS ELISA described here is the first differentiation ELISA for PRRSV serology based on recombinant antigen. It is convenient with respect to antigen production, and it is reliable, economical, and highly sensitive and specific. Thus, it is considered to be a powerful tool for routine diagnostics, epidemiological surveys, and outbreak investigations.

Newcastle disease (ND) is a highly contagious viral disease of birds particularly domestic poultry. Switzerland is currently declared free from ND; since vaccination is prohibited, the detection of antibodies against ND virus (NDV) results in the destruction of the respective flock (stamping-out policy). However, in 1995 and 1996, antibody-positive flocks were detected and sporadic ND outbreaks even occurred in Switzerland. Therefore, a serosurvey was done to look for evidence of NDV infections in Swiss laying-hen flocks. The survey was designed to provide 95% confidence of detecting at least one seropositive flock if the flock prevalence were 1%. Thirty blood samples from each of 260 commercial laying-hen flocks were collected during 1996 in a central poultry slaughterhouse. Sera were screened for NDV antibodies with a commercial blocking enzyme-linked immunosorbent assay (ELISA). Samples with a questionable or positive test result were retested with the same ELISA. A stochastic computer model was applied to define a cut-off number of test-positive samples to help to differentiate between true- and false-positive flocks and to estimate the true flock prevalence of infection. Four flocks were identified as NDV-seropositive and the NDV true seroprevalence among commercial laying-hen flocks in Switzerland was most likely between 1.35 and 1.55%. This indicates that Swiss laying-hen and parental flocks with more than 150 animals have been in contact with strains of NDV that cause subclinical infection in chicken, because no clinical symptoms have been observed. In this context, computer simulation was a useful technique to interpret survey results.