Using farmers' attitude and social pressures to design voluntary Bluetongue vaccination strategies.

Understanding the context and drivers of farmers' decision-making is critical to designing successful voluntary disease control interventions. This study uses a questionnaire based on the Reasoned Action Approach framework to assess the determinants of farmers' intention to participate in a hypothetical reactive vaccination scheme against Bluetongue. Results suggest that farmers' attitude and social pressures best explained intention. A mix of policy instruments can be used in a complementary way to motivate voluntary vaccination based on the finding that participation is influenced by both internal and external motivation. Next to informational and incentive-based instruments, social pressures, which stem from different type of perceived norms, can spur farmers' vaccination behaviour and serve as catalysts in voluntary vaccination schemes.

The dying of the light: crepuscular activity in Culicoides and impact on light trap efficacy at temperate latitudes.

The light trap is the tool of choice for conducting large-scale Culicoides (Diptera: Ceratopogonidae) vector surveillance programmes. Its efficacy is in doubt, however. To assess this, hourly changes in Culicoides activity over the 24-h diel were determined comparatively by way of light trapping and aerial sweeping, and correlated against light intensity. In the Netherlands, sweeping around cattle at pasture revealed that, in early summer, Culicoides are active throughout the diel, and that their abundance peaks during the crepuscular period and falls to a low during the brightest hours of the day. By contrast, the light trap was able to accumulate Culicoides only at night (i.e. after illuminance levels had dropped to 0 lux and midge activity had begun to decline). Although Culicoides chiopterus and species of the Culicoides obsoletus complex were similarly abundant around livestock, they differed critically in their hours of peak activity, being largely diurnal and nocturnal, respectively. This polarity helps to explain why, routinely, the C. obsoletus complex dominates light trap collections and C. chiopterus does not. Inability to accumulate Culicoides at light intensity levels above 0 lux means that, at ever-higher latitudes, particularly beyond 45° N, the progressive northward lengthening of the twilight period will have an increasingly adverse impact upon the efficacy of the light trap as a vector surveillance tool.

Culicoides (Diptera: Ceratopogonidae) and livestock in the Netherlands: comparing host preference and attack rates on a Shetland pony, a dairy cow, and a sheep.

Culicoides (Diptera: Ceratopogonidae) host preferences and attack rates were quantified in early summer at a dairy farm in the Netherlands using livestock tethered at pasture. Midges were aspirated hourly over seven consecutive hours (17:00-23:00) from a dairy cow, a Shetland pony, and a sheep and correspondingly yielded seventeen, thirteen, and nine species. Of the 14,181 midges obtained, approximately 95% belonged to the C. obsoletus complex, C. dewulfi, C. chiopterus, and C. punctatus that together include all proven or potential vectors for arboviral diseases in livestock in northwestern Europe. On average, 7.6 and 3.5 times more Culicoides were collected, respectively, from the cow and the Shetland pony than from the sheep. In descending order of abundance, the C. obsoletus complex, C. dewulfi, and C. chiopterus dominated attacks on all three hosts, whereas C. punctatus and C. pulicaris favored only the two larger hosts. Irrespective of the host species involved, the three body regions attracted the same component species, C. chiopterus favoring the legs, C. punctatus and C. achrayi the belly, and the C. obsoletus complex, C. dewulfi, and C. pulicaris the head, back, and flanks. That known and potential vectors for animal diseases feed indiscriminately on a broad range of mammal hosts means that all major livestock species, including equines, are rendered susceptible to one or more Culicoides-borne pathogens.

Mosquitoes and Culicoides biting midges: vector range and the influence of climate change.

Vector-borne animal diseases pose a continuous and substantial threat to livestock economies around the globe. Increasing international travel, the globalisation of trade, and climate change are likely to play a progressively more important role in the introduction, establishment and spread of arthropod-borne pathogens worldwide. A review of the literature reveals that many climatic variables, functioning singly or in combination, exert varying effects on the distribution and range of Culicoides vector midges and mosquitoes. For example, higher temperatures may be associated with increased insect abundance--thereby amplifying the risk of disease transmission--but there are no indications yet of dramatic shifts occurring in the geographic range of Culicoides midges. However, the same cannot be said for mosquitoes: over the last few decades, multiple Asian species have established themselves in Europe, spread and are unlikely to ever be eradicated. Research on how insects respond to changes in climate is still in its infancy. The authors argue that we need to grasp how other annectant changes, such as extremes in precipitation (drought and flooding), may affect the dispersal capability of mosquitoes. Models are useful for assessing the interplay between mosquito vectors expanding their range and the native flora and fauna; however, ecological studies employing classical mark-release-recapture techniques remain essential for addressing fundamental questions about the survival and dispersal of mosquito species, with the resulting parameters fed directly into new-generation disease transmission models. Studies on the eventual impact of mosquitoes on animal and human health should be tackled through large-scale integrated research programmes. Such an approach calls for more collaborative efforts, along the lines of the One Health Initiative.

Detection of serum neutralizing antibodies to Simbu sero-group viruses in cattle in Tanzania.

BACKGROUND: Orthobunyaviruses belonging to the Simbu sero-group occur worldwide, including the newly recognized Schmallenberg virus (SBV) in Europe. These viruses cause congenital malformations and reproductive losses in ruminants. Information on the presence of these viruses in Africa is scarce and the origin of SBV is unknown. The aim of this study was to investigate the presence of antibodies against SBV and closely related viruses in cattle in Tanzania, and their possible association with reproductive disorders. RESULTS: In a cross-sectional study, serum from 659 cattle from 202 herds collected in 2012/2013 were analyzed using a commercial kit for SBV ELISA, and 61 % were positive. Univariable logistic regression revealed significant association between ELISA seropositivity and reproductive disorders (OR = 1.9). Sera from the same area collected in 2008/2009, before the SBV epidemic in Europe, were also tested and 71 (54.6 %) of 130 were positive. To interpret the ELISA results, SBV virus neutralization test (VNT) was performed on 110 sera collected in 2012/2013, of which 51 % were positive. Of 71 sera from 2008/2009, 21 % were positive. To investigate potential cross reactivity with related viruses, 45 sera from 2012/2013 that were positive in SBV ELISA were analyzed in VNTs for Aino, Akabane, Douglas, Peaton, Sabo, SBV, Sathuperi, Shamonda, Simbu and Tinaroo viruses. All 45 sera were positive for one or more of these viruses. Twenty-nine sera (64.4 %) were positive for SBV, and one had the highest titer for this virus. CONCLUSIONS: This is the first indication that Aino, Akabane, Douglas, Peaton, Sabo, SBV, Sathuperi, Shamonda and Tinaroo viruses circulate and cause negative effect on reproductive performance in cattle in Tanzania. SBV or a closely related virus was present before the European epidemic. However, potential cross reactivity complicates the interpretation of serological studies in areas where several related viruses may circulate. Virus isolation and molecular characterization in cattle and/or vectors is recommended to further identify the viruses circulating in this region. However, isolation in cattle is difficult due to short viremic period of 2 to 6 days, and isolation in vectors does not necessarily reflect the situation in cattle.

An unrecognized species of the Culicoides obsoletus complex feeding on livestock in The Netherlands.

In studies on Culicoides attacking livestock in the Netherlands, we chanced upon a species of the Obsoletus complex that we do not recognize, but whose dark wing pattern is distinctive. Nine cytochrome c oxidase (CO1) sequences of our so-called 'dark obsoletus' support its status as a separate species, the sequences differing significantly from those representing Culicoides obsoletus (Meigen) (90-91% homology) and Culicoides scoticus Downes & Kettle (87-88% homology). In the last decade, several research groups in Europe have encountered 'mystery species' related to C. obsoletus and in some instances have made their sequences for various genetic loci available in GenBank. These include a CO1 series submitted from Sweden in 2012 (annotated as 'obsoletus 01, 02, or 03 MA-2012') and of which some share a 99% identity with our sequences for 'dark obsoletus'. Without doubt, the series from the Netherlands, along with a portion of the Swedish submissions, together represent a single species ('dark obsoletus'). Whether this species is referable to the Russian Culicoides gornostaevae Mirzaeva recorded recently from Norway, Sweden and Poland, and based solely upon the external morphology of the male, is not clear. The presence in Western Europe of multiple undescribed species related to C. obsoletus means that the taxonomy of this important vector complex is not fully resolved; consequently, we know little about these cryptic species with regard to seasonality, geographic range and host preference. This is undesirable given that Culicoides-borne arboviruses causing disease in livestock are moving more regularly out of the tropics and spreading north into temperate latitudes.

Why German farmers have their animals vaccinated against Bluetongue virus serotype 8: results of a questionnaire survey.

In response to the Bluetongue disease epidemic in 2006-2007, Germany started in 2008 a country-wide mandatory vaccination campaign. By 2009 the number of new outbreaks had decreased so that vaccination became voluntary in 2010. We conducted a questionnaire survey in cattle and sheep farms in three German federal states, namely North-Rhine Westphalia, Rhineland Palatinate and Saxony-Anhalt to estimate the vaccination uptake in 2010, the intention to vaccinate in 2011 and the main determinants of refusal or acceptance to do so. The results showed that 42.8% (40.6-45.1) of the cattle farmers and 33.8% (31.8-35.8) of the sheep farmers had their animals vaccinated in 2010, whereas 40.7% (38.5-43.0) of cattle and 37.93% (35.8-40.1) sheep farmers expressed their intention to vaccinate in 2011. The main reasons mentioned for having animals vaccinated against BTV-8 were ability to export animals, prevention of production losses, subsidized vaccination, and recommendation by the veterinarian. Motives for refusing vaccination were presumed low risk of infection, costs, absence of clinical BT symptoms, presumed negative cost-benefit ratio, and negative experience with previous vaccination events (side effects). We assume that in order to increase farmers' motivation to have their animals immunized against BTV-8, (1) the vaccination needs to be subsidized, (2) combined vaccines with several different BT serotypes or even other diseases should be available and (3) farmers need to be better informed about the safety and benefit of vaccination.

Schmallenberg Virus in Culicoides Biting Midges in the Netherlands in 2012.

A total of 130 pools of Culicoides biting midges collected between May and September 2012 in the Netherlands were assayed for Schmallenberg virus (SBV). The Culicoides midges were caught in the same area as where in 2011 a high proportion of Culicoides pools tested positive for SBV, in majority with a high viral load (Ct values between 20 and 30). Two of a total of 42 pools comprising 50 midges/pool of the Obsoletus complex from the 2012 collection tested weak positive (Ct values: 34.96 and 37.66), indicating a relatively low viral load. On an individual midge level, the proportion of SBV-infected Culicoides of the Obsoletus complex caught in the same area and in a comparable period of the year was significantly lower in 2012 (0.1% = 1 per 1050 tested) compared with 2011 (0.56% = 13 per 2300 tested).

Risk based surveillance for early detection of low pathogenic avian influenza outbreaks in layer chickens.

Current knowledge does not allow the prediction of when low pathogenic avian influenza virus (LPAIV) of the H5 and H7 subtypes infecting poultry will mutate to their highly pathogenic phenotype (HPAIV). This mutation may already take place in the first infected flock; hence early detection of LPAIV outbreaks will reduce the likelihood of pathogenicity mutations and large epidemics. The objective of this study was the development of a model for the design and evaluation of serological-surveillance programmes, with a particular focus on early detection of LPAIV infections in layer chicken flocks. Early detection is defined as the detection of an infected flock before it infects on average more than one other flock (between-flock reproduction ratio Rf<1), hence a LPAI introduction will be detected when only one or a few other flocks are infected. We used a mathematical model that investigates the required sample size and sampling frequency for early detection by taking into account the LPAIV within- and between-flock infection dynamics as well as the diagnostic performance of the serological test used. Since layer flocks are the target of the surveillance, we also explored whether the use of eggs, is a good alternative to sera, as sample commodity. The model was used to refine the current Dutch serological-surveillance programme. LPAIV transmission-risk maps were constructed and used to target a risk-based surveillance strategy. In conclusion, we present a model that can be used to explore different sampling strategies, which combined with a cost-benefit analysis would enhance surveillance programmes for low pathogenic avian influenza.

Response to an emerging vector-borne disease: surveillance and preparedness for Schmallenberg virus.

Surveillance for new emerging animal diseases from a European perspective is complicated by the non-harmonised approach across Member States for data capture, recording livestock populations and case definitions. In the summer of 2011, a new vector-borne Orthobunyavirus emerged in Northern Europe and for the first time, a coordinated approach to horizon scanning, risk communication, data and diagnostic test sharing allowed EU Member States to develop early predictions of the disease, its impact and risk management options. There are many different systems in place across the EU for syndromic and scanning surveillance and the differences in these systems have presented epidemiologists and risk assessors with concerns about their combined use in early identification of an emerging disease. The emergence of a new disease always will raise challenging issues around lack of capability and lack of knowledge; however, Schmallenberg virus (SBV) gave veterinary authorities an additional complex problem: the infection caused few clinical signs in adult animals, with no indication of the possible source and little evidence about its spread or means of transmission. This paper documents the different systems in place in some of the countries (Germany and the Netherlands) which detected disease initially and predicted its spread (to the UK) and how information sharing helped to inform early warning and risk assessment for Member States. Microarray technology was used to identify SBV as a new pathogen and data from the automated cattle milking systems coupled with farmer-derived data on reporting non-specific clinical signs gave the first indications of a widespread issue while the UK used meteorological modelling to map disease incursion. The coordinating role of both EFSA and the European Commission were vital as are the opportunities presented by web-based publishing for disseminating information to industry and the public. The future of detecting emerging disease looks more positive in the light of this combined approach in the EU.

Culicoides (Diptera: Ceratopogonidae) host preferences and biting rates in the Netherlands: comparing cattle, sheep and the black-light suction trap.

Host preference is an important determinant of feeding behaviour in biting insects and a critical component in the transmission of vector-borne diseases. The aim of the study was to quantify Culicoides (Diptera: Ceratopogonidae) host preferences and biting rates using tethered livestock at pasture (a dairy cow and a sheep) and to compare the numbers of biting midges aspirated off them to those captured simultaneously in a black-light suction trap acting as a surrogate host. Culicoides collections were made hourly over seven hours (from five hours before official sunset to two hours after) between 27 May and 19 June, 2013 at a dairy farm (eastern Netherlands). The study involved 13 replicates of a site × host randomised design. Culicoides collected by black-light suction trap and by direct aspiration were identified to species morphologically and age-graded. The C. obsoletus complex, C. dewulfi and C. pulicaris predominated on the back and flanks of the animals, C. punctatus on the belly, and C. chiopterus on the legs. Using comparable collection periods, 9.3 times (95% confidence interval: 8.6-10.0) more Culicoides were caught on the cow than on the sheep and 25.4 times (95% confidence interval: 18.4-35.1) less in the black-light suction trap compared to the sheep. Mean Culicoides biting rates on the cow across the 7-h collection period were 4.6, 3.5, 1.0, 1.0 and 0.5 min(-1) for C. dewulfi, the C. obsoletus complex, C. chiopterus, C. punctatus and C. pulicaris, respectively; for the sheep they were 0.6, 0.4 and 0.1 min(-1) for the C. obsoletus complex, C. dewulfi and C. punctatus, respectively. Though midges were aspirated off livestock during each of the seven hours, they only began to appear in the black-light suction trap 5h later, from sunset onwards. After sunset, its efficacy improved markedly, but occurred when midge activity overall had begun to decline. Though it was quite accurate in ranking Culicoides species abundance, the black-light suction trap proved to be of limited value for determining hours of peak biting activity, levels of abundance, and host preference, in Culicoides.

Expected utility of voluntary vaccination in the middle of an emergent Bluetongue virus serotype 8 epidemic: a decision analysis parameterized for Dutch circumstances.

In order to put a halt to the Bluetongue virus serotype 8 (BTV-8) epidemic in 2008, the European Commission promoted vaccination at a transnational level as a new measure to combat BTV-8. Most European member states opted for a mandatory vaccination campaign, whereas the Netherlands, amongst others, opted for a voluntary campaign. For the latter to be effective, the farmer's willingness to vaccinate should be high enough to reach satisfactory vaccination coverage to stop the spread of the disease. This study looked at a farmer's expected utility of vaccination, which is expected to have a positive impact on the willingness to vaccinate. Decision analysis was used to structure the vaccination decision problem into decisions, events and payoffs, and to define the relationships among these elements. Two scenarios were formulated to distinguish farmers' mindsets, based on differences in dairy heifer management. For each of the scenarios, a decision tree was run for two years to study vaccination behaviour over time. The analysis was done based on the expected utility criterion. This allows to account for the effect of a farmer's risk preference on the vaccination decision. Probabilities were estimated by experts, payoffs were based on an earlier published study. According to the results of the simulation, the farmer decided initially to vaccinate against BTV-8 as the net expected utility of vaccination was positive. Re-vaccination was uncertain due to less expected costs of a continued outbreak. A risk averse farmer in this respect is more likely to re-vaccinate. When heifers were retained for export on the farm, the net expected utility of vaccination was found to be generally larger and thus was re-vaccination more likely to happen. For future animal health programmes that rely on a voluntary approach, results show that the provision of financial incentives can be adjusted to the farmers' willingness to vaccinate over time. Important in this respect are the decision moment and the characteristics of the disease. Farmers' perceptions of the disease risk and about the efficacy of available control options cannot be neglected.

The Mondrian matrix: Culicoides biting midge abundance and seasonal incidence during the 2006-2008 epidemic of bluetongue in the Netherlands.

During the northern Europe epidemic of bluetongue (BT), Onderstepoort-type blacklight traps were used to capture Culicoides Latreille (Diptera: Ceratopogonidae) biting midges weekly between November 2006 and December 2008 on 21 livestock farms in the Netherlands. Proven and potential vectors for the bluetongue virus (BTV) comprised almost 80% of the midges collected: the Obsoletus complex, constituting C. obsoletus (Meigen) and C. scoticus Downes & Kettle (44.2%), C. dewulfi Goetghebuer (16.4%), C. chiopterus (Meigen) (16.3%) and C. pulicaris (Linnaeus) (0.1%). Half of the 24 commonest species of Culicoides captured completed only one (univoltine) or two (bivoltine) generations annually, whereas multivoltine species (including all BTV vectors) cycled through five to six generations (exceeding the one to four generations calculated in earlier decades). Whether this increment signals a change in the phenology of northern Europe Culicoides or simply is an adaptive response that manifests during warmer episodes, thus heightening periodically the incursive potential of midge-borne arboviruses, remains to be clarified. Culicoides duddingstoni Kettle & Lawson, C. grisescens Edwards, C. maritimus Kieffer, C. pallidicornis Kieffer and C. riethi Kieffer are new records for the biting midge fauna of the Netherlands. It is suggested that C. punctatus (Meigen) be added to the European list of vector Culicoides.

Principal climatic and edaphic determinants of Culicoides biting midge abundance during the 2007-2008 bluetongue epidemic in the Netherlands, based on OVI light trap data.

Palaearctic Culicoides midges (Diptera: Ceratopogonidae) represent a vital link in the northward advance of certain arboviral pathogens of livestock such as that caused by bluetongue virus. The effects of relevant ecological factors on weekly Culicoides vector abundances during the bluetongue virus serotype 8 epidemics in the Netherlands in 2007 and 2008 were quantified within a hurdle modelling framework. The relative role of meteorological parameters showed a broadly consistent association across species, with larger catches linked to temperature-related variables and lower wind speed. Moreover, vector abundance was found to be influenced by edaphic factors, likely related to species-specific breeding habitat preferences that differed markedly amongst some species. This is the first study on Culicoides vector species in the Netherlands identified during an entomological surveillance programme, in which an attempt is made to pinpoint the factors that influence midge abundance levels. In addition to providing key inputs into risk-mitigating tools for midge-borne pathogens and disease transmission models, the adoption of methods that explicitly address certain features of abundance datasets (frequent zero-count observations and over-dispersion) helped enhance the robustness of the ecological analysis.

Schmallenberg virus epidemic in the Netherlands: spatiotemporal introduction in 2011 and seroprevalence in ruminants.

This study aimed at estimating the Schmallenberg virus (SBV) seroprevalence in dairy heifers, non-dairy adult cattle, sheep and goats in the Netherlands after cessation of SBV transmission at the end of 2011. Archived serum samples from ruminants submitted to the GD Animal Health Service for monitoring purposes between November 2011 and March 2012 were selected and tested for presence of SBV-specific antibodies using an in-house ELISA. Animal seroprevalences were estimated at 63.4% in dairy heifers, 98.5% in adult non-dairy cattle, 89.0% in sheep and 50.8% in goats. Multivariable analyses were carried out to describe the relationship between potential risk factors and the ELISA outcome S/P%. The overall SBV seroprevalence in ruminants and ruminant herds in the Netherlands at the end of 2011 was high, with considerable differences between species and farm types. No gradient spatial pattern in final seroprevalence could be detected and therefore no suggestions about the site of introduction and spread of SBV in the Netherlands in 2011 could be made. In dairy heifers, it was shown that S/P% increased with age. In sheep, S/P% was lower in animals located in the coastal area. Whether herds were located near the German border did not affect the S/P% in sheep nor in dairy heifers. An attempt was made to gain insight in the spatiotemporal introduction of SBV in the Netherlands in 2011, by testing sheep serum samples from 2011. A seroprevalence of about 2% was found in samples from April, June and July 2011, but the ELISA positive samples could not be confirmed in a virus neutralization test. A clear increase in seroprevalence started at August 2011. From mid-August 2011 onwards, seropositive samples were confirmed positive by virus neutralization testing. This indicated the start of the epidemic, but without a clear spatial pattern.

Using egg production data to quantify within-flock transmission of low pathogenic avian influenza virus in commercial layer chickens.

Even though low pathogenic avian influenza viruses (LPAIv) affect the poultry industry of several countries in the world, information about their transmission characteristics in poultry is sparse. Outbreak reports of LPAIv in layer chickens have described drops in egg production that appear to be correlated with the virus transmission dynamics. The objective of this study was to use egg production data from LPAIv infected layer flocks to quantify the within-flock transmission parameters of the virus. Egg production data from two commercial layer chicken flocks which were infected with an H7N3 LPAIv were used for this study. In addition, an isolate of the H7N3 LPAIv causing these outbreaks was used in a transmission experiment. The field and experimental estimates showed that this is a virus with high transmission characteristics. Furthermore, with the field method, the day of introduction of the virus into the flock was estimated. The method here presented uses compartmental models that assume homogeneous mixing. This method is, therefore, best suited to study transmission in commercial flocks with a litter (floor-reared) housing system. It would also perform better, when used to study transmission retrospectively, after the outbreak has finished and there is egg production data from recovered chickens. This method cannot be used when a flock was affected with a LPAIv with low transmission characteristics (R(0)<2), since the drop in egg production would be low and likely to be confounded with the expected decrease in production due to aging of the flock. Because only two flocks were used for this analysis, this study is a preliminary basis for a proof of principle that transmission parameters of LPAIv infections in layer chicken flocks could be quantified using the egg production data from affected flocks.

African swine fever virus excretion patterns in persistently infected animals: a quantitative approach.

The continuing circulation of African swine fever (ASF) in Russia and in the Trans-Caucasian countries has led to increased efforts in characterizing the epidemiology of ASF. For a better insight in epidemiology, quantitative data on virus excretion is required. Until now, excretion data has mainly focused on the initial stages of the disease. In our study we have studied ASF virus (ASFV) excretion dynamics in persistently infected animals. For this purpose, virus excretion through different routes was quantified over 70 days after infection. Three virus isolates of moderate virulence were used: the Brazil'78, the Malta'78 (a low and a high inoculation dose) and the Netherlands'86 isolate. For each isolate or dose, 10 animals were used. All (Brazil'78 group), or three animals per group were inoculated and the other animals served as contact animals. It was shown that dose (Malta'78 low or high) or infection route (inoculated or naturally infected) did not influence the ASFV excretion (p>0.05). Nasal, ocular and vaginal excretions showed the lowest ASFV titres. Virus was consistently present in the oropharyngeal swabs, showing two peaks, for up to 70 days. Virus was occasionally present in the faeces, occasionally with very high titres. Viral DNA persisted in blood for up to 70 days. The results presented in this study show that a high proportion of persistently infected animals shed virus into the environment for at least 70 days, representing a possible risk for transmission and that should be considered in future epidemiological analysis of ASF.

Transmission characteristics of low pathogenic avian influenza virus of H7N7 and H5N7 subtypes in layer chickens.

Low pathogenic avian influenza virus (LPAIv) infections of H5 and H7 subtypes in poultry are notifiable to the OIE, hence surveillance programmes are implemented. The rate at which LPAIv strains spread within a flock determines the prevalence of infected birds and the time it takes to reach that prevalence and, consequently, optimal sample size and sampling frequency. The aim of this study was to investigate the transmission characteristics of an H7N7 and an H5N7 LPAIv in layer chickens. Two transmission experiments were performed, which consisted of 30 (first experiment) and 20 (second experiment) pairs of conventional layers, respectively. At the start of the experiments, one chicken per pair was inoculated with LPAIv and the other chicken was contact-exposed. Occurrence of infection was monitored by regularly collecting tracheal and cloacal swab samples, which were examined for the presence of virus RNA by RT-PCR. The results of the test were used to estimate the transmission rate parameter (ß), the infectious period (T) and the basic reproduction ratio (R(0)). In addition, egg production and virus shedding patterns were quantified. For the H7N7 virus, the ß, T and R(0) estimates were 0.10 (95% confidence interval (CI): 0.04-0.18) day(-1), 7.1 (95% CI: 6.5-7.8) days and 0.7 (95% CI: 0.0-1.7), respectively. With the H5N7 virus, only a few inoculated chickens (5 out of 20) became infected and no transmission was observed. This study shows that transmission characteristics of LPAIv strains may vary considerably, which has to be taken into account when designing surveillance programmes.

Transmission between chickens of an H7N1 Low Pathogenic Avian Influenza virus isolated during the epidemic of 1999 in Italy.

The transmissibility of an H7N1 Low Pathogenic Avian Influenza (LPAI) virus isolated from a turkey flock during the large epidemic in Italy in 1999, was experimentally studied in chickens. Four group transmission experiments were performed. Infection and transmission were monitored by means of virus isolation on swab samples and antibody detection in serum samples. From the results of these groups, we estimated the mean infectious period at 7.7 (6.7-8.7) days, the transmission rate parameter at 0.49 (0.30-0.75) infections per infectious chicken per day and the basic reproduction ratio at 3.8 (1.3-6.3). These estimates can be used for the development of surveillance and control programmes of LPAI in poultry.

Constructing naive Bayesian classifiers for veterinary medicine: a case study in the clinical diagnosis of classical swine fever.

For diseases of which the clinical diagnosis is uncertain, naive Bayesian classifiers can be of assistance to the veterinary practitioner. These simple probabilistic models have proven to be very powerful for solving classification problems in a variety of domains, but are not yet widely applied within the veterinary domain. In this paper, naive Bayesian classifiers and methods for their construction are reviewed. We demonstrate how to construct full and selective classifiers from a data set and how to build such classifiers from information in the literature. As a case study, naive Bayesian classifiers to discriminate between classical swine fever (CSF)-infected and non-infected pig herds were constructed from data collected during the 1997/1998 CSF epidemic in the Netherlands. The resulting classifiers were studied in terms of their accuracy and compared with the optimally efficient diagnostic rule that was reported earlier by Elbers et al. (2002). The classifiers were found to have accuracies within the range of 67-70% and performed comparable to or even better than the diagnostic rule on the available data. In contrast with the diagnostic rule, the classifiers had the advantage of taking both the presence and the absence of particular clinical signs into account, which resulted in more discriminative power. These results indicate that naive Bayesian classifiers are promising tools for solving diagnostic problems in the veterinary field.