No major outbreaks of pseudorabies virus in well-immunized sow herds

Transmission of Classical Swine Fever Virus in Cohabitating Piglets with Various Immune Statuses Following Attenuated Live Vaccine

Classical swine fever (CSF) is a systemic hemorrhagic disease affecting domestic pigs and wild boars. The modified live vaccine (MLV) induces quick and solid protection against CSF virus (CSFV) infection. Maternally derived antibodies (MDAs) via colostrum could interfere with the MLV's efficacy, leading to incomplete protection against CSFV infection for pigs. This study investigated CSFV transmission among experimental piglets with various post-MLV immune statuses. Nineteen piglets, 18 with MDAs and 1 specific-pathogen-free piglet infected with CSFV that served as the CSFV donor, were cohabited with piglets that had or had not been administered the MLV. Five-sixths of the piglets with MDAs that had been administered one dose of MLV were fully protected from contact transmission from the CSFV donor and did not transmit CSFV to the piglets secondarily exposed through cohabitation. Cell-mediated immunity, represented by the anti-CSFV-specific interferon-γ-secreting cells, was key to viral clearance and recovery. After cohabitation with a CSFV donor, the unvaccinated piglets with low MDA levels exhibited CSFV infection and spread CSFV to other piglets through contact; those with high MDA levels recovered but acted as asymptomatic carriers. In conclusion, MLV still induces solid immunity in commercial herds under MDA interference and blocks CSFV transmission within these herds.

The use of vaccines to control pathogen spread in pig populations

Vaccine efficacy has often been studied from the viewpoint of individual direct clinical protection. For several vaccines, a decrease in pathogen shedding in vaccinated animals has also been documented, which suggests that transmission between individuals has the potential to be reduced. In addition, vaccination induces an immune response in the host potentially decreasing susceptibility to infection in comparison with immunologically naïve animals. As a collective result of individual vaccinations, vaccine programmes generally have a wider impact on pathogen diffusion at the population scale. Beyond the individual protection conferred by mass vaccination campaigns, the indirect protection of non-immune individuals in contact with vaccinated ones also contributes to controlling pathogen spread at the population scale; a phenomenon known as herd immunity.

Pathogen spread within pig populations is strongly related to the required vaccine coverage at the population level and to pathogen characteristics in terms of diffusion (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {R}_0 $$\end{document}R0). Before setting up vaccination programmes, it is therefore necessary to have quantitative knowledge on vaccine efficacy as regards transmission reduction. These data can be obtained by carrying out experimental studies or observational protocols in real conditions. These quantitative data have mainly been estimated for major infectious diseases which have now been eradicated. A great gap in knowledge has however been identified for enzootic diseases which are daily impacting the swine sector as well as for the source of variation responsible for a decrease in vaccine efficacy as compared to assessments obtained in experimental conditions.

The impact of compartmentalised housing on direct encephalomyocarditis virus (EMCV) transmission among pigs; insight from a model

Although generally considered a rodent virus, pigs sometimes were suggested a potential reservoir host for encephalomyocarditis virus (EMCV), implying pig-to-pig transmission can cause major outbreaks in a pig population (basic reproduction ratio, R0>1). An earlier experimental study on EMCV transmission among pigs was inconclusive in this respect (R0≈1.24; CI 0.4-4.4). In this study we used a simulation model to extrapolate the experimental results to commercial, compartmentalised pig housings and tested to what extend contacts between pigs in different pens needed to be reduced in order to prevent major outbreaks in a compartment following a single introduction. The final size of simulated outbreaks was measured and the probability to observe outbreaks that affected at least 50 or 80% of the pens was calculated. Simulation scenarios compare one homogeneously mixing compartment (no fence) to epidemiological theory and an increasing effect of fencing on the pig-to-pig transmission between pigs in neighbouring pens. For any R0<1.24 the probability to observe outbreaks affecting more than 50% of the pens remained below 10% if compartmentalisation was introduced leaving per capita transmission rate unchanged. If fences also reduced contact transmission the probability to observe major outbreaks was below 50% for any R0<2.7. Only for R0>4, major outbreaks occurred with more than 50% chance even if only minimal contact between adjacent pens was allowed. In conclusion the results suggested that in a compartmentalised pig housing one single EMCV introduction is unlikely to cause a major outbreak by direct pig-to-pig transmission alone. Other mechanisms e.g. multiple introductions from a rodent reservoir may be required for large outbreaks to occur.

The effect of vaccination on foot and mouth disease virus transmission among dairy cows

The aim of this study was to quantify the effect of a single vaccination of dairy cows on foot and mouth disease virus (FMDV) transmission. To estimate if vaccination could significantly reduce virus transmission, we performed two replicates of a transmission experiment with one group of vaccinated and one group of non-vaccinated dairy cows (ten animals per group). Half of both groups were intranasally inoculated, with FMDV field isolate O/NET2001, and housed with the other half of the group (contact-exposed cows) from the next day onwards. Virus transmission was quantified by estimating the reproduction ratio R, which is the average number of secondary cases caused by one infectious animal. In the non-vaccinated groups all cows became infected and Rnv was significantly above 1. In the vaccinated groups infection was demonstrated in three inoculated cows, and no transmission was observed (Rv was 0, not significantly below 1). Transmission was significantly reduced in the groups of vaccinated cows when compared to the groups of unvaccinated cows. Our findings indicate that after a single vaccination cows are protected against infection of FMD and that most likely no virus transmission will occur within a vaccinated herd.

Determination of the effectiveness of Pseudorabies marker vaccines in experiments and field trials

The aim of vaccination in an eradication campaign is not only to induce clinical protection, but primarily to stop transmission of infections within and between herds by inducing herd immunity. Consequently, vaccines should be evaluated for their capacity to reduce virus transmission in the population. Glycoprotein E (gE) negative marker vaccines against Pseudorabies virus (PRV) infections in pigs have been evaluated this way in experiments and field studies. PRV infection in groups of (vaccinated) pigs was determined by measuring antibodies against gE of PRV from regularly taken serum samples. For the statistical analysis of the experiments a stochastic susceptible-infectious-removed (SIR) model was used. A measure for the transmission of virus is the reproduction ratio R, which is defined as the average number of secondary cases caused by one typical infectious individual. This implies that an infection will always fade out in a population when R < 1, but the infection can spread massively when R > 1. From several experiments it was shown that R < 1. Field studies showed that the R within herds was still > 1, but by reducing further contacts the R could be reduced to a value below one. This would imply that PRV could be eradicated by means of vaccination. In The Netherlands, an eradication campaign was launched in 1993, and in 2002 the virus was eradicated, as shown by a negligible number of gE-positive pigs. Farmers' organizations have to decide whether or not to stop vaccination.

Vaccination against foot and mouth disease reduces virus transmission in groups of calves

The aim of vaccination during an epidemic of foot and mouth disease (FMD) is not to induce clinical protection, but to reduce virus transmission. Since no quantitative data were available on the effectiveness of vaccination in cattle, we investigated whether a single vaccination against FMD could reduce virus transmission in groups of calves by estimating the reproduction ratio R, i.e. the average number of secondary cases caused by one infectious animal in a susceptible population. We performed two experiments with six groups of either four vaccinated or four non-vaccinated calves each. Vaccination was carried out with O(1) Manisa vaccine. Two weeks after vaccination, two calves per group were inoculated intra-nasally with FMDV field isolate O/NET 2001. The two other calves were contact-exposed to the inoculated calves. Contact infections were observed by clinical inspection, virus isolation and RT-PCR on heparinised blood, oro-pharyngeal fluid and probang samples and antibody response to non-structural proteins. In all six non-vaccinated groups, transmission to contact-exposed calves was recorded; in the vaccinated groups, virus transmission was observed to one contact-exposed calf. In the non-vaccinated groups R(c) was 2.52 and significantly above 1, whereas in the vaccinated groups R(v)=0.18 and significantly below 1, indicating that vaccination may successfully be applied as additional intervention tool to reduce virus transmission in a future epidemic of FMD.

Mathematical modelling of pseudorabies virus (syn. Aujeszky's disease virus) outbreaks aids eradication programmes: a review

Pseudorabies virus will be eradicated from the Netherlands if a typical infectious pig (Rind) infects, on average, less than one other pig. In this review, we used a stochastic SIR model to estimate Rind using data from the field and from experiments. Rind in sow herds was estimated to be significantly less than 1 and in rearing and finishing pigs Rind was higher than 1. However, if Rind is higher than 1, PRV can still be eradicated if one infectious herd infects less than one other herd during the period that the herd is infectious(Rherd <1). Some future developments in Dutch pig husbandry (e.g. group-housing of sows) and possible risks after halting vaccination are also quantitatively evaluated.

Experimental quantification of the transmission of Sarcoptes scabiei var. suis among finishing pigs

In this study, the rate of S. scabiei var. suis transmission among finishing pigs was quantified in a contact transmission experiment. Forty piglets originating from a mange free farrow-to-finish herd were randomly allocated to three groups and one S. scabiei var. suis infested finishing pig was subsequently added to each of these groups. After 35 days, the three seeder pigs were removed from the groups and the remaining 40 pigs were re-allocated to five pens. Ear scrapings, to be examined for mites, were collected from each pig on days 1, 14, 28, 42, 56, and 84 of the experiment. Blood samples, to be tested for antibodies against S. scabiei, were collected from each pig on days 0, 14, 28, 42, 56, 70, 84 and 112 after the introduction of the seeder pigs. From the results of the ear scrapings and the blood samples the number of susceptible (not infested) and infested pigs was derived at the time of each sample collection and the number of new infestations in the intervals between the sample collections. From these data the infestation rate parameter beta (average number of new infestations per infested pig per day) was estimated by use of a Generalised Linear Model (GLM) and accordingly, beta was estimated at 0.056 (95% CI: 0.037-0.085) infestations per infested pig per day.Next, by use of beta, the transmission of S. scabiei was simulated in a population of 100 finishing pigs for 100 days after the introduction of a single infested pig. For this purpose, 500 simulations were done. The 90% confidence interval of the number of infested pigs at day 100 ranged from 12 to 88 (median: 63). It was concluded that transmission of S. scabiei among finishing pigs is slow. Due to the presumed lower contact rate between sows as compared to finishing pigs, it is anticipated that transmission of S. scabiei among sows will even be slower than among finishers These findings are of particular interest for the development of surveillance programmes for S. scabiei free herds.

Vaccination and eradication programme against Aujeszky's disease in five Swedish pig herds with special reference to herd owner attitudes

A vaccination eradication programme against Aujeszky's disease (AD), based on the use of gE-negative killed vaccine, was carried out between 1987 and 1992 in 5 Swedish weaner pig producing or farrow-to-finish herds, with 63 to 398 breeding animals. All breeding animals were tested at the start and the end of the programme. Seroprevalence to Aujeszky's disease virus (ADV) ranged between 47% and 100% in the herds at the first test. During the programme, all breeding animals were vaccinated simultaneously every 4 months and ADV-free replacement animals were vaccinated shortly after arrival and boostered within a month. In one herd only, a limited number of fatteners were vaccinated. The herds were declared free (gE-negative) 12 to 53 months after the start of the programme. When all seropositive breeding animals had been culled, the programme ended after 2 negative tests of the breeding animals. Seroconversion was limited in all herds but one, where initially no isolation unit was available for replacement animals. The attitude of the herd owners towards the programme and the special conditions prevailing in the herds are discussed. It is suggested that vaccination may promote risk behaviour of herd managers.

Detection of pseudorabies virus DNA in individual single-reactor pigs found in certified pseudorabies-free herds

During monitoring of certified pseudorabies (PRV)-free herds to confirm their PRV -free status, occasional individual gE-seropositive pigs are detected. These single-reactor pigs remain gE-seropositive when further serum samples are collected and tested. For the eradication programme to proceed, it is important to determine whether these pigs are only false positives or are; in fact, infected with field PRV. The purpose of this study was to determine whether the polymerase chain reaction (PCR) could detect field PRVDNA in single-reactor pigs and so confirm positive reactions in the serologic monitoring programme. First, DNA samples of various tissues from 15 single-reactor pigs all from different herds were examined for field PRV by PCR. Additionally, serum samples from these pigs were analyzed in a gE-confirmation enzyme linked immunosorbent assay (gE-confirmation ELISA). PCR detected PRVDNA in five of the 15 pigs, and these results were confirmed by the gE-confirmation ELISA. The remaining 10 pigs that tested negative in the PCR also tested negative in the gE- confirmation ELISA. We conclude that PCR can be used to discriminate between true and false serological positive single-reactor pigs and, moreover, that the gE-confirmation ELISA confirms these PCR results.

Transmission of classical swine fever virus within herds during the 1997-1998 epidemic in The Netherlands

In this paper, we describe the transmission of Classical Swine Fever virus (CSF virus) within herds during the 1997-1998 epidemic in The Netherlands. In seven herds where the infection started among individually housed breeding stock, all breeding pigs had been tested for antibodies to CSF virus shortly before depopulation. Based upon these data, the transmission of CSF virus between pigs was described as exponential growth in time with a parameter r, that was estimated at 0.108 (95% confidence interval (95% CI) 0.060-0.156). The accompanying per-generation transmission (expressed as the basic reproduction ratio, R0) was estimated at 2.9. Based upon this characterisation, a calculation method was derived with which serological findings at depopulation can be used to calculate the period in which the virus was with a certain probability introduced into that breeding stock. This model was used to estimate the period when the virus had been introduced into 34 herds where the infection started in the breeding section. Of these herds, only a single contact with a herd previously infected had been traced. However, in contrast with the seven previously mentioned herds, only a sample of the breeding pigs had been tested before depopulation (as was the common procedure during the epidemic). The observed number of days between the single contact with an infected herd and the day of sampling of these 34 herds fitted well in the model. Thus, we concluded that the model and transmission parameter was in agreement with the transmission between breeding pigs in these herds. Because of the limited sample size and because it was usually unknown in which specific pen the infection started, we were unable to estimate transmission parameters for weaned piglets and finishing pigs from the data collected during the epidemic. However, from the results of controlled experiments in which R0 was estimated as 81 between weaned piglets and 14 between heavy finishing pigs (Laevens et al., 1998a. Vet. Quart. 20, 41-45; Laevens et al., 1999. Ph.D. Thesis), we constructed a simple model to describe the transmission of CSF virus in compartments (rooms) housing finishing pigs and weaned piglets. From the number of pens per compartment, the number of pigs per pen, the numbers of pigs tested for antibodies to CSF virus and the distribution of the seropositive pigs in the compartment, this model gives again a period in which the virus most probably entered the herd. Using the findings in 41 herds where the infection started in the section of the finishers or weaned piglets of the age of 8 weeks or older, and of which only a single contact with a herd previously infected was known, there was no reason to reject the model. Thus, we concluded that the transmission between weaned piglets and finishing pigs during the epidemic was not significantly different from the transmission observed in the experiments.

Diva vaccines that reduce virus transmission

This brief review deals with the effect of diva (Differentiating Infected from VAccinated individuals) vaccines (also termed marker vaccines) on transmission of herpesviruses and pestiviruses in swine and cattle. Pseudorabies and bovine herpesvirus 1 diva vaccines have been demonstrated to reduce transmission of wild-type virus in populations of pigs and cattle in the laboratory as well as in the field. A subunit diva vaccine based on the immunodominant E2 protein of classical swine fever virus that is expressed in the baculovirus system may reduce transmission of wild-type virus among pigs and also transmission from mother to foetuses. A similar diva vaccine against bovine virus diarrhoea infections protected sheep against transplacental transmission of antigenically homologous wild-type virus. Diva vaccines along with their companion diagnostic tests can play a role in control of infections, ultimately leading to eradication of viruses.

Implications derived from a mathematical model for eradication of pseudorabies virus

Simple mathematical models based on experimental and observational data were applied to evaluate the feasibility of eradicating pseudorabies virus (PRV) regionally by vaccination and to determine which factors can jeopardise eradication. As much as possible, the models were uncomplicated and our conclusions were based on mathematical analysis. For complicated situations, Monte-Carlo simulation was used to support the conclusions. For eradication, it is sufficient that the reproduction ratio R (the number of units infected by one infectious unit) is < 1. However, R can be determined at different scales: at one end the region with the herds as units and at the other end compartments with the pigs as units. Results from modelling within herds showed that contacts between groups within a herd is important whenever R between individuals (R(ind)) is > 1 in one or more groups. This is the case within finishing herds. In addition, if the R(ind) is more than 1 within a herd, the size of the herd determines whether PRV can persist in the herd and determines the duration of persistence. Moreover, when reactivation of PRV in well-vaccinated sows is taken into account, R(ind) in sow herds is still less than 1. In sow herds with group-housing systems, it is possible that in those groups R(ind) is > 1. Results from modelling between herds showed that whether or not Rherd is < 1 in a particular region is determined by two factors: (1) the transmission of infection between nucleus herds and rearing herds through transfer of animals and (2) contacts among finishing herds and among rearing herds. The transmission between herds can be reduced by reduction of the contact rate between herds. reduction of the herd size, and reduction of the transmission within herds.

Dynamics of pseudorabies virus infections in vaccinated pig populations: a review

This paper reviews our studies on the dynamics of pseudorabies virus (PRV) infections in populations of vaccinated pigs. By using mathematical models, experiments, and field observations, we have been able to quantify PRV transmission by the reproduction ratio R, which is defined as the average number of secondary cases caused by one infectious individual. If R is less than 1, PRV infections will fade out in the pig population and eradication is certain. Under experimental conditions, R of double-vaccinated pigs was estimated at 0.3. In the field, R was estimated at 0.7 among multiple-vaccinated breeding pigs, 3.4 among single vaccinated finishing pigs, and 1.5 among double-vaccinated finishing pigs. So far, no risk factors have been identified that explain the difference between the transmission among double-vaccinated pigs in the field and under experimental conditions. The implications of the transmission characteristics of the different types of pigs for the Dutch PRV eradication campaign are discussed. The structure of the PRV research programme described in this paper, in which knowledge of the interaction of the virus with individual pigs is extrapolated to the interaction of the virus with pig populations, can serve as an example for other research programmes that study infectious diseases.

Aujeszky's disease (pseudorabies) virus eradication campaign in The Netherlands

The Dutch Aujeszky's disease virus (ADV) eradication campaign is based on vaccination with glycoprotein E deleted vaccines. In the first stage of the programme, that was started in September 1993, the transmission of ADV must be reduced sharply. Subsequently, the remaining sources of virus need to be traced and eliminated. During the final stage, vaccination should be forbidden. This paper summarizes the observations made during a field study on the eradication of ADV by vaccination and reports the design and preliminary results of the first stage of the Dutch eradication campaign.

Computer simulation to support policy making in the control of pseudorabies

A further integration of international markets makes a coordinated policy against contagious animal infections increasingly important. In the future, stricter demands are to be expected concerning the control and eradication of such infections. To anticipate these demands, a computer simulation model is created in which scenarios can be evaluated with respect to epidemiological and economic effects of the infections and control strategies. In this paper, the simulation model is described for Pseudorabies in swine. In the model, the population of herds is subdivided into two main herd types: breeding and finishing. Each herd is in one of 24 states per herd type. The states are based on (1) the reproduction ratio R which is the number of secondary cases caused by one infectious herd, (2) the prevalence for each value of R and (3) the expected number of infectious animals in an infectious herd within each prevalence range and for each R. The different values of R are based on experiments and field data in which different vaccination strategies were used. The transition matrix with the probabilities of every transition from one state to another is calculated on a weekly base. With this matrix the distribution of herds over states from week to week is derived. To include a dynamic element in the transition probabilities, the number of newly infectious herds per week is a function of animal and other contacts, including aerial, material and personal contacts. Calculations show that the infection in the Dutch swine population will not disappear without vaccination. With a vaccination scheme in which sows are vaccinated 3 times per year and fattening pigs 1 time per cycle the infection will ultimately be eradicated, but 2 vaccinations per cycle for fattening pigs are needed to eradicate the infection within an acceptable timespan (i.e. 2 to 3 years). The latter strategy will become compulsory in the Netherlands from October 1st 1995.

The influence of maternal immunity on the transmission of pseudorabies virus and on the effectiveness of vaccination

The purpose of this study was to investigate whether maternal immunity could prevent transmission of pseudorabies virus (PRV) among pigs, and whether it reduced the effectiveness of a single or double vaccination with regard to the transmission of PRV. In five experiments, the transmission of PRV, expressed as the reproduction ratio R, was compared in groups of pigs with maternal immunity and in groups of pigs without maternal immunity. Transmission of PRV among unvaccinated pigs with maternal immunity (R = 0.2) was significantly lower than among pigs without maternal immunity (R = 6.3). Furthermore, maternal immunity in young pigs prevented transmission of PRV, as R was significantly below one. In once-vaccinated groups, PRV spread extensively among pigs with maternal immunity (R = 23), but did not spread extensively among pigs without maternal immunity (R = 0.6). In twice-vaccinated groups, transmission of PRV among pigs with maternal immunity (R = 0.6) did not differ significantly from the transmission of PRV among pigs without maternal immunity (R = 0.3). Thus, a single vaccination of pigs with PRV strain 783 at 10 weeks of age, when they still possessed maternal immunity, seemed not sufficient to prevent transmission of PRV. Virus transmission could be reduced, however, if maternally immune pigs were vaccinated twice at 10 and 14 weeks of age.