Consideration of different outbreak conditions in the evaluation of preventive culling and emergency vaccination to control foot and mouth disease epidemics

In recent foot and mouth disease outbreaks, many healthy animals have been culled to prevent disease transmission. Emergency vaccination is discussed as an alternative to culling of unaffected animals. A spatial and temporal Monte–Carlo simulation model was used to compare preventive culling and emergency vaccination. Different outbreaks are described using additional influence factors such as airborne spread, farm density, type of index-case farm and delay until establishment of the control strategies. The fewest farms were infected establishing a combined strategy including a 1 km preventive culling and 1–10 km emergency vaccination zone around each outbreak farm. Taking the number of culled and vaccinated farms into account, vaccination around the first diagnosed farm combined with the baseline strategy (culling of outbreak farms, protection and surveillance zone, contact tracing) is to be preferred. In the present study, emergency vaccination was an effective control strategy especially in densely populated regions.     

How do resources influence control measures during a simulated outbreak of foot and mouth disease in Australia?

An outbreak of foot and mouth disease (FMD) could seriously impact Australia's livestock sector and economy. As an FMD-free country, an outbreak would trigger a major disease control and eradication program that would include the culling of infected and at risk animals (‘stamping out’), movement restrictions and zoo-sanitary measures. Additional control measures may also include pre-emptive culling or vaccination. However, it is unclear what disease strategy would be most effective under Australian conditions and different resource levels. Using a stochastic simulation model that describes FMD transmission between farms in a livestock dense region of Australia, our results suggest that using current estimates of human resource capacity for surveillance, infected premises operations and vaccination, outbreaks were effectively controlled under a stamping out strategy. However, under more constrained resource allocations, ring vaccination was more likely to achieve eradication faster than stamping out or pre-emptive culling strategies.     

Potential impact of species and livestock density on the epidemic size and effectiveness of control measures for foot-and-mouth disease in Japan

The characteristics of a livestock area, including farm density and animal species, influence the spread of foot-and-mouth disease (FMD). In this study, the impact of livestock area on FMD epidemics was examined using an FMD transmission model. For this simulation, three major livestock areas were selected: the 2010 FMD epidemic area in Japan as the baseline area (BS), a cattle and pig mixed production area (CP) and a cattle production area (C). Simulation results demonstrated that under the 24-hr culling policy, only 12% of epidemics among 1,000 simulations were abated within 100 days in the CP area, whereas 90% of the epidemics ceased in the BS area. In the C area, all epidemics were successfully contained within 100 days. Evaluation of additional control measures in the CP area showed that the 0.5-km pre-emptive culling, even when only targeting pig farms, raised the potential for successful containment to 94%. A 10-km vaccination on day 7 or 14 after initial detection was also effective in halting the epidemics (80%), but accompanied a large number of culled or vaccinated farms. The combined strategy of 10-km vaccination and 0.5-km pre-emptive culling targeting pig farms succeeded in containing all epidemics within 100 days. The present study suggests the importance of preparedness for the 24-hr culling policy and additional control measures when an FMD outbreak occurs in a densely populated area. Considering the characteristics of the livestock area is important in planning FMD control strategies.     

The foot-and-mouth disease epidemic in The Netherlands in 2001

An outbreak of foot-and-mouth disease (FMD) in Great Britain was reported on 21 February 2001, followed by an outbreak of FMD in The Netherlands a month later. This Dutch index outbreak occurred on a mixed, veal-calf/dairy-goat farm in Oene, in the central part of The Netherlands. The most-likely route of infection was the import of Irish veal-calves to this Dutch herd via an FMD-contaminated staging point in France. With hindsight, more herds seemed to be infected by the time the index outbreak was confirmed. The regular EU control measures were implemented, in combination with pre-emptive culling of herds within 1km of each outbreak. Nevertheless, more outbreaks of FMD occurred. Most of the virus infections on those farms were "neighborhood infections". Because the situation seemed out of control locally and the destruction capacity became insufficient, it was decided to implement an emergency vaccination strategy for all biungulates in a large area around Oene to stop further spread of the virus. All susceptible animals on approximately 1800 farms in this area were vaccinated. All farms subsequently were depopulated, starting from 2 weeks after vaccination. In total, 26 outbreaks were detected (the last outbreak on 22 April 2001). In total, approximately 260,000 animals were killed.     

The role of movement restrictions and pre-emptive destruction in the emergency control strategy against CSF outbreaks in domestic pigs

Classical swine fever (CSF) outbreaks in domestic pig herds lead to the implementation of standard control measures according to legislative regulations. Ideal outbreak control entails the swift and efficient culling of all pigs on premises detected positive for CSF virus. Often all pig holdings around the detected cases are pre-emptively destroyed to exclude transmission into the neighbourhood. In addition to these measures, zones are defined in which surveillance and protection measures are intensified to prevent further distant disease spread. In particular, all movements are prohibited within standstill areas. Standstill also excludes the transport of fattened pigs to slaughter. Historical outbreaks provide evidence of the success of this control strategy. However, the extent to which the individual strategy elements contribute to this success is unknown. Therefore, we applied a spatially and temporally explicit epidemic model to the problem. Its rule-based formulation is tailored to a one-by-one model implementation of existing control concepts. Using a comparative model analysis the individual contributions of single measures to overall control success were revealed. From the results of the model we concluded that movement restrictions had the dominant impact on strategy performance suggesting a reversal of the current conceptual thinking. Additional measures such as pre-emptive culling only became relevant under imperfect compliance with movement restrictions. The importance of movement restrictions for the overall control success illustrates the need for explicit consideration of this measure when contingency strategies are being amended (e.g. emergency vaccination) and associated risks assessed.     

Risk based culling for highly infectious diseases of livestock

ABSTRACT: The control of highly infectious diseases of livestock such as classical swine fever, foot-and-mouth disease, and avian influenza is fraught with ethical, economic, and public health dilemmas. Attempts to control outbreaks of these pathogens rely on massive culling of infected farms, and farms deemed to be at risk of infection. Conventional approaches usually involve the preventive culling of all farms within a certain radius of an infected farm. Here we propose a novel culling strategy that is based on the idea that farms that have the highest expected number of secondary infections should be culled first. We show that, in comparison with conventional approaches (ring culling), our new method of risk based culling can reduce the total number of farms that need to be culled, the number of culled infected farms (and thus the expected number of human infections in case of a zoonosis), and the duration of the epidemic. Our novel risk based culling strategy requires three pieces of information, viz. the location of all farms in the area at risk, the moments when infected farms are detected, and an estimate of the distance-dependent probability of transmission.     

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

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

The classical swine fever epidemic 1997-1998 in The Netherlands: descriptive epidemiology

The objective of this paper is to describe the severe epidemic of classical swine fever (CSF) in The Netherlands in 1997–1998 under a policy of non-vaccination, intensive surveillance, pre-emptive slaughter and stamping out in an area which has one of the highest pig and herd densities in Europe.

The primary outbreak was detected on 4 February 1997 on a mixed sow and finishing pig herd. A total of 429 outbreaks was observed during the epidemic, and approximately 700 000 pigs from these herds were slaughtered. Among these outbreaks were two artificial insemination centres, which resulted in a CSF-suspect declaration of 1680 pig herds (mainly located in the southern part of The Netherlands). The time between introduction of CSF virus (CSFV) into the country and diagnosis of CSF in the primary outbreak was estimated to be approximately 6 weeks. It is presumed that CSFV was spread from The Netherlands to Italy and Spain via shipment of infected piglets in the beginning of February 1997, before the establishment of a total stand-still of transportation. In June 1997, CSFV is presumed to be introduced into Belgium from The Netherlands.

Pre-emptive slaughter of herds that had been in contact with infected herds or were located in close vicinity of infected herds, was carried out around the first two outbreaks. However, this policy was not further exercised till mid-April 1997, when pre-emptive slaughter became a standard operational procedure for the rest of the epidemic. In total, 1286 pig herds were pre-emptively slaughtered. (approximately 1.1 million pigs). A total of 44 outbreaks (10%) was detected via pre-emptive slaughter.

When there were clinical signs, the observed symptoms in infected herds were mainly atypical: fever, apathy, ataxia or a combination of these signs. In 322 out of 429 outbreaks (75%), detection was bases on clinical signs observed: 32% was detected by the farmer, 25% by the veterinary practitioner, 10% of the outbreaks by tracing teams and 8% by screening teams of the veterinary authorities. In 76% of the outbreaks detected by clinical signs, the farmer reported to have seen clinical symptoms for less than 1 week before diagnosis, in 22% for 1–4 weeks before diagnosis, and in 4 herds (1%) the farmer reported to have seen clinical symptoms for more than 4 weeks before diagnosis.

Transportation lorries played a major role in the transmission of CSFV before the primary outbreak was diagnosed. It is estimated that approximately 39 herds were already infected before the first measures of the eradication campaign came into force.

After the first measures to stop the spread of CSFV had been implemented, the distribution of the most likely routes of transmission markedly changed. In most outbreaks, a neighbourhood infection was indicated.

Basically, there were two reasons for this catastrophe. Firstly, there was the extent of the period between introduction of the virus in the region and detection of the first outbreak. As a result, CSFV had opportunities to spread from one herd to another during this period. Secondly, the measures initially taken did not prove sufficient in the swine- and herd-dense region involved.     

A decision-tree to optimise control measures during the early stage of a foot-and-mouth disease epidemic

A decision-tree was developed to support decision making on control measures during the first days after the declaration of an outbreak of foot-and-mouth disease (FMD). The objective of the tree was to minimise direct costs and export losses of FMD epidemics under several scenarios based on livestock and herd density in the outbreak region, the possibility of airborne spread, and the time between first infection and first detection. The starting point of the tree was an epidemiological model based on a deterministic susceptible–infectious–recovered approach. The effect of four control strategies on FMD dynamics was modelled. In addition to the standard control strategy of stamping out and culling of high-risk contact herds, strategies involving ring culling within 1 km of an infected herd, ring-vaccination within 1 km of an infected herd, and ring-vaccination within 3 km of an infected herd were assessed. An economic model converted outbreak and control effects of farming and processing operations into estimates of direct costs and export losses. Ring-vaccination is the economically optimal control strategy for densely populated livestock areas whereas ring culling is the economically optimal control strategy for sparsely populated livestock areas.     

Comparing the epidemiological and economic effects of control strategies against classical swine fever in Denmark

In 2006, total Danish pork exports were valued at €3.8 billion, corresponding to approximately 5% of the total Danish exports, and an outbreak of a notifiable disease would have dramatic consequences for the agricultural sector in Denmark. Several outbreaks of classical swine fever (CSF) have occurred in Europe within the last decade, and different control strategies have been suggested. The objective of this study was to simulate the epidemiological and economic consequences of such control strategies in a CSF epidemic under Danish conditions with respect to herd demographics and geography and to investigate the effect of extra biosecurity measures on farms. We used InterSpread Plus to model the effect of nine different control strategies: the minimum measures required by the EU plus depopulation of contact herds (EUplus), extra depopulation of neighbouring herds, extra surveillance within the protection and surveillance zones, extra biosecurity in SPF herds—or in all herds, vaccination of all pigs in the 1 or 2 km zones using live vaccine as a protective measure (vaccination-to-kill), vaccination of all weaners and finishers in the 1 or 2 km zones using an E2 marker vaccine as a suppressive measure (vaccination-to-live). Each epidemic was simulated to start in four different index herds: production herds located in low, medium and high pig density areas, respectively; and a nucleus herd in an area of high pig density. For each control strategy and index case, we calculated the size and duration of the epidemic, the number of depopulated and/or vaccinated herds and animals, the control costs borne by the public and the pig industry, respectively, as well as the loss of exports associated with the epidemic.The simulations showed that the EUplus strategy is the most effective of the evaluated strategies with respect to limiting the size, duration and cost of the epidemic, regardless of the index case. However, regarding the number of slaughtered animals, the vaccination-to-live strategies appeared to be more effective.Epidemics become larger and last longer if the index case is a nucleus herd. This implies that biosecurity in nucleus herds is extremely important to avoid transmission of CSF to these herds.Simulations showed that a Danish CSF epidemic will be moderate in most cases and will include fewer than 10 cases and last less than 2 weeks on average. However, for some iterations, long-lasting and large epidemics were observed. Irrespective of the size and duration, an epidemic is expected to be very costly due to the export losses.     

Evaluating control strategies for outbreaks in BHV1-free areas using stochastic and spatial simulation

Several countries within the EU have successfully eradicated bovine herpesvirus type I (BHV1), while others are still making efforts to eradicate the virus. Reintroduction of the virus into BHV1-free areas can lead to major outbreaks - thereby causing severe economic losses. To give decision-makers more insight into the risk and economic consequences of BHV1 reintroduction and into the effectiveness of various control strategies, we developed the simulation model InterIBR. InterIBR is a dynamic model that takes into account risk and uncertainty and the geographic location of individual farms. Simulation of a BHV1-outbreak in the Netherlands starts with introduction of the virus on a predefined farm type, after which both within-farm and between-farm transmission are simulated. Monitoring and control measures are implemented to simulate detection of the infection and subsequent control. Economic consequences included in this study are related to losses due to infection and costs of control. In the simulated basic control strategy, dairy farms are monitored by monthly bulk-milk tests and miscellaneous farms are monitored by half-yearly serological tests. After detection, movement-control measures apply, animal contacts are traced and neighbour farms are put on surveillance. Given current assumptions on transmission dynamics, we conclude that a strategy with either rapid removal or vaccination of infected cattle does not reduce the number of infected farms compared to this basic strategy - but will cost more to control. Farm type with first introduction of BHV1 has a considerable impact on the expected number of secondarily infected farms and total costs. To limit the number of infected farms and total costs due to outbreaks, we suggest intensifying the monitoring program on farms with a high frequency of cattle trade, and monthly bulk-milk testing on dairy farms.     

Vaccination of weaner pigs against classical swine fever with the subunit vaccine "Porcilis Pesti": influence of different immunization plans on excretion and transmission of challenge virus

Excretion and transmission of CSFV after vaccination with the CSF subunit marker vaccine "Porcilis Pesti" have been studied using the following different vaccination schedules: Group A--two vaccinations with an interval of 28 d, challenge 14 d after second vaccination (p.v2.); group B--two vaccinations with an interval of 14 d, challenge 14 d later; group C--two vaccinations with an interval of 28 d, challenge at time of booster vaccination; group D--two vaccinations with an interval of 14 d, challenge 7 d p.v2.; group E--single vaccination and infection 14 d later. After infection one sentinel pig was added to the vaccinated and infected pigs of each group. A single vaccination did not induce protective immunity against a CSFV challenge. Double vaccination at a four-week interval protected piglets from clinical infection, and neither viraemia and leukopenia nor virus excretion were detected if infected 14 d p.v2. Two vaccinations at a two-week interval followed by a challenge 7 d p.v2. led to a short viraemia on day 5 p.i. but without excretion of CSFV. Though all other vaccination schedules induced a reduction in virus shedding and a decrease in CSFV replication, in all these cases in-contact controls became infected. The results of transmission of CSFV are discussed in relation to a potential use of subunit marker vaccines in CSF control.     

CP7_E2alf oral vaccination confers partial protection against early classical swine fever virus challenge and interferes with pathogeny-related cytokine responses

The conventional C-strain vaccine induces early protection against classical swine fever (CSF), but infected animals cannot be distinguished from vaccinated animals. The CP7_E2alf marker vaccine, a pestivirus chimera, could be a suitable substitute for C-strain vaccine to control CSF outbreaks. In this study, single oral applications of CP7_E2alf and C-strain vaccines were compared for their efficacy to induce protection against a CSF virus (CSFV) challenge with the moderately virulent Bas-Rhin isolate, in pigs as early as two days post-immunization. This work emphasizes the powerful potential of CP7_E2alf vaccine administered orally by a rapid onset of partial protection similar to that induced by the C-strain vaccine. Furthermore, our results revealed that both vaccinations attenuated the effects induced by CSFV on production of the pig major acute phase protein (PigMAP), IFN-α, IL-12, IL-10, and TGF-β1 cytokines. By this interference, several cytokines that may play a role in the pathogeny induced by moderately virulent CSFV strains were revealed. New hypotheses concerning the role of each of these cytokines in CSFV pathogeny are discussed. Our results also show that oral vaccination with either vaccine (CP7_E2alf or C-strain) enhanced CSFV–specific IgG2 production, compared to infection alone. Interestingly, despite the similar antibody profiles displayed by both vaccines post-challenge, the production of CSFV-specific IgG1 and neutralizing antibodies without challenge was lower with CP7_E2alf vaccination than with C-strain vaccination, suggesting a slight difference in the balance of adaptive immune responses between these vaccines.     

Simulated effect of pig-population density on epidemic size and choice of control strategy for classical swine fever epidemics in The Netherlands

We examined the importance of pig-population density in the area of an outbreak of classical swine fever (CSF) for the spread of the infection and the choice of control measures. A spatial, stochastic, dynamic epidemiological simulation model linked to a sector-level market-and-trade model for The Netherlands were used. Outbreaks in sparsely and densely populated areas were compared under four different control strategies and with two alternative trade assumptions.The obligatory control strategy required by current EU legislation was predicted to be enough to eradicate an epidemic starting in an area with sparse pig population. By contrast, additional control measures would be necessary if the outbreak began in an area with high pig density. The economic consequences of using preventive slaughter rather than emergency vaccination as an additional control measure depended strongly on the reactions of trading partners. Reducing the number of animal movements significantly reduced the size and length of epidemics in areas with high pig density. The phenomenon of carrier piglets was included in the model with realistic probabilities of infection by this route, but it made a negligible contribution to the spread of the infection.     

Spatial and stochastic simulation to compare two emergency-vaccination strategies with a marker vaccine in the 1997/1998 Dutch Classical Swine Fever epidemic

Two alternative emergency-vaccination strategies with a marker vaccine that could have been applied in the 1997/1998 Dutch Classical Swine Fever (CSF) epidemic were evaluated in a modified spatial, temporal and stochastic simulation model: InterCSF. In strategy 1, vaccination would be applied only to overcome a shortage in destruction capacities. Destruction of all pigs on vaccinated farms distinguishes this strategy from strategy 2, which assumes intra-Community trade of vaccinated pig meat. InterCSF simulates the spread of CSF between farms through local spread and three contact types. Disease spread is affected by control measures implemented through different mechanisms. Economic results were generated by a separate model that calculated the direct costs (including the vaccination costs) and consequential losses for farmers and related industries subjected to control measures. The comparison (using epidemiological and economic results) between the different emergency-vaccination strategies with an earlier simulated preventive-slaughter scenario led to some general conclusions on the Dutch CSF epidemic. Both emergency-vaccination strategies were hardly more efficient than the non-vaccination scenario. The intra-Community trade strategy (vaccination-strategy 2) was the least costly of all three scenarios.     

Porcine circovirus type 2 (PCV2) infection decreases the efficacy of an attenuated classical swine fever virus (CSFV) vaccine

ABSTRACT: The Lapinized Philippines Coronel (LPC) vaccine, an attenuated strain of classical swine fever virus (CSFV), is an important tool for the prevention and control of CSFV infection and is widely and routinely used in most CSF endemic areas, including Taiwan. The aim of this study was to investigate whether PCV2 infection affects the efficacy of the LPC vaccine. Eighteen 6-week-old, cesarean-derived and colostrum-deprived (CDCD), crossbred pigs were randomly assigned to four groups. A total of 105.3 TCID50 of PCV2 was experimentally inoculated into pigs through both intranasal and intramuscular routes at 0 days post-inoculation (dpi) followed by LPC vaccination 12 days later. All the animals were challenged with wild-type CSFV (ALD stain) at 27 dpi and euthanized at 45 dpi. Following CSFV challenge, the LPC-vaccinated pigs pre-inoculated with PCV2 showed transient fever, viremia, and viral shedding in the saliva and feces. The number of IgM+, CD4+CD8-CD25+, CD4+CD8+CD25+, and CD4-CD8+CD25+ lymphocyte subsets and the level of neutralizing antibodies against CSFV were significantly higher in the animals with LPC vaccination alone than in the pigs with PCV2 inoculation/LPC vaccination. In addition, PCV2-derived inhibition of the CSFV-specific cell proliferative response of peripheral blood mononuclear cells (PBMCs) was demonstrated in an ex vivo experiment. These findings indicate that PCV2 infection decreases the efficacy of the LPC vaccine. This PCV2-derived interference may not only allow the invasion of wild-type CSFV in pig farms but also increases the difficulty of CSF prevention and control in CSF endemic areas.     

Modeling the spread and control of foot-and-mouth disease in Pennsylvania following its discovery and options for control

In this paper, we simulate outbreaks of foot-and-mouth disease in the Commonwealth of Pennsylvania, USA - after the introduction of a state-wide movement ban - as they might unfold in the presence of mitigation strategies. We have adapted a model previously used to investigate FMD control policies in the UK to examine the potential for disease spread given an infection seeded in each county in Pennsylvania. The results are highly dependent upon the county of introduction and the spatial scale of transmission. Should the transmission kernel be identical to that for the UK, the epidemic impact is limited to fewer than 20 premises, regardless of the county of introduction. However, for wider kernels where infection can spread further, outbreaks seeded in or near the county with highest density of premises and animals result in large epidemics (>150 premises). Ring culling and vaccination reduce epidemic size, with the optimal radius of the rings being dependent upon the county of introduction. Should the kernel width exceed a given county-dependent threshold, ring culling is unable to control the epidemic. We find that a vaccinate-to-live policy is generally preferred to ring culling (in terms of reducing the overall number of premises culled), indicating that well-targeted control can dramatically reduce the risk of large scale outbreaks of foot-and-mouth disease occurring in Pennsylvania.     

The 1997-1998 epidemic of classical swine fever in the Netherlands

In 1997, the pig husbandry in the Netherlands was struck by a severe epidemic of classical swine fever (CSF). During this epidemic 429 CSF-infected herds were depopulated and approximately 1300 herds were slaughtered pre-emptively. In addition millions of pigs of herds not CSF-infected were killed for welfare reasons (over crowding or overweight). In this paper, we describe the course of the epidemic and the measures that were taken to control it.The first outbreak was detected on 4 February 1997 in the pig dense south-eastern part of the Netherlands. We estimate that CSF virus (CSFV) had already been present in the country by that time for 5-7 weeks and that the virus had been introduced into approximately 39 herds before the eradication campaign started. This campaign consisted of stamping-out infected herds, movement restrictions and efforts to diagnose infected herds as soon as possible. However, despite these measures the rate at which new outbreaks were detected continued to rise. The epidemic faded out only upon the implementation of additional measures such as rapid pre-emptive slaughter of herds in contact with or located near infected herds, increased hygienic measures, biweekly screening of all herds by veterinary practitioners, and reduction of the transportation movements for welfare reasons. The last infected herd was depopulated on 6 March 1998.     

Vaccination against classical swine fever virus: limitations and new strategies

The most widely used vaccines for the control of classical swine fever (CSF) in countries where it is endemic are live attenuated virus strains, which are highly efficacious, inducing virtually complete protection against challenge with pathogenic virus. In the European Union (EU), the combination of prophylactic mass vaccination and culling of infected pigs in endemic regions has made it possible to almost eradicate the disease. However, it is not possible to discriminate between infected and vaccinated animals, thus hampering disease control measures that rely on serology. Therefore, vaccination was banned at the end of 1990 before the internal common market was established in the EU. Vaccination is allowed only in severe emergencies. In addition, there are strict restrictions on the international trade in pig products from countries using vaccination. To circumvent these problems, marker vaccines which allow differentiation of infected from vaccinated animals (DIVA) have been developed. There are several approaches, ranging from protective peptides, single expressed proteins, naked DNA and chimeric viruses. To date, two subunit vaccines based on the E2 glycoprotein are commercially available and have been tested extensively for their efficacy. The accompanying discriminatory tests are based on an ELISA detecting another viral glycoprotein, the E(rns). The subunit vaccines were found to be less efficacious than live attenuated vaccines. In addition, the currently available discriminatory tests do not provide high enough specificity and sensitivity. Although there is an urgent need for more advanced marker vaccines and better discriminatory tests, the development of new DIVA vaccines against CSF is hampered by the small market potential for these products.     

Transmission of classical swine fever virus by artificial insemination during the 1997-1998 epidemic in The Netherlands: a descriptive epidemiological study

In the course of the 1997-1998 CSF epidemic in the Netherlands, two semen collection centres (SCC) became infected. As an eradication strategy for an acute crisis situation, it was concluded that all semen of the boars at the SCCs collected and distributed in the risk period of 28 January to 7 March 1997 was potentially contaminated (suspect semen). As a consequence, a total of 1,680 pig herds, mainly located in the southern part of the Netherlands, were officially declared CSF suspect. The purpose of this study was to investigate whether infection of farms through contaminated semen played a significant role in the CSF epidemic. A total of 123 CSFV infected herds were identified, that had received suspect semen from one or both of the infected SCCs. In 87 out of these 123 infected herds, infection by way of artificial insemination (AI) could be excluded either according to the insemination information or the infection pattern observed. In only 21 herds, infection by way of AI was regarded as possible according to the insemination information and infection pattern. Owing to missing information, no conclusion could be drawn about the possibility of infection of 15 farms by way of AI. Thus, we conclude that at most 36 farms may have been infected through AI during the CSF epidemic in the Netherlands.