|Epidemiology and control of classical swine fever (CSF) in wild boar and potential use of a newly developed live marker vaccine. |
Summary of an EU research project (SSPE-CT-2003-501599) in the framework of the FP6 programme.
Classical swine fever virus (CSFV) is a recurring infection of domestic pigs and wild boar. Notwithstanding the relative success in eradicating the disease in certain parts of Europe and despite intensive efforts on national as well as on international level, the complete eradication of CSF in Europe has proved to be elusive. Additionally, the disease is still endemic in some new EU member states (Romania and Bulgaria) and sporadic outbreaks still occur in free areas. Although the specific role of wild boars as CSFV reservoirs needs to be further clarified it can maintain the infection in the environment and is probably responsible for its reoccurrence. The latter is very important as in the last decade, the wild boar population has increased all over Europe and wild boar density is recognised as one of the most relevant risk factor enabling endemic infections. Direct and indirect contacts between domestic and wild species and illegal swill feeding practices have been demonstrated as the main source for the spreading of the infection in domestic pigs. The relevant risk of introduction of CSFV from wild boar is clearly demonstrated by the facts that about 80% of primary CSF outbreaks in domestic pigs have occurred in regions where CSF in wild boar is endemic. Therefore, any program aimed at a permanent eradication or control at long term of CSF should include methodologies which allow not only an efficient approach in domestic animals but also in wild boar. However, the latter is hampered by the lack of knowledge regarding wild boar population dynamics and structure as well as epidemiological parameters influencing the course of the infection within it. By using existing literature or the development of new applications, different parameters were obtained and subsequently used to design a new mathematical model based upon a metapopulation principle. The model allows a better understanding of CSFV epidemiology in wild boar populations and the evaluation of the impact of control strategies, such as hunting and vaccination, on the course of the infection. Hunting showed to be an ineffective way to control the infection as only unrealistically intensive hunting efforts could eradicate the infection. Although in small populations (<1000 animals) a non-intervention policy revealed to be successful, vaccination was demonstrated to be an effective tool in controlling the CSFV infection as it always reduces the epidemic peak. The chance of successful eradication of the infection is determined by the percentage of the susceptible population that is vaccinated within a short range of time. While a 60% vaccination rate of the susceptible animals will lead to prompt eradication, 20% will increase the probability of endemic stability of the infection. The latter clearly shows that when the decision is made to vaccinate, it should be done in such a manner that a very large portion of the population is reached.
On the domestic pig level, the current control and eradication strategies for CSF management are based on stamping out the infection combined with pre-emptive culling. Notwithstanding the efficacy of this approach, there is an increasing ethical concern and public resistance, as numerous healthy, uninfected animals are killed and destroyed. Furthermore, this stamping-out policy is not suited for infection management in wild boar populations. An interesting alternative to this current strategy is vaccination. However, the contemporary available live vaccines as C-strain, although potent, resulting in a rapid onset and life-long immunity, do not allow serological differentiation from infected animals. In contrast, such a differentiation is possible with commercial E2-marker subunit vaccines, but their reduced efficacy in preventing horizontal and vertical CSFV transmission and the lack of an oral applicability limits it general application. Therefore there is a great need for a new type of vaccine which combines all the benefits (efficient and long-term protection, differentiation capability and oral application) but none of the disadvantages. This was addressed by the development of a live marker vaccine, whereby the E2-region of BVDV (CP7) and CSFV (C-strain) were replaced by the corresponding sequence of CSFV (Alfort 187) and BDV (Gifhorn) respectively. Although the CSFV-based candidates (e.g. pRiems_ABCgif) were safe to use, the protection granted by them was not always complete, especially when given orally, or differentiation with wild type CSFV was not unequivocally possible using commercial ELISA systems. Conversely, the BVDV-based candidate, CP7_E2alf, was found to induce complete sterile immunity in domestic pigs as well as wild boar independent of the application method. CP7_E2alf is not only the highly efficient, it is also safe to use. The latter was demonstrated as in none of the performed animal trials any significant adverse effects were observed in domestic pigs and wild boar. Similarly, no effects on farrowing or the birth performance of the piglets were found upon vaccination of pregnant sows. Furthermore, no serological or virological evidence could be found for the presence of the CP7_E2alf in young ruminants and rabbits upon oral application, and even intramuscular application to cattle and sheep did not result in detectable vaccine virus replication or shedding. The latter confirmed the changed cell tropism of CP7_E2alf from bovine to porcine previously observed in vitro. In summary, it can be stated that transmission of CP7_E2alf, even in field conditions, to other species is very unlikely. The field applications during CSFV vaccination campaigns in wild boar have encountered another problem, namely the limited uptake of baits by young animals. The latter is important because independently of the efficacy of the vaccine, this can lead to a part of the wild boar population, already more vulnerable to infection, remaining unprotected. This could contribute to the persistence of the infection. For this purpose new small spherical and cuboid baits were designed and constructed. The new 3 cm spherical bait clearly showed an improved uptake rate in young animals up to 3.5 months. However, even this new small bait was not taken up by animals younger than 3 months, probably due to the fact that they prefer suckling. This has important implications in any vaccination strategy as it has to be kept in mind that these young animals cannot be immunized in this way. Repeated baiting or baiting at a time point sufficiently long after farrowing (when all the gruntlings are older than 3 months) is therefore desirable.
As previously stated, the disadvantage of the contemporary live vaccines is the inability to distinguish vaccinated from infected animals. In order for the newly developed live marker vaccine to be useable in general control programs, novel accompanying diagnostic tools were developed and subsequently evaluated in an interlaboratory evaluation (ILE). At the start of the project it was decided to evaluate the DIVA potential of existing commercial ELISA systems. Similar to earlier initial results, differentiation between vaccinated and infected animals was possible for CP7_E2alf and to a lesser degree for the CSFV-based candidate, using existing commercial systems (ERNS-antibody ELSIA). During the last decade PCR-based techniques have become more and more important as a diagnostic tool, especially with the development of real-time RT-PCR. These techniques combine high throughput capacity with a superior sensitivity and reduced labour requirements, compared to virus isolation (VI). Such advantages are important during outbreaks because they allow an earlier detection of the infection (in this project real-time RT-PCR detected CSFV 1 to 2 days earlier than VI), and permit a larger number of samples to be analysed within shorter time. The latter is further accentuated by the ability to pool at least 5 samples for real-time RT-PCR evaluation without loss of sensitivity as demonstrated in this project. However, early detection is influenced by the choice of tissue used for analysis. The detection of the virus in tonsil preceded that of other tissues, including blood, and is therefore the most suited tissue for early as well as long term detection. On the other hand, blood can be used as an alternative on herd level if a larger number of samples are analysed to compensate for the lower probability of detecting the virus. The availability of accompanying genetic diagnostic tools is an important criterion for a more generalised use of a CSFV vaccine. Therefore classic gel-based and real-time RT-PCRs were developed that are able to differentiate between CSFV and the new vaccine candidate CP7_E2alf (genetic DIVA). Despite the diagnostic potential of PCR-based methods, some issues have arisen regarding samples which were scored positive by PCR but negative by VI. Although research in the field of CSFV was one of the first to report this problem; it is not restricted to it as it is now also described for other viruses. In addition to differences in detection sensitivity and antibody complexation, it was demonstrated, by using a newly developed RT-PCR panel, that the presence of viral genome fragments in the sample can equally be the cause of discrepant PCR and VI results.
In summary, the work performed during this CSF vaccine & wild boar resulted not only in an enhanced insight in the epidemiological situation of CSF in wild boar but also to a validated model allowing the evaluation of the impact of different control strategies on the infection course. In addition, with the development and the production under GMP-conditions of a novel potent and safe live marker vaccine and its accompanying genetic and serological DIVA tools, an interesting alternative can be presented to the current control and eradication strategies.
Frank Koenen1,Andy Haegeman1, Sándor Belák2, Katinka Belák2, Martin Beer3, Christoph Staubach3, Volker Moennig4, Guberti Vittorio5, Martin Hofmann6, Juan Plana-Durán7, Jose Manuel Sanchez-Vizcaíno8, Gábor Kulcsár9.
1) Veterinary and Agrochemical Research centre, Brussels, Belgium;
2) The National Veterinary Institute (Virology), Uppsala, Sweden;
3) The National Veterinary Institute (Pathology), Uppsala, Sweden;
4) Friedrich-Loeffler-Institute (Virology), Island of Riems, Germany;
5) Friedrich-Loeffler-Institute (Epidemiology), Island of Riems, Germany;
6) Institute of Virology, School of Veterinary Medicine Hannover, Hannover, Germany;
7) Istituto Nazionale per la Fauna Selvatica, Ozzano Emilia, Italy;
8) Institute of Virology and Immunoprophylaxis, Mittelhäusern, Switzerland;
9) Fort Dodge Veterinaria SA, Vall de Bianya (Girona), Spain;
10) Universidad Complutense de Madrid (Sanidad Animal), Madrid, Spain;
11) Central Agricultural Office, Directorate of Veterinary Medicinal Products, Budapest , Hungary
Project acronym: CSFVACCINE & WILD BOAR
Project’s official full title: Epidemiology and control of classical swine fever (CSF) in wild boar and potential use of a newly developed live marker vaccine
Research priority: New and more environment friendly production methods to improve animal health and welfare including research on animal diseases such as foot and mouth disease, swine fever and development of marker vaccines
European Commission scientific officer: Isabel Minguez-Tudela (Isabel.Minguez-Tudela@ec.europa.eu)
Dr Frank Koenen
Department of Virology
Section: Modelisation of Epizootic Diseases
Tel: + 32 2 379 05 18
Fax: + 32 2 379 06 70