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.