Marker vaccines and companion diagnostic tests for classical swine fever.

For Classical Swine Fever (CSF) a subunit vaccine consisting of the E2 protein is commercially available. The discriminatory ELISAs detect antibodies against another viral protein, the E(rns). As CSF has already been eradicated from many countries the use of a marker vaccine in these regions can only be contemplated as emergency vaccination after a new introduction of virus. Therefore, a Large Scale Marker Vaccine Trial was financed by the EU Commission and organised by the EU Reference Laboratory for CSF in 1999. When tested under the conditions of emergency vaccination, e.g. challenge before full immunity had developed, it was shown, that most CSF challenge infections took a subclinical course with reduced virus shedding. Transplacental transmission in pregnant sows could not be prevented after an application of a single vaccine dose. The most serious deficiencies have been found in the discriminatory ELISAs. Both available tests have shown deficiencies in sensitivity and specificity compared to conventional CSF antibody ELISAs. At the time, when the trial was performed, no confirmatory test was available to verify the results of the discriminatory ELISAs. Currently two new developments of marker vaccines for CSF are in progress. A chimaeric vaccine is based on infectious clones of the conventional live vaccine (C-strain) where a gene is replaced with the corresponding gene of the closely related pestivirus Bovine Viral Diarrhoea (BVD) virus. Conversely, the E2 gene of a BVD virus can be replaced by the E2 of a virulent CSF virus. The other principle is the construction of a DNA vaccine, expressing the E2 gene after entering the host cell. Deletion mutants of the E2 gene have also been constructed and tested for their induction of immunity. Both new developments are based on the same discriminatory tests as mentioned previously and developments of other principles for discrimination are rare.