A diagnostic assay to differentiate antibodies induced by foot-and-mouth disease virus (FMDV) infection from those induced by vaccination was developed. The test is an indirect-trapping ELISA which uses a monoclonal antibody to trap the non-structural 3ABC-FMDV polypeptide expressed in E. coli. Experimental and field sera from naive, vaccinated and infected cattle were examined. Using the established threshold of 0.20 optical density units, the sensitivity of the assay was 100%, as all the experimental post-infection sera (n degree = 137) gave values greater than this threshold, irrespective of the FMDV serotype used for the infection. In contrast, more than 99% of sera from vaccinated animals were negative (225 out of 228 primo-vaccinates and 159 out of 159 multi-vaccinates). A high degree of specificity was also confirmed by the finding that 99.5% (442 out of 444) of sera from naive animals gave negative results. Serum conversion against 3ABC was first detected 8 days post-infection and demonstrable levels of 3ABC specific antibodies were detectable at least 1 year post-infection. The described 3ABC-ELISA is safe, cheap and also easy to perform in large scale serological surveys. The high specificity and sensitivity makes this test an ideal tool for FMD eradication campaigns and control programs.

A radioimmunoprecipitation assay (RIPA) has been developed for detection of antibody to African swine fever virus (ASFV) and compared with the immunoblot assay with regard to sensitivity and specificity. Two hundred seven field sera, obtained from pigs in Spain from different geographic areas between 1975 and 1986, that were positive by ASFV enzyme-linked immunosorbent assay (ELISA) were also analysed by immunoblot assay and RIPA. By serum dilution experiments, the RIPA appeared at least as sensitive as the ELISA and immunoblotting tests, although ELISA and RIPA detected antibodies to ASFV earlier in natural infection than did the immunoblot assay, as disclosed by animal inoculation studies. The most antigenic ASFV-induced proteins in natural infection detected by RIPA were the viral proteins p243, p172, p73, p25.5, p15, and p12 and the infection proteins p30 and p23.5. In the immunoblot assay, the proteins that were most reactive with the same sera were the viral protein p25.5 and the infection proteins p30, p25, and p21.5. Only 1 serum, from an animal infected with ASFV, was negative by immunoblot assay but showed a positive result by RIPA. A modification of conventional RIPA was performed using a dot transference of immunoprecipitated proteins to a nitrocellulose filter. This modification simplified the conventional RIPA procedures by eliminating the electrophoresis of immunoprecipitated proteins without affecting sensitivity and specificity. The ease of use, specificity, and the sensitivity comparable to that of the immunoblot assay make the RIPA a useful confirmatory assay for sera that yield conflicting results in other ASFV antibody assays.

The epitope specificity of the protective immune response against swine transmissible gastroenteritis (TGE) has been investigated by using circulating and secretory antibodies. This study was carried out with sows vaccinated with TGEV or the antigenically related porcine respiratory coronavirus (PRCV). TGEV vaccination of sows resulted in greater lactogenic protection of suckling piglets against TGEV challenge and a higher secretory immune response than PRCV vaccination did. These differences in the immune response were conditioned by the route of antigen presentation as a result of the different tropism of each virus. Epitopes on S protein, and in particular those contained in its antigenic site A, were more immunogenic than epitopes on N and M proteins in both groups of vaccinated sows, as determined by a competitive radioimmunoassay. Minor differences in antibody response against the previously defined antigenic subsites Aa, Ab, and Ac were also detected, with subsite Ab being the most antigenic in both TGEVand PRCV-immune sows. These findings suggest that antigenic site A on S protein, involved in virus neutralization, is the immunodominant site in pregnant sows that confer lactogenic protection. They also validate, in experiments with secretory antibodies, the antigenic maps made with murine monoclonal antibodies. Therefore, this antigenic site should be considered for vaccine or diagnostic development.

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