A simpler, more sensitive competitive inhibition enzyme-linked immunosorbent assay for detection of antibody to malignant catarrhal fever viruses

An earlier competitive inhibition enzyme-linked immunosorbent assay (CI-ELI SA) was developed for detection of specific antibody against malignant cata rrhal fever (MCF) viruses (MCFV) in ruminants. In this study, the indirect CI-ELISA was improved by conjugating the monoclonal antibody 15-A directly with horseradish peroxidase and by developing a method of producing precoat ed, dried antigen plates. This new test is referred to as a direct CI-ELISA . The reformatted test yielded a significantly improved sensitivity, and th e time required was reduced to about one-sixth of the previous time. Of 37 MCF cases in cattle that were confirmed by histopathology and polymerase ch ain reaction (PCR) assay, 37 (100%) were positive by the new test, whereas the indirect CI-ELISA detected only 23 (62%). The direct CI-ELISA detected antibody to MCFV in 100% of 48 sheep that had been defined as infected with ovine herpesvirus 2 (OvHV-2) by PCR, whereas the indirect CI-ELISA detecte d only 41 (85%). Comparison of antibody titers measured by the 2 assays for sera collected from OvHV-2-infected sheep and from cattle, bison, and deer with clinical sheep-associated MCF revealed that the direct CI-ELISA offer ed a 4-fold increase in analytical sensitivity over the indirect format. Th e number of seropositive animals detected by the direct CI-ELISA among appa rently normal cattle and bison was 2-3 times greater than the number detect ed by the indirect CI-ELISA, indicating that a significant percentage of no rmal cattle and bison are subclincally infected with MCFV.