THE USE OF AN INDIRECT ELISA WITH PROTEIN G-PEROXIDASE CONJUGATE AND A BLOCKING ELISA TO DEMONSTRATE RECENT BLUETONGUE INFECTION IN SHEEP

The humoral immune response of sheep infected with bluetongue virus se rotypes 3, 9 and 16 was monitored by plaque inhibition (PI), blocking ELISA (BELISA) and indirect ELISA over a period of 63 days post-infect ion. Results indicated that testing of a single plasma or serum sample by both a BELISA and an indirect ELISA using a recombinant streptococ cal protein G (PrG) peroxidase conjugate enabled an assessment of the proximity of a recent infection based on the failure of PrG to bind ov ine IgM class antibodies. When BELISA and indirect ELISA results were expressed as a ratio, values indicative of recent infection (greater t han or equal to 5) were observed for an average duration of 16.5 days (range 8 to 23 days) following the initial detection of antibody by BE LISA. This approach has potential to improve diagnosis of a wide range of virus infections by providing an indicator for the relationship of serological status with a recent infection. However, where reinfectio n may occur, as with bluetongue virus, alternative methods may be requ ired for definitive diagnosis.