EVALUATION OF AN AGAR-GEL IMMUNODIFFUSION TEST TO DETECT INFECTION OFCATTLE WITH BLUETONGUE VIRUSES IN QUEENSLAND, AUSTRALIA

An agar gel immunodiffusion (AGID) test to detect group-specific antib odies to infection of cattle by bluetongue viruses was evaluated using field collected sera in Queensland, Australia. The AGID test was comp ared to the serum neutralisation (SN) test used to detect serotype-spe cific bluetongue virus antibodies, The AGID test was found to be highl y sensitive (95% confidence interval, 80.7-100%) but to have moderate specificity (95% confidence interval, 59.3-79.6%), relative to the SN test. The correlation between AGID and SN test results was 0.41 (P < 0 .001) Using likelihood ratio estimates the chance (posttest probabilit y) of an AGID test positive serum sample being positive (titre greater than or equal to 1:20) in the SN test was estimated to be 87% and 90% for 2 + and 3 + AGID test reactions, respectively. A serum sample was estimated to have a probability of 40% of being positive in the SN te st if it gave a 1 + reaction in the AGID test. Results suggest that th e AGID test should ideally be used as a screening test in serological surveys and surveillance programs. If the test is to be used for ident ification of individually infected cattle, a cutoff point of greater t han or equal to 2 + should be employed.