SENSITIVITY AND SPECIFICITY OF SEROLOGICAL TESTS FOR INFECTIOUS-BRONCHITIS VIRUS-ANTIBODIES IN BROILERS

Broilers with maternally-derived immunity (MDI) to infectious bronchit is (IB) were either spray-vaccinated with H120 at 1 day old, or not va ccinated, then challenged at 28 days with one of four different IBV se rotypes. Birds were bled frequently and the sera tested by agar gel pr ecipitation (AGP), haemagglutination inhibition (HI), 2 commercial ELI SAs, and virus neutralization (VN) to compare the sensitivity and spec ificity of the assays. The AGP detected a transient response to challe nge with a specificity of 100% and a sensitivity of approximately 40%. The ELISAs showed moderate sensitivity and high specificity with sera from non-vaccinated broilers, and high sensitivity and variable speci ficity with vaccinated birds. Depending on the cut-off value used, the specificity of HI tests was 55 to 100%, while the sensitivity varied widely, making identification of the serotype of an IB challenge unrel iable. In vaccinated broilers the sensitivity of the VN tests (used at 21 days post-challenge only) varied from 20 to 100%, while the specif icity was dependant on the cut-off value selected. Increases in HI, VN and ELISA titres in vaccinates were generally about half those in non -vaccinates. It is concluded that AGP and ELISA are adequate to detect antibody responses to IBV challenge in both vaccinated and non-vaccin ated broilers. In the ELISA, a cut-off value higher than that suggeste d by the manufacturers is preferred in vaccinated broilers. Similarly, a cut-off value of at least log(2)7 is desirable when attempting to u se HI for IB serotyping.