INFECTIOUS BURSAL DISEASE - INFLUENCE OF VVIBDV ON IMMUNITY TO RANIKHET-DISEASE AT THE FIELD LEVEL

The seroepidemiological data of Ranikhet disease (RD) and infectious b ursal disease (IBD), before (during 1991-92) and after (during 1993-94 ) the outbreak of very virulent form of IBD (vvIBD) in Tamil Nadu, wer e statistically analysed. During 1993-94 the half-life (t(1/2)) of RD- maternally derived antibodies (RD-MDA) was 3.2 days and the IBD-MDA wa s 4.11 days in clean premises. In the infected premises t(1/2) of the RD-MDA was 2.69 days and the t(1/2) of RD-vaccinal antibodies was 1.85 days. The observed RD vaccinal titres of samples collected during 199 3-94 are significantly lower than the statistically predicted HI-titre s (using 1991-92 titres as the base), for that age. Mathematical concl usions indicate that vvIBDV could be the cause of this perceived diffe rence in the RD-HI titre values.