Preliminary evaluation of the haemagglutination and haemagglutination inhibition tests for avian infectious bronchitis virus

Four of the 9 strains of infectious bronchitis virus which were concentrated and treated with phospholipase C type 1 showed haemagglutination activity. These strains, Holte, Massachusetts 41 (M41), H120 and Connecticut, were distinguishable by the haemagglutination inhibition (HI) test but showed much closer relationships than could be detected by the plaque reduction (PR) test. The four haemagglutinating strains were used to compare the HI and PR titres of 17 anti sera prepared against reference and field virus strains. In most cases titres were similar although there was a tendency for the HI titres to be higher than the PR titres especially with M41 antigen. HI titrations of the pooled sera from 20 birds infected with M41 showed a peak of activity at 14 days after infection which was not detected by serum neutralisation or complement fixation tests. HI titres of individual sera from birds infected 21 days previously with M41 virus compared favourably with serum neutralisation titres but showed little relationship to the complement fixation titres.