Reactivity of ostrich sera to Newcastle disease virus (NDV) by enzyme-linked immunosorbent assay (ELISA) and haemagglutination inhibition (HI) test were compared. Ten-month old ostriches seronegative by both tests were vaccinated with an oil-based NDV vaccine on days 0 and 11. Significant levels of reactive antibodies were first detected on day 11 by ELISA (sample/positive ratio > 0.2 in 11/20 birds; 55%) and HI (titre > 1/8 in 10/20 birds; 50%). At the end of the experiment (day 37) all birds had significant antibody levels by ELISA, but only 16/20 (80%) by HI test. There was a sigmoidal relationship (r= 0.62, 3rd degree polynomial) between antibody levels detected by ELISA and by HI test. Antibodies reactive with NDV in naturally exposed ostriches from Zimbabwe and Botswana were also detected by ELISA (112/165; 68%) and HI (85/165; 52%).
