B. Newcastle Disease Virus (ND)

One hundred and twenty eight groups of sera were tested by HI, mainly in 1993, and a further 613 were tested by Elisa (Idexx), mainly in 1994. The results are plotted against age in Figures 3.16 and 3.17. Strongly positive reactions due to maternal antibody (at 14 days) and due to the combined or separate effects of vaccination and field challenge (after 30 days) are evident in both tests. The regression lines are based on submissions from birds of 21 days or more of age to exclude the effect of maternal antibody. As was the case in broiler parents, the Elisa mean titre group figures tend to be higher than the HI GMT values. Positive reactions were more common in 1994 than in 1993, though this may have, in part, been due to increased testing. There is a clear trend towards increasing Elisa titres with age suggesting that lentogenic ND may be a contributor to respiratory problems on some farms. No evidence of mesogenic or velogenic ND has been seen on these farms.