Acute infectious bursal disease in poultry: protection afforded by maternally derived antibodies and interference with live vaccination.

Maternally derived antibodies (MDA) were found insufficient to protect broiler chicks against a highly pathogenic strain of IBDV during the growing period even if the parent flocks had been boostered at point of lay by using oil emulsion vaccines. Whatever the vaccination scheme of the parent flocks, maximum mortality was observed after a challenge performed at 38 days of age in broiler and layer chicks, suggesting that the offspring need to be vaccinated with live vaccines before that age. MDA were demonstrated to interfere with live vaccination but strain D78 was shown to be more efficient as it could establish an infection even at higher antibody levels than the other strains. Given this pathogenicity and the lack of uniformity in the transmitted immunity, there is always a critical period where immune and susceptible birds coexist in the same flock. This period seems to be broader when parental flocks were vaccinated with an inactivated oil emulsion vaccine before lay.