Field Assessment of an Immune-Complex Infectious Bursal Disease Vaccine in Chicks Born to Non-Hyperimmunized Broiler Breeders

Abstract

Infectious bursal disease (IBD) is a viral disease that affects chickens worldwide and it has a major economic impact on the modern poultry industry. It is caused by a birnavirus, called infectious bursal disease virus (IBDV), which is a bi-segmented, double stranded RNA virus, highly resistant in the environment. The RNA encodes for five viral proteins (VP1 to VP5) of which VP2 contains hypervariable portions that enable classifying strains into various antigenic and genetic groups; in addition, VP2 contains the majority of neutralizing sites. Depending on the pathogenic type, IBDV strains are classified as very virulent, virulent, and subclinical. Antigenically two serotypes (serotype 1 and 2) of IBDV exist, although antigenic variants are described within serotype 1 strains. Only the serotype 1 virus is pathogenic for chickens in which it can cause depression, diarrhoea, haemorrhages in muscles and proventriculus, and inflammation, necrosis and atrophy of the bursa of Fabricius depending on the virulence of the strain. Variant IBDV strains generally do not cause clinical signs or mortality, but can cause severe atrophy of the bursa without marked Prevention of infectious bursal disease (IBD) remains of paramount importance, given the dramatic losses it can induce in commercial chicken flocks. In the Netherlands, maternal immunity is regarded as low, because the parent stock flocks do not receive an inactivated vaccination prior to the onset of production. Therefore, the aim of this field study was to assess the consequences of hatchery vaccination against IBD using an immune-complex vaccine in broiler chicks that were provided with a limited amount of passive immunity. Three grow out cycles in two houses each (six flocks) were monitored after vaccination. The recorded parameters included (i) body and bursa weight, (ii) IBD antibody response, and (iii) histopathology and PCR from the bursa of Fabricius. Average growth was similar in all groups, although inferior to the breed’s standards. Average bursa weight faced some reduction as part of vaccine effect. Histopathology analysis of the bursa showed a transient inflammatory process, while PCR detected the vaccine strain only. Lastly, serology tests confirmed the successful immunization of all broiler flocks. The two commercial ELISA kits (Idexx and BioChek) failed to detect antibodies for a limited period of time, whereas virus neutralization (VN) test could detect antibodies for the entire broilers’ lifespan. Idexx and BioChek ELISA kits had the highest correlation coefficient (0.93), whereas the lowest correlation was found between Idexx ELISA and VN (0.68). BioChek ELISA and VN showed a correlation of 0.78. This is the first detailed investigation of immune-complex vaccine’s mechanism of action in field conditions in chicks with low maternal immunity at hatch (<4000) and using a comprehensive set of monitoring parameters. It also provides updated information about IBD serology tests.