Protection studies of recombinant HVT-IBD and/or live IBD vaccine against 14-day or 25-day AL2 challenge in commercial broilers

AL2 is the most prevalent IBD virus in U.S. broilers, accounting for as much as half of today’s field isolations. The newest recombinant rHVTIBD vaccine was introduced in 2021. Previous studies in broilers with no IBD maternal antibodies showed this vaccine gave high levels of protection (70-78%) against AL2 challenge by 18-19 days. This paper will present 2 studies conducted to measure AL2 protection in commercial broilers. Study Design: Ross 708 broilers from a single breeder source were given a full dose of rHVT-IBD and/or a live IBD vaccine in ovo and then raised in separate rooms on litter. At 14 days all vaccine treatments including controls were commingled. One room of birds was challenged at 14 days and the other at 25 days. Challenged birds received 3.5 EID50 AL2 by eye/ nose drop. At 7 days post challenge, bursameter scores were recorded and bursa to body weight ratios (B:BWs) were determined to calculate protection. Challenged birds with no bursal edema and a mean B:BW within 2 standard deviations of the non-challenged controls were considered protected. Histopathology scoring of protection will be presented once completed. Results: The AL2 challenge “took” well even at 14 days. All vaccine treatments had bigger bursas (less atrophy) than challenge controls after both 14-day and 25-day challenges. The live IBD treatment showed a modest but not significant improvement in bursa sizes and protection in both challenge intervals while the recombinant vaccine showed significant protection (67-69%) vs. challenge controls (0-3%). The use of both vaccines together resulted in further improvements in bursa size and protection (81%) against the 25-day challenge. Discussion: This pair of studies demonstrated that the newest rHVT-IBD vaccine can offer significant AL2 protection in broilers with marginal maternal immunity that would otherwise be susceptible to challenge by 14 to 25 days of age. While the live IBD vaccine only afforded modest levels of protection by itself, administering it along with the recombinant vaccine resulted in the highest bursa sizes and 25-day IBDV challenge protection—demonstrating the compatibility and potential utility in using these two vaccination strategies together in commercial broilers.

Key Words: IBDV, recombinant, live, vaccine, AL2.