Study investigating the interaction of IBDV (AL2) and IBV (DMV/1639) challenges in commercial broilers based on infectious bronchitis vaccination status

    

Infectious bursal disease virus (IBDV) infections within the first 2 weeks of age cause significant immune suppression while infections after 3 weeks are less severe and more transient. The purpose of this study was to see if a 3-week IBDV challenge could significantly reduce immunity against an infectious bronchitis virus (IBV) challenge given 4 days later. Study Design: Ross 708 broilers with high maternal antibodies to IBDV (7,480 Idexx-XR) were vaccinated at day of hatch (Vax) with a full dose of Mass + GA08 IB vaccines by coarse spray or (half) remained unvaccinated (NoVax), then placed into a dozen horsfal isolators. At 21 days of age, half of all birds were challenged with IBDV AL2 (3.5EID50) resulting in four groups: Vax/noIBD, NoVax/noIBD, Vax/AL2, NoVax/AL2. At 25 days of age all birds were challenged with 3.5EID50 DMV/1639. At 31 days, birds and bursas were weighed, bursas were measured using bursameters and tracheas were collected for real-time IBV PCR and histopathology scoring. All hypotheses were conducted at the p≤0.05 level of significance with the Shaffer simulated method used to adjust for multiple comparisons. Results: The 21-day AL2 challenge “took” as mean bursa to body weight ratios (B:BW) and bursameter scores were significantly lower than controls (1.8 vs. 1.0 and 5.8 vs. 4.4, respectively). Mass + GA08 vaccination resulted in significant reductions in IBV infections after DMV/1639 challenge based on Mean Ct (36.1 vs. 27.4) and Ct-35 protection (52% vs. 0%). Vaccinated birds also had significantly higher values if they were not also IBDV challenged based on Mean Ct (37.4 vs. 34.7) and Ct-35 protection (74% vs. 30%). Tracheal protection from inflammation and damage was also significant in IB vaccinates. AL2 challenge resulted in significantly enhanced IB lesions in non-vaccinated birds and numerically enhanced them in IB vaccinates. Conclusions: In this study, a 21-day IBDV challenge in commercial broilers with high day of age maternal antibodies resulted in bursal atrophy and led to increased tracheal inflammation and viral loads from an IBV challenge happening 4 days later. Given how crucial IBV control is today—especially during winter respiratory season—it’s easy to see why IBD vaccination of broilers continues to grow.

Key Words: IBD, AL2, vaccination, IBV, DMV/1639.