Characterization of infectious bursal disease viruses isolated from commercial chickens

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University of Delaware
Infectious bursal disease virus (IBDV) field isolates recovered in 2007 from Delmarva Peninsula broiler farms with a history of poor performance were characterized. The isolates originated from 3- and 4-week-old broilers having only passive breeder vaccinations. A 743 base pair fragment of the VP2 coding region of each of the field isolates was amplified by reverse transcriptase-polymerase chain reaction. VP2 sequencing and phylogenetic analysis showed that all of the isolates distributed into six different clades representing serotype 1 Delaware variant viruses. Two active vaccination-challenge experiments were performed to test the antigenic and immunogenic properties of the IBDV field isolates and examine their ability to break through immunity to a Delaware variant vaccine 89/03 and a classic vaccine PBG98. Five field isolates representative of each molecular clade were used as challenge viruses in three-week-old specific pathogen free (SPF) White Leghorn chickens vaccinated with the Delaware variant and classic vaccines. The five field isolates induced variant-like gross lesions in all non-vaccinated SPF chicks. However, a single subcutaneous injection of either the 89/03 or PBG98 live, attenuated vaccine was sufficient to protect chickens against challenge with the five field isolates. Virus neutralization (VN) using antiserum produced against the five field isolates showed no difference in neutralization of the 89/03 and PBG98 strains. This similarity in neutralization indicated that the two cell-culture-adapted strains may not differ enough on the basis of antigenicity to reveal a difference between the five IBDV field isolates. The relatedness of the field isolates to other known variant IBDV strains was evaluated using VP2 sequence analysis and two monoclonal antibody (MAb) methodologies, transfection and immunofluorescence and whole virus, with an antigen capture enzyme linked immunosorbent assay (AC-ELISA). Isolate 4813 was determined to be most related to Delaware variant E (Del E) based on a 99.5% identity at the amino acid level and its reactivity with monoclonal antibodies (MAbs) 63 and 67. Isolates 4947 and 4955 were determined to be most related to variant vaccine strain, RS593, based on a 98.1% identity at the amino acid level and similar MAb reactivity patterns. Isolate 5041 was found to be most antigenically similar to the GLS variant by the whole virus/ELISA MAb test, but only 96.8% related at the amino acid level. Isolate 5038 failed to react with MAbs used in either AC-ELISA experiment, which indicated that it may differ antigenically from the IBDV reference strains tested. Genomic characterization and comparison of the variable region of VP2 of the five field isolates revealed amino acid substitutions in the four hydrophilic peaks previously identified as being important for antigenic variation and the binding of neutralizing antibodies. However, most of the VP2 sequence changes observed for the five field isolates closely resembled those of other characterized variant strains. A passive immunity progeny challenge was performed to assess passive IBDV immunity in leghorn chicken progeny with maternal antibodies to IBDV. Ten-day-old chicks challenged with isolate 5038 were less protected than chicks challenged with Del E. This finding indicated that 5038 was able to break through maternally-derived immunity earlier than Del E. The findings of this study indicated that field isolates 4813, 4947, 4955, and 5041 shared sequence and antigenic similarities with Delaware variant strains of IBDV. The isolation of these viruses from commercial Delmarva broilers at 3-4 weeks of age is not surprising as maternal antibodies to the virus normally are sufficiently low to permit infection. On the other hand, solate 5038 appeared to be antigenically different than IBDV reference strains Del E, STC, GLS, RS593, and AL-2 based on MAb testing and antigenically different from Del E based on progeny challenge findings and warrants further investigation.