RAPID IDENTIFICATION OF THE MOST APPROPRIATE VACCINE STRAIN TO CONTROL INFECTIOUS BURSAL DISEASE

Daral J. Jackwood, Ph.D.

The Ohio State University/OARDC

Choosing a vaccine strain to use against pathogenic field isolates of infectious bursal disease virus (IBDV) has been difficult. Now it is possible to rapidly and inexpensively identify IBDV vaccine strains that have nucleotide sequence homology to newly isolated wild-type IBDV strains. This short communication describes how short sequences that encode neutralizing epitopes of an IBDV vaccine strain can quickly be compared to a wild-type pathogenic IBDV so the best nucleotide sequence match at critical neutralizing epitopes can be determined.

Problem: Recommending an IBDV vaccine that will protect a flock is difficult because the antigenicity of the pathogenic field isolate causing the disease is usually unknown.

Solution: Use real-time RT/PCR to identify nucleotide sequence homology between vaccine strains and wild-type pathogenic viruses. Comparing sequence homology between the pathogenic virus and each IBDV vaccine product will identify the vaccine that matches or most closely matches the nucleotide sequence of the pathogenic virus. Since only two or three short nucleotide sequences can be compared in this assay, it is important to select regions that are important in the development of immunity to the virus.

Sequence homology or mutations were detected using real-time RT/PCR and four mutation probes. The mutation probes were designed to match four different IBDV strains, STC, D78, Del-E and Bursine 2. The melting temperature (Tm) of the mutation probes is an indicator of sequence homology. An exact sequence match between the mutation probe and viral RT/PCR products was determined using each mutation probe and its homologous virus (Table 2). These homologous Tm values were determined for each of the four mutation probes (boxed values). Homologous Tm values were then compared to Tm values obtained for each of the IBDV strains examined in this study. A Tm within two standard deviations of the homologous Tm for a mutation probe indicated the sequences of the probe and the test virus, were identical (bracketed values). If a Tm for the test virus was lower than two standard deviations from the homologous Tm for that mutation probe, the sequences were considered to be different in at least one or more nucleotides.

The utility of the real-time RT/PCR assay lies in the identification of viruses with nucleotide sequences identical to the mutation probes. When Tm values between the mutation probe and test virus indicate similar sequences, it is appropriate to conclude the two viruses have a similar amino acid sequence and thus have an antigenically similar epitope. We chose to examine the hydrophilic B region epitope. This region is known to encode a neutralizing epitope of the virus. However, with this assay any region of the viral genome could be targeted. It would be possible for a vaccine manufacturer to prepare mutation probes for each of their IBDV vaccines and use these mutation probes to determine which vaccine most closely matches a wild-type pathogenic IBDV strain. This would provide direct evidence to poultry producers for selecting a vaccine strain with the most antigenic potential to control an IBD outbreak.

Table 1: Vaccine strains of IBDV.

ALohman Animal Health includes the following companies: Vineland Laboratories and Agri-Bio Corp

BSchering-Plough Animal Health includes the following company: American Scientific Labs, Inc.

CIntervet, Inc. includes the following companies: Tri-Bio Laboratories, Inc. and Intervet America, Inc.

Table 2: Mutation probe melting temperatures (oC) for IBDV strains.
 

                                                                                               Mutation Probes