Submitted to: American Association of Veterinary Laboratory Diagnosticians
Publication Type: Abstract Only
Publication Acceptance Date: July 28, 2010
Publication Date: November 11, 2010
Citation: Neill, J.D., Schefers, J., Chase, C.C., Ridpath, J.F. 2010. Vaccination of Cattle Persistently Infected with BVDV Does Not Cause a Change in the Consensus Sequence of the Structural Proteins of the Quasispecies [abstract]. American Association of Veterinary Laboratory Diagnosticians. p. 101. Technical Abstract: Bovine viral diarrhea virus (BVDV) is a ubiquitous viral pathogen of cattle worldwide. An interesting aspect of these viruses is the great amount of sequence diversity that exists amongst strains in circulation in livestock herds that can impact diagnostic testing. The driving force behind change in sequence is not known but the inaccurate replication of the genomic RNA by a viral RNA polymerase without proof-reading capabilities is believed to be a major source of errors. Additionally, immune pressure on structural proteins is also thought to play a role in generating differences between strains. To test the hypothesis that immune pressure on the virus results in the selection of new viral structural protein sequences from the quasispecies population, cattle persistently infected with BVDV (PI) were vaccinated with commercially available killed virus vaccines and the sequence of the structural proteins were determined at various times post-infection. To do this, six calves that were infected in utero with the same progenitor virus were divided at random into two groups of three animals, with a seventh calf acting as a non-vaccinated control. The first group of three animals received the recommended dose of Triangle 4 Plus that contained a BVDV type 1a virus while the second group received the recommended dose of Triangle 4 that contained BVDV types 1a and 2. The calves were vaccinated on days 0 and 21 and were bled on day 0 and then at weekly intervals until day 35. Viral RNA was purified from the serum using the Qiagen Viral RNA purification kit. The sequences encoding the structural proteins were amplified by PCR and were sequenced. The sequences of the structural proteins of the persistent viruses on Day 0 were amplified and compared. The seven calves used in this study were all derived from the same progenitor virus and thus all viruses had a very high degree of sequence similarity (> 99%). There were slight differences in nucleotide sequence noted but this consisted solely of point mutations. Next, the Day 35 viruses were subjected to PCR amplification and DNA sequencing in the same manner and these sequences were compared to those of Day 0. This comparison showed that there was no sequence differences found between the 2 bleeding dates in any of the viruses. The immune pressure placed on the viruses from vaccination did not result in changes in the structural proteins of these viruses. This was a surprising result because conventional wisdom stated that virus replicating in the presence of an immune response would develop protective changes in the nucleotide and amino acid sequences of the immunodominant proteins. This experiment was done using killed vaccines containing type 1a and type 2 viruses while the persistent viruses were type 1b. It is not known whether a type 1b killed vaccine would have more of an effect because a vaccine of this type was not available at the time of the experiment.