Shearer, Patrick Leslie (2009) Development of novel diagnostic and vaccine options for beak and feather disease virus (BFDV). PhD thesis, Murdoch University.
Beak and Feather Disease Virus (BFDV) is a circovirus which causes ill-thrift, feather loss and immunosuppression leading to secondary infections and eventually death in psittacine birds. The development of standardised reagents for the detection and characterisation of BFDV infections and for the production of protective vaccines has been difficult as no cell culture system has yet been found to grow the virus successfully in vitro. However, the development of consistent and effective diagnostic tests and vaccines is now more practical through the application of nucleic acid-based detection methods and recombinant technology.
A quantitative real-time PCR assay for the detection of BFDV DNA was developed, using primers designed to amplify a conserved 81 bp fragment of ORFV1 and SYTO9, a fluorescent intercalating dye, with assays run on a Corbett RotorGene 3000. A synthetic oligonucleotide was used to establish standard curves for the quantitation of viral load in both blood and feather preparations. The assay was very sensitive, with a detection limit of 50 copies/ìL. The assay was developed using BFDV-positive DNA extracts from the feathers of 10 different species of birds and validated with blood and feather samples from corellas vaccinated with an experimental BFDV vaccine, then challenged with live virus. Viral DNA was reliably detected in the blood of all control (non-vaccinated) birds and in some vaccinated birds. Contamination of the environment with the feather dander of BFDV-infected birds meant that HA feather preparations were unreliable for the detection and quantitation of viral excretion. Nonetheless, the assay should prove to be a useful and sensitive test for the detection of viral DNA in a range of samples in future investigations.
A recombinant BFDV capsid protein was also produced and a specific monoclonal antibody developed against it. The behaviour of the protein in haemagglutination (HA) assays and the behaviour of the monoclonal antibody in western blotting, immunohistochemistry (IHC), ELISA and haemagglutination-inhibition (HI) assays were characterised. The protein had the ability to agglutinate galah erythrocytes as per the wild-type virus and this agglutination was successfully inhibited by antibodies to wild-type BFDV from naturally immune psittacine birds. Furthermore, the protein self-assembled into virus-like particles as determined by electron microscopy. The antibody was specific for both the recombinant BFDV capsid protein and the whole virus and had similar optimal titres when used in western blotting and IHC. The antibody also had HI activity and detected BFDV virus from 3 genera of psittacine birds, including the recently described cockatiel BFDV isolate. A novel “blocking” (or “competitive”) ELISA (bELISA) for the detection of anti- BFDV antibodies in psittacine sera (Ab-bELISA) was also developed and validated with 166 samples from eastern long-billed corellas vaccinated with the recombinant capsid protein and challenged with live virus. The bELISA was found to be both sensitive and specific and correlated strongly with the HI test, thus it should have wide application for the serodiagnosis of BFDV.
A survey of cockatiels (n=88) housed at commercial aviaries was conducted to investigate whether BFDV infection occurs in cockatiels. All birds were diagnosed as being virus-free by PCR and HA and had no detectable antibody titre by HI assay. In addition to this, the genomes of two BFDV isolates obtained from diseased cockatiel feathers were sequenced and cross-reactivity assays performed using virus eluted from these feathers and sera from naturally immune psittacine birds. Serological cross-reactivity results and phylogenetic analysis of the nucleotide sequences indicated that the cockatiel virus isolates were serologically and genetically different to other BFDV isolates. This is the first report of an antigenically distinct BFDV in psittacine birds. Since the Ab-bELISA has a lower limit of detection than the HI assay, it was used to repeat the cockatiel sero-survey. No antibodies were detectable in any of the cockatiels tested and thus questions about the real prevalence of BFDV infection in cockatiels and the possible existence of a novel BFDV serotype adapted to cockatiels remain unanswered.
The successful control of PBFD in both pet and wild birds depends on the development of vaccines that incite a strong specific immune response and can be efficiently produced in large quantities. Recombinant BFDV capsid proteins have recently been considered as candidate vaccines against BFDV and recombinant techniques allow the development of other candidate vaccines, including DNA vaccines. In order to examine the potential of DNA vaccination as a strategy for the prevention and control of BFDV, two DNA vaccines, based on the nucleotide sequence encoding the capsid protein of BFDV, were developed using the mammalian expression vector pVAX1. The vaccine constructs encoding both the full length and NLS-truncated capsid protein resulted in protein expression both in vitro and in vivo. Protein was detected in COS-7 cells transfected with the constructs with an indirect immunocytochemistry assay using the monoclonal antibody described in Chapter 5. Protein was present in the nucleus of cells transfected with the vaccine encoding the full-length nucleotide sequence and in the cytoplasm of cells transfected with the vaccine encoding the NLS-truncated sequence as expected. Both DNA vaccine constructs induced detectable levels of anti-BFDV antibodies in vaccinated birds, determined using the Ab-bELISA described in Chapter 5. Thus, DNA vaccines similar to those presented here may have application in the prevention and control of BFDV and some options for the further development of these vaccines into effective methods for the control of BFDV are discussed.