An immunogenic thermostable novel spray dried vaccine against Psittacine beak and feather disease

Beak and feather disease virus (Circovirus parrot) associated Psittacine beak and feather disease (PBFD) is a major health concern for parrots and cockatoos worldwide that threaten several endangered psittacine species with extinction. Over the last decade, a recombinantly expressed circovirus capsid molecules that self-assembles into soluble virus like particles (VLPs) was identified as promising PBFD vaccine candidate. However, a practical and safe antigen delivery system for the vulnerable and nestling birds is a critical barrier for a robust vaccination regimen. We developed a novel spray dried PBFD vaccine in thermostable powder formulation for a minimally invasive non-parenteral delivery of circovirus antigen. Following a design of experiment approach, we optimised spray drying parameters such as inlet temperature, feed flow rate, nozzle gas flow rate and stabilizer concentration to achieve an optimum yield of homogenous powder with a desired macromolecular size range (>5µm). This spray dried formulation demonstrated high thermostability and shelf life of the capsid antigen with only single fold reduction of haemagglutination (HA) titre post spray drying process. Safety and suitable delivery routes for this vaccine formulation was tested in experimental chicken model followed by multiple modalities of immunodiagnostics for proof of seroconversion. Detectable seroconversion was observed after two booster doses (six-week post inoculation) in oculonasal and intramuscular routes, whereas per-cloacal route of delivery did not demonstrate positive seroconversion. Undesirable tissue damage or adverse vaccine reaction was ruled out by haematology and biochemistry assays as well as histopathological assessment of vaccine sites. This research developed an innovative delivery system for PBFD vaccination and highlighted suitability of spray dried formulation for VLP based subunit vaccine candidates with desired antigenicity, thermostability and shelf life which might prove beneficial for captive management practices as well as for future wildlife vaccination strategies against avian circoviruses. Further research including vaccination and challenge trials in natural psittacine hosts is required to determine the efficacy and protection against wild-type infections.