Use of the ferret model to investigate the transmission of human and swine influenza viruses

In this thesis, the ferret animal model has been used to investigate the transmission characteristics of human and swine influenza viruses. Australian swine viruses were identified in samples which were collected at piggeries in Western Australia and Queensland in 2012. These viruses were cultured and characterised via direct sequencing and BLAST analysis and two were identified as H1N2 viruses and one as a H3N2 virus. These swine viruses have been used in an in vivo ferret model, to determine the possibility of transmission events via contact transmission experiments. The infection and transmission events of swine viruses were compared to those caused by human influenza viruses. It was found that the swine viruses could infect a donor ferret and transmission to recipient ferrets occurred efficiently. The viable viral load in nasal washes collected from infected ferrets showed similar levels to the viral load in nasal washes collected from ferrets infected with human influenza viruses. These findings indicated that these viruses were probably capable of causing infections in the human population without significant further adaptation.
   
   The second theme to be investigated was the use of an aerosol transmission model to study the role of aerosols in transmission events. To obtain transmission in an aerosol model, virus particles must be transferred via small or large droplets across a wire barrier from an infected ferret to a naïve ferret, who subsequently suffers from the same influenza infection. Many aerosol studies use the ferret model to study the transmissibility of novel influenza viruses have been described, however transmission efficiency results vary depending on the different cage set-ups that are used. No commercially available ferret aerosol cage system is available therefore we established a new cage design to study aerosol transmission in the ferret and these cages were tested for their performance for containment and airflow characteristics as a prelude to performing aerosol transmission experiments.
   
   A pilot ferret transmission experiment showed aerosol transmission efficiency rates off 25% (1/4 naïve ferrets infected) when tested with a human H1N1 pdm09 virus which resulted in cage modifications to achieve a higher transmission efficiency. Following these modifications, a transmission efficiency of 100% (4/4 naïve ferrets infected) was achieved using the same H1N1 pdm09 virus. Further studies with H3N2 influenza viruses were performed, resulting in transmission efficiency of 0% and 50% indicating further improvements may be necessary for this subtype. Possible improvements include lowering the airflow speed and the minimization of floor space at the back of the cages to encourage the ferrets to move closer together. Aerosol transmission experiments with the Australian swine viruses to study the role of aerosols in transmission events were planned; however, these experiments could not be performed due to time constraints and the need for further cage modifications.
   
   In summary, the pathogenicity and transmissibility of Australian swine viruses in ferrets was successfully determined in the contact model. The major finding was that the Australian swine viruses were able to infect donor ferrets and transmit to naïve recipient ferrets as efficiently as recent human influenza viruses. The development of aerosol transmission caging for ferrets was achieved and the preliminary testing of these cages has been performed. Further optimization of these cages needs to occur before the Australian derived swine viruses can be robustly tested for their ability to transmit influenza via the aerosol route, the route that is thought to be most important in the spread of influenza in the human population.