Veterinary Science Collected Works - Theses

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    Decision Support Tools for Vector-borne Spread of Animal Disease
    Al Riyami, Shumoos Abdullah Said ( 2020)
    Bluetongue is a vector-borne, viral disease of ruminants that can cause substantial losses in productivity in farmed livestock. It is transmitted by Culicoides midges (Diptera: Ceratopogonidae) which means that the geographic distribution of disease is restricted to those regions where these vectors are present. In 2006 a virulent serotype of bluetongue (BTV-8) was detected in Northern Europe causing losses of livestock with knock-on economic and welfare effects. Bluetongue virus is endemic in Northern Australia, but clinical signs of disease have not been reported in either cattle or sheep. The risk of incursion of a virulent strain of bluetongue into Australia represents a constant threat, particularly the risk imposed by infected Culicoides midges blown from Indonesia to the northern borders of the Northern Territory. This thesis reports the results of a series of studies designed to enhance Australia’s preparedness for an incursion of a virulent serotype of bluetongue. This objective was achieved by design and implementation of facilities to allow the Australian Animal Disease Simulation model (AADIS) to simulate the spread of vector-borne diseases. In Chapter 3 we developed a model to allow the spatial and temporal distribution of Culicoides spp. across Australia to be estimated as a function of ambient temperature, prevailing wind conditions and the presence of cattle. In Chapter 4 a model of bluetongue virus vector-host transmission was developed, using the simulated midge population from Chapter 3 for the source population of insect vectors and details from the National Livestock Identification Scheme for the cattle host population at risk. A novel approach in Chapter 4 was the separation of host and vector populations into two ‘layers’ of different spatial format: a raster layer for the insect vector population and a spatial vector layer (i.e. points) to represent the cattle herd population at risk. This allowed our model to be readily adapted for other insect vector-borne diseases where the precise geographic location of a disease reservoir cannot be defined at the point location level. This approach would also be suitable for diseases with a wildlife reservoir such as wild boar in the case of African swine fever. In Chapter 5 we implemented the prototype models in Chapters 3 and 4 into the Australian Animal Disease Simulation (AADIS) model, taking advantage of AADIS’s existing architecture to account for the geographic distribution of the herd population at risk and direct animal movements between herds. Simulations were carried out using AADIS, allowing the number and geographic of distribution of infected herds to be estimated following an incursion of virulent bluetongue virus into a single herd. Large outbreaks of bluetongue were predicted in Southern Queensland and Northern New South Wales with larger outbreaks occurring when incursions occurred during the summer. Outbreaks in Northern New South Wales were smaller compared with the other areas, with a similar seasonal effect. In Chapter 6 the AADIS bluetongue model was re-parameterised using climate change temperature estimates provided by CSIRO (2020), providing an example of how the preparatory work presented in Chapters 3, 4 and 5 can be drawn together to support real-world animal health decision making. In Chapter 6 the geographic extent of bluetongue outbreaks in the area immediately surrounding the site of incursion were substantially different from those estimated under the non-climate change scenario. An important finding was that bluetongue was able to establish itself in previously non-endemic areas of the country after being seeded into those areas following incoming movement of infectious cattle. This has important implications for animal health policy: in the event of an incursion of bluetongue into Australia the single most important control measure is to limit the movement of cattle out of affected areas in order to prevent establishment of disease elsewhere. This recommendation becomes even more important under climate change scenarios because the extent of the country with suitable habitat for Culicoides spp. will be substantially greater than what it is today.