Mathematical modelling of complex host-parasite interactions under global climate change

Mathematical modelling in microbiology, especially simulating bacteria-phage interactions, has been a subject of extensive research. However, despite numerous previous publications, several fundamental questions regarding modelling this complex system remain unanswered. In particular, a key question is about the role of spatial heterogeneity of the environment in the dynamics of bacteria and phages, as well as their interactions. Another challenge is our current lack of understanding of how external forcing of the system, due to daily, seasonal, or global trends (climate change) would influence the long-term behaviour and prediction of the model. This thesis focuses on the mathematical modelling of host-parasite interactions under global climate change and its implications for Melioidosis, a significant infectious disease caused by the environmental bacterium B. pseudomallei, which is prevalent in Southeast Asia and Northern Australia. We utilized mathematical modelling to predict dangerous timings and locations, particularly for agricultural workers.

We collected temperature and UV index data and projected future trends using SARIMA and trigonometric models. Our projections indicate a slightly greater annual temperature increase than previous studies. We then analyzed the probability of controlling bacterial populations by phages using a model and historical data from high-risk provinces. Seasonal variations in population dynamics were observed, influenced by temperature and UV radiation. The density of susceptible bacteria was highest in the evening, warmer months, and correlated with temperature.

We also studied spatiotemporal dynamics in the upper layer of soil using an extended temperature-dependent lysogeny model. Lysogenic bacteria dominated the upper soil layers, while lytic bacteria were found near the surface. Soil mixing and excessive use of fertilizers were found to impact bacterial and phage densities, highlighting the need for revising the existing agricultural practices. Furthermore, the study emphasized the role of pesticides and environmental factors in bacteria-phage interactions. Understanding these dynamics is crucial for predicting disease spread and implementing preventive measures. Our findings provide insights into the effects of climate change and agricultural practices on Melioidosis dynamics, emphasizing the need to consider environmental factors in future studies.