Epidemic Modelling Studies of Hendra virus and Coronavirus in Australian Bats

Abstract

Bats (order Chiroptera) are known as natural reservoir hosts of many emerging zoonotic diseases. The increasing trend in outbreaks of bat-borne emerging zoonotic diseases in recent years poses serious risks to public health. Coronaviruses in bat populations have demonstrated their potential to bring about deadly pandemics, such as SARS (severe acute respiratory syndrome) and MERS (Middle East respiratory syndrome). Hendra virus in Pteropus spp. (fruit bats or flying foxes) is a lethal zoonotic virus that has repeatedly emerged to infect horses, leading to fatal human infections in eastern Australia. However, more research has been needed on mechanisms how bats maintain zoonotic pathogens in their populations and on factors that stimulate the reservoir hosts to excrete the pathogens. This knowledge would help understand the spillover mechanism and manage the diseases effectively in their natural reservoir hosts before the diseases spillover. This thesis explores the transmission dynamics of bat-borne viruses (coronavirus and Hendra virus) in their natural reservoir hosts of bats, by employing mathematical epidemic models to simulate the dynamics. Chapter 1 commences with the story of the emergence of Hendra virus. From the story, particular questions are extracted. I review the knowledge previously available to answer those questions and explain how approaches for mathematical modelling of infectious diseases can be used to study these topics. Relevant information on bat biology and ecology is suggested. Management strategies for bat zoonotic diseases are also previewed. Finally, the aims and structure of the thesis are outlined. Chapter 2 analyses the effect of persistent infection on coronavirus maintenance in a population of Australian bats (Myotis macropus). By using a previously performed capture-mark-recapture (CMR) study, more intensive mathematical methods were employed. The multi-model selection processes supported the notion that it is appropriate to divide coronavirus infectious bats into two groups of persistently infectious and transiently infectious bats, based on the infectious period. The epidemic models predicted that the grouping of bats increases the probability of coronavirus maintenance in the bat population. Chapter 3 explores the effects of maternally-derived immunity in seasonally breeding wildlife on epidemic patterns by using a system of Hendra virus infection in black flying foxes (Pteropus alecto). Deterministic models were used to simulate epidemics, which were characterised by a variety of timings of viral introduction and a range of pre-existing herd immunities. Waning maternally-derived immunity dispersed the timing of supply of susceptible individuals from births and losses of maternally-derived immunity and thereby diluted the effect of seasonal breeding on epidemics. The dispersion of timing increased the probability of viral persistence and contributed to shifting the timing of epidemic peaks further away from the peak of a birth pulse. Chapter 4 numerically examines whether a metapopulation of flying foxes (Pteropus spp.) can support the maintenance of Hendra virus. The implications of metapopulation structure of flying foxes on Hendra virus dynamics needs more investigations. A single population of flying foxes in the context of a metapopulation structure was stochastically simulated to repeat the cycle of viral extinction and recolonisation in the population. The simulation results predicted that viral recolonisation should occur more frequently than extinction in a colony in a metapopulation, supporting the hypothesis that the metapopulation structure of flying foxes can maintain long-term persistence of Hendra virus. Chapter 5 examines the effects of culling and dispersal of flying foxes on the spillover risk of Hendra virus. Metapopulation models were simulated stochastically using various culling and dispersal scenarios. The models used the most favourable possible assumptions about Hendra virus epidemiology for the application of these management strategies. Nevertheless, many scenarios were predicted to be counter-productive in reducing the spillover risk of Hendra virus. Even though the scenarios expected positive effects on decreasing the spillover risk, the degree of benefits was not realistic if the cost was considered. I, therefore, concluded that culling or dispersal were not effective strategies to manage Hendra virus spillover. Chapter 6 describes the findings provided in each chapter. Then, I discuss the findings, focusing on the viral dynamics in reservoir populations of emerging infectious diseases. Based on the dynamics, I suggest the disease management strategies. I discuss how to do proper modelling research using insufficient data on wildlife diseases. Finally, this chapter provides suggestions for further research.