Master´s Thesis Ecosystems, Governance and Globalisation Master´s programme 2007/09 New Zealand as a model for vector borne disease emergence Effects of social and environmental factors on dengue Malin Nordwall New Zealand as a model for vector borne disease emergence: effects of social and environmental factors on dengue Malin Nordwall Supervisors: Dr. Elisabet Lindgren & Dr. Mary McIntyre 1 Prologue Until recently, New Zealand’s geographic isolation has allowed a very unusual fauna to evolve in the near absence of native land mammals (Crump et al. 2001). About 80 million years ago New Zealand was separated from the super-continent Gondwana and the links that had earlier enabled mass migrations (Stevens 1988). The long isolation allowed for an extensive in situ evolution resulting in an almost 100% endemism at a species level (Daugherty et al. 1993), 80% of the plants (van Bunnik et al. 2007) and 35% of its bird species can only be found in New Zealand (Daugherty et al. 1993). The absence of mammalian predators has allowed traits such as flightlessness, no defensive behaviours, gigantism and low productive rates to appear (Atkinson and Cameron 1993). The first colonising Polynesians, ancestors of Maori, arrived about 1000 years ago (McCulloch 1988, Crump et al. 2001), this started off the first human induced extinction wave on New Zealand, exterminating the Haast’s eagle and the moas amongst others. The second wave was set off 200 years ago, when the first European settlers came (Diamond 1990). The extensive human disturbance on the country’s ecosystems has resulted in indigenous forest being reduced from 82% to 21% of the land surface area with large tracts of remaining mainly at higher altitude, natural freshwater wetlands have been reduced by 90%, more than 50% of the flora consists of non-native species (Department of Conservation 2000) and of 31 species of land mammals only two insectivorous bat species are native (King 1990). The native fauna of New Zealand never included animal hosts for human pathogen, but the introduction of exotic mammals gave an opportunity for zoonotic disease to establish. Emerging zoonoses has been successfully controlled and excluded from New Zealand by a strict quarantine system. However, New Zealand has been argued to have deferred impacts from infectious diseases that are emerging in other parts of the world (Crump et al. 2001). It is New Zealand’s isolation that makes it such an interesting place, not only to study the flora and fauna, but also for studying the emergence of infectious diseases. 2 Acknowledgements I would like to thank my supervisors Dr. Elisabet Lindgren and Dr. Mary McIntyre for their support and guidance. I would also like to express my gratitude to Mary for making it possible for me to carry out this research together with Wellington School of Medicine and Health Science, University of Otago, helping me orientating myself in New Zealand and making me feel welcome. Last but not least, I would thank Dr. Simon Hales for his help and expertise on the subject and Dr. James Stanley for guiding me through a statistical nightmare. 3 Abstract The geographic distribution of dengue fever has increased worldwide in recent years and is at present the most widespread vector borne viral disease in the world (Halstead 2002). Because of its rapid spread and increasing seriousness of its complications it is considered to be the most troubling vector borne disease (Wilcox and Colwell 2005, Phillips 2008). Dengue fever is the one vector borne disease that poses the greatest threat to New Zealand. Imported cases are being reported in ever increasing numbers and all the components for a mosquito borne disease cycle is already present (Ministry of Health 1997). Furthermore, New Zealand’s geographic isolation makes it a unique location for studying the emergence of vector borne diseases, such as dengue. The objective of this thesis was - by using case studies on dengue fever on a global scale and the potential emergence of the disease in isolated New Zealand as examples – to explore the interlinkages between global changes (climate change and rapid urbanisation), globalisation (rapid travel and trade), and their local impacts on vulnerability and health (i.e. changes in local climate, travel and trade patterns and demographic changes that affect emergence and transmission of disease). This was done by 1) reviewing the ecological and environmental conditions necessary for dengue transmission; 2) examining key social and environmental factors contributing to the recent global increase in dengue fever and dengue hemorrhagic fever (DHF) and 3) drawing projections to 2070 in order to build future scenarios for epidemic dengue risks in New Zealand. Regression analysis were used to analyse 16 years of area specific dengue rates from 232 geographical areas in relation to key social and environmental factors proposed to contribute to dengue emergence. The results were tested on the mainland of New Zealand in order to build future scenarios for epidemic dengue risks in New Zealand for 2070. The outcome from the regression analysis proved to have a good ability to predict dengue rates based on national characteristics and it predicted a nearly fourfold increase in risk of epidemic for New Zealand’s North Island based on climate projections for 2070. The projected increase in population density however, had much less of an effect on the perceived risk than the projected climate change despite an estimated increase of 33% in population density. 4 This is the first study that makes an attempt to measure the relative importance of different social and environmental variables proposed to contribute in the recent global increase in dengue. 5 List of Acronyms DHF Dengue Hemorrhagic Fever EID Emerging Infectious Diseases EIP Extrinsic Incubation Period GDP Gross Domestic Product IPCC Intergovernmental Panel on Climate Change IPO Interdecadal Pacific Oscillation MA Millennium Ecosystem Assessment NTD Neglected Tropical Diseases SOI Southern Oscillation Index WHO World Health Organization YoY Year on Year 6 Table of Contents Table of Contents ....................................................................................................................... 7 1. Introduction ............................................................................................................................ 9 1.1 Case studies, Global assessment and New Zealand focus: Problem statement .............. 11 1.1.1 Risk of dengue establishment in New Zealand ........................................................ 12 1.2 Objectives of the case studies ......................................................................................... 12 2. Theoretical framework ......................................................................................................... 13 2.1 Mechanisms for dengue emergence ............................................................................... 16 2.1.1 Climate change ......................................................................................................... 17 2.1.2 Population growth and urbanisation ........................................................................ 18 2.1.3 Travel and trade ....................................................................................................... 19 2.1.4 Socio-economic factors ............................................................................................ 20 2.1.5 Land use change ....................................................................................................... 21 3. Dengue overview .................................................................................................................. 23 3.1 The dengue transmission cycle ....................................................................................... 24 3.2 Dengue vector ecology ................................................................................................... 25 3.2.1 Aedes albopictus – the Asian tiger mosquito ........................................................... 25 3.2.2 Aedes aegypti – the primary dengue vector ............................................................. 26 3.2.3 Aedes polynesiensis – the Polynesian dengue vector .............................................. 27 3.2.4 Aedes notoscriptus – an urban dengue vector in New Zealand ............................... 27 3.3 Dengue flavivirus ........................................................................................................... 28 4. Potential introduction of dengue fever into New Zealand: risk assessment background ..... 29 4.2 Ecological features ......................................................................................................... 30 4.2 Socio-economic features ................................................................................................. 31 4.3 Dengue cases and intercepted dengue vectors in New Zealand ..................................... 34 4.4 Biosecurity - surveillance and exclusion programmes ................................................... 36 4.5 Previous risk assessments for epidemic dengue in New Zealand .................................. 38 4.5.1 The Hotspots system - spatial analysis of dengue fever risk to New Zealand ......... 39 5. Case study ............................................................................................................................ 42 5.1 Case study part I: The global context ............................................................................