Improving projections for the future of bluetongue and its vectors under scenarios of climate and environmental change

How will climate change affect the diseases that threaten our health and food security? We have good reason to believe that climate change will cause a number of infectious diseases to spread to new places or occur more often, particularly vector-borne diseases - those spread by arthropod (mostly insect) pests, such as malaria and dengue. This is because the arthropods that spread these diseases are themselves affected by climate and the environment they live in. While we recognise that climate change will affect vector-borne diseases, we currently have very limited ability to make predictions about what will actually happen in future - even to say which disease will threaten next. We cannot, therefore, give policy makers the information they need to be able to take necessary and timely measures.

Our main aim here is to develop a tool for exploring the nature of vector-disease outbreaks under future conditions of climate and environment, and to assess what interventions may be needed to contain them. We develop the tool for bluetongue (BT), a viral disease of sheep and cattle that is spread by tiny biting insects (midges). BT reappeared in Europe in 1998 after a gap of several decades and, in the next ten years, spread over most of the continent, including the UK in 2007, leading to the deaths of millions of animals, mostly sheep. This BT outbreak was unprecedented: the longest and largest on record; numerous countries, including Italy, France, Germany and UK, were affected for the first time; disease occurred much further north than ever before; and a large number of viral strains were involved. Furthermore, the disease continues to threaten: midges are hugely abundant on our farms, feeding on our animals, and for the majority of viral strains there is no vaccine immediately available for use.

We have chosen BT because it is considered a prime example of a disease that has emerged already in response to climate change. The tool we propose to develop is a novel mathematical model for the spread of BT between farms in GB, integrated with state-of-the-art climate model projections of the future, so that we can investigate the way in which the disease will spread under conditions of future climate (up to 2050), and what interventions may be required in the event of an outbreak.

It will not only be climate that changes by 2050 however. Other environmental changes may also affect BT, although we have only limited understanding of what these 'drivers' are and very little knowledge of how they will change. We will hold a workshop to solicit expert opinion about non-climate drivers and scenarios for how they might change in future; and we will then consider the effect of these changes within our modelling system.

Model-based predictions of the future are always uncertain. It is useful to try to measure the scale of this uncertainty, and also from where in the model it is arising, so as to better understand the limitations of the predictions and to guide further work. We will investigate whether the main source of uncertainty in our model arises because we do not understand BT well enough, or because we have insufficient clarity about the future conditions that BT will occur in.

Finally, we know already that some uncertainty arises in our disease model from limited knowledge of the biology of the insects that spread BT. We will trap Culicoides at 144 farms across England and Wales in order to map them, statistically analyse the results with weather and climate data to improve our models; and undertake detailed studies on a smaller number of farms to investigate how able the midges are to spread BT virus.

Bluetongue (BT) is a vector-borne disease of ruminants which, since 1998, has caused the deaths of millions of head of livestock in Europe. BT's emergence has been linked to climate change: first, by demonstrating overlap between the disease and regions of greatest warming; and second, by driving a model of BT's basic reproduction number (a measure of outbreak risk following viral introduction) with observed climate and successfully capturing many aspects of the disease's emergence.

While climate change is implicated in BT's past, we lack a framework for investigating the disease's future. The aim of this project is to integrate a newly developed BT transmission model (which is spatially explicit, has infection spread by recorded animal movements and vector dispersal, incorporates control measures and is climate-sensitive) with state-of-the-art climate-model projections of the future. We will then investigate disease dynamics in future years (to 2050), and the efficacy of control measures under conditions of future climate.

Non-climate drivers (environmental, demographic, husbandry etc) may also impact on BT over similar timescales. We will solicit expert opinion about these drivers and how they may change in future, and we will then consider future scenarios for non-climate drivers within our modelling system.

Models of the future are always uncertain. We will address the question of whether the larger uncertainty in our model arises from uncertainty in the disease model or from the uncertainty about our future climate.

Finally, we will reduce uncertainty in our disease model by field study of key aspects of vector biology. We will trap vectors at 144 farms across England and Wales in order to map them; develop climate-driven models for their density; and undertake detailed studies on a subset of farms to record data on vector biting rates, mortality rates and feeding preferences.

The future impact of climate change on health is of major interest to certain Government departments and national and international organisations, given their roles in horizon scanning and policy development for adaptation/mitigation of climate change impacts. BT does not affect directly human health but it is a threat to food security, and it is perceived as a prime example of how climate change might affect vector-borne diseases, both human and animal, in the future. Hence, interest in our work is not limited to veterinary and farming professionals. This is exemplified by some of the organisations which in recent years have commissioned inputs from us on BT, on climate change and disease, or on the spread of vector-borne diseases: the UK government's Foresight programme (2005), the UK's Health Protection Agency (2010), the World Bank (2011) and the US Department of Defense (2011) as well as non-governmental organisations (the Smith School of Enterprise and the Environment, Catholic Overseas Development Agency, Humanitarian Futures Programme).

We have also recently (September 2011) received funding from industry (Horse Race Betting Levy Board, £130,000 over 4 years) to train a veterinary resident in the epidemiology of a closely related disease to BT called African horse sickness (AHS), including adaptation of our basic farm-to-farm transmission model for this disease.

These or equivalent organisations will benefit from the proposed work, as per academics, in terms of understanding, methods and application. Understanding - by adding to the evidence-base that climate change and non-climate drivers can affect diseases. Methods - by providing a tool for quantitative assessment of climate change's future impact on a vector-borne disease, as well as the role of non-climate disease drivers. Application - by providing projections for the future of BT, along with the uncertainty surrounding the projections, under future conditions of climate and environment, and with the ability to investigate control options.

We therefore envisage the outputs of the proposal to be useful to a range of organisations in terms of preparedness for future vector-borne disease incidence and control. Hence, we intend that it will contribute in terms of enhancing the effectiveness of policy formulation, and to enhancing quality of life.