Health in a changing climate: the dynamic challenge of snake bite in South Asia

Snakebite is a neglected tropical disease (NTD) and despite being less well publicised and studied, has a far greater impact than many other NTDs such as dengue or leishmaniasis. There are estimates of up to 120,000 deaths per year globally with considerable additional morbidity resulting, for example, from limb damage or renal failure. This burden of disease is relatively hidden as snakebite not only predominantly affects poorer countries in the tropics and subtropics, but it also mainly affects the rural poor, particularly agricultural workers and subsistence farmers in lower-middle income countries (LMICs).

In many countries, snakebite is seasonal and is distributed unevenly across a country because of complex interactions between human behaviour, climate, varying geography and other factors affecting snake distributions. This means that there are substantial challenges in estimating the number of deaths and complications of snakebite. National surveys are rare because they are so difficult to carry out. The variation in numbers of snakebite across countries and over any one year means that it is difficult to estimate national and regional numbers from time-limited small local studies. This absence of accurate numbers for many countries and regions means that the problem does not get the international attention that it requires and makes it extremely difficult for local public health authorities to plan appropriate health services.

Improved methodological approaches for mapping snakebite risk are therefore urgently needed, particularly when considering the potential for global environmental and climate change to exacerbate snakebite impacts. Examples such as the peak of snakebite deaths in India during the monsoon and in Bangladesh during floods illustrate the potential for environmental factors to influence disease. In Sri Lanka, bite and envenoming patterns also vary between climatic zones, with bite and envenoming incidence changing with altitude and rainfall. Climate change is thus likely to be an important yet currently unrecognised contributor to altering snakebite risk in affected areas, potentially impacting on snakebite incidence by altering snake abdundance, distributions and behaviour, altering human abundance and behaviour or both.

This study aims to improve the ways in which the epidemiological burden of snakebite can be estimated and mapping risk using modelling of the interactions between snake and human distributions, behaviours and environmental factors and to investigate the extent to which climate and land-use change will impact upon this burden. The study will develop and validate methodologies using data from Sri Lanka and, in future, use these approaches to improve estimations of the snakebite burden and map risk over wider geographical areas in South Asia, and predict how they may change in the future. This approach will facilitate the diversion of appropriate resources towards addressing this major problem and will provide accurate information about the distribution of the snakebite burden at relevant scales to help health managers target resources appropriately and explore interventions that will help manage this risk and its associated socio-economic impacts in LMICs.

Snakebite is a neglected tropical disease (NTD) and despite being less well studied, has a far greater impact than many other NTDs. There are estimates of up to 120,000 deaths per year globally with major morbidity also resulting from limb damage or renal failure. This burden predominantly affects the rural poor, particularly agricultural workers and subsistence farmers in lower-middle income countries (LMICs) in the tropics/subtropics. There is considerable seasonal and geospatial variation of snakebite because of complex interactions between human behaviour, climate, geography and other factors affecting snake distributions. Burden estimates are difficult: health system statistics are flawed, national surveys are logistically difficult to do and complex variation makes it difficult to estimate national burdens from small local studies. Variation also means that changing environmental, ecological and human conditions could influence this burden.

This new collaboration brings together experts in snakebite, epidemiology, ecology and environmental change science to develop spatially explicit predictions for snakebite risk in a changing climate or land-use environment. We will first update our 2008 global burden of disease study and risk factor analysis to provide the global and regional context for subsequent national-scale research. Then, using Sri Lanka as a study system, we aim to better understand the spatial and temporal variation in snake bite by focusing on the human-snake contact rate. To do this, we will a) develop high resolution distribution models for important venomous snake species and identify how their distributions are shaped by environmental variables, b) generate a snake bite exposure index and validate it with existing detailed epidemiological data on snake bite, envenoming and mortality patterns and c) undertake activity profiling to assess the impact of climate change and land-use on snake and human behaviours and upon these models.

Snakebite is predominantly a problem of the rural poor in the poorest LMICs (1). The greatest burden occurs in South and South-East Asia, sub-Saharan Africa (particularly West Africa) and parts of the South American continent. This application is focused on Sri-Lanka and South Asia, but the methodologies that will be developed will be applicable to all regions where there is a substantial snakebite problem. Snakebite has both significant health consequences (morbidity and mortality) and socio-economic consequences for health systems and households. This application is therefore clearly is line with the principles of ODA.

The project will make important contributions to the study and understanding of the epidemiology of snake bite and establish a novel agenda for understanding and potentially addressing the consequences of global change. Specific outcomes/deliverables will include:

a) Development of a collaborative network across South Asia to facilitate research and follow-on studies and produce an expert opinion report
b) Publication of a 10 year update to the global burden of snake bite study using new methodology
c) Robust global predictive models describing spatial variation in the snake bite burden
d) A venomous snake spatio-temporal occurrence database for South Asia (VenBank)
e) Venomous snake spatio-temporal models for Sri Lanka
f) Socio-ecological snake bite risk models for Sri Lanka
g) Projections (simulations) of future snake bite burden under scenarios of global climate and land-use change.

These outputs will have impact in a number of ways.

The development of regional partnerships, collation of available data and validation of our approach will allow us to begin to apply our methods from Sri Lanka to other countries where data are currently more limited. Our methods will be transferrable, open-source, replicable and adaptable to other geographic regions. Improved spatially-explicit models offer numerous opportunities to improve management and data collection for future risk assessment and reduction activities, including enhancing surveillance (e.g., by establishing sentinel surveillance nodes in high risk areas), optimising interventions (e.g., directing protective equipment and anti-venom supplies to areas of high risk), informing healthcare decision making (e.g., identifying locations for establishing snake bite management centres), or for systematically obtaining additional samples from under-sampled locations to reduce uncertainty and improve representativeness of burden estimates. Better understanding of the main drivers behind human snakebite risk may ultimately allow discussion and design of focused preventative interventions.

Internationally, updated burden estimates and change scenarios will help to support international efforts to raise the profile of snakebite as a neglected tropical disease. These efforts have recently commenced, sparked by a global crisis in antivenom supply, and culminating in discussion of the issue at the World Health Assembly (May 2016). Deficiencies in burden estimation and snakebite epidemiology were recently identified as a barrier to progress at an international consensus meeting at Hinxton in 2015 and this study will help to address this. Better scientific understanding of the health impacts of climate change also help to garner global support for meeting the COP21 targets and other parallel post-2015 health, environmental, and sustainable development targets and for implementing WHO's Generic framework for control, elimination and eradication of neglected tropical diseases.

1) Harrison RA, Hargreaves A, Wagstaff SC, Faragher B, Lalloo DG. Snake envenoming: a disease of poverty. PLoS Negl Trop Dis. 2009 Dec 22;3(12):e569. doi: 10.1371/journal.pntd.0000569