Month: February 2015

Hard Evidence: will climate change affect the spread of tropical diseases?

Asian Tiger Mosquito
The Asian Tiger Mosquito (Aedes albopictus) can transmit dengue fever

The Climate and Environment at Imperial blog has moved. View this post on our new blog 

By Dr Paul Parham, Honorary Lecturer in Infectious Disease Epidemiology

Many tropical diseases such as malaria, Chagas disease and dengue are transmitted to humans via mosquitoes and other carriers known as vectors. These vector-borne diseases continue to have a major impact on human health in the developing world: each year, more than a billion people become infected and around a million people die. In addition, around one in six cases of illness and disability worldwide arise from these diseases.

Malaria arguably continues to attract the most attention of all the vector-borne diseases by virtue of causing the greatest global disease burden. However, others such as dengue are not only resurgent in some regions, but threaten a vast proportion of the world’s population.

Climate change remains a substantial threat to future human health and since the behaviour of disease carriers like mosquitoes is known to be extremely sensitive to temperature and rainfall, it seems unquestionable that climate change will affect many, if not all, of these diseases. What is less clear, however, is the extent to which climate increases the risk of becoming infected in certain regions compared to other factors such as poverty or fragile health systems.

In addition, although the number of new cases of diseases such as malaria appears to be declining worldwide, it is still increasing in many regions for a variety of reasons; the continued spread of insecticide resistance, changes in land use, and difficulties in maintaining political interest pose considerable challenges. Which of these factors will be most influential over the coming decades remains up for debate and one that was raised in a special edition of Philosophical Transactions B.

Changes in risk

The latest research, however, is clear and consistent in many of its findings. Different diseases, transmitted by different vectors, respond in different ways to changing weather and climate patterns. Climate change is very likely to favour an increase in the number of dengue cases worldwide, while many important mosquito populations that are able to transmit devastating diseases are changing in their distribution.

The latest maps show that many areas of Europe (including the UK) could become increasingly hospitable for mosquitoes that transmit dengue over the coming decades (the map below shows a projected change in suitable habitat for the Aedes albopictus mosquito). Similarly, other mosquito range expansions are likely to occur in the US and eastern Asia. If dengue and/or chikungunya are imported into these regions, there will be a considerable increase in the worldwide number of vulnerable individuals.

European map of simulated Aedes albopictus habitat suitability based on one future climate projection for the period 2045-2054.
European map of simulated Aedes albopictus habitat suitability based on one future climate projection for the period 2045-2054.

It is also clear that small changes in these so-called risk maps can have very large public health impacts. Tick-borne diseases (such as Lyme disease) are also predicted to expand in range as climate changes. Although, as before, plenty of other factors are likely to contribute, meaning that direct causation is very hard to attribute.

It is important to remember too that climate change is not just global warming; the latter refers to an increase in global mean temperatures, but there is also an overwhelming body of evidence demonstrating that rainfall is at least as important for many vector-borne diseases. Rainfall episodes have also been shown to provide a very good early-warning sign a few months in advance for outbreaks of West Nile Virus.

New research on African anti-malaria mosquito control programmes that involve spraying houses (to kill indoor mosquitoes) and distributing bed nets also shows that both temperature and rainfall can influence the degree to which programmes decrease new infections and, crucially, their cost-effectiveness. However, whether or not this is substantial enough to affect regional policy decisions about scaling up mosquito control programmes depends on factors such as how rapidly insecticide resistance emerges, the human immune response to malaria, and country-specific conditions.

In terms of malaria elimination in Africa, adopting the same approach in all affected regions is unlikely to be the best way forward. However, there is some new evidence to suggest that if efforts continue to be concentrated on scaling-up current intervention programmes in regions close to elimination, the longer-term effects of climate change will become far less important. Indeed, one of the most effective ways of protecting human health against climate change in the long-term is to further strengthen current disease control efforts.

Mathematical models

As with the formulation of public health policies to deal with diseases such as Ebola, flu, and HIV, mathematical models are valuable tools that are widely used to make predictions about how different carrier-borne diseases are likely to respond to climatic changes. How reliable these predictions are is an important question and, like many areas of science, include unavoidable uncertainties. For example, people may change their behaviour and actions as climate change evolves – for example by migrating to other areas – which evidently makes forecasting more difficult.

New evidence has also shown that disease vectors may evolve in under a decade to changes in temperature, which conflicts with many current models that assume climate change only affects their ecology, not their evolution. Predictions that might be affected by climate change must therefore not only take account of these uncertainties, if they’re to be more reliable and useful, but also recognise that these predictions cannot strictly be disproved until the future arrives.

This remains a very active research field, but considerable progress in our understanding has been made over the last ten to 15 years. Better data on the links between vectors, diseases they carry and the environment is definitely required, as are better ways of quantifying disease risk for different populations and different diseases.

 Seven steps to understanding climate impacts and assessing risks. Philosophical Transactions B

Seven steps to understanding climate impacts and assessing risks.
Philosophical Transactions B

Future challenges

Many diseases have received very little attention so far on how climate change may affect future trends. One example is onchocerciasis (river blindness), for which tentative predictions suggest that we might expect substantial increases in the number of disease vectors in certain African regions over the coming decades.

Almost all models are currently based on single diseases, but many populations are unfortunately burdened with multiple diseases at any one time; understanding how climate change affects interactions between these diseases has attracted little attention to date.

One other important challenge for the field is the mismatch between the data current global climate models are able to provide and the information required by local public health officials to make more informed decisions; continued improvements in computing power are essential to progress. The predictions of our current models are not perfect and improvements in our understanding are certainly required.

To date, we have tended to react to disease outbreaks as they occur, but we need an increased focus on being more proactive; we cannot stop outbreaks of many of these diseases, but proactive risk management is less expensive (and more effective) than responding after a crisis. Ultimately, the challenge is not to address specific health risks due solely to climate change, but instead to ensure sustained progress is made towards decreasing the number of deaths and cases of these diseases for future generations.

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The global health benefits of tackling climate change

The Climate and Environment at Imperial blog has moved. View this post on our new blog 

by Professor Paolo Vineis and Pauline Scheelbeek, School of Public Health
Cycling

It is sometimes claimed that addressing climate change with proper policies is too expensive and could lead to a further decline in the economy. However, the co-benefits of implementation of climate change mitigation strategies for the health sector are usually overlooked. The synergy between policies for climate change mitigation in sectors such as energy use (e.g. for heating), agriculture, food production and transportation may have overall benefits that are much greater than the sum of single interventions (Haines et al, 2009). Here we describe a few examples of climate change mitigation strategies that have important co-benefits for global health.

  1. Reducing CO2 admission by promotion of active transport

The transportation sector is often the single largest source of greenhouse gas emissions in urban areas. Policy makers have tried to reduce these emissions by discouraging car travel and promoting other means of (active) transport. Active transport, such as cycling and walking, increases daily physical activity. Physical inactivity is one of the leading causes of non-communicable diseases all over the world. It has been estimated that the combination of active travel and lower-emission motor vehicles would give large health benefits, notably from a reduction in the number of years of life lost from coronary heart disease (10-19% in London, 11-25% in Delhi according to Woodcock et al, 2009). Also obesity, which is increasing dramatically all over the world, particularly in children, could effectively be reduced by a more active lifestyle. A 30-minute walk per day could – on many occasions – be enough to even out slight caloric excess.

  1. Domestic energy management & reduction of cooking/heating emissions

Improving heating and cooking systems – for example by making them more efficient – reduces energy consumption. Improved models of stoves (electrical vs biomass) allow a 15-times reduction in the emission of particles and other pollutants, thus contributing to decreased emissions in the atmosphere. Especially in developing countries – where old stoves are common – these improvements could also have a considerable positive impact on health: cooking on simple wood or coal stoves currently forms a major source of indoor pollution and increases risk of certain chronic diseases, such as chronic obstructive pulmonary disease (COPD). The potential effectiveness of this strategy was shown by Wilkinson et al: they calculated that if 150 million low-emission cookstoves were introduced in India, this could lead to the prevention of an estimated 1.3 million deaths from COPD and hundreds of thousands of deaths from other diseases such as coronary heart disease (Wilkinson, et al 2009). Air pollution is one of the biggest environmental causes of death worldwide, with household air pollution accounting for about 3·5-4 million deaths every year (Gordon et al, 2014).

  1. Reductions in CO2 through reduced meat production

meat production diagramMeat production is highly inefficient energetically: it requires an extremely high use of water and land per unit of meat.  One fifth of all greenhouse gases worldwide are related to methane production from livestock farms.  Reduction of meat intake by consumers would lower meat production and is therefore often promoted as climate change mitigation strategy. The figure shows that a high intake of meat is also associated with increased disease risk, in particular for certain cancers and cardiovascular disease (WCRF, 2007). Reduced meat consumption would therefore also have a major impact on public health. It has been estimated that a 30% reduction in livestock production in the UK would reduce cardiovascular deaths by 15% (Friel et al, 2009).

 

  1. Low carbon energy production

Non-renewable energy production, for example coal burning, is a major contributor to worldwide greenhouse gas emissions. Many countries have adopted policies to reduce polluting energy production and stimulate production of (renewable) energy through cleaner sources. For example, since 2000, the government in the Chinese Shanxi province has promoted several initiatives (including factory shutdowns) with the goal of reducing coal burning emissions. The annual average particulate matter (PM10) concentrations decreased from 196 μg/m3 in 2001 to 89 μg/m3 in 2010, which – as a matter of fact – is still very high for Western standards. It has been estimated that the DALYs (Disability-Adjusted Life Years) lost in Shanxi had decreased by 56.92% as a consequence of the measures (Tang et al, 2014).  The IPCC 5th assessment report stresses that the main health co-benefits from climate change mitigation policies come from substituting polluting sources of energy for renewable and cleaner sources, with a considerable effect on the improvement of air quality.

 

Practical conclusions

The co-benefits from climate change mitigation for the health sector have not yet been completely identified and quantified. The topic does not appear on the priority list of political discourse: relevant sectors, including those involved in non-communicable disease prevention (Pearce et al, 2014), transportation, agriculture, food production and climate change (Alleyne et al, 2013), still work separately, while collaboration would improve the synergy between health improvement and climate change mitigation and maximise benefits for both.

 

References

Alleyne G, Binagwaho A, Haines A, Jahan S, Nugent R, Rojhani A, Stuckler D; Lancet NCD Action Group. Embedding non-communicable diseases in the post-2015 development agenda. Lancet. 2013 Feb 16;381(9866):566-74. doi: 10.1016/S0140-6736(12)61806-6

Friel S, Dangour AD, Garnett T, Lock K, Chalabi Z, Roberts I, Butler A, Butler CD, Waage J, McMichael AJ, Haines A.  Public health benefits of strategies to reduce greenhouse-gas emissions: food and agriculture.  Lancet 2009; 374: 2016-25.

Gordon SB, Bruce NG, Grigg J, Hibberd PL, Kurmi OP, Lam KB, Mortimer K, Asante KP, Balakrishnan K, Balmes J, Bar-Zeev N, Bates MN, Breysse PN, Buist S, Chen Z, Havens D, Jack D, Jindal S, Kan H, Mehta S, Moschovis P, Naeher L, Patel A, Perez-Padilla R, Pope D, Rylance J, Semple S, Martin WJ 2nd. Respiratory risks from household air pollution in low and middle income countries. Lancet Respir Med. 2014 Oct;2(10):823-60. doi: 10.1016/S2213-2600(14)70168-7

Haines A, McMichael AJ, Smith KR, Roberts I, Woodcock J, Markandya A, Armstrong BG, Campbell-Lendrum D, Dangour AD, Davies M, Bruce N, Tonne C, Barrett M, Wilkinson P. Public health benefits of strategies to reduce greenhouse-gas emissions: overview and implications for policy makers. Lancet. 2009 Dec 19;374(9707):2104-14. doi: 10.1016/S0140-6736(09)61759-1. Epub 2009 Nov 26.

Pearce N, Ebrahim S, McKee M, Lamptey P, Barreto ML, Matheson D, Walls H, Foliaki S, Miranda J, Chimeddamba O, Marcos LG, Haines A, Vineis P. The road to 25×25: how can the five-target strategy reach its goal? Lancet Glob Health. 2014 Mar;2(3):e126-8. doi: 10.1016/S2214-109X(14)70015-4.

Tang D, Wang C, Nie J, Chen R, Niu Q, Kan H, Chen B, Perera F; Taiyuan CDC. Health benefits of improving air quality in Taiyuan, China. Environ Int. 2014 Dec;73:235-42. doi: 10.1016/j.envint.2014.07.016. Epub 2014 Aug 27.

Wilkinson P, Smith KR, Davies M, Adair H,  Armstrong BG, Barrett M, Bruce N, Haines A, Hamilton I, Oreszczyn T,  Ridley I, Tonne C and Chalabi Z. Public health benefits of strategies to reduce greenhouse-gas emissions: household energy. 2009. The Lancet, 374: 9705 (P1917 – 29)

World Cancer Research Fund. Recommendations Booklet. Available from:  http://www.wcrf.org/

Woodcock J, Edwards P, Tonne C, Armstrong BG, Ashiru O, Banister D, Beevers S, Chalabi Z, Chowdhury Z, Cohen A, Franco OH, Haines A, Hickman R, Lindsay G, Mittal I, Mohan D, Tiwari G, Woodward A, Roberts I. Public health benefits of strategies to reduce greenhouse-gas emissions: urban land transport. Lancet. 2009 Dec 5;374(9705):1930-43. doi: 10.1016/S0140-6736(09)61714-1. Epub 2009 Nov 26.