Climate Change and Vector-Borne Disease in Humans in the UK

11 Apr 2019 01:40 PM

A POST note that summarises the latest data on vector-borne disease in the UK, explores how climate may influence the geographical distribution of species, examines the consequences for public health, and highlights potential adaptation and mitigation strategies.

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Vector-borne diseases (VBDs) are transmitted by organisms, such as ticks and mosquitoes. The UK has both native and non-native vectors that are distributed across the country. The distribution of these species is changing across Europe, and within the UK. The causes of why the distribution is changing are complex and interlinked but climate change plays a key role, especially in mosquito distribution. Ticks are more susceptible to changes in landscape and host population density. These factors are correlated with the spread of Lyme disease, a major tick-borne disease in the UK. Surveillance coverage and methods vary for different vector species and there is consensus that UK surveillance requires improvement. In addition to surveillance, addressing knowledge gaps and managing vector populations, alongside improved public awareness are important factors when tackling vector-borne diseases.  

VBDs are spread by vectors, which are organisms that carry and transmit disease between humans, animals and plants. VBDs range from malaria to Lyme disease and account for approximately 17% of infectious disease and 700,000 deaths globally. The severity of VBDs varies depending on the disease but can result in severe symptoms if not treated. For example, the mosquito-borne disease West Nile virus (WNV) has caused around 1,600 deaths in the US. For tick-borne diseases, Lyme disease is the most common VBD in the UK with Public Health England (PHE) reporting an estimted 3,000 cases annually. Due to the global health implications of VBDs, the World Health Organization has highlighted international interest in predicting and limiting geographical distributions of vectors.  

There are many different vectors capable of spreading disease, however, ticks and mosquitoes are the vectors of most concern in the UK. The UK has 36 native mosquito species and most do not transmit diseases, however, some Culex mosquitoes are capable of transmitting West Nile virus. Invasive mosquitoes, such as Aedes albopictus, are capable of transmitting dengue and chikungunya diseases and Ae. albopictus was recorded in southern England in 2016, 2017 and 2018, despite it originating in southeast Asia. For tick vectors, Ixodes ricinus is the greatest public health concern for the UK, as it is the vector for the Borrelia bacteria that causes Lyme disease. This tick is found all across the UK and can also act as a vector for tick-borne encephalitis (TBE). TBE, however, is not present in the UK although it has been recorded in several European countries including Switzerland and the Netherlands.  

Recently, there have been changes in vector distribution across the UK. This includes identification of invasive species, such as Ae. albopictus, in Southern England as well as changing patterns of tick distributions. The causes of these changes are complex, however, climate change is thought to be a main contributory factor. Climate change causing higher temperatures, speed up mosquito development allowing for potentially earlier infection. Ticks, however, are longer-lived and feed sparingly during their life, meaning temperature has less of an effect than mosquitoes, however they are affected by other climate variables such as moisture levels.  

Currently, the UK has a Tick Surveillance Scheme (2005) run by Public Health England (PHE) to record and provide information about native and non-native invasive tick species. PHE also runs a mosquito reporting system and the public is also encouraged to submit mosquitoes for identification. A major concern for stakeholders is that the current level of national surveillance is inadequate, and that monitoring should be expanded. This includes adopting further citizen science approaches, enhancing public awareness as well as address more fundamental research gaps of the biology of VBDs. In general, however, vectors and the environment are adapting continuously so future guidelines will also need to adapt. This is often difficult to predict, but increasing surveillance and research aids this process.  

Key points: 

• VBDs are diseases that are transmitted by vectors, organisms that carry and transmit diseases between humans, animals and plants. VBDs account for 17% of infectious diseases and have a significant health impact globally.  
• Vectors vary in the diseases they transmit. Mosquitoes and ticks are the vectors most concerning to the UK. Mosquitoes recorded in the UK, such as Ae. albopictus are capable of transmitting dengue fever and chikungunya fever. Ticks endemic to the UK can transmit Lyme disease and tick-borne encephalitis, a severe viral infectious disease. 
• Vector establishment does not always mean spread of the VBDs they can carry, as other factors such as vector biting behaviour, affects their capacity to transmit disease. Currently, no mosquito-borne diseases of humans are circulating in the UK. Lyme disease is the most common VBD in the UK.  
• Vector distribution is complex and affected by a variety of factors including globalisation and socio-economic factors. Climate change is also thought to be a main contributory factor.  
• Higher temperatures as a result of climate change impacts mosquito development and affects transmission of VBDs. Infected ticks are less dependent on temperature but are affected by other climate variables such as moisture levels. Modelling predictions suggest that further climate changes coupled with other factors such as globalisation of trade could increase the UK’s susceptibility to other VBDs.  
• Tick and mosquito surveillance is conducted by Public Health England, but several stakeholders believe that the current level is inadequate and should be expanded.  
• Expanding citizen science approaches, as well as increasing public awareness and health professional awareness would also help in combating VBD incursion into the UK.  

Acknowledgments

POSTnotes are based on literature reviews and interviews with a range of stakeholders and are externally peer reviewed. POST would like to thank interviewees and peer reviewers for kindly giving up their time during the preparation of this briefing, including:

• Dr Rhian Rees-Owen, Department for Business, Energy and Industrial Strategy
• British Veterinary Association 
• Dr Debbie Hemming, Met Office* 
• Dr Helen Roberts, Dr. Chris Browne, Department for Enviroment, Food & Rural Affairs*
• Professor Steven Lindsay, Durham University
• Professor James Logan, London School of Hygiene and Tropical Medicine
• Dr Jolyon Medlock, Public Health England*
• Dr Bethan Purse, Centre for Ecology and Hydrology* 
• Professor Matthew Baylis, University of Liverpool*
• Professor Heather Ferguson, University of Glasgow*
• Dr Madeleine Thomson, Columbia University
• Dr Barbara Tschirren, University of Exeter* 
• Professor Richard Birtles, University of Salford*
• Lydia Franklinos, University College London* 
• Dr Caroline Millins, University of Glasgow 
• Dr Lauren Cator, Imperial College London*  
• Dr Simon Carpenter, The Pirbright Institute*
• Dr Anne Cruickshank, Royal College of General Practitioners* 
• Lyme Disease Action*
• Gemma Holmes, Committee on Climate Change
• Oxitec

*Denotes those who acted as external reviewers of the briefing.