The ecological and environmental drivers of vector borne disease

21 May 2024

The ecological and environmental drivers of vector borne disease

Reported by Amber Khan, Clinical Medical Student, University of Cambridge

In February 2024, Dr Jolyon Medlock, Head of Medical Entomology at the UK Health Security Agency, met with the Cambridge Zero Policy Forum to discuss ecological and environmental drivers of vector borne disease. Attended by a multidisciplinary group of academics, the roundtable discussion centred on the links between climate and environmental change, habitat management, and vector borne disease, exploring vector ecology and ecosystem dynamics, national surveillance and local intervention, and stakeholder and public perspectives.

Vector borne disease

Vector borne disease describes infectious disease caused by pathogens (viruses, bacteria, or parasites) transmitted by vectors, of which hematophagous (blood-feeding) arthropods such as ticks and mosquitos are the most notable. The geographic and seasonal distribution of vector populations, the pathogens they carry, and subsequent human disease incidence, is influenced by complex social and environmental factors. Of particular relevance to climate discussions, drivers such as land use change, changing climate and whether patterns, and effects such as biodiversity loss and human mobility, may all contribute to shifting patterns of disease from pre-existing tropical and subtropical endemism.

Dengue, transmitted by Aedes aegypti (and to a lesser extent Aedes albopictus) mosquito, and tick-borne encephalitis, transmitted by tick, are some of the most important emerging vector borne diseases in Europe, highlighted in the Human Animal Infections and Risk Surveillance (HAIRS) national security risk assessments. These, along with tick borne Lyme, formed the basis of discussion.

Shifting patterns across Europe

Within the last 20 years, rising temperatures have enabled northward expansion of ectothermic (cold-blooded) arthropods and associated pathogens across Europe, shifting transmission cycles with correlating rises in disease incidence and local transmission. One of the northernmost outbreaks reported was that of Aedes albopictus borne dengue in Paris (October 2023), and with previously eradicated primary vector Aedes aegypti recolonising south-eastern Europe, an epidemic is expected in coming years. Climate thresholds and climate-based models hence play an important role in predicting risk and informing local risk-based surveillance and intervention decision making, encouraging local authority and community engagement. Both Aedes aegypti and Aedes albopictus being invasive species (non-native to the UK), surveillance focuses on species identification via trapping at key points of entry and urban centres, as well as citizen science. Interventions include biosecurity measures and wetland management guidance.

Ecosystem drivers in the UK

In addition to climate, ecosystem dynamics can drive shifting patterns of vector borne disease, as illustrated in the discussion of tick-borne Lyme. During the COVID19 pandemic, reduced human activity led to expansion of deer populations in some areas. With tick populations dependent on large mammals such as deer, changes in deer numbers are critical for impacting on tick densities. Smaller hosts such as woodland and game birds, and small mammals are important in driving the transmission of causative Borrelia bacteria. In 2023, various factors combined to increase tick densities and this was reflected in the increase in disease incidence. With the species already endemic, surveillance methods include working with national parks to monitor infection rates and characterise transmission cycles (such as of different Borrelia species, Borrelia garinii and Borrelia afzelii) via surveys and sequencing, working with farmers and veterinarians via livestock hosts as sentinels, as well as public health bodies to monitor changes in distribution via tick bite and disease incidence. Again, such evidence helps to inform local risk-based interventions, from forestry management to public health and education campaigns such as the Tick Awareness Toolkit.

Surveillance and intervention challenges

In such health paradigms, surveillance and interventions act at a range of levels, from environment and habitat management, to animal host and vector population control, to public health and healthcare services. A range of challenges were highlighted, and potential solutions discussed.

1. Methods and technology

Identification and characterisation of multiple interacting vectors and pathogens presents logistical challenges. Dengue vector surveillance via trapping, for instance, relies on accurate identification of vectors Aedes aegypti and Aedes albopictus from some 36 native mosquito species. The effects on transmission cycles of complex interactions between dengue primary and secondary vectors, Aedes aegypti and Aedes albopictus, and Lyme Borrelia pathogens, Borrelia garinii and Borrelia afzelii, requires further understanding. Here, technology for detecting a range of mosquito species is key to facilitating integrated expert and public efforts, from advances in sequencing and the potential for emerging environmental DNA (eDNA) methods, to the use of mobile phones in citizen science.

2. National and regional priorities

With successful surveillance programmes dependent upon diagnostic and intervention capacity, challenges include limited awareness and diagnosis of often non-specific symptoms of emerging vector borne diseases within primary care, and preparedness to respond to such emerging public health issues within the wider healthcare system. Evidence, from climate and habitat suitability modelling to serosurveillance, is used to generate risk assessments, identifying risk in regions or populations. Such risk assessments play a key role in targeting national political and local authority priorities and funding, for effective surveillance and intervention. This could translate to better education of the topic in both environmental health and healthcare spaces, and future scenario workshops for preparedness in policy spaces.

3. Multiple stakeholders

Participants advocated for diverse stakeholders, including healthcare, veterinary, commercial agriculture and environmental sectors, policy makers, and local communities. Classical conflicts of interest between commercial and environmental sectors were noted, complicated by political influence and power dynamics. For instance, early warning systems for vector borne disease in unmanaged land lacked the incentive for commercial investment in that land. Limits of health considerations even within the environmental sector, as illustrated in the absence of vector borne disease risk from Environmental Impact Assessments, also highlighted the need for more coordinated and connected efforts.

4. Narratives and public engagement

Multiple examples were drawn upon to illustrate the role of narratives in public engagement and ultimately success of interventions. Natural control of Aedes aegypti dengue vectors via Wolbachia involves introduction of the endosymbiont bacteria to produce vector population and pathogen transmission suppression. Globally, this has seen mixed responses, from successful public awareness campaigns in Indonesia to ecological concerns and fear-based opposition in France. Whilst sterile insect technique (SIT) was favoured in the latter case, decreased competition resulted in paradoxically increased fitness, necessitating pesticide boosting (BSIT).

In the UK, tick reporting was greatest in areas with least previous exposure and thereby existing awareness, where instead narratives of fear predominated, possibly exacerbated by media. Local authority and community concerns regarding tick exposure in urban green spaces highlighted the importance of balancing communication of risk with benefits of biodiversity, both for vector and pathogen control and human health more broadly. We may learn from Scandinavian countries, where tick prevalence and associated public awareness is already well established. Similarly in the Cambridgeshire fenlands, wetland restoration, combined with urban expansion in flood zones, has resulted in fear regarding malaria risk. Art and social projects conveying mosquitos as pollinators and an important part of the ecosystem, and biodiversity as natural control for vector borne disease, emphasise narratives of awareness and interconnection in place of fear and elimination.

Take home message

Vector borne disease presents a complex and dynamic planetary health challenge, solutions for which will require a shift from silos to systems thinking, global collaboration, interdisciplinary efforts, and cross-society engagement.

Image by Hans Eiskonen on Unsplash