IN our column today we will examine the impact of climate change on health. The rapid increase of greenhouse gas emissions from burning fossil fuels has driven the widespread effects of climate change.

Since 1900 the global mean temperature has risen by 1.1°C, with most of the change happening in the last 50 years. Regions with extreme climates, such as highlands and polar regions, have been affected the most, while temperatures in tropical regions are approaching the thermal limits of many organisms. Based on current national policies and actions, it is expected that the temperature will rise by 2.5°C to 2.9°C or more by the end of the century.

Climate change and its manifestations, including changes in precipitation and flooding in some areas and drought in others, have significant implications for vector-borne diseases. The effects on pathogens, vectors, and hosts can make it challenging to attribute changes in the distribution and frequency of diseases to climate change, as other factors such as changes in land use, the abundance of reservoir hosts, and control measures also play a role. However, it is clear that the components of vector-borne diseases, including pathogens, vectors, and reservoir hosts, are highly sensitive to changes in the environment. The behaviour, physiologic characteristics, and life history of vectors and pathogens, as well as the abundance and behaviour of reservoir and definitive hosts, can all be impacted by rising temperatures. Interactions between temperature, vectors, and pathogens can increase the risk of disease spreading and spilling over to humans. Thermal performance curves can be used to predict the effects of rising temperatures on vector-borne diseases by illustrating the ways in which temperature impacts the physiological traits of pathogens, vectors, and reservoir hosts.

The Intergovernmental Panel on Climate Change has high confidence that the prevalence of vector-borne diseases has increased in recent decades, and that malaria, dengue, Lyme disease, and West Nile virus infections in particular are expected to continue rising if action is not taken to adapt and strengthen control strategies.

In this first part of a two-part series we will examine climate-sensitive vector-borne diseases, including malaria, dengue, Lyme disease, and West Nile virus.

CLIMATE-SENSITIVE VECTOR BORNE DISEASES

Malaria

Malaria, caused by the plasmodium species and transmitted through infected female Anopheles mosquitoes, remains the deadliest and most studied, climate-sensitive, vector-borne disease. Despite control efforts, over 600,000 deaths were attributed to malaria in 2020, primarily among pregnant women and young children in Africa. In many regions malaria is seasonal or epidemic, responding to short-term changes in rainfall, humidity, and temperature. Rising temperatures have been linked to the spread of malaria to higher elevations in Colombia and Ethiopia while droughts are increasing, potentially reducing the prevalence of malaria in certain regions. However, broader effects of climate change on local livelihoods, food security, and migration may increase population vulnerability to the disease and undermine control efforts.

Dengue

Dengue, the most common mosquito-borne viral disease worldwide, has seen a substantial expansion of its geographic range in recent decades, driven by declining vector control programmes and increasing global trade and travel. With an estimated 390 million cases occurring each year in over 100 countries, the four serotypes of dengue virus are transmitted between humans through infected female mosquitoes, most commonly Aedes aegypti and Aedes albopictus. Water storage containers, which are commonly used in regions without piped water, can become mosquito breeding sites, driving epidemics. By 2030 the dominant cause of expansion of these vectors is predicted to be climate change. The differential ability of Aedes aegypti and Aedes albopictus to survive normally lethal temperatures may influence their roles in future outbreaks.

Lyme disease

Lyme disease is the most common tick-borne illness worldwide, with an estimated seroprevalence of 14.5 per cent; the reported prevalence is highest in the temperate regions of central and western Europe and East Asia. Without early treatment, infection can cause debilitating, multi-systemic chronic disease. Worldwide, Lyme disease involves four dominant tick species, although generally only one tick species is cause for concern in any given region. Wide-ranging reservoir hosts — including mammals (eg, mice and squirrels), lizards, and birds — are part of the ecologic complexities of this disease; however, humans play no role in ongoing transmission. The life cycle and prevalence of tick vectors are strongly influenced by the abundance of reservoir hosts and the ambient air temperature.

Insurance records indicate that 470,000 cases of Lyme disease were diagnosed and treated in the United States during the period from 2010 to 2018, as compared with 329,000 cases during the period from 2005 to 2010. Lyme disease is most common in the Northeast and rare in the Southeast. Although tick vectors are found in both regions, variations in the host preferences of the ticks (e.g., lizards or mice), in the host-seeking behaviour of the ticks, and in the tick density, help to explain this geographic pattern. The increases in Lyme disease cases in the Northeast are largely attributed to the recovery of white-tailed deer populations, which are critical hosts for adult stages of the tick vector; however, increased human–tick interaction owing to the extended summer season resulting from climate change also contributes to the increases in cases. Warming temperatures have been associated with the expansion of Ixodes ticks into Canada and Norway, with a corresponding increase in cases of Lyme disease.

West Nile virus infection

West Nile virus causes potentially fatal neuroinvasive disease in humans and animals worldwide. The virus is part of a complex ecosystem that is centred around a bird–mosquito transmission cycle involving more than 300 bird species and at least 65 mosquito vectors. Mammals, including humans and horses, can be incidentally infected. Human infections are mostly asymptomatic but can cause life-threatening illness in rare cases, predominantly in older adults, and in immunocompromised persons.

West Nile virus, which was first identified in the United States (in New York City) in 1999, is the leading cause of mosquito-borne disease in the continental United States. During the period from 1999 to 2016 nearly 7 million persons were infected. The observed air temperature that results in a peak incidence of the virus among humans across the country was found to be 24°C, which closely matches the temperatures (which ranged from 24°C to 25°C) that were predicted by mechanistic models that were based on vector and pathogen thermal performance curves. Warming temperatures are expected to shift transmission of this disease northward, as is already occurring in Europe; local transmission was recently discovered in Germany after unusually warm weather.

Dr Ernest Madu, MD, FACC and Dr Paul Edwards, MD, FACC are consultant cardiologists for the Heart Institute of the Caribbean (HIC) and HIC Heart Hospital. HIC is the regional centre of excellence for cardiovascular care in the English-speaking Caribbean and has pioneered a transformation in the way cardiovascular care is delivered in the region. HIC Heart Hospital is registered by the Ministry of Health and Wellness and is the only heart hospital in Jamaica. Send correspondence to info@caribbeanheart.com or call 876-906-2107