When we think of mosquito-borne diseases such as malaria or dengue fever, our minds first jump to areas on the African continent, Asia or South America. However, we are seeing more and more cases of these arboviruses reported further above and below the equator in areas where previously these mosquitos did not occupy.
The World Health Organisation (WHO) warned on 5 April that due to the planet’s increasing temperatures, we could see mosquito-borne arboviruses spreading further and further away from their current territories, leading to future global outbreaks of these diseases.
Climate change goes beyond ice caps melting and drought in the deserts; the effects of climate change can be felt much closer to home through disease - often an overlooked aspect impacted by global warming. With changes in temperature and weather patterns, studies predict that much of Europe, America and Canada could be affected by vector-borne outbreaks in the near future. Researchers at Imperial College London modelled life-cycle completions for Aedes aegypti against time, when under different environmental conditions which may become a reality due to climate change. The results indicated that by 2030, Aedes aegypti could potentially invade China, North America and regions of Europe. Whilst it is not unusual for some of these vector-borne diseases to be reported in these regions, public health agencies reported a sharp rise in cases compared to previous years. For example, in the Americas, the number of Dengue cases in 2022 was 2.8 million, compared to 1.2 million cases reported in 2021. The intensification of disease transmission is supported by many articles expressing concern about mosquito-transmitted viruses due to the sharp rise in cases in 2022 across multiple nations compared to previous years.
Changes in the environment are not only temperature-related. Increasing temperature in conjunction with rainfall increases humidity and can be a perfect pairing for breeding sites. In addition, as we see more significant droughts, individuals will be inclined to invest in water storage which can lead to more ideal breeding sites for mosquitoes. Due to climate change, changes in how we use land can also indirectly enhance disease transmission and human exposure to vector-borne diseases such as land use, urban development, water storage and land irrigation. This is particularly worrying for the vector Aedes aegypti, a mosquito which can spread several infectious diseases, including Zika, Dengue, Chikungunya, and yellow fever.
In 2022, the number of Dengue cases in the Americas was 2.8 million, compared to 1.2 million cases reported in 2021.
A growing threat of tropical disease outbreaks in not-so-tropical regions is alarming to public health authorities. Learning from past outbreaks, we can see how quickly these diseases can spread and their detriment to populations. For example, the explosive outbreaks when arboviruses are first introduced into naïve populations, like chikungunya and Zika in the Americas. Furthermore, the risk of viruses circulating through communities is heightened by the lack of herd immunity within the population, as many have not been exposed to these diseases before. Invasive species of mosquitoes such as Aedes albopictus have been recorded in the UK over consecutive years, even though they are native to southeast Asia, highlighting the extent of climate change for changes in vector distribution.
Vector-borne diseases account for 17% of the world’s infectious diseases, and the climate threat is not just towards mosquito-borne diseases but other ectotherms, such as ticks. Climate change has meant ticks are now a problem all year round in the UK, particularly in woodland areas. We have seen milder winters and increased rainfall; the increased humidity is perfect for ticks to thrive. This increases the risk of transmission of tick-borne diseases such as Lyme disease and tick-borne encephalitis. In the UK, Two cases of tick-borne encephalitis were reported in Autumn 2022, the first reported since 2019. Scientists have suggested that the occurrence of the disease may be linked to climate change.
Many vector-borne diseases threatening new borders are on the WHO neglected tropical disease list. Surprisingly, these diseases have lacked engagement and support in research and policy agendas. More must be done to prioritise funding and open discussion for global intervention to support the control, prevention, and eradication of these diseases in endemic areas. Public health authorities should reflect on previous outbreaks to enhance surveillance and control methods. Building on current surveillance infrastructures across nations, collaboration could tackle vector-borne diseases in the face of climate change, reducing the risk of these diseases affecting endemic territories and naïve populations.