Climate change and diseases: How does it end?

people wearing masks

Typically, the effects of climate change are discussed regarding rising sea levels and habitat loss. However, there is a direct relationship between climate and human disease, and is it not good. Photo credit: Gabriella Clare Marino via Unsplash

Global temperatures have increased over 1˚C since pre-industrial levels, with the ten warmest years on record all occurring since 2010, and there is a further predicted 1.5-2˚ C rise by 2100. The most significant consequence of climate change – rising temperatures — is reported to increase the frequency of natural disasters, resource conflicts, and extreme weather events. However, perhaps our chief concern of climate change should be the alteration of the temporo-spatial distribution and increased prevalence of fatal infectious diseases. Vector-borne diseases (VBDs) are particularly susceptible to these environmental changes. Without robust preventative measures, a future pandemic, which may be unanticipated and intensified because of climate change, could be detrimental to global stability and cause many fatalities.

…perhaps our chief concern of climate change should be the alteration of the temporo-spatial distribution and increased prevalence of fatal infectious diseases.

VBDs contribute to 17% of all infectious diseases and cause 700,000 deaths annually, with malaria alone infecting 249 million people in 2022. The March 2023 report from the Intergovernmental Panel on Climate Change (IPCC) described an increased incidence of VBDs, with almost 80% of the global population being at risk from VBDs. Globally, the most common vector (an organism that carries infectious diseases between species) is the mosquito, although ticks, fleas, and lice may also be vectors. Climate change affects VBDs through meteorological and environmental changes, influencing changes in the transmission dynamics, geographic spread, and re-emergence of VBDs, as more areas are hotter, and hotter for longer periods of time. If left unchecked, could this lead to the development of a non-resolvable pandemic?

There is no simple way to analyse the relationships between rising temperatures, mosquito populations, and disease incidence. Furthermore, there is no simple method of distinguishing the effects of climate factors from non-climate factors on the changes in disease prevalence; however, most studies indicate a positive correlation between increasing temperature and mosquito-borne diseases. This is due to an accelerated lifecycle and a shortened incubation period.  Interestingly, some studies show a decreased incidence of VBD compared to previous levels as extreme temperatures and weather have the potential to limit vector development and survival. It would be naïve to assume that changing temperature is the sole variant – other factors that are secondary to climate change, such as immigration (e.g. due to conflicts or flooding), changing of the vector habitat, and diminished immunity (due to food shortages or heat stress) can positively influence VBDs and their impact, although detrimental to the population if government responses to outbreaks remains inadequate.

Aedes aegypti (Yellow Fever Mosquito) is a primary vector for a range of diseases, such as dengue fever, yellow fever, chikungunya, and Zika fever, and can be found in tropical, subtropical, and temperate regions. Insect vectors cannot regulate their internal temperature (poikilotherms) and are therefore susceptible to changes in climate and the environment. As a result of the increased temperatures over the last 100 years or so, it was estimated that the world became 1.5% more suitable per decade for Aedes aegypti, between 1950 and 2000, and this is predicted to increase to 3.2-4.4% per decade by 2050. Presumably, this would be accompanied by an increase in outbreaks of its dependent diseases and fatalities.

Deforestation is another environmental change that contributes to global warming. Warming can indirectly affect disease vectors, but it can also directly affect the temporo-spatial distribution and behavioural aspects of vectors and the diseases. For example, the Amazon rainforest is an extensive carbon sponge, storing 150 billion metric tons of carbon, aiding in the regulation of Earth’s temperature. Deforestation is a well-documented contributing factor to climate change, and also leads to wildlife migration into urban areas, thus allowing for closer human-pathogen interactions. This is known to increase the incidence of disease, and therefore fatalities, and not just from geographical proximity. A currently unexplained phenomenon has been observed, in which mosquitoes have a higher bite frequency in deforested areas compared to forested ones.

The effects of climate change on the spread and incidence of infectious diseases, particularly VBDs, have already started to occur, and it is anticipated that these effects will worsen. Therefore, it is no longer a case of future contingency planning; international governments and non-governmental organisational bodies need to take immediate action to implement effective infectious disease strategies, at local and national levels. These must include prevention, surveillance, and rapid responses to outbreaks. These measures should be in conjunction with the restoration and protection of ecosystems, such as the Amazon rainforest.          

The effects of climate change on the spread and incidence of infectious diseases, particularly VBDs, have already started to occur, and it is anticipated that these effects will worsen.

Climate change doesn’t just have environmental impacts. It also has detrimental consequences on national economies, in particular low- and middle-income countries. These are often countries affected by VBDs. For example, Brazil accounts for over 70% of the Americas’ reported Dengue Fever cases. Without adequate prevention and action, it is predicted that South America could experience around $17 trillion in losses by 2070. Inadequate infectious disease control is common in poor economies due to a lack of direct funding or increased conflict

In nations that are politically unstable or economically fragile, increased incidence and lengthened transmission season can overwhelm existing prevention strategies or control measures. Therefore, they should be considered during pandemic forecasting and planning. Peru is a tropical country with previous experience with Dengue infection, however, the effects of environmental changes and extreme weather overwhelmed the measures implemented in this middle-income country to deal with such endemics. During 2023, Peru experienced a period of localised El Niño which also happened to coincide with Cyclone Yaku. Consequently, the country suffered the worst dengue outbreak in its history. This outbreak was thought to be driven by the warm and wet conditions of El Niño. This, along with the rainstorm conditions of the cyclone, which are not commonly seen in that part of the Pacific, generated favourable conditions for the Aedes aegypti mosquitoes. This outbreak led to the resignation of Peru’s Health Minister, a state of emergency declared in 18 of 24 states, 223 deaths (of 137,539 infections), and an exceeded hospital capacity, all key indicators that more robust pandemic preparations are needed. Furthermore, COVID-19 caused an international public health emergency for over 3 years, sadly demonstrating that even when countries do identify a risk, inappropriate preparedness is generally lacking.

In 2017, WHO approved the Global Vector Control Response 2017-2030 to guide countries in strengthening and improving their vector control. However, it is indisputable that the COVID-19 pandemic demonstrated that many countries lacked sufficient endemic/pandemic response plans. The WHO guide should be seen as a beneficial reference for governments worldwide, however, individual governments need to produce appropriate planning tailored to countries, accounting for financial constraints and the influence of local climate changes.

Overall, I think climate change has the potential to bring about the extinction of humans, not because of rising sea levels or severe weather events, but because of inadequate international and governmental response to changes in disease expansion and proliferation, and the probable ensuing epidemics and pandemics. There is a clear need for immediate action to improve this.