How climate change affects vector-borne diseases

Vector-borne diseases are caused by pathogens that have been transmitted from an infected animal to a human by a vector (a living organism, like a tick or mosquito). Examples of vector-borne diseases include dengue, Zika virus, malaria, Chikungunya and yellow fever.

Vector-borne diseases account for more than 17% of all infectious diseases and cause more than 700,000 deaths annually. 

Virtually all vector-borne diseases have a climate dimension.

The pathogens, vectors and hosts associated with vector-borne diseases are highly responsive to the environments they inhabit. This means that changes in temperature and precipitation as a result of climate change can have significant impacts on the spread of vector-borne diseases.

There are three key ways climate change affects vector-borne diseases:  

1. More places will become suitable for vectors. Warmer temperatures can increase the geographic spread of where vectors – like mosquitoes and ticks – can survive and breed. Increased rainfall can increase the amount of standing water, creating more breeding areas for many vectors. And droughts can support breeding by forming pools of standing water from previously flowing water.
2. Warmer climates extend the disease transmission season. Climate change is improving the climatic and environmental conditions for the transmission of many diseases. This may also lead to an increase in the duration of disease transmission seasons.
3. Temperature change can affect the behaviour of vectors. For example, increased temperatures change the biting behaviour of mosquitoes, reducing the effectiveness of barriers such as bed nets. 

However, it can be challenging to attribute these impacts to climate change, as other factors are also at play. For example, changes in land-use, control measures and human movement can also influence the distribution of vectors and spread of disease.

According to a 2022 study in The Lancet Planetary Health, an additional 4.7 billion people may be at risk of the vector-borne diseases malaria and dengue by 2070.

The geographic distribution of dengue, a mosquito-borne viral infection, has expanded globally since the 1990s. And while this can be largely attributed to human movement so far, by 2030, one of the dominant causes of expansion of these vectors is predicted to be climate change.

The Intergovernmental Panel on Climate Change 2022 report stated that, “dengue risk will increase with longer seasons and a wider geographic distribution in Asia, Europe, Central and South America and sub-Saharan Africa, potentially putting additional billions of people at risk by the end of the century."

Similarly, Zika is a vector-borne disease that is predicted to spread as a result of climate change. More than 1.3 billion new people could be exposed to Zika by 2050 as warmer temperatures make more environments suitable for disease transmission.

However, while some areas are warming to suitable conditions for vectors to thrive, others will become too hot for them to survive.

Laboratory experiments found that Aedes aegypti (a species of mosquito) larvae die when the water temperature surpasses 34°C, and the adults start to die when the air temperature is above 40°C. To survive, mosquitoes have been discovered shielding in small-scaled environments – like cement tanks, tires or household pitchers – where ambient temperature change does not prevail.

Mosquitoes are not the only concerning vector.

Ticks are another vector capable of transmitting pathogens, including zoonotic diseases like Lyme disease and tick-borne encephalitis. Tick expansion is promoted by the warmer winters in the last decade due to global warming.

Ticks that carry the virus that causes tick-borne encephalitis have moved into northern subarctic regions of Asia and Europe. In 2019, tropical disease-carrying ticks were also found to have survived in Germany through the winter months. This discovery could indicate that these ticks are moving towards establishing themselves in new northern territories.

Climate change is increasing the risk of vector-borne diseases. The sooner we act to mitigate the impacts of climate change – by transitioning from using fossil fuels to clean, renewable energy – the better off we’ll be in the future. 

But mitigation alone will not be enough. There are other actions we must take to adapt to the risks: 

• provide universal access to care and disease management 
• improve disease surveillance, including health education and community surveillance for early outbreak detection 
• minimise vector exposure (for example, by using window and door screens, protective clothing, insecticide or habitat avoidance) 
• accelerate vaccine development with new technologies 
• prioritise wetland management and elimination of vector breeding sites in the vicinity of populations 
• investigate novel vector control methods (for example, the Wolbachia-infected mosquitoes) 

This article was first published on 12 May 2022.