How global change is affecting the threats from mosquito-borne diseases

Mosquitoes are one of the most widespread pests in the world. They’re present in virtually every environment and on every continent, except Antarctica. They occur at altitudes up to 5,500m and have been found in caves and mines down to 1,250m below sea level.

Out of the 3,500 mosquito species worldwide, relatively few transmit dangerous diseases to humans. However, these diseases are a threat to the majority of the human population and cause infections in 700 million people each year. These species include the following.

• Aedes aegypti, Ae. albopictus, and other Aedes spp: dengue, Chikungunya, West Nile virus, yellow fever, Zika (and many other arboviruses), lymphatic filariasis/elephantiasis (parasitic roundworm)
• Anopheles species (60+ species transmit diseases): malaria (parasitic protozoan), lymphatic filariasis (parasitic roundworm), West Nile virus
• Culex species: West Nile virus, lymphatic filariasis/elephantiasis (parasitic roundworm)
• Haemagogus species; Sabethes species: yellow fever

In developed countries of Europe and North America, mosquitoes are currently mainly a biting nuisance for outdoor activities around homes and businesses. Diseases such as malaria and yellow fever were eradicated in the twentieth century in these countries when their vector mosquito species were wiped out. In many parts of the world, however, mosquito-borne diseases are an increasing threat to the health of millions and the invasive Aedes species that are the vectors for several viral diseases have returned to the US and Europe.

The great progress with malaria over recent decades has been threatened by the COVID-19 pandemic and South America and southeast Asian countries are suffering from large outbreaks of dengue, possibly exacerbated by the pandemic.

Outbreaks of dengue at July 2020. Source: European Centre for Disease Prevention and Control

The population and distribution of mosquitoes are influenced by many factors, as are the pathogens that they can carry. Here’s a brief look at some of these influences.

Climate change

Climate change is causing record high temperatures in many parts of the world and is expected to increase the populations of and the range of some mosquito species in temperate areas, especially Aedes species. Even the tropical species Aedes aegypti can survive low and freezing temperatures for short periods in the egg stage, which means it could spread farther than previously thought into Europe and the US. Areas with increased rain and flooding will have more breeding sites for mosquitoes.

In areas that already have hot climates, however, increasing temperature can reduce the amount of rain and sites with water and also adversely affect their reproduction, growth and survival. Due to the nature of biological systems, small changes in temperature can have a large effect on survival and transmission of disease. The effects of climate change have to be seen in the context of other global factors undergoing change.

Land use changes

For a large part of their life, mosquitoes are aquatic animals, filtering microbes in water or feeding on decaying vegetation in their larval stage. Any changes to land use can have a dramatic effect on access to the aquatic environments that particular species favour.

Deforestation

Forest encroachment and clearance is associated with the introduction of new diseases to human populations, including Ebola, HIV, Zika and chikungunya and other arboviruses. Removing tree cover affects the amount of sunlight, nutrients and acidity of pools of water and can favour some species. It can also bring the human population into close contact with new species or diseases that are carried by the local mosquito population and disease reservoirs in the local wild animal population.

Agriculture

Water management practices in agricultural areas and livestock production can have an impact on mosquito populations by providing more breeding and feeding opportunities. Irrigation and building of dams can increase the breeding sites for some species – but also adversely affect others.

Culex quinquefasciatus, which is widespread in tropical and subtropical regions and is a vector of lymphatic filariasis and several arboviruses such as Zika and West Nile virus, breeds in clean water. Culex tritaeniorhynchus, on the other hand, which is a vector of Japanese encephalitis, prefers stagnant water.

Livestock provide a source of blood meals for mosquitoes and can be a reservoir for some human diseases. Pigs are affected by the Japanese encephalitis virus, which can be transmitted to humans by mosquitoes. Rice farming and pig rearing in developing Asian countries such as Bangladesh, Indonesia and Pakistan are seen as a high risk for the spread of the virus.

Urbanisation

Urbanisation is having a major impact on land use in developing countries, but also, to a lesser extent, in developed countries. Rural populations are migrating to cities, where rapid expansion of residential and industrial building provides breeding and feeding opportunities for a wide range of urban pests, including some mosquito species.

The invasive Aedes aegypti and Ae. Albopictus are ideally suited to the micro water sites created around homes, recreational and industrial areas and in discarded man-made products. They originated in forest habitats where they breed in holes in trees, rock pools and rain water collected in the structure of plants.

Socioeconomic factors

Income is one of the most important factors in the susceptibility of people to many mosquito-borne diseases. Populations with low income have a far greater prevalence of malaria. When new land is cleared for paddy farming, there’s a surge in mosquito-borne infections at first and then a reduction, called the “paddy paradox”. This is thought to be a result both of the change in habitats favouring different mosquito species in the area and the increased income from the crops allowing the farmers to have better living conditions and healthcare.

Travel and trade

Increased global travel and trade have enabled the spread of mosquito-borne diseases and their vector species. The ease of international travel has allowed both infected people and mosquitoes to carry diseases to new areas or to reintroduce them to areas where they had previously been eradicated. Refugee movements after wars also reintroduced malaria to many countries 

Aedes aegypti and Ae. Albopictus were spread out of their native areas first by local urbanisation then to global urban hubs by international trade, where they found ideal environments for breeding.  These mosquito species now have their widest distribution ever recorded.

Mosquito control

Mosquito control in the varied habitats of the human environment requires coordinated effort from local and national government agencies, businesses and homeowners. A wide range of tools is available for controlling mosquitoes at their different life stages. These range from reducing the breeding and resting sites, maintaining physical barriers to sophisticated traps, adulticides and larvicides. Scientists in many disciplines are constantly developing new ways of mosquito control and prevention of diseases, such as releasing genetically modified mosquitoes that cannot breed.