Mosquitoes around the world
Aedes aegypti and Aedes albopictus are primarily mosquitoes of the tropical and sub-tropical parts of the world. However, both species have taken advantage of human trade and travel to expand their distribution ranges considerably.
A brief history
Ae. aegypti originated in sub-Saharan Africa, and likely arrived in the Americas on trade ships sailing from West Africa. Following their arrival in the New World, Ae. aegypti is thought to have spread west across the Pacific into Asia during the late 19th Century and into Australia, rather than arriving in Asia through an eastward migration from East Africa.1
By contrast, Ae. albopictus is an Asian mosquito with an original range that spanned from China and northern Japan to tropical Asia. It is thought to have spread to islands in the Indian Ocean with migrants coming from Asia in the 17th and 18th centuries, and later spread eastward across the Pacific Islands. As we heard in the previous step, the arrival of Ae. albopictus in the southern United States in the 1980s is associated with the global trade in used tyres, which were frequently found to contain both eggs and larvae of this species.2 More recently, imports of bamboo from sub-tropical China have been blamed for infestations in the Netherlands.
A global picture of the distribution of Ae. aegypti and Ae. albopictus today is revealed through national entomological surveys and published datasets, which are combined with models of environmental and land-cover information to give maps that predict the probability of occurrence of these species.
Temperature is the most important predictor of whether or not Ae. aegypti and Ae. albopictus will be found in a particular area.3 The warm climate of the tropical and sub-tropical regions of the world, as well as the high humidity and rainfall, is beneficial to the lifecycle of these mosquitoes. Accordingly, Figure 1 shows that Ae. aegypti is concentrated in South America, sub-Saharan Africa, Southern Asia and Australia.
Colder temperatures prevent the establishment of this species at higher latitudes and at high elevations, so few areas of Europe and temperate North America provide a suitable habitat. Elevations above 1000 meters have been found to be unfavourable to Ae. aegypti in India, but in Mexico they have been commonly encountered up to 1,700 m, and even above 2,000 m.4
Ae. albopictus is distributed in many of the same territories as Ae. aegypti, particularly in South America.5 The ability of this species to tolerate lower temperatures is reflected in a greater colonisation of the United States and southern Europe, as well as northern China, Southern Brazil, and Japan (Figure 2). In Africa there is relatively sparse sampling, so it remains uncertain whether areas that are predicted to be suitable habitats have actually been colonised.3
The expansions and contractions in the ranges of these species in the future will likely be impacted by control efforts and changes in climate. Interestingly, modelling studies using climate projections have indicated an overall contraction in the area of land that is considered climatically suitable for Ae. aegypti in the future.6 Unfortunately, the situation is far from straightforward, with some areas expected to see an increase in disease transmission by these mosquitoes and others a decline. It is important to note that while climate change may directly influence the distribution range of Ae. aegypti, there are confounding factors associated with human responses to climate change. In Australasia, for example, projections of rainfall reductions in southeast Australia have led to the installation of large numbers of domestic water storage containers, and these could provide breeding grounds that allow Ae. aegypti to expand out of Queensland into southern Australian’s urban regions.7 Furthermore, changing patterns of urbanisation, population density, poverty, and international travel and migration, will all play their part in the future distribution of the species.
© London School of Hygiene & Tropical Medicine