Anthropogenic greenhouse gas emissions have caused the mean global temperature to increase by 1 °C above preindustrial levels. The impacts of a 1 °C rise have been profound, including a decrease in the number of cold days and nights, an increase in the number of warm days and nights, an increase in extreme heat events, decreased snow cover, and accelerating sea-level rise. Global warming has exhibited considerable heterogeneity, with greater warming over land than over the oceans, the greatest warming occurring in the Arctic, and evidence for greater warming in winter versus summer and in nighttime versus daytime. Increased evaporation from warming has resulted in complex, region-specific changes in the hydrological cycle; while there has been an increase in overall global precipitation, some areas have become wetter and others have become drier. Both wet and dry regions, however, have experienced an increase in extreme precipitation events.
If current greenhouse gas emission trends continue, the mean global temperature could increase by 4 to 5 °C above preindustrial levels by the end of the century, which would result in dramatic intensification of the changes already observed. According to the authoritative Intergovernmental Panel on Climate Change (IPCC), “Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems”. The impact of climate change on the incidence, transmission season duration, and spread of vector-borne diseases represents a major threat.
Vector-borne diseases
A vector is an organism (most often an arthropod) that transmits an infectious pathogen from an infected human or animal host to an uninfected human. The World Health Organization identifies the major global vector-borne diseases as malaria, dengue, chikungunya, yellow fever, Zika virus disease, lymphatic filariasis, schistosomiasis, onchocerciasis, Chagas disease, leishmaniasis, and Japanese encephalitis. Other vector-borne diseases of regional importance include African trypanosomiasis, Lyme disease, tick-borne encephalitis, and West Nile fever. Tropical and subtropical low- and middle-income countries bear the highest burden of vector-borne diseases. Eight vector-borne diseases are considered to be neglected tropical diseases.
Humans serve as the primary host for some vector-borne diseases, including malaria, dengue, chikungunya, and Zika virus disease, whereas other vector-borne diseases have more complex transmission dynamics, with both human and non-human hosts. For example, for Lyme disease, small mammals and birds serve as competent (reservoir) hosts (they are infected by the tick vector and they can infect ticks); deer and other large- and medium-sized mammals serve as incompetent hosts (they provide adult ticks with blood meals but do not become infected), and humans serve as dead-end hosts (they are infected by ticks but do not infect ticks). For the West Nile virus, a wide range of bird species serves as reservoir hosts with a range of competence, whereas humans, horses, and other mammals serve as dead-end hosts.
Among vector-borne diseases, malaria is the major killer, causing an estimated 620,000 deaths in 2017 (most occurring in Africa), followed by dengue, with an estimated 40,500 deaths (most occurring in Asia). The estimated number of incident cases in 2017 was 209 million for malaria and 105 million for dengue. While most other vector-borne neglected tropical diseases are seldom fatal, they involve chronic infections that confer substantial disability. In 2017, an estimated 65 million people were living with lymphatic filariasis, 143 million with schistosomiasis, 21 million with onchocerciasis, 6.2 million with Chagas disease, and 4.1 million with leishmaniasis.
References
Rocklöv, J., Dubrow, R. Climate change: an enduring challenge for vector-borne disease prevention and control. Nat Immunol 21, 479–483 (2020). https://doi.org/10.1038/s41590-020-0648-y
