There are many types of mosquitoes, but a few in particular are primarily responsible for the spread of some of the most significant human diseases: the Anopheles, Aedes and Culex mosquitoes.
Anopheles mosquitoes (multiple species) are known to transmit malaria in different parts of the world, including Anopheles gambiae (pictured left). Every year, malaria alone kills an estimated 400,000 people. Anopheles mosquitoes also transmit lymphatic filariasis and are the main vectors for the o'nyong'nyong virus. Female Anopheles mosquitoes can be distinguished from female Aedes and Culex by structures called palps on the head which are as long as the proboscis, and by the presence of black and white scales on the wings.
Aedes mosquitoes transmit multiple arboviruses including yellow fever, dengue, zika, and chikungunya. Whilst yellow fever has a high mortality rate there are relatively few cases and an effective vaccine is available. Dengue by contrast is not as deadly, but is prolific. One modelling estimate indicated that 390 million dengue infections occur around the world of which 96 million presented clinically [ii]. Some of the cases develop into severe dengue, or dengue haemorrhagic fever, a more severe form of the disease which can be lethal. Dengue cases are doubling globally every decade and is set to be a significant public health threat in the near future. While the Aedes aegypti mosquito, pictured left, can be recognised by white markings on the legs and a marking in the form of a lyre on the upper surface of its thorax. Other important species such as the Aedes albopictus do not possess these distinctive markings.
Culex mosquitoes transmit diseases such as lymphatic filariasis, and some arboviral diseases such as West Nile virus and viruses causing encephalitis.
In addition to Aedes and Anopheles mosquitoes, Culex mosquitoes transmit diseases such as lymphatic filariasis, and some arboviral diseases such as West Nile virus and viruses causing encephalitis.
Only female mosquitoes feed on blood and are therefore able to transmit diseases to humans. They use the blood as a source of protein for their eggs.
The main methods of controlling mosquitoes, especially those transmitting malaria, include using insecticide-treated nets (ITNs) and spraying residual insecticides inside houses on walls and ceilings where mosquitoes rest after feeding. Other methods include removal or treatment of standing water where mosquitoes lay their eggs as a means of larval control.
Repellents can be used on the body and clothing as personal protection to prevent mosquitoes from biting.
However, mosquitoes are very good at adapting to new environments and interventions and continue to evolve resistance against a variety of widely-used insecticides as well as changing behaviours such as feeding habits (feeding outside and earlier) to avoid bed nets and insecticide-sprayed homes.
The future of vector control
Although ITNs and indoor residual spraying (IRS) are considered core interventions especially for malaria vector control and have resulted in huge reductions in infections and disease incidence, they mainly target indoor-biting and/or indoor-resting mosquitoes. In areas where the local vectors mostly bite or rest outdoors, we need to find alternative vector control tools to supplement ITNs and IRS and there are some interventions such as larval source management targeting the immature stages of mosquitoes and other tools currently in development and testing. These cover a wide range of technologies, including genetic methods, mosquito-specific parasites to reduce breeding capacity, drugs harmless to cattle or humans but that kill mosquitoes when they feed on such treated hosts, mosquito-specific toxic baits and new repellent products for personal protection and reduction of bites. In the majority of malaria endemic countries, the continued use of proven interventions such as ITNs and IRS still remains effective in reducing transmission and prevention of infection.
[i] J. D. Gillett with illustrations by Judith G. Smith (1972). Common African Mosquitoes and their Medical Importance. London: William Heinemann Medical Books Ltd.
[ii] Bhatt S, Gething PW, Brady OJ, Messina JP, Farlow AW, Moyes CL. The global distribution and burden of dengue. Nature, 2013; 496.