Description of the condition

The four main infections spread by Ae. albopictus and Ae. aegypti, dengue, yellow fever, chikungunya, and Zika virus, cause considerable morbidity, mortality, and healthcare expenditure in low‐ and middle‐income countries (LMICs).

The origin of Ae. aegypti is probably the African subcontinent. It subsequently spread throughout tropical, subtropical, and temperate climates. This species is currently found in Africa, South and Central America, parts of North America, the Middle East, Southeast Asia, and Oceania including Northern Australia (Powell 2013). It is mainly an urban mosquito that flies low and bites at dusk. These mosquitoes have a high vectorial capacity for dengue, chikungunya, Zika, and yellow fever (Anderson 2007; Manore 2017). Furthermore they have a propensity to breed in containers as well as small spaces, such as between large leaves and stems of vegetation. Ae. aegypti female mosquitoes feed preferentially on humans who are within enclosed spaces, such as houses (Scott 2010).

Ae. albopictus originated in Asia and subsequently spread globally, probably due to human migration, travel, and urbanization. It tolerates broader ranges of temperature and cool climates. This has enabled this mosquito species to survive in more temperate climates. The current distribution of Ae. albopictus mosquitoes is throughout Africa, Asia, South America, and in Pacific and Indian Ocean islands (Hawley 1988). Ae. albopictus is a daytime feeder that has a more cosmopolitan and semi‐urban presence and prefers environments with vegetation (Smith 2016b). Ae. albopictus also breeds in artificial containers and prefers to feed indoors.

Description of the intervention

Mosquito‐borne viral infections can be prevented by boosting host immunity (vaccines) or controlling the vector. Different forms of vector control has been implemented against Aedes spp. with varying success. The principal intervention categories are as follows.

• Chemical interventions: insecticides, chemical larviciding.

• Habitat management.

• Non‐chemical larviciding: larvivorous fish, oil coating, and mass trapping of larvae.

• Population replacement methods.

• Genetic techniques.

Insecticides that belong to the chemical classes of pyrethroids, carbamates, organophosphates, and organochlorines can be used for chemical control of mosquitoes (van den Berg 2012). However, organophosphates and pyrethroids (for example, permethrin, deltamethrin, cyfluthrin) are mainly used against Aedes spp. mosquitoes. Insecticides can be used against adult mosquitoes and larvae in forms of space treatment, indoor residual spraying, insecticide‐treated bed nets, and as larvicides (Smith 2016b). These insecticides act on various stages of the mosquito life cycle, such as larval and adult forms. Chemical larviciding is predominantly with Bacillus thuringiensis israelensis (Bti), which is a grampositive, spore‐forming bacterium that is pathogenic to mosquitoes (Goldberg 1977).

Habitat (source) management attempts to reduce mosquito breeding sites by removing potential breeding sites (Gubler 1996). Many countries that face regular epidemics try to enforce these measures by public education or by punitive methods through the legal systems (for example, fines or imprisonments) (Mendes 2014).

Non‐chemical methods of larviciding includes oil coating, mass trapping of larvae, and the use of larvivorous fish (Hurst 2012; Seng 2008). Population control methods, such as the use of Wolbachia spp. as well as genetic manipulation of mosquito populations (for example, introduction of sterile males) are emerging methods of vector control (Alphey 2013; Kean 2015).

How the intervention might work

Insecticides have been used extensively for vector control and have shown to be effective. Dichlorodiphenyltrichloroethane (DDT) was successfully used to eradicate Ae. aegypti from 19 countries between 1947 and early 1960s (Smith 2016b). Eventually DDT resistance emerged and this compound was phased out in mosquito control measures.

Pyrethroids are neurotoxins that prolong the opening of voltage‐gated sodium ion channels of the mosquito (Lund 1981). Historically, pyrethroids were successful in controlling mosquito populations but resistance due to a) mutations in the voltage‐gated sodium ion channel and b) increased detoxification by the mosquito cytochrome p450 monooxygenase system have limited their use (Smith 2016b).

Organophosphates, such as fenthion and malathion, are also neurotoxins that work by irreversibly blocking insect acetylcholinesterase. However, resistance of the Aedes mosquito to commonly used organophosphates has been reported (Polson 2010). The two principal groups of organochlorine insecticides are the DDT‐type compounds and the chlorinated alicyclics. DDT‐type compounds exert their action by preventing deactivation of the sodium channel after depolarization of neurons in the peripheral nervous system, while the chlorinated alicyclics act by inhibition of neurons secreting gamma‐aminobutyric acid (GABAergic neurons) (Coats 1990).

Chemical larviciding involves the use of Bti bacteria. Bti exerts its lethal effects by producing toxic proteins, which are subsequently ingested by the larvae of the target organism (Lacey 2007). These toxins are activated in the gut of the larvae and cause extensive disruption of cell membranes of the organism causing cell death. These toxic proteins demonstrate significant species specificity and do not exert toxic effects on non‐target organisms (Saik 1990).

Habitat management works by reducing mosquito breeding grounds. Countries use a combination of methods, such as grassroots community involvement, coordination of public health systems, health education, and punitive measures in vector control (Mendes 2014). However, by nature, Aedes mosquitoes prefer both urban and semi‐urban areas. In urban areas, breeding places are difficult to eliminate due to overcrowding, pollution, suboptimal waste water drainage, and inappropriate garbage and waste disposal. In semi‐urban areas, there are plenty of natural water collection sites that are also difficult to eliminate.

Non‐chemical methods, such as oil coating and vector trapping, interrupt the life cycle of the vector, thus limiting propagation. In larval traps, which are decoy breeding surfaces, the collected larvae can be destroyed by physical (for example, drying) or other means (chemicals). The utility of oil coating in larval control is limited for Aedes spp. mosquitoes. Larvivorous fish, such as Gambusia affinis, have been known to ingest mosquito larvae and pupae and lower the density of immature vector forms in water bodies. They are successfully used to control larvae of the Anopheles mosquito (Pyke 2005).

Wolbachia spp. are used as a population control method for the Aedes mosquito. Wolbachia is an intracellular, maternally‐inherited, endosymbiotic bacteria found in insects. Wolbachia causes reproductive modifications, such as cytoplasmic incompatability, which results in the generation of unviable offspring when an uninfected female mates with a Wolbachia‐infected male (Zabalou 2004). There is also evidence that It limits dengue and chikungunya viral replication within the vector (Moreira 2009).

Genetic manipulation encompasses several modalities for vector control. The sterile insect technique involves creation and release of sterile males into the environment, reducing the reproductive potential of the target wild population thus eventually reducing the population size. Sterility is induced traditionally by radiation and lately by genetic manipulation. These methods cause death of mosquito offspring following mating. Modifications of this system such as Release of Insects carrying a Dominant Lethal (RIDL) are also in use (Alphey 2013). Genetic strategies for generation of refractory insects include the expression of specific antibodies, peptides, or manipulating cell signalling. For example, RNA manipulation techniques are currently under investigation to reduce arbovirus replication within vectors (Alphey 2013).

Why it is important to do this review

As mentioned above, viral infections spread by Aedes spp. cause significant morbidity and mortality globally. Despite having an effective vaccine for yellow fever, the annual estimated mortality of yellow fever still exceeds 30,000 people (WHO 2016). Although not as deadly as yellow fever, the morbidity caused by dengue infection is also high, with an estimated 60 million infections per annum (Stanaway 2016). The annual incidence of dengue has more than doubled per decade since reliable statistics became available from the latter part of the 20th century (Stanaway 2016). These statistics are only for symptomatic cases and do not include the asymptomatic cases that can also serve as reservoirs for infection at the community level.

While vaccinations (when available) help to reduce the impact of infections, this alone is not enough to stem the existing burden of disease as shown in the case of yellow fever. For dengue, the efficacy of the new vaccine remains to be clarified. For many other viral infections for which Aedes spp. serve as a vector, vaccines are unavailable. Hence it is essential to evaluate the value of vector control measures as a means of reducing disease morbidity and mortality. This evaluation should be in the form of a systematic review to weigh the evidence more objectively for each of the many options available. The efficacy of vaccines can be tested relatively easily due to the low number of choices available and clear outcome measures. However, given the heterogeneity of vector control measures and the difficulty of measuring the impact, a systematic review on vector control is a challenging task. Nevertheless, evidence from such an analysis will be a useful adjunct to decision‐making and for assessing cost effectiveness by public health authorities combating these diseases.