Endectocides

Endectocides are classified as systemic drugs with both endoparasitocidal and ectoparasitocidal activity (Foy et al. 2011). Drugs used against endoparasites such as Avermectin and Ivermectin have long been known to have a killing effect on a number of blood-sucking arthropods. Endectocides hold several advantages over traditional insecticides—these interventions travel with the host and do not rely on time or place to be effective. The primary advantage is that this strategy will be effective against endophagic, exophagic, and zoophilic vectors as well as crepuscular and night-biting vectors (Foy et al. 2011). In addition, the mode of action for endectocides ensures that there is a low probability of cross-resistance with current insecticidal strategies (Strycharz et al. 2008). This increases their usage where pyrethroid-based interventions are threatened by insecticide resistance. Last, these systemic drugs have the potential to inhibit the development of the malaria parasite, thus making them a viable target to combat drug resistance in the malaria parasite. Although endectocide use is an attractive option for malaria control, issues of cost (Burnham and Mebrahtu 2004; Goldman et al. 2007), long-term human use (Duke et al. 1990; Guzzo et al. 2002), and resistance build-up in both endo- and ectoparasites (Bourguinat et al. 2007) have been raised. Alternatives to this approach are to use treated livestock as the delivery medium for the endectocide to host-seeking vectors. Such approaches would again be focused on zoophagic vectors but would reduce health impacts associated with maintaining human populations on long-term drug therapy.

Focusing solely on these drugs ability to reduce vector abundance in intervention programs may be too simplistic (Wilson 1993), and these strategies need to be assessed for their ability to affect all variables associated with vectorial capacity.

Push–Pull Strategies

Personal protective measures have shown usage for preventing malaria (Hill et al. 2007, 2014; Syafruddin et al. 2014) and for reducing the overall intensity of outdoor biting (Goodyer et al. 2010). Using interventions focused on repelling mosquitoes from an individual, however, have prompted concerns that diverted vectors may result in higher attack rates on unprotected populations (Maia et al. 2013). Therefore, a strategy that relies on killing mosquitoes and reducing community-level risk is much more desirable (Howard et al. 2000).

One novel strategy currently being developed uses a push–pull approach that seeks to exploit the complementary effects of repellents and traps. Developed initially as a way to control agricultural and urban pests (Cook et al. 2007), push–pull interventions work by combining the repellency action of one component and the attractiveness of another to elicit movement away from a protected resource and toward a trap for subsequent removal from the environment (Pyke et al. 1987; Cook et al. 2007; Kitau et al. 2010; Reddy and Guerrero 2010; Paz-Soldan et al. 2011; Menger et al. 2014; Wagman et al. 2015). Accordingly, push–pull strategies for the control of mosquito vectors of human disease would use repellents to deter host-seeking mosquitoes from treated spaces toward a baited trap resulting in their capture and removal from the peridomestic environment thereby decreasing population densities for added community protection and/or personal protection of hosts in the outdoor environment (Cook et al. 2007; Kitau et al. 2010; Paz-Soldan et al. 2011). Although still in the proof-of-concept phase, preliminary studies have been encouraging (Kitau et al. 2010; Menger et al. 2014; Wagman et al. 2015), showing reduced biting rates and house entry.

Spatial Repellents

The term spatial repellent is used here as a general term to refer to chemical products designed to release volatile chemicals into the air and elicit a range of insect behaviors induced by airborne chemicals that result in a reduction in human–vector contact (Achee et al. 2012a; WHO 2012a). A large number of products are commercially available that use chemical actives registered as spatial repellents (U.S. Environmental Protection Agency [EPA]). These products range in cost and sophistication from expensive heat driven electrical outlet plugins used in the Europe to inexpensive mosquito coils that are widely used throughout Africa and Asia.

Spatial repellents can induce mosquitoes to move away from a chemical stimulus, interfere with their host detection (attraction–inhibition) and/or feeding response (WHO 2012a) and consequently can operate on all adult behaviors that incorporate movement. These effects have been measured in laboratory studies (Grieco et al. 2005; Suwannachote et al. 2009), in phase II testing under experimentally controlled conditions (Grieco et al. 2000, 2007; Ogoma et al. 2012), and in field settings (Pates et al. 2002; Kawada et al. 2004a,b; Lucas et al. 2007) against Aedes spp., Anopheles spp., and Culex spp. of varying insecticide resistance profiles. There is also evidence from a phase III study in Indonesia that spatial repellents can impact malaria incidence (Syafruddin et al. 2014).

The role for spatial repellents in modern vector control can best be conceptualized for transmission settings in which IRS and/or LLINs may not offer full protection or have reached their efficacy limits, especially in areas with residual transmission or areas where elimination is proposed. Control of malaria in these areas will require new approaches and this may be where spatial repellency would be most effective (Achee et al. 2012b; Ogoma et al. 2012, 2014). Spatial repellents may show effect against insecticide-resistant populations and have the potential to limit the spread of insecticide-resistant alleles because of reduced selection pressure when considering the nonlethality of effect (WHO 2012a). Spatial repellents could be offered as stand-alone tools where no other interventions are currently in use; or, most likely, combined with existing interventions to augment efficacy of these other tools.

Attractive Toxic Sugar Baits

Attractive toxic sugar baits (ATSBs) represent a new tool for the indoor and outdoor control of mosquito disease vectors. Mosquitoes are killed when they are attracted to and feed on toxic sugar meals that are either sprayed on plants or used in bait stations. This intervention targets a toxin susceptible vector population while sugar feeding. Because sugar feeding has been shown to occur repeatedly and often in the adult mosquito, the potential effects of this intervention on a vector population may be dramatic.

The use of sugar feeding to reduce mosquito populations was first reported in 1965 (Lea 1965) and then in several other studies (Schlein and Pener 1990; Robert et al. 1997; Muller and Schlein 2006; Xue et al. 2006; Schlein and Muller 2008; Muller et al. 2010b,c) in Aedes and other Culicine mosquitoes as well as sand flies. Effects on anopheline populations were shown in Israel and Mali (Muller et al. 2008, 2010b). This approach may be used in either of two ways, that is, direct mortality induced by feeding on the bait, and/or, the bait may be used to disseminate mosquito pathogens or toxins (Schlein and Pener 1990; Allan 2011).

Attractive toxic sugar bait solutions are composed of sugar, an attractant such as a flower scent, and an oral toxin. Toxins tested include malathion (Lea 1965), boric acid (Xue and Barnard 2003), spinosad (Muller and Schlein 2006), fipronil (Xue et al. 2008), as well as several other classes of insecticide (Muller and Schlein 2006). Toxins, such as boric acid, hold the added advantage of being safe for the environment. Attractants focus on locally acquired sugars, juices and fruit—as mosquitoes may be selective to carbohydrate choices originating from their geographic range (Grimstad and DeFoliart 1974; Muller and Schlein 2005; Muller et al. 2010a).

Sugars derived from plants are an integral component of mosquito nutrition and provide energy for survival, flight, and enhance fecundity and vectorial capacity (Nayar and Sauerman 1971, 1975a,b; Foster 1995; Briegel 2003; Gu et al. 2011). Both male and female mosquitoes may feed on sugar sources several times a day—primarily after emergence and then as required (Reisen et al. 1986; Foster 1995; Foster and Takken 2004; Gary and Foster 2006). Because sugar feeding occurs more often than blood feeding and continues throughout the life of the insect, this behavior represents an opportunity in a mosquito life cycle where a vector intervention may be placed. An accumulative effect of ATSBs on a mosquito population was shown for an area with readily available alternate sugar sources; however, the overall population level lethality was delayed relative to sugar poor areas (Beier et al. 2012). This suggests that ATSBs may be highly effective in arid, sugar-poor environments.

The recurrent nature of sugar feeding suggests a large ATSB effect on vectorial capacity (Garrett-Jones and Shidrawi 1969; Gu et al. 2011). A single application of ATSBs affected mosquito density, parity, survival, and hence vectorial capacity (Garrett-Jones and Shidrawi 1969; Beier et al. 2012). ATSBs in conjunction with other interventions like bednets might show an exponential effect as energy-deprived mosquitoes (e.g., those that are unable to acquire a blood meal because of bednets) seem to take more and larger sugar meals (Stone et al. 2012). A primary drawback of this strategy is the possibly effect on nontarget organisms—the killing of other sugar feeding insects (honey bees and pollinators in particular).

Altthough ATSB approaches are being developed and tested, they represent new powerful tools for the control of malaria vectors, especially because this method is simple, inexpensive, and environmentally friendly, and has been shown to be highly effective for mosquito control.

Genetic Control

Genetic control of malaria vector populations can be described as the dissemination of genetic or inheritable factors toward the decrease of disease and/or target vector populations. These control strategies rely on the dispersal of a modified organism for the purpose of mating with wild-type populations and are therefore species-specific. These strategies are expected to function synergistically with current and other proposed disease-intervention programs.

Curtis first presented the concept of genetic control in 1968 (Curtis 1968). Recent advances in molecular biology have resulted in the germ-line transformation of several anopheline species (Catteruccia et al. 2000; Grossman et al. 2001; Perera et al. 2002; Lobo et al. 2006). These systems have shown a reduction in vector competence (James et al. 1999; de Lara Capurro et al. 2000; Ito et al. 2002; Bian et al. 2013; Wilke and Marrelli 2015) and have further progressed to developing drivers that can disseminate these systems into natural populations (Burt 2003; Deredec et al. 2008; Windbichler et al. 2011; Bian et al. 2013; Gantz et al. 2015; Hammond et al. 2016). Toward testing and wild release, large sets of molecular markers and genome sequences have been produced that can be used to study population structure and gene flow (Thomas et al. 2000; Black and Lanzaro 2001; Donnelly et al. 2001; Walton et al. 2001; Holt et al. 2002; Neafsey et al. 2015).

Genetic control can be broadly separated into germline transformation of mosquitoes, involving the germline manipulation of a genome and paratransgenesis—which works with transformation of obligate symbionts (Alphey et al. 2002). Paratransgenesis involves genetically modified organisms that can colonize vector species (Wilke and Marrelli 2015). Genetically modified symbiotic bacteria are reintroduced into the vector species where they express the genetic trait (Beard et al. 1993; Conte 1997; Favia et al. 2007; Coutinho-Abreu et al. 2010). These genetic traits may be a pathogenic effect in the host, interfering with reproduction, reducing vector competence (Bian et al. 2013) or the reproductive pathway (Yoshida et al. 2001; Chavshin et al. 2012; Bongio and Lampe 2015). Wolbachia have recently shown the ability to confer fitness costs on Anopheles vectors (Joshi et al. 2014) and inhibit Plasmodium infection (Jin et al. 2009; Kambris et al. 2010; Hughes et al. 2011). However, some studies have shown the opposite, where Plasmodium infections were enhanced in the presence of Wolbachia (Hughes et al. 2012). This finding necessitates a clearer understanding of underlying processes. Various other symbiotic bacteria species have been identified in anophelines and may be used for paratransgenesis, the choice of which is determined by the approach being used (Beard et al. 1993; Yoshida et al. 2001; Gonzalez-Ceron et al. 2003; Lindh et al. 2005; Favia et al. 2007; Wilke and Marrelli 2015). An advantage of paratransgenesis over that of genetic transformation is that a transgeneic strain is required for every species or reproductively isolated strain in the latter, while the same paratransgenic system may be used for multiple species as long as the bacteria being used can survive and the system function in the vector species used (Sayler and Ripp 2000; Riehle and Jacobs-Lorena 2005; Riehle et al. 2007; Wilke and Marrelli 2015).

Suppression and replacement strategies: Outcomes of genetic control strategies include (1) both population suppression, where the number of vectors in a target population is reduced, and (2) population replacement, where a trait is spread in the natural vector population toward the reduction of vectorial capacity at some point in the vector life cycle (Alphey et al. 2002; Wilke and Marrelli 2012). The “release of insects with dominant lethality” (RIDL) system uses a dominant lethal gene with a female-specific promoter (Heinrich and Scott 2000; Thomas et al. 2000; Atkinson et al. 2007). When genetically modified, males mate with wild females, sex-specific lethality results in inviable female progeny. Several genes and lethal mechanisms may be used with this system (Fortini et al. 1992; Alphey 2002). Population-replacement strategies have resulted in several anopheline species being genetically modified toward the disruption of Plasmodium transmission. Anopheles stephensi was transformed to express a peptide that blocked the majority of oocyte development (Ito et al. 2002). Other studies that have shown the ability of germline transformation to reduce or inhibit malaria transmission include the expression of venom phospholipase (Zieler et al. 2001; Moreira et al. 2002), single-chain antibodies (Isaacs et al. 2011) and other antimalaria genes (Meredith et al. 2011).

Self-limiting and self-sustaining strategies: Self-limiting as well as self-sustaining strategies may be used (Alphey et al. 2002). A strategy that incorporates a strong fitness penalty will result in the rapid reduction of the target population by natural selection. Sterile insect technique (SIT) (Alphey 2002) is a highly effective, area-wide method where periodic mass releases of irradiated and sterile males are required to maintain the selection pressure. Sterile-male methods incorporate the release of sterile males, which, because of infertile mating, results in population suppression (Helinski et al. 2008). Genetic sexing mechanisms that are self-sustained and stable offer a major benefit more than traditional SIT techniques that rely on radiation-based sterilization (Heinrich and Scott 2000; Thomas et al. 2000; Alphey 2002). In general, population replacement strategies are self-sustaining, whereas population suppression strategies are self-limiting (Alphey et al. 2002; Jasinskiene et al. 2007; Wilke and Marrelli 2012). Self-sustaining paratransgenic systems such as Wolbachia invade and are maintained in the population once initially established in the target (Alphey et al. 2002; Wilke and Marrelli 2012). Genetic transforming self-sustaining strategies usually consist of two components—a genetic refractory mechanism that enable either population suppression or refractoriness to disease transmission, as well as a gene-drive system (Sinkins and Gould 2006) that disseminates the transgene cargo into the population toward the disruption of disease transmission (Zieler et al. 2001; Ito et al. 2002; Moreira et al. 2002; Isaacs et al. 2011; Meredith et al. 2011). There are several gene-drive systems that include the use of selfish genetic elements like transposons (Burt 2003; Chen et al. 2007; Sethuraman et al. 2007), meiotic drive genes (Lyttle 1991), and homing endonuclease genes (HEGs) (Burt 2003; Deredec et al. 2008).

Transposable elements are able to move within a genome and increase their number (Scott et al. 2002). The spread of the P element in Drosophila melanogaster is an example of the spread of a transposon in a population (Kidwell 1992; Engels 1997). Although common in malaria vectors (Holt et al. 2002) and having been used in germline transformation (Catteruccia et al. 2000; Grossman et al. 2001; Perera et al. 2002), there have not been any studies showing their use as a gene-drive system in Anopheles vectors (Sinkins and Gould 2006).

Meiotic drive, usually in males, occurs when a heterozygous locus segregates at a greater-than-expected frequency (Lyttle 1991; Sinkins and Gould 2006) through various mechanisms.

HEGs are selfish genetic elements naturally found in microbes (Burt and Koufopanou 2004; Stoddard 2005). They encode an endonuclease that recognizes and cleaves specific DNA sequences of ~20–30 nucleotides that usually is present only once in the genome. This gene is inserted into the cleaved sequence. In a heterozygous genome, the chromosome without the endonuclease gets cleaved and the broken chromosome gets repaired using the homolog containing the HEG thereby propagating itself in the genome. In principle, an HEG with a transgene cargo is able to cleave a highly conserved target gene and should be capable of population invasion from a low starting frequency (Deredec et al. 2008) with germline incorporation of an HEG transgene. HEGs may be used in two ways (Burt and Koufopanou 2004; Deredec et al. 2008), that is, an HEG can be engineered to recognize a specific nuclear gene/sequence where, on insertion, it would knockout the gene. An HEG construct may be engineered to recognize and insert into a repeat sequence on the X chromosome, be linked to meiosis-specific control sequences, and inserted on the Y chromosome. The HEG bearing Y would propagate in the population biasing the sex ration toward males (Galizi et al. 2014).

Multiple genetic control strategies are presently being investigated and some have successfully been tested in the field with other field tests ongoing.