Malaria vector control: from past to future
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- Raghavendra, K., Barik, T.K., Reddy, B.P.N. et al. Parasitol Res (2011) 108: 757. doi:10.1007/s00436-010-2232-0
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Malaria is one of the most common vector-borne diseases widespread in the tropical and subtropical regions. Despite considerable success of malaria control programs in the past, malaria still continues as a major public health problem in several countries. Vector control is an essential part for reducing malaria transmission and became less effective in recent years, due to many technical and administrative reasons, including poor or no adoption of alternative tools. Of the different strategies available for vector control, the most successful are indoor residual spraying and insecticide-treated nets (ITNs), including long-lasting ITNs and materials. Earlier DDT spray has shown spectacular success in decimating disease vectors but resulted in development of insecticide resistance, and to control the resistant mosquitoes, organophosphates, carbamates, and synthetic pyrethroids were introduced in indoor residual spraying with needed success but subsequently resulted in the development of widespread multiple insecticide resistance in vectors. Vector control in many countries still use insecticides in the absence of viable alternatives. Few developments for vector control, using ovitraps, space spray, biological control agents, etc., were encouraging when used in limited scale. Likewise, recent introduction of safer vector control agents, such as insect growth regulators, biocontrol agents, and natural plant products have yet to gain the needed scale of utility for vector control. Bacterial pesticides are promising and are effective in many countries. Environmental management has shown sufficient promise for vector control and disease management but still needs advocacy for inter-sectoral coordination and sometimes are very work-intensive. The more recent genetic manipulation and sterile insect techniques are under development and consideration for use in routine vector control and for these, standardized procedures and methods are available but need thorough understanding of biology, ethical considerations, and sufficiently trained manpower for implementation being technically intensive methods. All the methods mentioned in the review that are being implemented or proposed for implementation needs effective inter-sectoral coordination and community participation. The latest strategy is evolution-proof insecticides that include fungal biopesticides, Wolbachia, and Denso virus that essentially manipulate the life cycle of the mosquitoes were found effective but needs more research. However, for effective vector control, integrated vector management methods, involving use of combination of effective tools, is needed and is also suggested by Global Malaria Control Strategy. This review article raises issues associated with the present-day vector control strategies and state opportunities with a focus on ongoing research and recent advances to enable to sustain the gains achieved so far.
Malaria vectors and their distribution
All human malaria is transmitted by female mosquitoes of genus Anopheles, and not all anophelines are reported as vectors of malaria. Anopheles mosquitoes belong to phylum Arthropoda, class Insecta, order Diptera, sub-order Nematocera, family Culicidae, sub-family Culicinae, genus Anopheles, and sub-genus Cellia. There are approximately 460 recognized species so far, of which over 100 can transmit human malaria (Wikipedia 2009).
Vector species prevalence in South-East Asia region
Malaria vector control
The historic successful eradication of malaria in various parts of the world is achieved mainly by vector control (Harrison 1978). In addition, the Global Malaria Control Strategy emphasizes the need for selective and sustainable preventive measures for reducing malaria transmission (WHO 1993). The options available for present day vector control efforts mainly include chemical, biological, natural plant products, and environmental management. Immense literature is available for malaria vector control, and a WHO manual on vector control prepared by Rozendaal (1997) is highly informative, in which various methods are given on use of insecticides, insecticide-treated materials, biological control agents, insect growth regulators (IGRs), environmental management, and personal protection methods against mosquito vectors. World Health Organization Pesticide Evaluation Scheme (WHOPES) is involved in the development of new tools and methods for malaria vector control, regular updation of knowledge, and in the support in selection of safe and judicious use of public health pesticides by member states and other stakeholders.
Control of adult mosquitoes is the most important facet of controlling vector-borne diseases. It is accomplished by application of chemical pesticides against adult-stage mosquitoes.
Application of insecticides remains the primary control tool in the majority of vector control programs throughout the world since early nineteenth century (Breman 2001). The use of chemicals to control insects dates back to classical period in Greece and Rome. As early as in the eighteenth century, pyrethrum (Persian insect powder), and during the nineteenth century, many compounds were discovered as conventional pesticides, viz., mercuric chloride (1860), paris green (1865), phenol and cresols (1867), naphthalene (1882), bordeaux mixture (1983), rosin-fish oil soap (1886), calcium arsenate (1907), and nicotine sulfate (1909) (Raghavendra and Subbarao 2002). In the twentieth century, after the discovery of the insecticidal potential of dichlorodipehnyltrichlroethane (DDT), a new era of insect control began (Hassall 1982). DDT was the first synthetic organic insecticide used for effective vector control with reasonable success. Dash et al. (2007a,b) has clearly discussed in an editorial on various issues related to DDT and its ban and importance in vector control. In brief, environmental protection agency (EPA) declared the ban of DDT in 1972, owing to ecological considerations and opened up a debate between pro and against groups of the ban. However, the ban exempts its use in public health emergencies like outbreaks of malaria. The concern of its impact on human health is due to the prevalence of its residues in various tissues mostly in fat depots and vital organs. Available epidemiological studies do not conclusively prove that the compound has any direct association with liver, pancreatic, or breast cancers.
The Stockholm Convention on persistent organic pollutants (POPs) in 2001 identified DDT as one of the 12 POPs. But DDT ban had certain restrictions for countries that notify to the secretariat for its continued use. The restriction permits indoor residual sprays (IRS) of DDT in malaria control as per the WHO specifications for its production and following safety precautions for its proper use and disposal and phasing out is delayed until an effective, affordable, and safe alternative is available. According to WHO, 31 countries opted for exemption from total DDT ban until 2001 including China, Republic of Russia, and India, and recently, South Africa, Swaziland, Mozambique, and Ecuador followed the suit (DDT-Wikepedia-encyclopedia 2007). This reflects the confidence of member states on use of DDT for vector control in spite of reported resistance in malaria vectors though Stockholm conference on May 17, 2004 urged complete elimination of DDT production.
Based on the increasing scientific evidences, finally, WHO in September 2006 gave a clean bill to use of DDT to combat malaria in Africa and other areas where the vectors are still susceptible to DDT (WHO 2006a).
However, the debate on the use of DDT is still continuing and will continue until a more effective, affordable, and safe alternative tool is made available.
Indoor residual spray (IRS)
Insecticides recommended for indoor residual spraying against malaria vectors (Source: WHO/CDS/NTD/WHOPES/GCDPP/2006.1)
Insecticide compounds and formulations
Chemical type (2)
Mode of action
Duration of effective action (months)
Contact & airborne
Pirimiphos-methyl WP, EC
Contact & airborne
Contact & airborne
Contact & airborne
Alpha-cypermethrin WP, SC
Deltamethrin WP, WG
Lambda-cyhalothrin WP, CS
Insecticides suitable for application as cold aerosol ULV sprays or thermal fogs for mosquito control (Source: WHO/CDS/NTD/WHOPES/GCDPP/2006.1)
Dosage of a.ia (g/ha)
Insecticide-treated nets (ITNs)
WHO-recommended insecticide products for the treatment of mosquito nets for malaria vector control
1. Conventional Treatment (Source: WHO/CDS/NTD/WHOPES/GCDPP/2006.1)
Dosage (mg/m2 net)
Suspension concentrate 10%
Emulsion, oil in water 5%
Suspension concentrate 1%; Water dispersible tablet 25% and WT 25% + binder 3
Emulsion, oil in water 10%
Capsule suspension 2.5%
Emulsifiable concentrate 10%
2. Long-lasting treatment
Status of WHO recommendation
Lambda-cyhalothrin 10% CS + binder Target dose of 50 mg/m2
Long-lasting insecticidal materials (LMs)
WHO-recommended long-lasting insecticidal mosquito nets for use in public health
Status of WHO recommendation
Deltamethrin coated on polyester
Alpha-cypermethrin incorporated into polyethylene
Alpha-cypermethrin coated on polyester
Deltamethrin incororated into polyethylene
Permethrin incorporated into polyethylene
Deltamethrin coated on polyester
Deltamethrin coated on polyester with strengthened border
Combination of deltamethrin coated on polyester with strengthened border (side panels) and deltamethrin and PBO incorporated into polyethylene (roof)
In western Kenya, use of permethrin-impregnated bed nets reduced incidence of P. falciparum infections by 40–48% in children of less than 6 years age (Beach et al. 1993). In rural area of Zanzibar with very high perennial transmission, permethrin-impregnated bed nets led to 74–78% reduction in the weekly rate of reinfection with malaria parasites in all age groups (Stich et al. 1994). Sreehari et al. (2009) reported that 100% mortalities of A. culicifacies and A. stephensi exposed from Olyset® nets, Permanets®, K-OTab1-2-3®, and conventionally treated deltamethrin net (CTDN) unwashed net. All hand-washed LLINs produced mortality of >80% up to 20 washes in both A. culicifacies and A. stephensi. In case of CTDN, >80% mortality was reported up to three washes in both species. In case of machine-washed nets, >80% mortality was reported only up to 10–15 washes in all the LLINs and up to one wash on CTDN in both the above mosquito species. In 2009, Ahmed and Zerihun (2010) examined the Ugandan program of net distribution with pro-poor policy. Inequity was observed in the number of LLINs possessed by the households and in the knowledge on its use and maintenance. However, Gerstl et al. (2010) reported that in Bo and Pujehun districts distribution managed by Me´decins Sans Frontie`res (MSF) achieved good usage with freely distributed LLINs. Russell et al. (2010) reported the findings of use of long-lasting insecticide treatment kits, co-packaged with bed nets in communities of Namawala and Idete villages in southern Tanzania, a 4.6-fold reduction in entomological inoculation rate (EIR) in comparison to the use of untreated nets alone and with 79% reduction in the density of Anopheles gambiae s.s and 38% of A. arabienesis. Recently, Govella et al. (2010) established mathematical model of mosquito behavior and malaria transmission to illustrate how ITNs can achieve suppression of malaria transmission, even where mosquito evade them and personal protection is modest.
In addition to the LLINs, treatments of screens, curtains, canvas tents, plastic sheet, tarpaulin, etc., with insecticides may provide a cheap and practical solution for malaria vector control. Treated screen and curtains can be as effective as mosquito nets. Different types of long-lasting insecticide impregnated materials are under field trials in different countries. The residual insecticides in insecticide-treated wall lining (ITWL) are durable and maintain control of insects significantly longer than IRS and may provide an effective alternative or additional vector control tool to ITNs and IRS (Munga et al. 2009). In Pakistan, use of permethrin-impregnated top-sheets and chaddars has been shown killing effects on malaria vectors (Rowland 1999). Deltamethrin at 265 mg a.i./m2 incorporated Zerofly® plastic sheets have been successfully used in temporary tents and shelters or as internal lining of tents (Mittal et al. 2007). Deltamethrin-impregnated tarpaulins showed good potential for malaria prevention in displaced populations of Afghan refugee camp in Pakistan (Graham et al. 2002). However, permethrin-sprayed canvas tents in Pakistan showed decay of residue within a few months of spraying of inner surfaces (Bouma et al. 1996). The ITWL and curtains retain effective levels of residual insecticide for significantly longer periods than conventional IRS and are materially durable (Arnez et al. 2009). Field trials were conducted at Anwona and Mmemiriwa villages located at Ghana on residual activity of deltamethrin-impregnated durable residual wall lining (DL) against susceptible A. gambiae s.s even 3 weeks after installation on both cement and mud surfaces, found 100% mortality on both surfaces using WHO cone bioassay kits (Stiles-Ocran et al. 2009). In addition, field trials were conducted with DL on mud, concrete, and wood walls in different countries namely Benin, Equatorial Guinea, Kenya, Angola, Mali, Ghana, South Africa, Vietnam, and India in 2008 to assess the advantages of DL over traditional IRS programs and feasibility of use of the materials and user preference across a variety of housing conditions (Allan et al. 2009). Studies have shown that permethrin impregnation of battle-dress uniforms (at 0.125 mg/cm2), and application of DEET provides a complete protection against bites of insects. Field trials carried out in villages near Ouagadougou, Burkina Faso using permethrin-(lg a.i./m2) impregnated curtains on doorway, window(s), and the space under the eaves was effective against A. gambiae Giles s.l. and Anopheles funestus Giles for about a year resulting in complete reduction in indoor-resting mosquitoes (Majori et al. 1987). More field studies are needed to establish the utility of these interventions at personal and household levels.
Non-chemical control method
A traditional approach in mosquito control is the use of ovitrap. Torres-Estrada et al. (2007) reported that Anopheles pseudopunctipennis gravid females attracted and deposited more eggs in cups containing natural algae in water from breeding sites than in cups containing artificial (nylon rope) life-like algae in water from the corresponding natural breeding site, or in cups containing natural algae in distilled water, and also found that Spirogyra majuscula organic extracts at concentrations of 0.1%, 0.01%, and 0.001% attracted more oviposition of A. pseudopunctipennis. Laboratory studies on A. gambiae ss Giles conducted by Sumba et al. (2004) concluded that it preferred to oviposit on unmodified substrates from natural larval habitats containing live microorganisms rather than on sterilized ones, because microbial populations in breeding sites produce volatiles that serve as semiochemicals for gravid A. gambiae. Recently, ovitrap laced with insect growth regulator are also being used to stop the development of hatched larvae.
Insecticides suitable as larvicides for mosquito control. (Source: WHO/CDS/NTD/WHOPES/GCDPP/2006.1)
Dosage of a. i.a (g/ha)
125–170 g/ha of formulated product in open bodies of water.
Water dispersible granules
1–5 mg/l for container breeding mosquitoes.
Insect growth regulator
142–190 l/ha; 19–47 l/ha if a spreading agent is used
Insect growth regulator
Insect growth regulator
Insect growth regulator
Emulsifiable concentrate, granules
The principle of chemical larviciding is to eliminate or reduce the vector population by killing the larvae. Theoretically, larval control measures could contribute toward malaria elimination; however, high coverage of breeding sites is required to achieve significant impact, which is a major operational and logistical challenge in many ecological settings (Pampana 1969). Chemicals like petroleum oils (Gratz and Pal 1988), Paris Green (copper acetoarsenite) (Rozendaal 1997) has been used successfully as larvicides. Although inexpensive and highly effective, use of Paris Green is no longer recommended because of its toxicity of arsenic metal in the formulation. Several organophosphate larvicidal formulations has been used routinely since 1960s for malaria vector control in several countries including India, Mauritius, and Oman (Kumar et al. 1994; Gopaul 1995; Parvez and Al-Wahaibi 2003). In India, in urban areas, the major vector control strategy is larviciding of breeding habitats with organophosphate insecticides namely temephos and fenthion (Sharma et al. 1996), and recently, fenthion has been withdrawn for use in vector control in India due to development of insecticide resistance. In particular, temephos exhibits very low mammalian toxicity (Gratz and Pal 1988; FCCMC 1998). Synthetic pyrethroids are also effective but are reported highly toxic to aquatic non-target organisms, especially fishes (Chavasse and Yap 1997; WHO 2006c), and hence, not used for larval control.
There is a growing interest in the use of very safe IGRs that are emerging as promising vector control compounds for mosquito control with specific action and are relatively safe to non-target organisms (Mulla and Darwazeh 1979; Schaefer et al. 1984; Mulla et al. 1985, 1986). Treated larvae generally die during the ecdysis, this is because of inhibition of chitin deposition; the larvae do not have the rigidity to shed the old cuticle (Mulla 1995). A single treatment of pyriproxyfen at the rate of 0.01 mg a.i/l has inhibited adult emergence for a period of 190 days and reduced the adult population (78%) in the riverbed pools of Sri Lanka (Yapabandara and Curtis 2004). One hundred percent inhibition of adult emergence was observed in the laboratory evaluation of Hilmilin (diflubenzuron) at a dose of 0.0125 ppm against fourth-instar larvae of A. culicifacies (Ansari et al. 2005). An eco-friendly formulation Starycide 480 SC (Triflumuron-OMS-2015) was evaluated against anopheline mosquito and resulted in complete inhibition of adult emergence (Batra et al. 2005). Recently, a new ethoxylated alcohol formulation that results in monomolecular film on breeding waters was found effective for larval control of anophelines (Batra et al. 2006; Bukhari and Knols 2009).
Non-chemical control methods
Bacterial pesticides. Two bacterial species, Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus (Bs), have been widely demonstrated to be effective larvicides against mosquitoes. Protoxins in parasporal crystals, and the spore coat of these bacteria, function as stomach poisons in the mosquito larval midgut. Since the discovery of the mosquito larvicidal activity of Bti (serotype H-14) in 1977, several formulations of Bti are used against different mosquito larvae such as Anopheles albimanus, A. sinensis, A. stephensi, A. gambiae, A. maculatus, A. maculipennis, and A. sundaicus complexes (Lacey and Lacey 1990; Becker and Margalit 1993; Das and Amalraj 1997; Mittal 2003). The advantages of biological larval control agents in comparison to chemical control are (1) effectiveness at relatively low doses, (2) safety to humans and non-target wildlife, (3) low cost of production, and (4) lower risk of resistance development (Yap 1985). Presently, Bti tablet and granular formulations are available that can be used in small container for larval abatement. Efficacy of aqueous suspension of Bti (Vectobac 12 AS) was investigated in a laboratory and field conditions against A. culicifacies and A. stephensi and found effective (Mittal 2003). Larvicidal efficacy of Bacticide ® WP (wettable powder) and VectoBac ® 12AS (liquid formulation) was investigated in different habitats against anopheline mosquitoes (Haq et al. 2004). The mode of action of Bt delta endotoxin involves the synergistic interaction of four toxic crystal proteins. After ingestion of sporulated Bt, the parasporal crystals are solubilised in the larval alkaline midgut followed by proteolytic activation of the soluble insecticidal crystal proteins by proteases. The toxin binds to a receptor on the midgut cell wall resulting in pores in the gut cell membrane, followed by destruction of the epithelial cells (Cooksey 1971; Norris 1971; Fast 1981; Huber and Lüthy 1981; Lüthy and Ebersold 1981; Smedley and Ellar 1996); which leads to death of the larva by hemosepticemia.
Other biocontrol agents. The main biological control agents that have been successfully employed against anophelines are predators, particularly fishes like Gambusia, Tilapia, Poeciellia, and a recent report of Aphanius dispar dispar. Insect-killing viruses are another group of natural enemies of mosquitoes that seem highly effective because they are species-specific, non-toxic to humans, and easy to distribute (Ren et al. 2008). Other microbial pathogens of mosquitoes include protozoa and microsproidia. Several fungal pathogens Metarhizium anisopliae, Beauveria tenella, Lagenidium giganteum, and Chrysosprium lobatum have shown promise as larvicides (Scholte et al. 2003a), although commercial formulations are not presently available for mosquito control (Scholte et al. 2004). Scholte et al. (2003b) reported that adult A. gambiae is susceptible to Beauveria bassiana and M. anisopliae, and Mohanty et al. (2008) reported that M. anisopliae 892 was effective against larvae of A. stephensi. Other biological control agents include the nematode Romanomermis culcivorax and the Azolla plant (Lacey and Lacey 1990). Natural predators of mosquito larvae include the tadpole stages of amphibians (Mokany 2007), which consume mosquito larvae, while frogs can reduce mosquito population by preying on adult mosquitoes (Raghavendra et al. 2008), planktivorous fishes, dragonfly naiads, hemipteran water bugs, such as Notonecta and Anisopus (Blaustein 1998; Hampton 2004), dytiscid beetles, such as Rhantus, Ilybius, and Agabus spp. (Lundkvist et al. 2003; Aditya and Saha 2006), malacostracans, anostracans, cyclopoid copepods, and triclad flatworms (Kumar and Hwang 2006). The cyclopoid copepod Mesocyclops thermocyclopoides (Kumar 2003; Kumar and Rao 2003) and the dytiscid beetles Colymbetes, Ilybius, Rhantus, and Agabus spp. (Lundkvist et al. 2003) were shown to prefer mosquito larvae over other arthropod prey. Triops granatius had the ability to decimate larvae of A. gambiae in temporary breeding sources around huts in a village in Somalia (Maffi 1962). In Latin America, larvae of malaria vectors were found to be absent in the aquatic habitat that contained Mesocyclops longisetus (Jennings et al. 1995; Nam et al. 1998).
Natural plant products
Phytochemicals derived from various botanical sources have provided numerous beneficial uses ranging from pharmaceuticals to insecticides (Sukumar et al. 1991). Studies on the efficacy of natural plant products as larvicides during the last decade have indicated them to be possible an alternative to synthetic chemical insecticides (Vasudevan et al. 1997; Pathak et al. 2000). Minjas and Sarda (1986) reported that larvicidal efficacy of crude aqueous extract of fruit pods of Swartzia madagascariensis against A. gambiae, and similar observations were made by Sujatha et al. (1988) with petroleum ether extract of six plants Acorus calamus, Ageratum conyzoides, Annona squamosa, Bambusa arundanasia, Madhuca longifolia, and Citrus media against A. gambiae. Crude extract of Yucca aloifolia was studied and found toxic to A. stephensi (Thomas et al. 1994). Raghavendra et al. (2009a) tested the larvicidal efficacy of the extracts of ripe fruits of Solanum nigrum against A. culicifacies species A, C, and A. stephensi and found effective. Laboratory evaluation of crude extract of fruit of Solanum xanthocarpum (Singh and Bansal 2003) and aqueous extract from the roots of Hibiscus abelmoschus (Dua et al. 2006) were found to be toxic to the larvae of A. culicifacies. Crude extract of flower, leaf, and stem of Spilanthes acmella L. plants were tried against A. stephensi Liston and found that the LC50 and LC90 values of flower extract were more than the leaf and stem extracts against A. stephensi (Pandey et al. 2005). Studies have been carried out on the larvicidal activity using the flower head extract of Spilanthes acmella Murr. against Anopheles spp. (Pendse et al. 1946; Saraf and Dixit 2002). The hexane extract of flower heads of the three species namely S. acmella L var oleraceae Clarke, S. calva L., and S. paniculata Wall ex DC have shown variable mortality against A. stephensi Liston and A. culicifacies (Pandey et al. 2007). Laboratory bioassay of latex from the unripe fruits of Carica papaya was carried out against A. stephensi and the LC50 and LC90 value was 0.013% and 0.062%, respectively (Thomas et al. 2004). Larvicidal activity of the emulsified neem oil formulation was observed against late instars of A. stephensi larvae in tanks and pits, and 100% reduction was found (Dua et al. 2009). However, very few plant products have shown promise for use in large-scale vector control in field, e.g., neem oil.
Personal protection methods
Various methods for protection from mosquito bites, such as repellent oils, smoldering coils, vapourizing mats, repellent creams, liquid vapourizer, etc., are used worldwide (Feles et al. 1968; Charlwood and Jolley 1984; Coene et al. 1989). Effectiveness of these methods lasts for 5–7 h with 60–80% protection (Curtis et al. 1989; Ansari et al. 1990). Synthetic pyrethroids, such as allethrin, bioallethrin, and related chemicals used in these repellents are generally safe, but their prolonged use may be harmful (Lui et al. 1987). Oils extracted from plants are being used for repelling mosquitoes in several countries (Curtis et al. 1990). Essential oils extracted from Mentha piperita gave 84.5–100% protection against A. culicifacies during the whole night landing collection (Ansari et al. 1999). The efficacy of turmeric, gingili, and mustard oil was demonstrated as early as 1947 against A. fluviatilis (Ansari and Razdan 1995). Variable degree of bioefficacy was reported from neem oil extracted from seeds of Azadirachita indica (Sharma et al. 1993a,b), oils of aromatic grasses citronella, and lemon grass oils (Cymbopogon nardus and Cymbopogon citratus) with repellent action against mosquitoes (Osmani et al. 1972; Ansari and Razdan 1995). Smoke produced by burning neem oil mixed in kerosene in lamps had also shown protection from mosquito bites (Sharma and Ansari 1994). Nagpal et al. (2001) reported that the neem cream had a strong repellent action against A. culicifacies. Mats impregnated with neem oil are found effective in repelling mosquitoes (Sharma et al. 1993a,b). Amer and Mehlhorn (2006a) tested different essential oils stored in light and dark conditions against different mosquito species and found larvacidal toxicity. Amer and Mehlhorn (2006b) also reported larvicidal activity of Thymusserpyllum against A. stephensi Liston with LC50-10 ppm after 24 h of exposure. Similarly, Massebo et al. (2009) tested essential oils of 11 plants in Ethiopia for larvicidal activity against A. arabiensis and found oils of Chenopodiul ambrosioides Linnaeus and Ocimum lamiifolium Hochst to be relatively more effective than others. Uma devi et al. (2010) reported that methanolic leaf extract of Artemisia parviflora (APLE) was found effective against A. stephensi larvae and pupae. In addition to different repellants, odor baits are likely to be acceptable to communities in disease-endemic areas (Takken and Knols 2009). Odor baits and repellents require long-lasting slow-release formulations that do not interact with the environment. DEET (N, N-diethyl-3-methylbenzamide) is an effective and well-known mosquito repellent, as evidenced by entomological research (McCabe et al. 1954; Schreck 1977; Curtis et al. 1990; Gupta and Rutledge 1994; Barnard 2000). However, DEET distributed to one member only of a family of Indian villages produced no evidence for reduction in malaria transmission (Vittal and Limaye 1984), and another study on school children in Tanzania did not show a significant effect (Curtis et al. 1994).
The insect repellent DEET (N, N-diethyl-3-methylbenzamide), is in use since last 50 years, with 200 million people using it worldwide to reduce their risk of vector-borne diseases (Moore and Debboun 2007). DEET was developed by the US Army in 1946 and was registered for use by the general public in 1957, and is still considered to be very safe and effective as mosquito repellent, as evidenced from several decades of entomological research (McCabe et al. 1954; Schreck 1977; Curtis et al. 1990; Gupta and Rutledge 1994; Barnard 2000). An early study with DEET in some Indian villages showed no reduction in disease transmission (Vittal and Limaye 1984) and similarly in schoolchildren in Tanzania (Curtis et al. 1994). In addition to this, a community-randomized trial with DEET in South America showed no effect on malaria incidence rates (Kroeger et al. 1997), while a recent outbreak of malaria in a village in South Africa was controlled by use of DEET as repellent among the affected population (Durrheim and Govere 2002). Another study in Thai–Myanmar border with multidrug resistant strains of P.falciparum malaria, use of a mixture of a root paste made from pulp of the wood apple tree “Thanaka” (Limonia acidissima,) and DEET by Karen pregnant women found to reduce mosquito bites (Lindsay et al. 1998). McGready et al. (2001a) reported lower accumulation of DEET in cord blood (8%, n = 50, 95% confidence interval 2.6–18.2) applied regularly to Karen pregnant women during the second and third trimesters of pregnancy without adverse effects on survival, growth, and development at birth and up to 1 year, indicating considerable safety. A randomized controlled trial of an insect repellent in an area of low malaria transmission at the Thai–Burmese border showed a non-significant reduction of 28% in the incidence of P falciparum infection (McGready et al. 2001b). Two DEET alternatives are recommended by the Centers for Disease Control and Prevention (CDC) against ticks on human skin that were also approved by US Environmental Protection Agency (EPA): IR3535 (3-[N-butyl-N-acetyl]-aminopropionic acid, ethyl ester) and the piperidine, Picaridin (1-piperidine carboxylic acid) (EPA 2008). Later, CDC recommended Picaridin as mosquito repellent. A study showed that DEET and Picaridin exert repellent and deterrent effects on the behavior of A. stephensi (Klun et al. 2006). A field trial conducted to test the impact of burning 1% neem oil in kerosene lamps showed reduction in A. culicifacies s.l. from 64.8% to 14.06% in indoor resting density (Ansari and Razdan 1996). More research using new chemicals and natural plant products are in offing.
The concept of modifying vector habitat to discourage larval development and/or human–vector contact is generally referred to as environmental management (Singer et al. 2005). The techniques of environmental management are generally grouped into three main categories—environmental modification, environmental manipulation, and modification of human habitations/behaviors (WHO 1982; Ault 1994). The feasibility of flushing streams to control A. culicifacies Giles was examined in five river systems in rural Sri Lanka in the late 1930s (Konradsen et al. 2004), and again, in one stream in the mid 1990s (Konradsen et al. 1998). Sharma et al. (2008) recently reported that due to the construction of the dam in San Dulakudar village, Sundargarh District, Orissa, India, resulted in impounding of water in a small reservoir, thereby preventing flow of water both above and below the dam, thus, making it unfavorable for breeding of A. fluviatilis that generally prefers to breed in slow-flowing water streams, and further reported that decline in malaria incidence among children of the 1–5 years age group. Open water marsh management (OWMM) involves the use of shallow ditches to create a network of water flow to drain the mosquito habitats within marshes and to connect the marsh to a pond/canal, in which fish will feed on mosquito larvae. This reduces the need for other control methods using insecticides. Rotational impoundment management (RIM) involves the use of large pumps and culverts with gates to control the water level within an impounded marsh. Gates in the culverts are used to permit fish, crustaceans, and other marsh organisms to enter and exit the marsh. RIM was used on the east coast of Florida for larval control. Supporting social, economic, and environmental developments have indirect beneficial effects on malaria transmission. For example, in Tanzania, the switch from thatched to corrugated iron roofs profoundly reduced mosquito entry and biting in houses (Schofield and White 1984; Lindsay et al. 2003). In Indonesia, the environmental changes that led to periodic drainage of rice fields and cleaning of fish ponds contributed to the disappearance of malaria in Java (Takken et al. 1990). Today, 80 years after implementation, these effects are still visible and is becoming increasingly clear that malaria control can be most effective if several tools are applied simultaneously, complementing each other (Nyarango et al. 2006). In India, environmental management has been successfully listed as a vector control strategy in urban areas, several industrial complexes, seaport areas, and railways, etc., (IDVC Project 2007).
Integrated vector management (IVM)
Chemical control (use of pesticides) is still the most important element in the integrated approach to vector control. However, due to the severe demerits like health hazards and damage to the environment and local ecology, integrated Vector management (IVM) provide a great opportunity where chemicals are used as final option to bring the malaria transmission below threshold level. The renewed attention to IVM enables the integration of several tools to achieve a stronger impact (WHO 2008). IVM is an environmentally sensitive approach for mosquito control, which uses ecological methods of insect control including the insecticides as a last resort. WHO has issued a Global Framework for Integrated Vector Management (IVM), stressing the importance of evidence-based combinations of vector control methods (WHO 2004). It also encourages effective coordination of the control activities of all sectors that have an impact on vector-borne diseases, including health, water, solid waste and sewage disposal, housing and agriculture (WHO 2004). Environmental and biological control of mosquitoes were successful in larval abatement in a variety of geographical and ecological settings (Scholte et al. 2007). Integration of several intervention tools, coupled with new prospects of effective malaria vaccines, has led to the resolve of Bill and Melinda Gates to call for a new global initiative for the eradication of malaria.
Genetic manipulation of the disease-vector species is considered as a probable vector control strategy, owing to certain successes in insect pest management. However, the molecular technological advances started only after the complete genome sequences are available (Alphey et al. 2002). Increasing number of successfully sequenced insect genomes, coupled with the availability of powerful bioinformatic tools, is helping to formulate, test, and evaluate many genetic modification methodologies and protocols. Genetic engineering and sterile insect technique are two very potential and tested genetic modification techniques.
Genetic engineering of mosquitoes
Transgenic technology acts as an alternative strategy to the conventional vector control methods that allow modification of genetic structure of the living organism by the introduction of foreign DNA. Thus, in-principle introduction of anti-parasite macromolecules stably into the mosquito genome can limit the malaria transmission (Catteruccia et al. 2000). In a more simplified way, the technology allows producing the genetically modified vectors that are either resistant/refractory to the parasite infection (Riehle et al. 2003) or decreased life span of the vector mosquitoes. The methodologies for producing transgenic organisms can be classified into biological and physical categories (Atkinson et al. 2001). Biological methods include use of viruses, symbiotic microorganisms, mobile genetic elements, etc. While physical methods include, microinjection, biolistics (Huynh and Zieler 1999; Mialhe and Miller 1994; Thomas et al. 2001), electroporation (Leopold et al. 1996), etc., to deliver the foreign macromolecules into the target tissue. Of these biolistics and electroporation methods are of less importance (Takken and Scott 2003).
There has been considerable progress over the last decade towards developing genetic engineering tools and potential molecules like the refractory genes, promoters, regulatory elements, and effector molecules for creating refractory mosquito (Riehle et al. 2003). Transposable elements, such as piggyBac, minos, mariner, and hermes have been successfully used to transform mosquitoes (Catteruccia et al. 2000; Grossman et al. 2001; Nirmala et al. 2006; Riehle et al. 2003; Takken and Scott 2003). Densovirus applications have envisaged generation of stable transgenic mosquitoes for vector control (Carlson et al. 2006). Furthermore, the microinjection is a fool-proof standardized technique and is extensively used to transfect the germ lines through embryos in some insect species (Soreq and Seidman 1992; Terenius et al. 2007). Recently, Corby-Harris et al. (2010) reported that modulation of Akt gene that is involved in various cellular signaling at different levels, has increased the resistance against the malaria parasite invasion, and further decreased the life span of the mosquito, A. stephensi.
List of some of the candidate gene promoters that are found to express specific to tissue and external stimuli
Name of the gene/promoter
Midgut epithelial cells-blood inducible
AgAper1- a peritrophic matrix protein
Midgut epithelial cells
Shen and Jacobs-Lorena 1998
Vitellogenin (Vg) promoter and Difensin A (DefA)
Fat bodies and haemocoel
James et al. 1989
Up-regulation- blood inducible
van Huesden et al. 1998
Midgut gene- blood inducible
Smartt et al. 1998
Barillas-Mury et al. 1991
Ferdig et al. 1996
Juhn and James 2006
Sterile insect technique
Sterile insect technique (SIT) provides another ecologically safe program (Benedict and Robinson 2003), in which rearing, sterilization, and release of genetically modified male mosquitoes result in reduction in female population (Catteruccia et al. 2005b). This technique is suggested in relatively isolated area and with a single vector species. Conventionally, using chemicals or radiations and more recently, genetic engineering methodologies (introduction of principle that can cause sex-dependent sterility) are used to produce sterile insects. It is the first biological modification which was tested and was found successful. The major issues of SIT are (1) establishment of sophisticated production units, (2) production of large-scale sterile insects, (3) requirement of highly trained human resource, (4) sexing of mosquitoes among others. Despite these limitations, it is the only program that has solely achieved the eradication of some parasitic diseases of veterinary importance, and equally, not yet that effective for parasites of public health importance (Lindquist et al. 1992; Bowman 2006; Wyss 2000; Ragheb 2007) For sorting/sexing the mosquitoes at earlier stages, molecular fluorescent markers and methods have been developed and tested (Catteruccia et al. 2005a; Handler 2002; Horn et al. 2002). In-principle, the objective include the identification of genes that kill the mosquito sex-dependently. Studies should target to understand the biology of the spermatogenesis and sex determination (Catteruccia et al. 2005b).
A synthetic system has been conceptualized and proved for rapid spread of transgene into the wild population. This system segregate in non-mendelian fashion—first discovered in Tribolium castaneum and later designed and successfully tested in Drosophila (Chen et al. 2007a). As early as 1970s, SIT has been tested against the A. stephensi, in India and A. albimanus in El Salavador (Ragheb 2007) but failed in delivering intending purpose in India due to some non-technical reasons (Klassen 2009). In contrast, El Salavador could successfully control malaria in some parts of the country, and large-scale implementation was not possible due to civil war (Klassen 2009; Ragheb 2007). Pilot studies with mosquitoes are currently being undertaken in Sudan, with assistance from the International Atomic Energy Agency (Knols et al. 2007).
Other genetic control techniques like cytoplasmic incompatibility, incompatibility due to chromosomal factors, chromosome translocations, conditional lethal, meiotic drive, compound chromosome, etc., are proposed for alternative vector control strategies but were not extensively tested (Pal and LaChance 1974).
Success of transgenic technology in vector control also depends upon community perception, political will, media involvement and policy decisions, and implementation (Alphey et al. 2002). However, successful control of screwworms in America (Wyss 2000) again opened up opportunities for effective vector control using GMVs, leading to eradication of melon fly from Okinawa and control of tsetse fly in Africa. However, using these new technologies in field to control/eradicate the malaria, risk assessment to the ecology and environment need to be assessed (for detailed explanations about different transformation systems and possible threats please refer [Atkinson et al. 2001; Hoy 2003; Jacobs-Lorena 2003; Scott et al. 2002; Jackson et al. 2002]). Catteruccia et al. (2005a) proposed a method EFGP based on β2-tubulin promoter system for sexing mosquitoes in third instars makes the task more convenient and less work-intensive.
Technologically, reasonable advances have occurred, and standard procedures are available to conduct risk assessment studies due to risks associated with migration of mosquitoes. As many of the transgenic proponents for the vector control aim in stable genome integration and virtually permanent establishment of transgene, it may also raise doubt about the possibility of formation of separate ecological niches by the transformed insects (Hoy 2003). In that case, it will be called “man made new species”. Thus, the issues like how transgenic mosquitoes, if found to be harmful, removing these from the environment need to be thoroughly considered. Moreover, malaria vectors are region-specific, so the transformation technology and population genetic studies should be standardized and conducted according to the conditions and specifying to local species. This is largely due to the fact that at least half of the important vectors of malaria belong to sibling (or cryptic) species complexes whose members are isomorphic (Collins and Paskewitz 1996). Once the transformation technology is customized, the type of marker gene, vector for transformation, and effective doses of radiations, etc., need to be standardized along with extensive population genetic studies that could give insights into the feasibility of movement of transgene or success of SIT. Curtis and Townsont (1998) have suggested a methodology that could be applicable to the long term variation studies pertaining to the transgenic technology assessment for vector control. For instance, use of SIT in the urban localities where peri-urban areas and the surrounding villages have prevalence of other species, eg., A. stephensi, is a major urban vector in South India where surrounding peri-urban areas are prevalent in A. culicifacies. Making rapid spread of transgene into the desired population is a challenging task (Ito et al. 2002). Following Mendelian inheritance, spread of genes will take longer periods than required (Scott et al. 2002). To answer this, two methodologies have been proposed: (1) introducing the foreign DNA into the vector through obligate parasite-(paratransgenesis); and (2) introducing through transposable element constructs (Beerntsen et al. 2000). Future research in this area should focus on optimizing refractory genes to effectively confer resistance to human malaria. Other methods for generating refractoriness involve using antibodies that kill parasites within the mosquito and discovering genes that govern refractoriness in natural populations, and cost of transgene also need to be assessed (Catteruccia et al. 2003). It is important to ascertain that the genetically modified vectors do not harbor any unwanted pathogens (Beerntsen et al. 2000). One of another major area of research is to find out simple, cost effective, and technologically less demanding procedure for sex selection/identification. Finally, though the current position of genetic engineering demands considerable efforts to achieve success against numerous above-described hurdles, surely, the final payoff is larger (Ito et al. 2002).
Few success stories
Many success stories of disease control using individual or combination of different strategies are reported. The integrated malaria control program in Dar-es-Salaam, Tanzania increased community acceptance of the malaria control program through the use of expanded polysterene beads (EPBS) specifically for control of nuisance-biting mosquitoes (Castro et al. 2004). Intensified malaria control interventions in Betul (Madhya Pradesh, India), using IRS and larvivorous fish, speak of a success story of malaria control (Singh et al. 2006). A sociological study of public perceptions of a microbial larvicide-based mosquito control program in urban Burkina Faso found good approval of the program by the people owing to perceivable reduction in nuisance-biting mosquito (Samuelsen et al. 2004). In Indonesia, the environmental changes that led to periodic drainage of rice fields and cleaning of fish ponds contributed to the disappearance of malaria in Java (Takken et al. 1990), and even, 80 years after implementation, these effects are still visible. It is becoming increasingly clear that malaria control can be most effective if several tools are applied simultaneously, complementing each other (Nyarango et al. 2006). Barat (2006) provides an account of four highly notable malaria success stories, in Eritrea, Brazil, India, and Vietnam by the distribution and retreatment of ITNs, environmental management, and health education of community on malaria. Malaria control intervention in Burundi is another malaria control success story (Beier 2008). A pilot study by WHO to control malaria in Sri Lanka, where A. culicifacies is the major vector, involved farmers in farmer's field schools successfully to control malaria (WHO 2007b). In Indonesia, in the 1970s, synchronization of rice culture by the supply of uniform quality of seed, timing of sowing, and harvesting of short duration rice variety with rotation of two rice crops with non-rice crop, proved effective to control rice pests and malaria vector A. aconitus. Prawn and fish culture, requiring high salinity, proved injurious to A. sundiacus, and these agricultural practices controlled malaria as a collateral benefit in addition to increased rice productivity (SEARO 2009). In India, in the 1980s in Kheda district, marshy land was reclaimed by improving and by lowering the water table that resulted in less breeding of A. culicifacies vector populations.
In China (Henan Province), malaria was reduced to 99% after the year 1965 and maintained up to 1990 as a result of IRS, locally produced high-quality drugs, social mobilization, mosquito nets, and artemisinin-based traditional medicine. In Vietnam, numbers of malaria-related death toll were decreased by 97% between 1992 and 1997 using similar strategy as in China (http://www.who.int/inf-new/mala1.htm). However, noteworthy success is achieved in eradicating the parasitic diseases of veterinary importance using GMO, Cochliomyia hominivorax, NewWorld screwworm from southern states of USA, Mexico and Central America (Wyss 2000; Bowman 2006), and Libya (Lindquist et al. 1992) using SIT program. The SIT was also used successfully in controlling Tsetse fly (Glossina spp.) and Med fly (Mediterranian fly-Ceratitis capitata) (Ragheb 2007).
Some issues in present-day vector control
Malaria vector control has become less effective in recent years, partly due to poor use of alternative vector control tools, inappropriate use of insecticides, lack of an epidemiological basis for interventions, inadequate resources and infrastructure, and weak management (WHO 1995). Change in environmental conditions and the behavioral characteristics of certain vectors have further added to the failures (WHO 1995). Ecological changes driven by deforestation, human migration, and unmanaged urbanization have increased the densities of human hosts and vector-breeding sites in some malarious regions (Gratz 1999; Robert et al. 2003). Furthermore, ecological succession of vectors led to appearance and disappearance of malaria vectors necessitating appropriate strategies for effective vector control. Adding to this presence of sibling species complexes of vectors has complicated the situation. In this scenario, a need for correct identification of vectors is of paramount importance as misidentification of vectors will lead to failure of vector control as was reported in Vietnam, misidentification of A. minimus as A. varuna led to the suggestion of wrong vector-control methods (van Bortel et al. 2001). Current developments in genome analysis and barcode genes have further facilitated the identification of new species, but accurate morphological identification of field-collected specimens is necessary before applying modern molecular and computational phylogenetic techniques to establish the taxonomic relationships (Raghavendra et al. 2009b).
In addition to this, due to widespread insecticide resistance in vectors and non-availability of new and effective insecticide molecules in near future, management of insecticide resistance gained importance. Successful implementation of ITNs in a community-based intervention program is hampered by several technical, operational, economical, and social factors. Misuse of the interventions such as distributed bed nets, which were utilized for catching fish in streams, has resulted in the intended ineffectiveness (Singh et al. 1994). This highlights the need for involvement of individuals and community for effective vector control. IVM is based on the premise that effective control is not the sole preserve of the health sector but requires the collaboration of various public and private agencies, and more importantly, community participation (WHO 2004). Large-scale environmental modification efforts (mainly contain the larval propensity) would probably need to be planned and coordinated through a government agency and community involvement at all the stages from planning to maintenance would increase the sustainability of malaria control efforts. The deployment of larvivorous fish into human-made containers, particularly water storage tanks, require extensive community education and involvement, as the fish may be killed easily by householders due to lack of appropriate knowledge (Fletcher et al. 1992; Mohamed 2003) or can be a socio-cultural taboo.
To date, among all, two operation scale interventions, IRS, and long-lasting insecticidal-treated nets (LNs) are considered effective for reducing malaria transmission. Recently, use of other long-lasting insecticide-treated materials (LMs) that include sheets, curtains, and wall lining were found effective. Non-chemical methods that include use of biocontrol agents and environmental management methods have also shown promise. However, malaria vectors have developed resistance to different insecticides, and hence, there is a growing realization that alternate methods are needed for effective vector control operations. Effectiveness of interventions also depends on the behavior of mosquitoes, and hence, should be vector species-specific. Lack of wide range of effective insecticides and fast development of resistance to available insecticides being in use in both agriculture and public health, retards the effectiveness of vector control program. Effective IRM not only results in increased shelf life of existing insecticides but also delay onset of resistance. Many vector control programs are more successful with involvement of the local communities in the ongoing vector control operations.
Recently, genetic control by sterile males or disease non-transmitting biting mosquitoes is gaining importance but needs thorough put basic operational research and appropriate ethical considerations. Effective management of available methodologies and tools proved to be successful in controlling malaria in at least five countries, viz., China (Henan Province), Brazil, India, Eritrea, and Vietnam. Hence, rather than claiming on non-availability of new tools and technologies, the need is to learn from success strategies rather than putting repeated and futile efforts in finding out more alternative technologies that may have the same fate as others. One should rather focus on acquiring the information regarding the cause for failure of malaria control locally. Some of the causative issues are lack of education, proper coordination between the policy makers and various organizations and institutions, apathy of community and health workers, and associated technical factors, such as development of insecticide resistance in vector and drug resistance in parasite, can make the situation even more serious. There is much more need to be done in bringing the genetic transformation technology into field operational program. However, SIT, also being a genetically modified technology, has shown promise in control of vector species in many instances. Each and every old, newly implemented, and developing alternative technologies could be deficient in one or more issues. Hence, sustainable control of malaria disease requires integration of various methodologies into most appropriate program called “integrated vector management”. Though IVM is touted as eco-friendly and a program for sustainable management of malaria disease, it is still largely limited to academics only. Thus, great endeavors are needed to be put towards sketching the ideal IVM models to meet the purpose.
In addition to the above-discussed strategies, the interest of scientist's world over is drawing attention to evolve novel methods, and one such method under consideration is manipulation of the life cycle of the mosquitoes by developing evolution-proof insecticides. These are to avoid the development of insecticide resistance and to stop its propagation in the population. The principle of this is in contrast to existing strategies wherein older mosquitoes get targeted with late life acting insecticides (LLA). LLAs act to control mosquitoes that have completed two or more gonotrophic cycles and are expected to reduce the number of infectious bites and avoid development of host resistance. Insecticide with irritancy (excito-repellancy), fungal biopesticides, and biocontrol agents, such as Wolbachia and densoviruses, can target older mosquitoes (Read et al. 2009). Furthermore, these can be used optimistically in integration with other interventions in IVM strategies.