Aproaches of Controlling and Preventing Malaria Transmission

Predators, particularly fishes like Gambusia, Tilapia, Poeciellia and Aphanius dispar dispar. Even viruses can be used to kill mosquitoes. These viruses are beneficial because they are species-specific, non – toxic to humans and easy to distribute. Protozoans, microsporidia, Metarhizium anisopliae, Beauveria tenella, Lagenidium giganteum, and Chrysosprium lobatum. All of the pathogens mentioned above are effective larvicides. Scholte et al. reported that adult A. gambiae is susceptible to Beauveria bassiana and M. anisopliae, and M. anisopliae 892 was effective against larvae of A. stephensi.

Other biological control agents include the nematode Romanomermis culci-vorax and the Azolla plant. Tadpole stages of amphibians are one of the natural predators of mosquitoes as they feed on mosquito larva. Frogs, planktivorous fishes, dragonfly naiads, hemipteran water-bugs, such as Notonecta and Anisopus, dytiscid beetles, such as Rhantus, Ilybius, and Agabus spp. malacostracans, anostracans, cyclopoid copepods, and triclad flatworms can also control malaria as they feed on adult mosquitoes. According to a study conducted by Jennings et al.1995 showed that larval stages of mosquitoes are absent in the aquatic habitats where Mesocyclops longisetus is present.

Natural plant products

Phytochemicals are the alternatives to the synthetic chemicals insecticides. Few products which can be used as a phytochemical are the crude aqueous extract of fruit pods of Swartzia madagascariensis against A. gambiae, and petroleum ether extract of six plants Acorus calamus, Ageratum conyzoides, Annonasquamosa, Bambusa arundanasia, Madhuca longifolia, and Citrus media against A. gambiae. Crude extract ofYuccaaloifolia is toxic to A. stephensi. The crude extract of the fruit of Solanum xanthocarpum and aqueous extract from the roots of Hibiscus abelmoschus were found to be toxic to the larvae of A.culicifacies. Flower head extract of Spilanthes acmella Murr. against Anopheles spp.

Personal protection methods

There are numerous methods of protection from mosquitoes like repellent oils, smoldering coils, vaporizing mats, repellent creams, liquid vaporizer, etc. Their effectiveness lasts for 5-7 hours and they offer 60-80% protection. Allethrin and bioallethrin, significant constituents of synthetic pyrethroids are toxic on prolonged use. Essential oils extracted from Mentha piper is 84.5-100% effective against A. culicifacies and turmeric, gingili, and mustard oil is effective against A. fluviatilis. Other oils which are useful are neem oil from seeds of Azadiracta indica, oils of citronella and lemongrass oils. Neem oil is found to be most effective among all other oils. Neem oil mats are also highly effective against mosquitoes. Odorbaits are highly beneficial in endemic areas as they have a long-lasting effect and slow release of their contents. One of the popular odorbaist is DEET- N, N-diethyl-3-methylbenzamide. Around 200 million people are using DEET for the past 50 years as a mosquito repellant. Malaria outbreak in South Africa is controlled by using DEET among the affected population.

Environmental management

Environment management is the modification in the vector habitat, by creating the environment which is not suitable for the larval maturation. There are three main groups under environment management like environment modification, environment manipulation and modification of human habitats (WHO, 1982). In open water marsh management (OWMM), shallow ditches are created in the marshes, which allows the draining of small habitats of mosquitoes into large ponds or canals. In these canals or ponds, fish feed on these larvae. As a result, the adult mosquitoes are reduced. Another strategy is rotational impoundment management (RIM). In this strategy, large pumps and culverts with gates are used to control the level of water in the marsh. The gates allow the fish, crustaceans and other native marsh organisms to commute. This strategy is successful in Florida. Development in social, environmental and economic fields has an indirect effect on malaria transmission. In Tanzania, switching of the roofs from thatched to corrugated iron showed a reduction in indoor mosquitoes. Regular drainage of rice fields and cleaning of fish ponds showed the disappearance of malaria in Java, Indonesia.

Future trends

In the integrated vector control approach, many tools are used together in controlling malaria transmission. Chemical control is mostly used at the last stage due to its hazards and damage to the ecosystem. Chemical methods of vector control are used to bring malaria under threshold level during outbreaks. Integrated vector control approach is eco-sensitive. It includes co-ordination from several sectors including health, water, solid waste and sewage disposal, housing and agriculture (WHO2004). Integrated vector control approach along with malaria vaccines is highly effective in eliminating malaria.

Genetic manipulation

Availability of the complete genome sequences of the mosquitoes helped in the genetic manipulation. Many powerful bio-informatic tools coupled with successfully sequenced insect genomes are helping to reduce the rates of malaria transmissions. Genetic engineering and sterile insect technique are the two tested genetic manipulation techniques.

Genetic engineering

In genetic engineering technology, foreign DNA (anti-parasite macromolecules) are introduced into the mosquito genome, which makes them resistant to the parasites or decreases the lifespan of vectors. These modified vectors are called transgenic organisms. They can be produced in two ways: biological and physical. To deliver the foreign DNA into the mosquitoes, physical methods like, microinjection, biolistics, electroporation and biological methods like viruses, symbiotic micro-organisms, mobile genetic elements can be used.

Over the last decade, many tools like refractory genes, promoters, regulatory elements, and effector molecules for creating refractory mosquito, Transposable elements, such as piggyBac, minos, mariner, and hermes are developed which contributed to the progress. Micro-injection is one of the standardized technique and is helpful in modifying the genome through embryos. Modification of the Akt gene, which is involved in the signaling activities has decreased the malarial parasite invasion by increasing its resistance, and by decreasing its lifespan. Though transgenic technology is an alternative to current vector control modes, it comes with a limitation of stable germline integration of transgene. As an alternative to transgenic technology, paratransgenesis has emerged. In this technique, foreign DNA is introduced into the mosquito system through obligate endosymbiont microorganisms, and vertical transmission is possible. It is currently used in anophelines mosquitoes.

A study conducted by Beard et al. in 1998 showed that paratransgenesis technology is successful in controlling transmission of Trypanosoma cruzi by Rhodniusprolixus. Identification, isolation of new target genes can be achieved by DNA recombinant technology and RNAi in combination with genetic transformation. It is challenging to develop transgenic variants for all vectors and the limitation is its introduction into the wild population.

Sterile insect technique

Sterile insect technique is the rearing, sterilization, and release of genetically modified male mosquitoes to control the female population. Sterile insects can be produced by using chemicals, radiations, and genetic engineering technology. Few significant concerns in sterile insect technique are the placement of equipped production units, large-scale production of sterile insects, trained human resources, and sexing of mosquitoes. Molecular fluorescent markers are needed for sorting the mosquitoes at their early stages. In this technique, the primary objective is to identify genes that kill the mosquito sex-dependently. Pilot studies with sterile mosquitoes are going on in Sudan with the help from International Atomic Energy Agency.

There are few other genetic control techniques like cytoplasmic incompatibility, incompatibility due to chromosomal factors, chromosome translocations, conditional lethal, meiotic drive compound chromosome, etc. But these techniques are not widely tested. While using any of the new technologies, risk assessment and environment need should be taken care. Due to the development of transgenic technology, one of the concerns which arise is whether the genetically modified insects are eco-friendly or not. If they turned out to be harmful, methods to control the deleterious effects should be known on beforehand. Malaria vectors are region specific, so the technology used should be according to the local conditions. The technology used for sorting mosquitoes based on sex is costly, so the current research is going on to find a cost-effective and technologically less demanding solution.

Few success stories

Integrated malaria control program in Tanzania had shown an increased acceptance of the malaria control program in the community by using polystyrene beads (EPBS). In India, use of indoor residual sprays and larvicidal fish had shown a reduction in mosquitoes. In Indonesia, a disappearance of mosquitoes was achieved by the periodic draining of rice fields and cleaning of fish ponds. Effective malaria control can be achieved by using several tools simultaneously or by complementing each other. In China, 99% off malaria was reduced by using IRS, locally produced high-quality drugs, social mobilization, mosquito nets, and artemisinin-based traditional medicine. Vietnam mortality rate due t malaria was reduced by 97% by following similar techniques like China. Significant 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, and Libya using SIT program. The SIT was also used successfully in controlling Tsetse fly (Glossinaspp.) and Medfly (Mediterranean fly-Ceratitis capitata).

Conclusion

Indoor residual sprays (IRS) and long-lasting insecticide-treated nets (LNs) are most effective till now in vector control. Currently, long-lasting insecticide-treated materials like sheets, curtains, and wall linings are useful in reducing malaria transmission. Other methods like non-chemical methods and environmental management also shown good results. The main problem with the vector control through the above methods is the development of resistance by mosquitoes. Various other factors like the behavior of mosquitoes, locations should also be considered. All the vector control measures should be species specific. Local community involvement helped in the success of many malaria vector control programs.

Recently, sterile mosquitoes and genetic manipulation of the mosquitoes is gaining more importance but needs some ethical considerations. China, India, Vietnam, Brazil, Eritrea are successful in controlling malaria transmission by using vector control tools. Instead of putting so many efforts in discovering new solutions to malaria control, it is better to depend on the existing strategies. If present strategies cause failure, then it is necessary to identify the causes for it. Some of the causes of failure are lack of education, proper coordination between the policy makers and various organizations and institutions, the apathy of community and health workers, and associated technical factors, such as the development of insecticide resistance in vector and drug resistance in the parasite. Even though genetic modification technology is available, it is hard to implement on large scale. However, sterile insect treatment has shown a success. Each technique has both advantages and disadvantages. No single technique is 100% effective in vector control. So, to eliminate malaria, it is necessary to implement integrated vector management, which is eco-friendly.