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Bio-efficacy of plant secondary metabolites against mosquito vectors

Public health is of primary importance globally since it contributes to the socio-economic progress of any country. The World Health Organization (WHO) defined health as “a state of complete physical, mental and social well-being”. Diseases are one of the important factors which deteriorate good health, among which vector borne diseases contributes to more than 17% of the total communicable diseases, taking millions of lives annually and bringing more than two third of the world population in the risk zone. These vector borne diseases still exist as a major threat to public health in almost all tropical, sub-tropical countries and many other parts of the world particularly in underdeveloped countries (WHO, 2017). Organisms belonging to phylum Arthropoda serve as perilous carriers for many human and animal parasites viz. protozoans & helminthes and pathogens viz. viruses & bacteria which are responsible for causing mild to severe deadliest diseases.

Among arthropods, mosquitoes, belonging to family Culicidae of order Diptera, carry out the role of being crucial vector for the transmission of various life threatening diseases such as malaria, filariasis, dengue fever, chikungunya, West Nile viral fever, Eastern equine encephalitis, Japanese encephalitis etc. and thus remain a major global economic burden (Benelli, 2016). Besides spreading the diseases, mosquitoes also creates lot of nuisance by biting and sucking blood, and causes hypersensitive or allergic type of reactions on human skin producing symptoms like redness and edema (Peng et al., 1999). With increasing population, human exposure to unhygienic conditions is also increasing. These unhygienic environments, along with the sudden changing climatic conditions arising due to global warming, serve as favourable habitat for the proliferation of mosquitoes which in turn facilitates the transmission of endoparasites within the human population leading to disease outbreaks (Das et al., 2012; Wink, 2012).

Malaria and lymphatic filariasis are two important human parasitic diseases spread by different species of mosquitoes. Malaria, caused by protozoan parasite Plasmodium and spread mainly by Anopheles mosquito, adversely affects millions of people in Africa, Asia and the Americas and more dominantly in underdeveloped poorer countries (WHO, 2018b). Malaria more significantly hampers the growth and development of children and laborers and thus hinders the socioeconomic development of relative countries (Bucker et al., 2013). Lymphatic filariasis or elephantiasis is caused by nematode parasites which are transmitted mainly by mosquito vector Culex. It severely troubles over 120 million people inhabiting around 83 endemic countries (Bockarie et al., 2009) and as per the WHO (2018) report, more than 850 million people are currently at risk of infection. Lymphatic filariasis is considered as one of the neglected tropical diseases (NTDs) for being generally not a fatal disease despite producing severe morbidity or disfigurement. Similarly, dengue, an arboviral disease transmitted through Aedes mosquitoes has become an important matter of concern to public health with millions of infections annually (WHO, 2009). Over the last few decades dengue infections has increased many fold globally in urban and semi urban tropical zones (WHO, 2012) which may be attributed to increasing mosquito population.

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Looking at the devastating number of incidences of mosquito borne diseases, vector control becomes the foremost tool for prevention (Benelli, 2015a). Methods interrupting the disease transmission cycle of parasites or pathogens between mosquitoes and human provide a strong base to tackle the vector borne diseases. By targeting either the adult mosquitoes or the larval forms, the transmission cycle of disease can be broken (Joseph et al., 2003). By far, implementation of synthetic insecticides such as DDT, organochlorines, organophosphates, carbamates etc. and usage of insecticide treated nets (ITNs) has been the major tool for controlling the mosquitoes (Ghosh et al., 2012). But, consistent and indiscriminate application of these insecticides developed widespread multiple insecticide resistance among mosquitoes (Pani et al., 2015). In addition, they also generated striking and permanent toxic effects on the environment and non target organisms (Seetharaman et al., 2017). These drawbacks have reinforced the researchers to explore the alternative least toxic and more eco-friendly methods to control the mosquitoes and ultimately the diseases.

In this scenario, biological control of mosquitoes, such as, using aquatic larval predators, plant derived products, microbe derived products, endophyte derived products, has gained much attention towards itself (Singh and Prakash, 2009; Vivekanandhan et al., 2018). The natural products or bio-active compounds derived from living organisms offer several benefits over the synthetic insecticides such as target specificity, more efficiency, less effects on non target organisms, least environment toxicity due to their bio-degradable nature (Adeyemi, 2010). These features have led researchers to carry out a large number of investigations in this area. Currently, utilization of plant derived products or plant extracts is one of the most promising approaches for controlling mosquitoes (Dubey, 2011). This approach is actually not new. Conventionally plants or their parts have been used for repelling insects in various forms such as, fumigating by burning plants or their parts, hanging bruised plant parts, sprinkling either the plant extracts or leaves directly etc. (Karunamoorthi et al., 2009). But, discovery of DDT in 1939 sidetracked traditional practices. Later, due to the issues created by the synthetic insecticides, exploration of floral biodiversity became center of attraction in search for safer bio-insecticides (Ismaan, 1997).

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Plants produce a large number of phytochemicals that are mainly secondary metabolites. Initially categorized as biologically insignificant, these secondary metabolites or bioactive compounds have grown keen interest among the ecologists and pharmacologists since few decades (Wallace, 2004). In addition to being the key components of defense mechanisms against pests, pathogen and the herbivores, these compounds are known to possess several other biological functions such as to attract pollinators, to protect themselves from stress, as allelopathic agents (Ahmed et al.”,2017). Secondary metabolites show diverse and significant pharmacological activities such as anti-oxidative, anti-allergic, antibiotic, hypoglycemic, anti-parasitic, anti-fungal, anti-inflammatory, herbicidal, and insecticidal (Bamola et al., 2018).

Many research studies have been done to explore the efficacy of these compounds against the mosquitoes. Nikkon et al. (2011) investigated the mosquitocidal potential of Tagetes erecta and reported that chloroform extract of T. erecta flowers had strong Culex quinquefasciatus larvicidal potential. Similarly, Ramkumar et al. (2016) also studied the larvicidal and adulticidal effects of leaf extracts of Glycosmis pentaphylla against Anopheles stephensi, Cx. quinquefasciatus and Aedes aegypti. Shaalan et al. (2005) reviewed the various aspects of plant extracts on the mosquitoes such as extraction and fractionation effects, effects on growth and reproduction, mutualistic effects of constituents of extracts, effects on non target organisms and some other promising phytochemical research approaches. The plant extracts and the essential oils derived from plants are reported to be potent against the mosquito vectors by acting as ovicidal, larvicidal, pupicidal, adulticidal, growth and reproduction inhibitor, oviposition deterrent, and/or adult repellents (Hag et al., 1999; park et al., 2005; Evergetis et al., 2009; Kalaivani et al., 2012; Giatropoulos et al., 2013; Benelli, 2015b; Pavela and Benelli, 2016).

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The extracts or essential oils derived from plants contain a mixture of compounds which may act on the mosquitoes in a mutualistic manner and so development of resistance among the mosquitoes is unlikely. Since plants contains numerous and wide range of secondary metabolites like alkaloids, flavonoids, terpenoids, coumarins, tannins, alkenes, alkynes, fatty acids, phenols and lignans, a large number of these active compounds may be untapped. So there is always a possibility for a new phytochemical to be explored, which in future may emerge as a suitable potential and safer alternative mosquitocide.

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