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Herbal insecticides as ‘cure’ to resistant mosquitoes

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Garlic, ginger and turmeric as natural options against resistant mosquitoes CREDIT: https://www.dreamstime.com

*First genetically modified mosquitoes released in U.S.
*Avocados offer route to better leukaemia treatment
*Intermittent fasting reduces hypertension, says study

Before now, researchers had validated local herbs such as scent leaf, neem tree, Lantana camara, cloves, peels of citrus fruits, bush tea, thyme, lemon grass, and eucalyptus that could be effectively used in mosquito repellents.

To overcome growing resistance to World Health Organisation (WHO)-endorsed insecticides such as pyrethroids by vectors such as mosquitoes, scientists have advanced novel herbal-derived silver nanoparticles, garlic, ginger and turmeric.

A pyrethroid is an organic compound similar to the natural pyrethrins, which are produced by the flowers of pyrethrums. Pyrethroids are used as commercial and household insecticides.

The study titled “Biocontrol of mosquito vectors through herbal-derived silver nanoparticles: prospects and challenges” was published in the journal Environmental Science and Pollution Research.

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Mosquitoes spread several life-threatening diseases such as malaria, filariasis, dengue, Japanese encephalitis, West Nile fever, chikungunya, and yellow fever and are associated with millions of deaths every year across the world.

However, insecticides of synthetic origin are conventionally used for controlling various vector-borne diseases but they have various associated drawbacks like impact on non-targeted species, negative effects on the environment, and development of resistance in vector species by alteration of the target site.

Plant extracts, phytochemicals, and their nano-formulations can serve as ovipositional attractants, insect growth regulators, larvicides, and repellents with least effects on the environment. Such plant-derived products exhibit broad-spectrum resistance against various mosquito species and are relatively cheaper, environmentally safer, biodegradable, easily accessible, and are non-toxic to non-targeted organisms.

Ovipositional is pertaining to the laying of eggs through an ovipositor. Ovipositor is a tubular organ through which a female insect or fish deposits eggs.

The current knowledge of phytochemical sources exhibiting larvicidal activity and their variations in response to solvents used for their extraction is underlined in the study. Also, different methods such as physical, chemical, and biological for silver nanoparticle (AgNPs) synthesis, their mechanism of synthesis using plant extract, their potent larvicidal activity, and the possible mechanism by which these particles kill mosquito larvae are discussed.

In addition, constraints related to commercialisation of nano-herbal products at government and academic or research level and barriers from laboratory experiments to field trial have also been discussed. According to the study, this comprehensive information can be gainfully employed for the development of herbal larvicidal formulations and nano-pesticides against insecticide-resistant vector species in the near future.

Another study titled “Plant extracts as a source of bio-insecticide for mosquito control, review” was published in the journal International Journal of Mosquito Research by researchers from the Department of Biology Education, Federal College of Education (Technical), Potiskum Yobe State; and Department of Biological Sciences, Bayero University, Kano.

This work reviewed the adulticidal activities of some plants extracts and oils: Lemon grass (Cymbopogon citratus), Spear mint (Mentha piperita), Lemon basil (Ocimum citriodorum), Coffee senna (Senna occidentalis), Purple dead-nettle (Lamium purpurium), and Ginger (Zingiber officinale) leaves extracted with different solvents and were established as effective bio-insecticide against different genus and species of mosquitoes which can be used as an alternative means of controlling the population of mosquito vectors that would be environmentally safe and support the ecosystem.

Adulticides are insecticides used by mosquito control programmes to kill adult mosquitoes.

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According to the established results reviewed in this work, different solvent extracts of Lemon grass, Spearmint and Coffee senna were observed to possess high adulticidal activities against different species of mosquitoes.

The researchers concluded: “Therefore, we conclude that the activities of plant extract against mosquito species depend on the solvent used in extracting the phytochemicals responsible in the responses. We recommended the use of these plants extract as bio-insecticide against mosquito vectors.”

The researchers concluded: “In conclusion, various plants extracts and essential oils have been established to cause mosquito mortality due to the presence of phytochemicals in the plants. This review found that the activity of the plant extracts is relative to the type of extracting solvent used which is responsible for the bioactivity of the extract against various species of Anopheles, Aedes and Culex mosquitoes. All the reviewed plants show the bioactivity at different percentage of mortality and at different doses.”

They recommended: “We recommended these plants extract and essential oils that should be used to formulate bio-insecticide against different species of mosquito vectors for effectiveness in control and intervention measures. More plant species should be screened to identify a large number of plants that could be potentially useful in mosquito control in the attempt to avoid resistance in mosquitoes as it observed in the synthetic insecticides.”

Another study published in the journal Biomedical Sciences reviewed the efficacy of plant-based repellents against Anopheles mosquitoes.

The researchers from the School of Health Sciences, Kirinyaga University, Kutus, Kenya, concluded: “Therefore, the review showed, essential oils and extracts of some plants could be formulated for the development of eco-friendly repellents against Anopheles species. Plant oils may serve as suitable alternatives to synthetic repellents in the future as they are relatively safe, inexpensive, and are readily available in many parts of the world.”

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They noted: “Mosquitoes are usually targeted using insecticides, insect growth regulators, and microbial agents. Indoor residual spraying and insecticide-treated bed nets. However, these strategies have negative effects on human health, the environment and induce resistance in a number of species. Eco-friendly tools have been recently implemented against mosquito vectors, including plant-based insecticides. To date few studies have adopted World Health Organization (WHO) Pesticide Evaluation Scheme guidelines for repellent testing against mosquitoes.

“A total of 27 trials met the inclusion criteria. The highest repellency effect against mosquitoes was conferred by citronella, followed by Ligusticum sinense extract, pine, Dalbergia sissoo, and Rhizophora mucronata oils with 100 per cent protection for eight to 14 hours. Furthermore, essential oils from plants such as lavender, camphor, catnip, geranium, jasmine, broad-leaved eucalyptus, lemongrass, lemon-scented eucalyptus, amyris, narrow-leaved eucalyptus, carotin, cedarwood, chamomile, cinnamon oil, juniper, cajeput, soya bean, rosemary, niaouli, olive, tagetes, violet, sandalwood, litsea, galbanum, and Curcuma longa also showed greater than 90 per cent repellency within eight hours against different species of Anopheles….”

Also, after a decade of fighting for regulatory approval and public acceptance, a biotechnology firm has released genetically engineered mosquitoes into the open air in the United States for the first time. The experiment launched this week in the Florida Keys — over the objections of some local critics — tests a method for suppressing populations of wild Aedes aegypti mosquitoes, which can carry diseases such as Zika, dengue, chikungunya and yellow fever.

Oxitec, the firm based in Abingdon, United Kingdom (U.K.), that developed the mosquitoes, has previously field-tested the insects in Brazil, Panama, the Cayman Islands and Malaysia.

But until now, owing to a circuitous series of regulatory decisions and pushback from Florida residents, no genetically engineered mosquito had been trialled in the United States — even though the country previously allowed tests of a genetically engineered diamondback moth (Plutella xylostella) in New York and an engineered pink bollworm (Pectinophora gossypiella) in Arizona, both developed by Oxitec.

Researchers and technicians working on the project will release bioengineered male Aedes aegypti mosquitoes, which don’t bite, to mate with the wild female population, responsible for biting prey and transmitting disease. The genetically engineered males carry a gene that passes to their offspring and kills female progeny in early larval stages. Male offspring won’t die but instead will become carriers of the gene and pass it to future generations. As more females die, the Aedes aegypti population should dwindle.

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Also, for the first time researchers demonstrate that intermittent fasting can reduce hypertension by reshaping the gut microbiota in an animal model.

At Baylor College of Medicine, Dr. David J. Durgan and his colleagues are dedicated to better understand hypertension, in particular the emerging evidence suggesting that disruption of the gut microbiota, known as gut dysbiosis, can have adverse effects on blood pressure.

The researchers also have shown that transplanting dysbiotic gut microbiota from a hypertensive animal into a normotensive (having a healthy blood pressure) one results in the recipient developing high blood pressure.

Meanwhile, a compound in avocados may ultimately offer a route to better leukemia treatment, says a new University of Guelph study.

The compound targets an enzyme that scientists have identified for the first time as being critical to cancer cell growth, said Dr. Paul Spagnuolo, Department of Food Science.

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Published recently in the journal Blood, the study focused on acute myeloid leukemia (AML), which is the most devastating form of leukaemia. Most cases occur in people over age 65, and fewer than 10 per cent of patients survive five years after diagnosis.

Leukemia cells have higher amounts of an enzyme called VLCAD involved in their metabolism, said Spagnuolo.

“The cell relies on that pathway to survive,” he said, explaining that the compound is a likely candidate for drug therapy. “This is the first time VLCAD has been identified as a target in any cancer.”

His team screened nutraceutical compounds among numerous compounds, looking for any substance that might inhibit the enzyme. “Lo and behold, the best one was derived from avocado,” said Spagnuolo.

Earlier, his lab looked at avocatin B, a fat molecule found only in avocados, for potential use in preventing diabetes and managing obesity. Now he’s eager to see it used in leukemia patients.

Currently, about half of patients over 65 diagnosed with AML enter palliative care. Others undergo chemotherapy, but drug treatments are toxic and can end up killing patients.

Referring to earlier work using avocatin B for diabetes, Spagnuolo said, “We completed a human study with this as an oral supplement and have been able to show that appreciable amounts are fairly well tolerated.”

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