Researchers have 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. CHUKWUMA MUANYA, Assistant Editor writes.
Malaria is becoming untreatable due to growing resistance of the parasite, Plasmodium falciparum, to the drug of choice, Artemisinin Combination Therapy (ACT). Also, the vector, mosquito, female Anopheles mosquito, has become resistant to most available insecticides. However, most of the conventional insecticides are also associated with adverse health effects such as cancer and respiratory problems.
Health experts are worried. Why? According to the World Health Organisation (WHO), mosquitoes are one of the deadliest animals in the world. Their ability to carry and spread disease to humans causes millions of deaths every year.
Mosquitoes are the most deadly vectors of parasites that cause diseases such as malaria, yellow fever, Zika, dengue, chikungunya and filariasis. Zika, dengue, chikungunya, and yellow fever are all transmitted to humans by the Aedes aegypti mosquito.
In 2015 malaria alone caused 438 000 deaths. The worldwide incidence of dengue has risen 30-fold in the past 30 years, and more countries are reporting their first outbreaks of the disease. More than half of the world’s populations live in areas where this mosquito species is present.
A recent study by the Nigerian Institute for Medical Research (NIMR) Yaba, Lagos, confirmed that mosquitoes have defied available insecticides and concluded that mosquito-borne diseases are on the prowl in Nigeria.
But researchers have identified natural mosquito repellents without the side effects associated with conventional insecticides.
Scientists have formulated mosquito repellent cream using Ocimum gratissimum and Lantana camara extracts.
The study was published in Journal of Insect Science.
The researchers determined the repellent activity of creams formulated with methanol crude extract (MCE), hexane fraction (HF), and ethyl acetate fractions (EAFs) of Ocimum gratissimum and Lantana camara leaves in single and combined actions against female Aedes aegypti. Evaluation was carried out in the net cages (30 by 30 by 30 cm) containing 60 blood-starved female mosquitoes each and was assayed in the laboratory condition following World Health Organization 2009 protocol. All formulations (single and mixture) were applied at two, four, six, and eight mg/cm2 in the exposed area of human hands. Only acetone plus white soft paraffin served as negative control and odomos (12 per cent DEET) as positive control.
N,N-Diethyl-meta-toluamide, also called DEET is the most common active ingredient in insect repellents but with unpleasant side effects.
According to the study, all the formulations presented good protection against mosquito bites without any allergic reaction by the human volunteers. The repellent activity was dependent on the strength of the extracts and fractions. Among the tested formulations, the maximum protection time was observed in MCE (120 min) and EAF (150 min) of O. gratissimum; MCE:MCE (150 min) and HF:HF (120 min) mixtures of both plants. In addition, MCE:MCE and HF:HF mixtures from both plants showed possible synergistic effect.
The researchers concluded: “From the results, the combination of Ocimum gratissimum and Lantana camara to formulate natural mosquito repellent using small amount of extracts can be encouraging to be an alternative to conventional DEET.”
Commonly called scent leaf or sweet basil, Ocimum gratissimum (Lamiaceae) is an aromatic perennial herb wildly grown in Nigeria. It is known as Nchuanwa in Igbo, Effirin in Yoruba, and Dai doyatagida in Hausa. The plant is used as food spice and for the treatment of many infections. In Nigeria, the repellent activity of ointments formulated with Ocimum gratissimum oil has been reported against Aedes aegypti and Culex quinquefasciatus mosquitoes.
Lantana camara is a flowering ornamental plant belonging to family Verbenaceae. In Nigeria, Lantana camara has as local names Ewonadele in Yoruba, Kimbamahalba in Hausa, and Anya nnunu in Igbo. The plant is reported to have antibacterial, antifungal, wound healing, antimotility, antiulcerogenic, hemolytic, antihyperglycemic, antifilarial, antiinflammatory, embryo toxicity, antiurolithiatic, anticancer and antiproliferative, antimutagenic, antioxidant, and mosquitoes-controlling activities.
Another recent study published in Journal of Natural Products evaluated chemical evaluation of mosquito repellent formulation prepared from the essential oil of plants.
The researchers from the Nigeria Natural Medicine Development Agency (NNMDA), Victoria Island, Lagos; and Chemistry Department, University of Ibadan, Ibadan, Oyo State, extracted and then formulated in a complex solution the essential oil of leaves and peels of Ocimum gratissimum, Cymbopogon citratus (lemon grass), Hyptis suaveolens (bush tea), Eucalyptus globulus, Azadirachta indica (neem tree) and Citrus sinensis (orange).
Constituent analysis of the repellent formulation was conducted using Gas Chromatography (GC). A total of seven compounds were detected representing 99.92 per cent of the essential oil components of the repellent formulation. The major component present was limonene (34.91 per cent) and the other main components were salanine (4.34 per cent), citral (22.64 per cent), 1,8 – cineole (7.00 per cent), citronella (17,49 per cent), caryophyllene (11.25 per cent) and azadirachtin (2.29 per cent).
The researchers concluded: “Constituents of the essential oil in the herbal mosquito repellent formulation has potential as natural mosquito repellent and may therefore serve as an alternative to commercially available synthetic repellents.”
Azadirachta indica also known as neem is a member of the miliaceae family and a botanical cousin of mahogany. This plant is reputed to be responsible for the pesticidal, larvicidal, antifeedant or repellant action on various insects. The Azadirachtine is found in all parts of the plants but in a higher concentration in the seed. Studies have shown that neem is a natural mosquito repellant plant.
Traditionally, Azadirachta indica is used as oil cake obtained from seeds, used as fertilizer and manure. The Green twigs are used as toothbrushes and as prophylactic mouth and teeth compliance. The leaves are kept in suitcases to repel insects and to preserve woollens. Seeds yield famous magose oil of disagreeable garlic like flavor. The oil is said to be effective in treatment of leprosy and skin diseases. It is also used as a cure for management of dog disease. The leaves in poultice are used for healing of wounds. The oil from neem seed provides significant protection from various mosquitoes. Research has proven that besides azadirachitin, salanim, gedunin, azadinone, nimbin, nimbidine, nimbicidine, nimitinol, are also important liminods, which act as an excellent effect on insects and pest.
Cymbopogon citratus (Lemon grass) oil has been known as effective insect repellent; its effectiveness in mosquito repellence has also been documented.
A study aimed at testing the mosquito repellent property of Cymbopogon citratus on human volunteers and mosquitoes in captivity, to determine its suitability as natural product-based mosquito repellent was published in International Journal of Research –Granthaalayah.
The researchers concluded: “This study reports the repellent and insecticidal properties of lemon grass oil against malaria mosquitoes in the study area; and contributes to the growing literature on safe bio-pesticides for disease control. While commercial formulations should be expected, local use of this and other plants essential oil is recommendable to prevent man-vector contact in local communities where transmission rates are high but healthcare interventions are lacking.
“While this study did not consider chemical composition and characterization of C. citratus essential oil, indeed, in-depth and continuous studies in the area of health and environmental safety of bio-pesticides are of immediate necessity in the midst of current global awareness for natural products.”
A study published in Parasitology Research explored the effects of orange peel ethanol extract of Citrus sinensis (orange) on larvicidal, pupicidal, repellent and adulticidal activity against three different mosquitoes- Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus. The results suggest that the orange peel extracts of C. sinensis have the potential to be used as an ideal eco-friendly approach for the control of the vector control programmes.
Indeed, many natural repellents have been studied over the years. The use of essential oils from plants seems to be the most promising natural mosquito repellent.
According to a report by Medical Health Today, lemon eucalyptus oil is one of the most promising natural insect repellents on the market.
The active ingredient in the oil is the only natural repellent that the United States Centers for Disease Control and Prevention (CDC) recommend to prevent mosquito bites. The difference between lemon eucalyptus oil and chemicals like N, N-Diethyl-3-methylbenzamide (DEET) is how long the protection lasts.
One study found that lemon eucalyptus essential oil gives better protection than DEET when used properly. This study was carried out in an area where malaria was present.
A 30 percent application of lemon eucalyptus oil would need to be applied to the skin at least three times a day to stay effective.
Citronella has also become a common name in natural bug repellents. Citronella is found in almost every natural insect repellent around. Citronella prevents mosquito bites by acting on the receptors in a mosquito’s antennae.
One study found that concentrations of 50-100 percent citronella oil were needed to avoid mosquito bites completely. Many concentrations above 10 percent are effective as well, just not for as long.
The oil from the Syzygium aromaticum (clove) plant also makes a great natural mosquito repellent. One study found that clove oil gave protection for longer than some of the most effective essential oils.
Clove oil can repel mosquitoes for twice as long as many other essential oils.
Medical Health Today noted: “Here is an example of a simple natural mosquito repellent recipe. The use of apple cider vinegar helps to kill bacteria on the skin that the mosquitoes are attracted to. The high concentrations of essential oils repel them.
“In an eight-ounce spray bottle, combine the following; four ounces apple cider vinegar, three ounces water, 100 drops citronella essential oil, 50 drops lemon eucalyptus essential oil, and 50 drops clove oil.
“Users should shake well before applying. The mixture should be applied to a small area of skin first to test for sensitivity. Users should then apply to clothes and skin often when traveling in areas where mosquitoes live.”