Indigenous knowledge and plant species used as mosquito repellents in the West Nile Subregion, Uganda | Tropical Medicine and Health

Documented plants with mosquito repellent properties

Although several plant species with insect-repelling properties exist in the tropics [21], they have mostly remained unexplored, with only a few studies documenting the medicinal plant species used as repellents in Africa in general and in Uganda specifically. Limited documentation has prevented most of these plant species from being scientifically evaluated for their effectiveness.

Compared with our study, where 42 plant species were documented as being used traditionally for repelling mosquitoes in the West Nile Subregion, Uganda, Pavela and Benelli [31] conducted a review of plants used to repel mosquitoes across Africa and reported that natives in Africa (Ethiopia, Kenya, Nigeria, South Africa, and Tanzania) traditionally used 64 plants from 30 families. In addition, six plant species specifically used for repelling mosquitoes were previously documented in Cegere, northern Uganda [32]. Similarly, Shibeshi et al. [20] documented 19 plant species used by the local community as mosquito repellents in Seweyna district, Ethiopia, whereas Mavundza et al. [33] documented 23 plant species used in the uMkhanyakude district of KwaZulu-Natal province, South Africa. Furthermore, Pålsson and Jaenson [34] documented eight plant species used for repelling mosquitoes in the Oio region of Guinea-Bissau. Seyoum et al. [35] reported eight plant species from Kenya, and Havyarimana et al. [36] reported 12 from Burundi. Kweka et al. [37] reported essential oils from two plant species, Ocimum suave and Ocimum kilimandscharicum, from Northeast Tanzania, which are used as mosquito repellents.

The most commonly used plants for repelling mosquitoes in our study were Azadirachta indica A., Boswellia papyrifera (Caill.), Aeschynomene americana L., Mesosphaerum suaveolens (L.) Kuntze (Syn: Hyptis suaeolens), and Ocimum gratissimum L. These plants contain phytochemicals that exhibit mosquito-repelling properties. For example, two different Boswellia species, Boswellia microphylla and Boswellia neglecta, are among the most commonly used mosquito repellent plants in Ethiopia [20]. The same Boswellia species documented in our study (Boswellia papyrifera) was also used by the Oromo people in Ethiopia [38]. Another interesting overlap was the use of a different species of Ocimum (Ocimum ellenbeckii) and two different Acacia species (now Vachellia), Acacia mellifera Benth and Acacia bussei Harms.ex. Joste in Ethiopia, whereas V. seyal and V. siberiana were documented in this study.

In Kenya, other than L. camara, the other commonly documented mosquito repellent medicinal plant species (Ocimum americanum L., Tagetes minuta, A. indica, and H. suaveolens) [35] or members of the same genus (O. americanum) were also used as mosquito repellents in this study. In addition, M. azedarach as well as Balanites maughamii Sprague and Balanites aegyptiaca, were documented in South Africa as mosquito repellents [33], and M. azedarach was used among the Oromo people in Ethiopia [38]. The similarity in usage across countries supports the ethnobotanical use of these medicinal plants for mosquito repellency by the West Nile community in Uganda.

Similarly, in Guinea-Bissau, West Africa, Pålsson and Jaenson [34] documented some of the most commonly used mosquito repellent plant species, such as Hyptis suaveolens Syn: Mesosphaerum suaveolens, Daniellia olieri, Elaeis guineensis Jacq., Parkia biglobosa (Jacq. Benth.), Azadirachta indica, Eucalyptus sp., Ocimum canum Sims, and Senna occidentalis (L.) Link. Interestingly, except for Parkia biglobosa, all the other identified plant species or their genera are also frequently used in our study area as mosquito repellents (O. canum and S. occidentalis).

Methods of preparation and drug administration

The methods of preparation and/or use of the mosquito repellent plant species recorded in this study are similar to those reported in previous studies. The most typical traditional method of preparation and administration for plants is the burning of the materials to produce smoke indoors. Similarly, in northern Uganda, the same practice was reported, with many plant species used as mosquito repellents. For example, the peels of Musa spp., Ocimum forsskaolii, and Manihot esculenta are sun-dried and burned inside a house to produce smoke that repels mosquitoes [32]. The same practice was also reported in Guinea-Bissau [34] and in Ethiopia among the Oromo ethnic group, where, for example, C. citratus is prepared by smoking [38]. The widespread use of the traditional method, which involves burning plant parts to smoke the houses to repel mosquitoes, could be linked to the volatile nature of the essential oils found in the various parts of the plants used. As the parts burn, the essential oils evaporate into the smoke and come into contact with the mosquitoes flying in the surrounding area, killing or repelling them. Moreover, some of the plants documented in this study, such as C. citratus, were freshly prepared. In Guinea-Bissau, West Africa, mosquito repellents are also used fresh, probably because of the volatile phytochemicals they contain [34]. In contrast, both C. citratus and M. suaveolens were also planted around homesteads or houses to repel mosquitoes [34].

Remarkably, the most popular methods that do not involve directly using plants to repel mosquitoes include burning either dry cow dung or dry goat droppings to produce smoke. This is because smoke from cow dung contains mixtures of chemicals that irritate or block the sensory receptors of the mosquitoes [39]. This practice has also been reported in India [40] and parts of Ethiopia [41].

Efficacy of the plants used

Some of the plant species used as repellents are aromatic in nature, meaning that they release essential oils. These natural mosquito repellents could help address the high levels of insecticide resistance [19]. Plant-derived essential oils and extracts can be used in the formulation of environmentally friendly repellents. Plant-based oils are often effective alternatives to synthetic repellents because they are generally safe, cost effective, and widely available in many regions worldwide [19]. Several studies have shown the effectiveness of essential oils from various plants as mosquito repellents. For example, oils from citronella, clove, eucalyptus, geranium, lavender, peppermint, and other species have been tested for repellency in the laboratory against Aedes aegypti [42] and other species, confirming their efficacy [21]. However, Maia et al. [43] concluded in a Cochrane review that there is insufficient evidence to determine whether topical or spatial repellents can prevent malaria.