Chemical composition, toxicity and mosquito repellency of Ocimum selloi oil

Ocimum spp. (Lamiaceae) and their essential oils have been traditionally used to kill or repel insects, and also to flavor foods and oral products, in fragrances, in folk medicine and as condiments. In Brazil, Ocimum selloi has been used to treat stomachaches and as an anti-inflammatory remedy. This study was performed to provide data on the chemical composition, acute toxicity, mutagenicity, skin irritant potential and mosquito repellency of Ocimum selloi oil. GC/MS analysis of Ocimum selloi oil revealed that its major constituents were methyl-chavicol or estragole (55.3%), trans-anethole (34.2%), cis-anethole (3.9%) and caryophyllene (2.1%). Ocimum selloi oil given by gavage to adult Swiss Webster mice produced no adverse effects at doses as high as 1250 mg/kg body weight. Deaths and symptoms (e.g. hypoactivity, ataxia and lethargy) were observed at doses ≥1500 mg/kg body weight, being females apparently more susceptible than males. Genotoxicity of Ocimum selloi oil was evaluated in the Salmonella/microsome assay without and with S9 mixture. The oil, tested up to the toxicity limit (500–700 μg/plate), was not mutagenic to tester strains TA97a, TA98 and TA100. None of 30 volunteers of either sex exposed to undiluted Ocimum selloi oil (4-h patch test) showed a positive skin irritant reaction. A field test (six volunteers, each individual his/her own control) was carried out to evaluate mosquito (Anopheles braziliensis) repellency of Ocimum selloi oil diluted in ethanol (10% v/v). The median number of mosquito bites on volunteers’ skin-recorded for 30 min after application of Ocimum selloi oil (2, range 0–3) was much lower than that noted after application of the solvent alone (19.5, 3–25) (Wilcoxon test, P<0.01). In conclusion, results showed that Ocimum selloi oil is an effective mosquito repellent that presents a low acute toxicity, poses no mutagenic risk and seems not to be irritating to human skin.