Exploring oleic acid for anopheles arabiensis mosquito larvicidal applications

Abstract

Malaria is a major economic and medical burden in countries where it is endemic. The world health organization (WHO) recommends indoor residual spraying (IRS) of insecticides and sleeping under insecticide treated nets (ITNs) to prevent malaria transmission. Larviciding is being considered as an additional control measure. Oleic acid (cis–9–octadecanoic acid), a plant-derived fatty acid has been found to be effective larvicide for Aedes and Culex mosquito species. However, it had not yet been explored against An. (anopheles) arabiensis, the principal malaria vector in South Africa. This presented the research opportunity which was pursued and reported on in this study.

Initial laboratory bioassays were performed to determine the efficacy of oleic acid on 3rd and 4th An. arabiensis instar. Statistical analysis indicated that oleic acid, as a free-standing oil, had an LC50 of 13 ppm and an LC90 of 31 ppm after 48 h exposure at 95% confidence interval. Rancimat analysis showed that both curcumin and eugenol, natural antioxidants, provided effective protection for oleic acid against oxidative degradation. Curcumin proved to be a better antioxidant than eugenol. The induction period (IP) exceeded 15 h at a dosage of 0.25 wt.%. The corresponding value for eugenol was IP ≈ 7 h at 0.75 wt.%. Attempts were made to develop dosage forms for the oleic acid. These included, two porous matrices, i.e., activated charcoal and spent-coffee biochar. The latter was prepared by pyrolysis at 500 ºC. SEM revealed a highly porous biochar with a honeycomb-like structure.

Raman analysis confirmed successful carbonization as the spectrum was similar to that recorded for the activated charcoal. Both matrices were impregnated with oleic acid in a vacuum chamber. It proved possible to impregnate spent coffee biochar with up to 60 wt.% oleic acid. In the case of activated charcoal, a loading of up to 50 wt.% proved possible.

Oleic acid was also intercalated into layered double hydroxide (LDH) by hydrothermal reconstruction of calcined LDH. Intercalation of oleate anions into Mg/Al LDH was confirmed by XRD, FTIR, and TGA analysis. TGA analysis showed a loading corresponding to 25 wt.% oleate in the LDH. The XRD diffractogram proved intercalation of the oleic acid since the d-spacing increased from 0.76 nm for pristine LDH, to 3.65 nm for the intercalate. SEM showed that the plate-like morphology of the LDH was recovered following intercalation. Oleic acid was also incorporated into thermoplastic starch by twin screw extrusion. This was a nanocomposite containing nanocellulose and nano-clay at 5 wt.% nano-clay. The inclusion of the nano-clay was necessary to incorporate the oleic acid into the thermoplastic starch during processing. Loadings of up to 10 wt.% oleic acid were achieved using this extrusion-based process. FTIR confirmed the presence of oleic acid in the extruded starch strands.

Lastly, 35 wt.% oleic acid-in-water emulsions were prepared using a modified sorbitan monooleate-based emulsifier. The average particle size was determined to be 211 ± 80 nm. The emulsion was relatively stable as the zeta potential was −56 ± 10 mV, measured after four weeks of ageing.

The utility of these dosage forms was evaluated using large glass containers filled with 3 L of water. Sufficient dosage form was added to release up to 500 ppm of oleic acid into the water. Water was sampled from the bottom of the containers and laboratory bioassays were performed on a weekly basis. Only in the case of the oleic acid-in-water emulsion was an acceptable mortality of the larvicides attained, i.e., > 90%. However, it took three weeks before this was achieved. None of the other dosage forms proved effective. Potential reasons for this failure are discussed.