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Degradation of insecticides used for indoor spraying in malaria control and possible solutions

Mthokozisi M Sibanda1, Walter W Focke1*, Frederick JWJ Labuschagne1, Lumbidzani Moyo1, Nontete S Nhlapo1, Arjun Maity2, Herminio Muiambo13, Pedro Massinga13, Nico AS Crowther4, Maureen Coetzee5 and Gordon WA Brindley1

Author Affiliations

1 Institute of Applied Materials, Departments of Chemistry and Chemical Engineering, University of Pretoria, Lynwood Road, Pretoria 0002, South Africa

2 Polymers and Composites, MSM - CSIR, 1 Meiring Naude Road, Brummeria, Pretoria 0001, South Africa

3 Department of Chemistry, Eduardo Mondlane University, P.O. Box 257, Maputo, Mozambique

4 Department of Statistics, University of Pretoria, Lynwood Road, Pretoria 0002, South Africa

5 Malaria Entomology Research Unit, School of Pathology, Faculty of Health Sciences, University of the Witwatersrand and the National Institute for Communicable Diseases, 1 Modderfontein Road, Sandringham 2131, South Africa

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Malaria Journal 2011, 10:307  doi:10.1186/1475-2875-10-307

Published: 18 October 2011

Abstract

Background

The insecticide dichloro-diphenyl-trichloroethane (DDT) is widely used in indoor residual spraying (IRS) for malaria control owing to its longer residual efficacy in the field compared to other World Health Organization (WHO) alternatives. Suitable stabilization to render these alternative insecticides longer lasting could provide a less controversial and more acceptable and effective alternative insecticide formulations than DDT.

Methods

This study sought to investigate the reasons behind the often reported longer lasting behaviour of DDT by exposing all the WHO approved insecticides to high temperature, high humidity and ultra-violet light. Interactions between the insecticides and some mineral powders in the presence of an aqueous medium were also tested. Simple insecticidal paints were made using slurries of these mineral powders whilst some insecticides were dispersed into a conventional acrylic paint binder. These formulations were then spray painted on neat and manure coated mud plaques, representative of the material typically used in rural mud houses, at twice the upper limit of the WHO recommended dosage range. DDT was applied directly onto mud plaques at four times the WHO recommended concentration and on manure plaques at twice WHO recommended concentration. All plaques were subjected to accelerated ageing conditions of 40°C and a relative humidity of 90%.

Results

The pyrethroids insecticides outperformed the carbamates and DDT in the accelerated ageing tests. Thus UV exposure, high temperature oxidation and high humidity per se were ruled out as the main causes of failure of the alternative insecticides. Gas chromatography (GC) spectrograms showed that phosphogypsum stabilised the insecticides the most against alkaline degradation (i.e., hydrolysis). Bioassay testing showed that the period of efficacy of some of these formulations was comparable to that of DDT when sprayed on mud surfaces or cattle manure coated surfaces.

Conclusions

Bioassay experiments indicated that incorporating insecticides into a conventional paint binder or adsorbing them onto phosphogypsum can provide for extended effective life spans that compare favourably with DDT's performance under accelerated ageing conditions. Best results were obtained with propoxur in standard acrylic emulsion paint. Similarly, insecticides adsorbed on phosphogypsum and sprayed on cattle manure coated surfaces provided superior lifespans compared with DDT sprayed directly on a similar surface.

Keywords:
Indoor residual spray; DDT; pyrethroid; carbamate; stabilization