PSI Researching Natural Repellent To Curb Malaria

Day-biting mosquitoes beware! PSI and the London School of Hygiene and Tropical Medicine (LSHTM) are completing the first phase of product development for a natural plant-based mosquito repellent in Bolivia, and hungry mosquitoes may have to look elsewhere for their dinner.

Insecticide-treated nets (ITNs) have proven to be highly effective in Africa, owing to the fact that the important malaria vector species are late-night feeders and most of the population can be protected by sleeping under ITNs during periods of peak biting activity. However, in other parts of the world, vector behavior is quite different.

In Asia, for example, major vectors such as Anopheles dirus, An. minimus and An. sinensis often feed before people have gone to bed. In the Bolivian Amazon the major vectors, A. darlingi and An. albitarsis, have an initial, intense peak in feeding activity in the early evening, from 6 to 10 p.m., followed by all-night, lower-intensity feeding. Studies in neighbouring countries in South America find similar feeding patterns. A means of protection is needed that can be used outdoors before people go to bed.

Malaria in Asia and the Americas cannot be controlled by available measures. The solutions that worked in the Northern Hemisphere, area spraying and breeding site drainage, are not tenable for the watery ecosystems of the Amazon and Asia. ITNS are necessary to prevent night-time malaria transmission but are not sufficient because they do not protect against early evening transmission. House spraying, mats, coils and other such devices are not useful outdoors.

PSI is operating a three-year ITM project in Bolivia funded by the U.K.'s Department of International Development, and the grant provided for the identification and development of innovative products to cover this dusk high-transmission window. To that end, PSI invited LSTHM researchers to evaluate the potential for natural, plant-based insect repellents as a means of protection against malaria vector mosquitoes. A number of natural, plant-based repellents are available, including lemon eucalyptus extract (Eucalyptus maculata citriodora) traditionally used in China, neem tree extract (Azadirachta indica) used in India, and a combination of plant extracts registered with the U.S. Environmental Protection Agency under the trademark Treo (that includes geraniol, rhodinol, terpinol, citronella and Chinese euchrist) used in the United States for several years.

The use of repellents in combination with ITNs is expected to be complementary, with the repellent affording protection during early-evening feeding and ITNs during the late-night feeding. Indeed, it may be that where the vectors feed in the early evening this is the only means of securing a high level of malaria reduction.

Last year the LSTHM/PSI team conducted field research in the northern provinces of Bolivia, an area bordering Brazil and part of the Amazon headwaters ecosystem that extends north through Peru, Columbia, Ecuador and Venezuela, and is known for mosquitoes and malaria. The project conducted phase I laboratory tests and a phase II entomological study with three candidate plant-based repellents to evaluate efficacy in preventing mosquito feeding under field conditions, with excellent results. The team then conducted a phase III epidemiological assessment to evaluate health impact using the most efficacious repellent, PMD, in conjunction with ITNs in a blind, placebo-controlled trial. The results were encouraging and further funding is being sought by LSHTM and PSI to extend the phase III evaluation through the 2002 malaria season. The project also conducted consumer and market research and initiated brand development, including designing preliminary logo prototypes and selection of a brand name.

Although not part of the original work plan, the project took the opportunity to conduct laboratory tests with the Aedes mosquito species, which transmits dengue, and entomological field tests with the sand fly vectors of leishmaniasis, an obscure but unspeakably horrible jungle disease, and also had good results.

In order for repellents to be accepted as suitable protection, it is imperative that robust evaluation takes place. This is the first such clinical study of repellents, and as such should be considered significant. It would be difficult to find a more innovative product that meets a more pressing need. The conclusion of this study could open the door to utilization of this new technology to prevent the spread of malaria over the coming decade.

—Sharon Slater