Journal of the American Mosquito Control Association, l2(2):243-246, 1996 Copyright © 1996 by the American Mosquito Control Association, Inc.

EVALUATION OF A EUCALYPTUS-BASED REPELLENT AGAINST ANOPHELES SPP. IN TANZANIA

J. K. TRIGG
Department of Medical Parasitology, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT United Kingdom

ABSTRACT A eucalyptus-based insect repellent (PMD) with the principal active ingredient p-men-thanc-3,8-diol)] was evaluated in the field in comparison with deet. In human landing catches in Tanzania, 3 formulations of PMD were tested against Anopheles gambiae and An. funestus. Repellents, applied to the legs and feet at doses chosen as used in practice, gave complete protection from biting for between 6 and 7.75 h, depending upon the formulation type, with no significant difference between PMD and deet in terms of efficacy and duration of protection.

Repellents have long been used in protection against biting insects, with the main motivation being avoidance of nuisance. However, through their reduction in man-vector contact, repellents can also be regarded as important tools in the prevention of vector-borne disease.

For travellers to tropical countries, malaria continues to pose a serious health risk: in the United Kingdom in 1994 there were 1,887 im-ported cases of malaria, 11 of which resulted in death (Anonymous I994¹). The rapid spread of resistance to antimalarial drugs has increased the importance of supplementary preventative mea-sures. The World Health Organization (1995) currently advises "all travellers should be told that protection from biting mosquitoes is their first line of defence against malaria". The ar-mory of personal protection includes the use of bed nets, suitable clothing, and repellents. Since 1957, the most commonly used insect repellent formulations have contained diethyl methyl ben-zamide (deet), which is effective against a broad spectrum of insects. There are disadvantages as­sociated with the use of deet, which stem prin-cipally from its activity as a solvent of paints, varnishes, and some plastics and synthetic fab­rics. There have also been concerns over the tox­icity of deet (Miller 1982, Roland ei al. 1985), although serious adverse effects are rare (Veltri et al, 1994, Osimitz and Grothaus 1995).

In the search for effective alternatives to deet there has been much interest in natural plant ex­tracts (Opoku et al. 1986, Curtis et al. 1990). The eucalyptus-derived repellent quwenling was reported to have largely displaced the commer­cial market for di-methyl phthalate in China. (Curtis et al. 1990), although it was reported to be somewhat less effective than deet (Schreck and Leonhardt 1991, Collins et al. 1993). Qu-wenling is made from the waste distillate after extraction of oil from the lemon eucalyptus plant (Eucalyptus maculata citriodon) and the princi­pal active component is p-menthane-3,8-diol (Schreck and Leonhardt 1991).

A preliminary laboratory evaluation of a new repellent, PMD, similar to quwenling (Trigg and Hill 1996), reported this compound to be almost as effective as deet against Anopheles gambiae Giles and also to be effective against midges, ticks, and the stable fly. The repellent (trade name Mosi -guard Natural, MASTA. London, United Kingdom) is produced utilizing lemon eucalyptus oil itself, via an extraction process developed at University College, London. The active component (50%) is principally p-men-thane-3,8-diol with additional isopulegol and ci-tronellol and the repellent is formulated as a pat­ented mixture of isomers of each.

In this paper a field comparison of PMD with deet against 2 African malaria vectors, Anoph­eles fane stun Giles and An. gambiae is reported.

Study area: A field trial was undertaken in June 1995 in the village of Mkuzi, Tanga Re­gion, Tanzania. The village has mostly wattle and daub houses and suitable larval habitats for An, gambiae and An. funestus.

Repellents and application: Three formula­tions of PMD (50% Al), a pump spray, a stick, and a gel. were evaluated for efficacy and lon­gevity in comparison with a spray formulation of deet (50% Al. MASTA) against natural pop­ulations of mosquitoes. For each trial, 6 expe­rienced insect collectors participated in human night-biting catches having been offered malaria prophylaxis and treatment with sulfadoxine pyr­imethamine in the event of contracting malaria infection. On any one day, 2 subjects applied

Table 1. Total number of Anopheles gamhiae and An. funestus collected by individual subjects with repellent-treated or untreated skin and protection time until first bite in minutes (PT) over a 3-day period for each PMD formulation tested in comparison with deet spray.

The day on which cm e was iJimc is shown in deet, 2 applied PMD, and 2 remained as untreated controls. Repellents were applied as evenly as possible to the legs and feet from the knee downwards. It was decided to assess the repel­lents as used in practice and the choice of dos­age was therefore calculated on the basis of what the team members considered adequate to give even and comfortable coverage of their legs. The doses were determined by weighing the contain­ers before and after repellent application and taking the average of all applications.

Repellent was applied 5 h prior to commenc­ing the night catch after preliminary trials indi­cated that both deet and PMD gave at least 5 h repellency. In the interim period, subjects were careful to keep their clothing away from the treated skin.

Test procedure: In a method adopted from Curtis et al. (1987), subjects sat separately on benches or chairs spaced approximately 10 m apart for a period of 4 h each night from 22.00 h until 02.00 h. Using flashlights and test tubes they caught those mosquitoes that had landed on the skin and were clearly probing to feed he tore transferring them into labelled paper cups to be counted and identified later.

Subjects remained seated in the same position on 3 consecutive nights for each phase of the trial; treatments were rotated nightly. This meant that each subject experienced each treatment: a formulation of PMD, deet, and control; the ro­tation compensated for any positional differ­ences in the number of mosquitoes, and personal differences in persistence of repellent, catching ability, and/or attractiveness to mosquitoes.

The procedure outlined in the Materials and Methods section resulted in the repellents being applied at rates of 0,8 g/leg for PMD spray and deet spray, 2.0 g/leg for PMD gel, and 1.3 g/leg for PMD stick. The results of the night-biting catches are shown in Table 1. Anopheles funes-tus was the most common species biting during the study (69.3% of all mosquitoes collected), followed by An. gambiae (29.7%), and 1 % other species. When analyzing these data, two likely sources of background variation were consid­ered: 1) day to day variation in the number of mosquitoes caught by the controls, and 2) vari­ation between persons in attractiveness to mos­quitoes (Curtis ct al. 1987)—a fact apparently illustrated in this data set by person B who was consistently a high scorer regardless of treat­ment. This was confirmed by a 2-way analysis of variance, allowing for treatments, which showed a highly significant between-person variation in the number of mosquitoes caught (/■' = 10.32, F < 0.001).

To compensate for these factors, the number of bites each day on individuals treated with re­pellent were first subtracted from the mean con­trol catch on that day. Data were then analyzed using the nonparametric Wilcoxon signed rank test, pairing the corrected PMD and deet data for each person. This test was also applied to the time until first bite data (i.e., the time interval between repellent application and the first bite recorded), again pairing the deet and PMD data for each subject.

The result was a nonsignificant difference be­tween PMD and deet both in terms of efficacy (P > 0.05) and longevity (P > 0.05) of repel­lency against total anopheline biting for each of the formulations of PMD tested.

At the chosen dose rates, all repellents tested provided greater than 6 h protection from biting.

Table 1. Extended.

Although the gel formulation of PMD gave the longest mean protection time, it is likely that this was due to the application dose (chosen by the team) being the highest of all formulations at 2.0 g/leg. In contrast, where PMD spray and deet spray were compared at the same dose of 0.8 g/ leg, the average number of bites and average protection time were very similar.

Between-species differences in sensitivity to the repellents may have been expected as this has been widely documented (Rutledge et al. 1978, Robert et al. 1991). However, a species by repellents chi-square test using the PMD and deet spray formulation data only (as these were applied at the same dose of 0.8 g/leg) showed that there was no difference in sensitivity to PMD and/or deet spray between An. gambiae and An. funestus (χ² = 0.005, P = 0.94).

The present study and laboratory investiga­tions (Trigg and Hill 1996) have demonstrated that PMD is an effective repellent against ano-pheline mosquitoes. As an effective repellent of the malaria vectors An. gambiae and An. funes­tus, use of PMD can be regarded as supplemen­tary to bed nets and other measures such as screened windows in the armory of personal protection against the disease. The level and du­ration of protection by PMD is comparable to that afforded by deet. The repellent PMD has a lemon/menthol smell and, unlike deet, does not possess undesirable solvent properties. Acute toxicological studies have demonstrated minimal toxicity (oral LD₅₀ 2,408 mg/kg and dermal LD₅₀ >2,000 mg/kg in rats).

It is concluded that PMD is an effective al­ternative to deet with potential as a means of personal protection against mosquito vectors of disease.

I thank Ali Mtango, Lucy George, William Chambika, Isaya Kibwana, and Stephen Mkon-gewa of Ubwari Field Station, Muheza, Tanza­nia, for their participation in the night catches. I am grateful to Jo Morris, Chris Curtis, and Nigel Hill for statistical advice and suggestions for the manuscript.

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