Jounr.rel oF THE AuBRrclN Mosqurro CoNtnor, AssocrerroN V o L . 5 , N o . I

LABORATORY EVALUATION OF CONTROLLED-RELEASEREPELLENT FORMULATIONS ON HUMAN VOT,TITNERS UNDER

THREE CLIMATIC REGIMENS1P

RAJ K. GUPTA AND L. C. RUTLEDGE

Letterrnan Army Institute of Research, presi.dio of san Francisco, cA 94129-6g00

ABSTRACT. Six controlled-release personal-use topical insect/arthropod repellent formulations ofdiethylmethvlbenzamide (deet) were evaiuated in an environmentafchambiron "irrr"iu"* lo, ,"p.ii"r"vagainst the mosquitoes Aedes .aegypti and Ae. taeniarhyrrchu.s ;;a;r;;;-.hmatic re!ime;'.;--t;;;;variable high humidity- (tropical environment), basic consiant high humidiJy (forested and wet environ-ment) and basic hot (hot-dry environment)..The best protectio*n under ali the climatic ."gi-;;;;;;provided bv the Biotek formulation. In a tropic-al environment, ro-" for-ntaiions induced iior" llti"gfrom mosquitoes than the concurrent untreaied control in the iate h;;-;ilh; testing. n"p"iter,"y *u!not directly related to the deet concentration in the various controlled-release repelleni ror-uttiorrs. -_

INTRODUCTION

Inorganic and botanical materials such asalum, citronella and pennyroyal have been usedas insect repellents since ancient times. Al-though these materials are highly effective, theylast for only a short time on the skin. The firstpractical synthetic repellent was developed bythe Standard Oil Development Company in1929. This material, dimethyl phthalate, is ef-fective for about two hours, depending upon theamount applied, number and kinds of insectspresent, time of day, weather, activity of theuser and other factors. The next major advancein repellent technology was the discovery ofdiethylmethylbenzamide (also known as dieth-yltoluamide, "deet") by the U.S. Department ofAgriculture in 1954. Deet is still regarded as thebest broad spectrum repellent known, but itsshort protection/repellency time has been rec-ognized as a deficiency for many years.

In recent years the insect repellent programat Letterman Army Institute of Research(LAIR) has been directed towards the exploita-tion of modern sustained-release technology(Reifenrath and Rutledge 1983, Mehr et al.1985) to develop a controlled-release insect re-pellent formulation for topical use that will pro-vide extended protection against biting arthro-pods, be safe and pleasant to use and be com-patible with plastics, synthetic fabrics andsimilar materials. The sustained-release tech-nology is already widely employed in the for-

t Opinions and assertions contained herein are theprivate views of the authors and are not to be con-strued as official or as reflecting the views of the U.S.Department of the Army. Use of trade names does notconstitute an official endorsement or approval of theproducts mentioned.

2 Human subjects participating in this study gavefree and informed voluntarv consent.

mulation of drugs, fertilizers, pesticides, per-fumes, toiletries and other products.

Basic research at LAIR in the late 1920s andearly 1980s established the physical parametersand theoretical framework which demonstratedthe feasibility of polymer and microcapsulemechanisms to release deet at a predeterminedrate. The formulations tested in those earlvstudies utilized microcapsule and polymer sys-tems designed to provide continuous long-termrelease of the active ingredient, including micro-capsules and microparticles, and film-formingpolymers. In microcapsule formulations, the ac-tive ingredient is contained in tiny capsulesproduced by coacervation, interfacial polymeri-zation, extrusion and other processes, The re-Iease rate is determined by the size and numberof the microcapsules, the composition and thick-ness of the microcapsule walls, the concentra-tion and properties of the excipient, and otheradditives used. These formulations may alsocontain free active repellent in addition to thatcontained in the microcapsules. In polymer sys-tems, the active ingredient is formulated with apolymer that will form a thin film over the skin.This film acts as a reservoir for the active ingre-dient and slows its absorption and evaporation.In microparticulate controlled-release systems,the active ingredient is absorbed on the surfaceof microparticles and released slowly over time.

In the present study (phase I), we evaluatedsix prototype Extended Duration Topical In-sect/Arthropod Repellent (EDTIAR) formula-tions in an environmental chamber for effec-tiveness against laboratory reared mosquitoesunder three different climatic regimens: basicvariable high humidity (tropical environment),basic constant high humidity (forested, rainyand wet environment) and basic hot (hot-dryenvironment). The results obtained from thisstudy were used in conjunction with other data,including cosmetic evaluation and field testsagainst natural populations, to select two can-

JounNlr, oF THE AvnnrclN Moseurro CoNrnoL Assocrerron VoL.5 , No. 1

EASIC CONSIANT HIGH HUIAIDIIY

E (HOURS)

Fig. 2. Performance of repellent formulations inforested/wet environment against Aedes aegypti.

10 hr and then declined again. These increasesmay be attributed to the fluctuations in therepellent release rates at and after that time.The analysis of variance indicated that the ED-TIAR formulations provided significantly dif-ferent repellencies. The interactions betweenEDTIAR formulation and time (interval afterapplication) was significant at 57o level of con-fidence, implying that EDTIAR formulationsprovided significantly different repellenciesafter different time intervals.

Figure 3 shows the results from tests againstAe. aegyptiunder the basic variable high humid-ity environment. All except 3M and Javelinformulations provided l00Vo protection frombites for 6 hr. The Biotek formulation providedthe best protection under this climatic regimen.The analysis of variance indicated that therewas significant difference in the duration ofprotection provided by various EDTIAR for-mulations. Bend, SRI and 3M repellent formu-Iations were attractant to mosquitoes at 12 hr.This effect occurred only in the 12th hr of test-ing when the repellent residues were presumablylowest and only under this climatic regimen.The attractancy of such residues has also beenobserved in other studies (Gupta et al. 1987,Mehr et al. 1985, Potapov et al. 1977). Thiseffect may be attributed to the greater perspir-ation induced by the increased temperature andhumidity to which the volunteers were exposedin the Iast 6 hr of testing, which reduced theresidue ofthe repellent on the skin to a level atwhich attraction occurs.

In the hot-dry environment, all the EDTIARformulations except javelin provided 95% pro-tection or better for 4 hr against Ae. aegypti asshown in Fig. 4. There were statistically signif-icant differences among the EDTIAR formula-tions and among the time intervals after appli-cation. The interaction between the EDTIARformulations and the test days was also signifi-

cant, implying that the protection provided bythe repellent formulations was different on thedifferent days of testing.

Against Ae. taeniorhynchus, alI the EDTIARformulations provided 95% or better protectionfor 6 or more hr except that the protectionprovided by the Javelin formulation was lowerat 4 hr. The protection under the three climaticregimens is summarized in Table 1. Under thebasic variable high humidity climatic regimen,Ae. taeniorlqtnchus were attracted by the 3Mand SRI formulations at 10 hr but were repelledagain at 12 hr.

Attempts to extend the persistence of repel-lents with natural polymers such as shellac andgum tragacanth and powders such as zinc oxide,talcum powder, bentonite, and china clay weremade as early as 1947 by Christophers. In 1970,

BASIC VARIABIE HIGH HU'IAIDIIY

o 2 1 6 8 t 0 t 2

Ttr E (HOURS)

Fig. 3. Performance of repellent formulations intropical environment against Aedes aegypti.

BASIC HOT

o 2 1 6 8 l O t 2

ilME (HOURS)

Fig. 4. Performance of repellent formulations inhot-dry environment against Aedes aegypti.

Markina et al. associated the loss of repellentby evaporation directly to physical exertion andthe presence or absence of film-forming mate-rials in the formulation. A year later, Dremovaet al. (1971) reported increased persistence and

Iz

A

4

zu4

a

uz

c4

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I

4

a-t HElcoxqF----.€ rENoo----a lDrflo-----o .lavEtNt s - 3 it- - _r 5ll

MARCH 1989 CoNrnor,lnt-Rnl.elsn REPELLENTS

Table 1. Effectiveness of EDTIAR formulations against Aedes taeniorhynchus in various climatic regimens.

Hrs of 95% or better Protection

55

EDTIARformulations

Forested/wetenvironment

Tropicalenvironment

Hot-dryenvironment

10688

10L2

86

L21068

86

10l2b12t2

BendSRIHerconJavelin3MBiotek

'50% protection at 10 hr.b 83% protection at 4 hr.

less loss of repellent with the fat-based creamscontaining such film-forming agents as ethylcellulose and liquid silicone. In 1983, Reifenrathand Rutledge evaluated several film-formingformulations containing silicone and acrylatepolymers and observed significant improvementin duration of protection. Mehr et al. (1985) alsodemonstrated that the protection period of deetcan be effectively extended through controlled-release techniques.

In the present study, all of the six EDTIARformulations provided extended repellencyagainst mosquitoes as compared to simple deetformulation in ethanol which provided repel-lency from 2 to 4 hr (Buescher et al. 1983). Thisstudy also showed that the repellency was notdirectly related to the deet concentration in thevarious EDTIAR formulations, thus supportingthe report of Smith (1970) that several con-trolled-release formulations containing less re-pellent were as persistent as the higher strengthrepellent.

Until recently, the controlled-release technol-ogy had not been applied to the problem ofpreventing arthropod/insect-borne diseases byincreasing the effectiveness and persistence ofthe arthropod/insect repellents. This technologymay play an important role in future sincean increasing number of arthropods/insects arebecoming resistant to pesticides. The presentrepellent formulations, as a form of personalprotection, are an inexpensive and practicalmeans of reducing the biting activity and pre-venting arthropod-borne disease transmission.These controlled-release formulations providelonger protection and, according to other datanot included in this report, are compatible withmodern-day plastics and are easily accepted bypeople as compared to the full/higher strengthrepellents. EDTIAR formulations may be anexcelient alternative to present-day chemicalvector control strategies.

ACKNOWLEDGMENT

The authors gratefully acknowledge COL J.F. Reinert for the critical review of this manu-

script and his support for the Repellent Re-search Program at the Letterman Army Insti-tute of Research.

REFERENCES CITED

Abbott, W. S. 1925. A method of computing the effec-tiveness ofan insecticide. J. Econ. Entomol. 18:265-267.

Buescher, M. D., L. C. Rutledge, R. A. Wirtz and J.H. Nelson. 1983. The dose-persistance relationshipof deet against Aedes aegypti. Mosq. News 43:364-366.

Christophers, S. R. 1947. Mosquito repellents, being areport of the work of the mosquito repellent inquiry'Cambridge 1943-5. J. Hyg. 45:17 6-231'

Dixon, W. J., M. B. Brown, L. Engleman, J. W. Frane,M. A. Hill, R. L Jennrich and J. D. Toporek. 1983.BMDP statistical software. University of CaliforniaPress, Berkeley, 733 pp.

Dremova. V. P., V. V. Markina and N. A. Kamennov.19?1. How evaporation and absorption rates affectthe formulation of various insect repellents. Int.Pest Control 13:13-16.

Gupta, R. K., A. W. Sweeney, L. C. Rutledge, R. D.Cooper, S. P. Frances and D. R. Westrom. 1987.Effectiveness of controlled-release personal-use ar-thropod repellents and permethrin impregnatedclothing in the field. J. Am. Mosq. Control Assoc.3:556-560.

Markina. V. V.. V. P. Dremova and N. A. Kamennov.19?0. Dynamics of removal of diethyltoluamidefrom treated skin, Maslo.- Zhir. Prom. 36:30-32 (InRussian).

Mehr, Z. A., L. C. Rutledge, E. L. Morales, V. E.Meixsell and D. W. Korte. 1985. Laboratory evalu-ation of controlled-release insect repellent formu-lations. J. Am. Mosq. Control Assoc. 1:143-147.

Potapov, A. A., V. V. Vladimirova and G. V. Kashaeva.1977. Studies of the properties in insect attractingand repelling preparations. Med. Parazitol. Parszit.Bolezni. 46:331-335 (In Russian).

Reifenrath, W. G. and L. C. Rutledge. 1983. Evalua-tion of mosquito repellent formulations. J. Pharm.Sci. 72:169-173.

Smith, C. N. 1970. Repellents for anopheline mosqui-toes. Misc. Publ. Entomol. Soc. Am. 7(1):99-117.

Standard methods for laboratory testing of non-com-mercial mosquito repellent formulations on the skin.1983. Standard E951-83, Annual Book of ASTMStandards, Am. Soc. for Testing and Materials,Philadelphia, PA.