4. Appropriate technology for vector control: impregnated bed nets, polystyrene beads and fly traps

144

TRAN~ACTIOW~ OFTHEROYALSOCIETY OFTROPICAL MEDKXNEANDHYGIENE (1994) 88, 144-146

Approaches to vector control: new and trusted 4. Appropriate technology for vector control: polystyrene beads and fly traps C. F. Curtis

impregnated

bed nets,

London School of Hygiene and Tropical Medicine, Keppel Street, London, ?VClE 7HT, UK

Abstract There are social, economic and entomological problems with conventional insecticidal spraying methods for vector control. There is therefore interest in alternative technologies, especially the impregnation of bed nets with pyrethroid insecticides against malaria vectors. This method is cheap, socially acceptable and effective where bed nets are already widely used and where malaria transmission is not very intense. In holoendemic areas, and where people consider bed nets unaffordable, there are still unanswered questions. Whether pyrethroid resistance will be selected in anophelines also deserves more attention than it has so far attracted. Where Culex mosquitoes breed in confined sites such as pit latrines, the application of floating layers of polystyrene beads is a long-lasting and effective control method. There is increasing evidence that house flies are important agents in the mechanical transmission of diarrhoea due to ShigeUa. Simple fly traps can be an effective way of controlling house fly populations and this deserves comprehensive testing in tropical countries. Coverage of anti-vector spraying programmes has dropped in recent years for a variety of reasons, including (i) cuts in public expenditure due to enforced programmes of economic ‘structural adjustment’, (ii) refusals of householders to allow access to spray personnel because of failure of the spraying programmes to control nuisance insects, (iii) diversion of public health insecticides on to the ‘black market’ for use in agriculture, (iv) resistance of some vector populations to DDT and/or organophosphates, and (v) outdoor resting of some vector species. Effective and sustainable alternative technologies are required which communities can apply for themselves against vector and nuisance insects with minimum input from outside (CURTIS, 1991). Bed nets and screening of rooms have long been considered useful against malaria (MANSON, 1900; ROSS, 1910) because of the late night, indoor, biting habits of many of the important vector species. However, the torn nets often found in villages are known by medical entomologists as good places to find blood-fed Anopheles mosquitoes. The improvement of the functioning of netting or bed nets bv imoregnation with reucllents. DDT, or organophosphites ‘wai tried in the ‘193Os, ‘1940s &d 197Os, but it was only with the coming into use of persistent pyrethroids, effective at low dosages and with low mammalian toxicity, that impregnation of bed nets and curtains has come into its own. At rhe International Congress of Tropical Medicine in Pattaya, Thailand in 1992, there were 21 contributions on this subject (CURTIS, 1993), which suggests that, while we are waiting for an effective vaccine or means of driving genes for malaria refractoriness through wild vector populadons, this method is likely to bc widely adopted. In China the use of impregnated bed nets has already reached the scale of millions of oeoole beine orotected and some of the most convincing hat; on its e?f&tiveness have been collected isee dara of Li Zuzi and Lu Baolin in CURTIS, 1991; CURTIS? 1992). In the warmer parts of China bed net owner&p is already very widespread and it is calculated to he SO-75% cheaper to treat the existing nets in a house with deltamethrin annuallv. with a dose of about 15 mgim*, than to spray the walls-and ceiling of the same house with DDT. Communities have been found willing to participate and to pay part of the cost of such orogrammes, in contrast to unwillingness to let urofessidnalsprav personnel into houses, as-in many other countries. Orie of the factors which has restricted net treatment in China has been shortage of deltamethrin. Thus substitution of house spraying with this pyrethroid would have been out of the question, as much larger quantities would have been required because of the much larger areas involved. Bed netting is an appropriate ma-

terial to treat because (i) it does not absorb and mask insecticides as some building materials do, and (ii) mosquitoes are attracted to bed nets by the carbon dioxide and body odour emitted by the sleepers under the nets, so they act like baited traps. This was found to be important-against the notorio&ly endophagic but exophilicvector A. dim in Hainan Island, China, where the normal summertime malaria peak was elimmated in villages in which the nets were treated with deltamethrin, whereas in control villages, in which the existing nets-were untreated but routine DDT house spraying continued, malaria incidence rose year by year (see data of Li Zuzi in CURTIS. 19911. It is to be honed that similar

imnrove-

ments &ill be iegistered in other areas with A. diruf, such as Vietnam and Thailand, but it must be admitted that, though several impregnated net trials in these countries have resulted in reductions in malaria or its consequences such as anaemia in pregnancy, the results in these countries so far have not been strikingly better than with spraying (see summary by CURTIS, 1993). In the Solomon Islands impregnated bed nets arc decisively better than DDT house spraying in lowering malaria incidence :KEKE, 1992). In Africa the most remarkable success with bed net impregnation has been in The Gambia where, in combination with chemoDrouhvlaxis, a reduction in total annual child mortality 0?6&~was ;ecorded after impregnation of bed nets with permethrin by the primary health care system (ALONSO et al., 1991). In many other African countries bed net usage is less common than in The Gambia and malaria transmission is more intense. Even under these conditions the effect of bed nets in reducing the vectorial capacity of the mosquito population may exceed 90% (MAGESA et al., 1991) and the incidence of new infections may be reduced by 7545% ~MSUYA & CURTIS. 1991; STICH et al.. in mess). The la&r protection fact&s cohpare favou;abli wiih that of the best currently available malaria vaccine (VALERO et al., 1993). However, it remains co be seen what effect impregnated bed nets can have on the morbidity and mortality due to malaria in holoendemic areas. Pavment for nets in areas with low existing net usaee will Ge a problem, but it should be emphasizeYdthat dGable nets can be nurchased in bulk for T-X$3-4 and low income urban African populations already spend considerable sums on cost-ineffective mote&on against mosquitoes such as aerosol cans of insecticide anicoils (DESFONTAINE et al., 1990; Z~suu et al., 1991). Thus, means must be sought to enable spare c&h to ‘be saved up, and the very poor subsidized, to allow net purchase. It has been possible to organize a village fund in a low income Tanzanian village to pay for the permethrin for 6 monthly re-impregnation of nets (T. Wilkes & J. My-

145 amba, unpublished observations). Despite numerous trials with impregnated bed nets, the technology has not yet been perfected and work continues, bv bioassay and in experimental huts, to compare the sustained killing power and bite-preventing effects of different nvrethroids and different fabrics (LINES et al.. 1987; GO&OL etal., 1992; CURTISet al., 1492). ’ Pyrethroid resistance should be of major concern, whether these insecticides are to be used on nets or for house spraying. Fortunately, it appearsthat indnopheles, unlike house flies, existing DDT resistance mechanisms generally do not give cross resistance to pyrethroids (MALCOLM, 1988; R. Rwegoshora, personal communication). However, larval and/or adult selection of pyrethroid resistance has been effective in A. stephensi (see reoort bv H. Laddoni & H. Townson auoted bv MALCOLM, 1988). Occupied nets attracted o&y ferna& mosquitoes, and this reduces the effective selection pressure. In Sichuan, China, where up to 2.25 million nets have been treated annually for 5 years, tests of the 2 local vector specieswith the World Health Organization recommended discriminating dose gave 100% kill (CURTIS, 1992; Chen Huailu et al., paper in preparation). However. VULULE et al. (1993) reoorted an aonreciable increase l’n the 50% lethal time’ aftkr only one-iear’s use of impregnated bed nets or curtains in individual Kenyan villages. My colleagues and I are studying the protective effect of existing resistance genes in anophelines with mosquitoes flying freely in the laboratory and a human subject under an impregnated net. We are hoping to devise mixtures which will delay evolution of resistance on the analogy of multi-drug therapy against mycobacterial diseases. The protective effect of impregnated bed nets against nuisance insects such as bedbugs may be a major factor in gaining acceptability of this technique. It is therefore important to monitor, and if possible manage, resistance in these insects aswell asin anophelines. Repellents can give supplementary protection against biting before people go to bed or after they get up. Repellents are expensive if proprietary brands are used, but if bulk-purchased deet (diethyltoluamide) is applied to ankle bands and if these are kept in a plastic bag except when needed, one can obtain 95% protection for 100h of use from one impregnation at very low cost (T. Wilkes & C. Curtis, unpublished observations). Somepeople react badly to deet (see, e.g., TENNENBEIN,1987) and there is therefore interest in finding effective substitutes. The Chinese eucalyptus-based product quwenling is nearly as effective as deet against A. gambiae on skin (COLLINSet al., 1993), but it is not very effective on anklets (T. Wilkes & C. Curtis, unpublished observations). Culex quinquefasciatus, the main urban nuisance mosquito and filariasis vector, is not easily killed bv insec&ide deposits on walls or-bed nets. Ithas been controlled with oil films on the water in oit latrines and blocked drains (BURTON, 1967). Orgaiophosphates, such as chlorpyrifos, were more long lasting and convenient to distribute than oil. However, there is now a major organophosphate resistance problem in these mosquitoes in many countries. Floating layers of expanded polystyrene beads have a similar suffocating effect on larvae and pupae as does oil, but the beads last for many years. Ail 550 pit latrines treated in 1988 in Makunduchi, Zanzibar (MAXWELL et al., 1990) still retained their beads in 1991. However, some latrines lost their beads and had to be re-treated in 1992 after exceptional flooding. It is the likelihood of flowing or floodinn of water bodies which sets limits to the applicability ofjhe polystyrene technique. In Makunduchi virtually the only breeding sites were normally stagnant pits; 98% reduction in the adult mosquito population in bedrooms was achieved by searching out and treating all the wet pits. This was integrated with a single mass treatment of the human population with diethylcarbamazine, which reduced the microfilarial rate from 49%

before treatment in 1988 to 10% immediately afterwards (MAXWELL et al., 1990). The most recent figure is 3%. suggesting that &ith sustainable vector contril prevent: ing re-infection this chronic diseasemay be dying out in the town (Maxwell et al., paper in preparation). Polystyrene beadsare also being used in water tanks in India against A. stephensi (seeCHANDRAHASet al., 1987) and will be tested against A. culicifacies in disused gempits in Sri Lanka. In most tropical situations the available mosquito breeding places include open sites such as drains or marshes: in Tanzania, control of C. quinquefasciatus in pits is being integrated with experimental use of the larvicidal bacterium Bacillus sphaericus (RAGOONANANSINGHet al., 1992) or the pupation inhibitor pyriproxyfen (SUZUKIet al., 1989; D. Chavasseet al., paper in preparation) in the open sites. Not enough attention is paid to the possibility of mechanical transmission of pathogens by insects. Anv role for insects in the transtiission of himan immun& deficiencv virus (HIV) is generallv discounted ~MIIKE. 1987). fioweverj recently BRA~DNER et al.‘ (1992j showed that Stomoxys stable flies could transmit HIV from one cell culture to another as a result of regurgitation after interrupted feeding. Until recently it was considered likely that bedbugs had a role in hepatitis B virus transmission. However, LINDSAY (1992) reported no impact of intensive bedbug control on the rate of seroconversion in Gambian villages. Diarrhoea kills considerably more children than does malaria. It has long been known that many gut pathogens can be found on houseflies. Furthermore, control of these insects (LINDSAY et al., 1953; COHENet al., 1991; LEVINE & LEVINE, 1991) has been shown to lower the incidence of diarrhoea due to Shigella. COHENet al. (1991) used fly traps, comparable in some respects with odour-baited tsetse traps (see Laveissiere, Vale & Gouteux in CURTIS, 1991) or the fly-trapping vent pipes of ventilated improved pit latrines (see P. Morgan in CURTIS,1991). There is a strong casefor a trial of intensive use of house fly traps in tropical food markets, with monitoring of Shigella incidence among the regular customers of these markets in comparison with those who buy their food at markets with uncontrolled fly populations . References Alonso, P. L., Lindsay, S. W., Armstrong, J. R. M., Conteh, M., David, P. H., Fegan, G., de Francisco, A., Hall, A. J., Shenton, F. C., Cham, K. & Greenwood, B. M. (1991). The effect of insecticide treated bednets on mortality of Gambian children. Lancer. 337.1499-1515. Brandner, G., Kloft, k. J., Schlager-Vollmer, C., Platten, E. & Neumann-Opitz, P. (1992). Preservation of HIV infectivity during uptake and regurgitation by the stable fly. AIDS Forschunp. 5,253-256. Burton;G.‘J. (1967). Observations on the habits and control of Culex pipiensfatigans in Guyana. Bulletin of the World Health Organization, 37,317-323. Chandrahas, R. K. & Sharma, V. P. (1987). Small scale field trials with polystyrene beads for the c&trol of mosquito breeding. IndianJournal ofMalariology, 24, 175-180. Cohen, D., Green, M., Block, C., Slepon, R., Ambar, R., Wassermann, S. S. & Levine, S. (1991). Reduction of transmission of shigellosis by control of houseflies. Lancer, 337, 991-997. Collins, D., Brady, J. & Curtis, C. F. (1993). Assessment of the efficacy of Quwenling as a mosquito repellent. Phytotherapy Research, 7,17-20. Curtis, C. F., editor (1991). Control of Disease Vectors in the Community. London: Wolfe. Curtis, C. F. (1992). Spraying bednets with deltamethrin in Sichuan, China. Tropical Diseases Bulletin, 89, Rl-R6. Curtis, C. F. (1993). Workshop on bednets at the International Congress of Tropical Medicine. Japanese Journal of Sanitaary Zoology, 44,65-68. Curtis, C. F., Myamba, J. & Wilkes, T. W. (1992). Various pyrethroids on bednets and curtains. Memorias do Institute OsWaldo Cruz, 87, supplement 3, 363-370. Desfontaine, M., Gelas, H., Cahon. H.. Gorrhomou. A.. Kouka Bemba, c. & carnevale, %‘. (1990). E;alu%ion de; practiques

146 et des co&s de lutte antivectorielle g l’&helon familial en Afriaue Centrale II. Ville de Douala (Cameroun). iuillet 1988. Ahales de la Societt Belge de Mgdeciie Tropicale;%, 137-144. Gokool, S., Smith, D. F. & Curtis, C. F. (1992). The use of PCR to help quantify the protection provided by impregnated bednets. Parasitology Today, 8? 347-350. Kere, N. K. (1992). Permethrin tmpregnated bednets and DDT residual spraying: multi centre comparative trial. PhD thesis, University of London. Levine, 0. S. & Levine, M. M. (1991). Houseflies as mechanical iectors of shigelldsis. Review if Infectious Diseases, 13, 688-696. Lindsay, S. W. (1992). The possibility of transmission of the human immuno-deficiency virus or hepatitis B virus by bedbuss or mosauitoes. Abstracts. XIII International Conmess for Tripical Medicine andMalaria;

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Lines, J. D., Myamba, J. & Curtis, C. F. (1987). Experimental hut trials of permethrin-impregnated mosquito nets and eave curtains against malaria vectors in Tanzania. Medical and Veterinary Entomology, 1,37-5 1.

Magesa, S. M., Wilkes, T. J., Mnzava, A. E. I’., Njunwa, K. J., Myamba, J., Kivuyq, M. D. I’., Hill, N., Lines, J. D. & Curtis, C. F. (1991). Trial of pyrethroid impregnated bednets in an area of Tanzania holoendemic for malaria. 2. Effects on the malaria vector population. Acta Tropica, 49! 97-108. Malcolm, C. (1988). Current status of pyrethrold resistance in anoohelines. Parasitology Today, 4.5 13-515. Man&, P. (1900). Expezmentai proof of the mosquito-malaria theory. Lancet, ii, 923-925. Maxwell, C. A., Curtis, C. F., Haji, H., Kisumku, S., Thalib, A. I. & Yahva, S. A. (19901.Control of Bancroftian filariasis bv inteerati&thera& wi& vector control usine oolvstvrene biads i’; wet-pit la&es. Transactionsof the R&i1 $ococ$tyof Tropical Medicine and Hygiene, 84,709-714. Miike, L. (1987). Do Insects Transmit Aids? Washington: Office of Technology Assessment,US Congress. Msuya, F. H. M. & Curtis, C. F. (1991). Trial of pyrethroid im-

pregnated bednets in an area of Tanzania holoendemic for malaria. 4. Effect on incidence of malaria infection. Acta Tropica, 49? 165-171. Ragoonanansmgh, R., Njunwa, K. J., Curtis, C. F. & Becker, N. (1992). A field study of Bacillus sphaericus for the control of culicine and anopheline mosquito larvae in Tanzania. Bulletin of the Society of Vector Ecology, 17,45-50.

Ross, R. (1910). The Prevention of Malaria. London: John Murray. Stich, A. H. R., Maxwell, C. A., Haji, A. A., Haji, D. M., Machano. A. Y.. Mussa. I. K.. Matteelli. A.. Haii. H. & Curtis. C. F. ~1994).‘Insect&le-irnbregnated Led’nets reduce malaria transmission in rural Zanzibar. Transactionsof the Royal Society of Tropical Medicine and Hygiene, 88,150-154. Suzuki, H., Okazawe, T., Kere, N. & Kuwada, H. (1989). Field evaluation of a new insect growth regulator, pyriproxyfen, against Anopheles farauti in the Solomon Islands. JapaneseJournal

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Valero, M. V., Amador, L. R., Galindo, C., Figueroa, J., Bello, M. S., Murillo, L. A., Mora, A. L., Patarroyo, G., Rocha, C. L., Rojas, M., Aponte, J. J., Sarmiento, L. E., Lozada, D. M., Cornell, C. G., Ortega, N. M., Rosas, J. E., Alonso, P. L. & Patarroyo, M. E. (1993). Vaccination with SPf66, a chemically synthesised vaccine, against Plasmodium falciparum malaria in Colombia. Lance?, 341,705-710. Vulule, J. M., Beach, R. F., Aueli, F. K., Roberts, J. M. & Mwangi, R. W. (1993). Reduced susceptibility of Anopheles gambiae to permethrin in association with village-wide use of permethrin-impregnated bednets and curtains in Western Kenya. Abstracts of 14th Annual Medical Scientific Conference Of KEMRI and KETRI, Nairobi, p. 48. Zandu, A., Malengreau, M. & W&y, M. (1991). Practiques et depenses pour la protection contre les moustiques dans les m&ages & Kinshasa, Zaire. Annales de la Soci&tt Belge de Mddecine Tropicale, 71,259-266.

Announcements The Dr Albert Dubois Prize for Tropical Pathology The third quinquennial award of this prize will be made in 1994 by the Fondation Docteur A. Dubois, Acadgmie Royale de MIdecine de Belgique. The prize is worth 500 000 Belgian francs (about 12 500 ECU), and is intended to promote clinical, experimental or epidemiological research in tropical human and veterinary pathology. Entries should be submitted not later than 31 May 1994 to 1’Academie Royale de Medecine de Belgique, 1 rue Ducale, B-1000 Brussels, Belgium. Further information can be obtained from the Secretary of the Fondation at the same address.

Short Course in Advanced Epidemiological Methods London School of Hygiene and Tropical Medicine S-4 September 1994 This intensive course is intended for those who already have a working knowledge of basic epidemiology and statistics, and will cover both the classical methods of analysis and the more recently developed regression techniques, as well as considering aspects of study design. The course fee is &lOOO. Course brochures and application forms are available from Dr Margaret A. Parker, Assistant Registrar (Student Services), Short Courses, Registry, London School of Hygiene and Tropical Medicine, Keppel Street, London, WClE 7HT, UK; Telephone (+44) (0) 71927 2074; Fax (+44) (0) 71323 0638.