Communicable Diseases

Dengue/DHF

Dengue Bulletin Volume 23, December-1999

Space Sprays for the Control of Aedes aegypti in South-East Asia and the Western Pacific

By
Norman G. Gratz
4 ch du Ruisseau, 1291 Commugny, Switzerland

Introduction

Virtually every urban vector control programme in the South-East Asia and Western Pacific regions makes extensive use of insecticidal space sprays. These are mostly intended to achieve rapid control of the adult populations of the principal mosquito vector of dengue, Ae. aegypti, especially during the epidemic outbreaks of dengue haemorrhagic fever. As there have been questions raised in the American Region with regard to the efficacy of this type of control, this paper will consider the methods and materials used in Asia and review the results obtained in field trials and operational conditions for the control of adult Stegomyia.

The principles of Aedes aegypti control

Aedes Stegomyia populations can be controlled either by disposing of the containers which are their larval habitats - ‘source reduction’ - by applying larvicides to the containers or by space sprays to control adult mosquitoes.

Despite the desirability of eliminating the places in which mosquitoes breed, it is unlikely that the number of larval habitats can be reduced to levels which will control mosquito populations under the ecological conditions of southern Asia. A large proportion of the Ae. aegypti breeding throughout the region takes place in containers used for storing water for household purposes; these cannot, of course, be readily or easily disposed of. Attempts to control such sources have not been successful under operational conditions of the region^(1,2). In areas lacking piped water supply to the houses or in which there is only an irregular provision of water, storage of water for household use remains a necessity. In addition, municipal services in the region are often unable to cope with the disposal of waste containers in which much mosquito breeding also occurs. The cooperation of the community in preventing mosquito breeding is sought but is difficult to obtain under existing conditions. Inhabitants throughout the region have also been urged to cover their water jars to prevent oviposition but it now appears that covered containers are likely to have more larvae in them than those without covers(^3,4).

The enormous number of containers in urban areas in which Ae. aegypti may develop, makes control by larviciding costly and difficult. Even if done very carefully, larviciding will have only a delayed effect on the densities of the adult vector populations and would thus not be a satisfactory means of controlling an epidemic outbreak of DHF. Furthermore, there is a declining acceptance by inhabitants of the application of chemical larvicides to household water containers.

Residual sprays, as used in mainly rural areas against Anopheles vectors of malaria, would be far too costly to apply in the vast numbers of rooms in buildings in urban areas. Most control operations have therefore little choice other than the use of adulticides as space sprays; most control organizations will continue to rely on them as no immediate alternative appears to be feasible or available. It is, therefore, important to make sure that adulticide applications are as efficient as possible.

Kilpatrick et al. (1970)⁽⁵⁾ has observed that epidemics of certain vector-borne diseases can be stopped or drastically curtailed if the infected vector population is reduced or eliminated. He and his colleagues carried out a series of field trials in Thailand which demonstrated rapid and effective control of adult mosquito populations by ultra low volume applications and these will be reviewed.

Methods of applying space sprays

Thermal fogs

Thermal fogs are produced by equipment in which an insecticide dissolved in an oil with a suitably high flash point is vapourized when injected into the high-velocity stream of hot gases. Malathion has been the most commonly-used insecticide, usually applied by hand-carried Swingfog thermal generators, or, for larger areas, by vehicle-mounted generators. The hand-carried foggers usually have a pulse-jet engine (WHO 1990)⁽⁶⁾. Applications should be done early in the morning before thermal convection currents lift the fog from the ground level. Adult mosquito populations will generally recover rapidly unless the foggings are repeated; programme using thermal fogs should repeat applications every four days to maintain Ae. aegypti at low levels.

Thermal fogging is widely used by vector control organizations throughout the region. These thermal fogs are highly visible and inhabitants of the treated area perceive their application as an effort by the authorities to reduce mosquito populations. Unfortunately, thermal foggings are often applied late in the day (at a time when more people can see them) rather than early in the morning, they are not repeated frequently enough, and their efficacy is not evaluated to determine when retreatments are necessary. Nevertheless, if properly applied, thermal fogging can provide effective, if only short, periods of control. Applications of malathion 4% and fenitrothion 1% resulted in good reductions of natural infestations of mosquitoes in Bangkok⁽⁷⁾; even better results were obtained with thermal fogs of pirimifos-methyl in Malaysia⁽⁸⁾. Use is also being made of pyrethroids such as resigen, permethrin, cypermethrin and lambdacyhalothrin which can produce a rapid knock down of adult mosquitoes.

Ultra low volume (ULV) fogs

Also called "cold fogs" as no heat is used to produce them, ULV application equipment uses technical or high concentrations of insecticides which produce large numbers of droplet particles into the air, each of which, when of the right diameter, carries a dose lethal to the mosquito upon which it impinges. Mount et al. (1968)⁽⁹⁾ emphasized that ULV aerosols are advantageous as they have no need of oil solvents or carriers as with thermal fogging, the amount of spray solution or mixture that has to be applied is much smaller than thermal fogs, and ULV applications have no need of mixing. Early field trials carried out in the USA showed ULV applications to be at least as effective or more effective against both caged and free-flying Ae. taeniorhynchus⁽⁹⁾.

In order to determine if ULV applications would be effective against Ae. aegypti in South-East Asia, trials were carried out in Thailand, first by aerial applications. The largest trial covered 18 km² of the city of Nakhon Sawan with some 9,000 houses, which were treated by two applications, four days apart, of 95% malathion as an ULV spray at a concentration of 438 ml/ha by a C47 aircraft. The Ae. aegypti landing counts before treatment were 8.6/man hour and premise indices were as high as 94%. The landing rate was reduced by 95% to 99% after the applications and remained low for ten days⁽¹⁰⁾. The extent of the reduction of the mosquito populations was rapid and quite impressive; however, the aerial applications were expensive and depended on the availability of a large spray aircraft and a highly trained crew. Studies were therefore made on the efficacy of ULV ground application methods.

Pant et al. (1971)⁽¹¹⁾ evaluated the efficacy of ground-applied ULV malathion aerosols in Bangkok using a Leco ULV generator. Unlike most previous ULV trials which had been carried out in open fields, these trials were carried out among the buildings in two small residential suburbs of Bangkok. Later the large suburb of Huay Kwang was treated several times and, finally, the whole city of Sri Racha was treated three times. It was found that excellent control of adult mosquitoes could be obtained at a dosage of 438 ml/ha of fenitrothion concentrate. Two ULV applications carried out three days apart enabled the adult Ae. aegypti population to be reduced by 99%, and it was three weeks before it regained its pre-treatment level.

These trials were followed by a study of the effect of ULV applications to the interior of houses⁽¹²⁾. A small hand-carried ULV machine was used to apply fenitrothion at 5.4 ml/house in the first application and, two weeks later, at 13.2 ml/house. After the second application, Ae. aegypti densities in the treated houses were very low for a period of two to three months.

Because of the persistent effect of this type of treatment, a further trial was carried out in Bangkok in an area of 1,300 houses and 10,000 inhabitants using a vehicle-mounted ULV LECO machine. Five sequential applications of fenitrothion ULV concentrate were made at intervals of 11 to 49 days and an effective control of Ae. aegypti populations was obtained for 4 days to 4 months⁽¹²⁾. The authors observed that sequential ULV applications could suppress vector populations throughout the rainy season and these had operational and cost advantages over larviciding.

Later, sequential applications were made in the Suitisan area of Bangkok, covering an area of 20 ha with 1,500 houses and 11,500 inhabitants. Two indoor ULV applications of fenitrothion were made at a rate of 0.1 ml/m³ of room space 14 days apart. A complete control of Ae. aegypti as measured by the landing rates and oviposition traps lasted for 6-7 months after the treatment; densities were substantially reduced up to a year, and recovery was slow even 16 months after the applications⁽¹³⁾.

The use of ULV applications in urban areas, especially the operational-scale field trials carried out in the South-East Asia and Western Pacific regions, was extensively reviewed by Gratz (1991)⁽¹⁴⁾, who emphasized their utility for the control of epidemic outbreaks of arboviruses.

For reasons which are difficult to perceive, little use has been made in Thailand of these ULV application procedures which were shown to be both effective and economical. By 1993, vector control budgets had been reduced despite the continuously rising trend of DHF cases in the country. Because of the increased trend, a reappraisal of the dengue vector control methods in use and the adoption of ULV adulticide control methods that have been shown to be effective in large-scale trials would seem to be timely⁽¹⁵⁾.

Field trials with ULV application of newer pyrethroid insecticides have also shown considerable promise against Ae. aegypti adults^(16,17).

It has been shown that adulticiding against Ae. aegypti in the South-East Asia and Western Pacific regions, if properly carried out, can be highly effective; ULV concentrates can achieve an immediate and persistent control, particularly if sequential applications are made. Further, large-scale operational trials and demonstrations of the methods and materials shown to be successful should be encouraged. Vector control programmes must also train their control personnel in the most efficient methods of the application of adulticides and their evaluation along with other methods of control.

Wherever possible, integrated control making use of all appropriate and feasible methods should be carried out against Ae. aegypti populations. Inhabitants of dengue endemic areas should be encouraged to dispose of undesired containers wherever possible; where this does not achieve the aim of reducing Ae. aegypti adult populations, well-directed, efficient adulticiding should be carried out.

References

1.      Boonluan Phanthumachinda, Wirat Samutrapongse, Prakong Phan Urai Somkiat Boonabuncha, Banyong Matcum (1987). Approaches for community participation in Aedes aegypti control, Phanus Nikhom district, Chonburi province. Dengue Newsletter, 13:53-61.

2.      Manasthitpong N, Srisawat P. Nilapan S, Burapangkul C. (1991). A comparative study of Aedes aegypti control models in Chaiyapoom province. Communicable Dis. J 17(3):200-206.

3.      Pattamaporn Kittayapong, Strickman D. (1990). Containers with Aedes mosquito larvae in implications for community-based control. South-East Asian J. Trop. Med. Pub. Hlth 21(4):712.

4.      Pattamaporn Kittayapong, Strickman D. (1993). Distribution of containers- inhabiting Aedes larvae at a dengue focus in Thailand. J. Med. Entomol. 30(3):601-606.

5.      Kilpatrick JW, Tonn RJ, Sujarti Jatanasen (1970). Evaluation of ultra-low-volume insecticide dispensing systems for use in single-engined aircraft and their effectiveness against Aedes aegypti populations in Southeast Asia. Bull. Wld Hlth Org. 42:1-14.

6.      WHO (1990). Equipment for Vector Control, 3^rd edition, WHO, Geneva.

7.      Lofgren CS, Scanlon JE, Voraphon Israngura (1967). Evaluation of insecticides against Aedes aegypti and Culex pipens quinquefasciatus in Bangkok, Thailand. Mosq. News 26(2):118-121.

8.      Maccallum Deighton J, Cheong W, Tan Seng Jen (1974). Thermal fogs of pirimiphos-methyl against adult mosquitoes in West Malaysia. Mosq. News 34(2):163-166.

9.      Mount GA, Lofgren CS, Pierce NW, Husman CN (1968). Ultra-low-volume non-thermal aerosols of malathion and naled for adult mosquito control. Mosq. News, 28(1):99-103.

10.   Lofgren CS, Ford HR., Tonn RJ, Sujarti Jantanasen (1970). The effectiveness of ultra-low-volume applications of malathion at a rate of 6 US fluid ounces per acre in controlling Aedes aegypti in a large-scale test at Nakhon Sawan, Thailand. Bull. Wld Hlth Org., 42:15-25.

11.   Pant CP, Mount GA, Sujarti Jatanasen, Mathis KL (1971). Ultra-low- volume ground aerosols of technical malathion for the control of Aedes aegypti L. Bull. Wld Hlth Org., 45:805-817.

12.   Pant CP, Nelson MJ, Mathis HL (1973). Sequential application of ultra-low-volume ground aerosols of fenitrothion for sustained control of Aedes aegypti. Bull. Wld Hlth Org., 48:455-459.

13.   Pant CP, Mathis HL, Nelson MJ and Phanthumachida B. (1974). A large-scale trial of ultra-low-volume fenitrothion applied by a portable mist blower for the control of Aedes aegypti. Bull. Wld Hlth Org. 51:409-415.

14.   Gratz NG (1991). Emergency control of Aedes aegypti as a disease vector in urban areas. J. Am. Mosq. Control Assoc., 7(3):353-365.

15.   Gratz NG (1993). Lessons of Aedes aegypti control in Thailand. Med. Vet. Entomol, 7:1-10.

16.   Lee HL, Khadri MS, Chiang YF. (1997). Preliminary field evaluation of the combined adulticidal, larvicidal, and wall residual activity of ULV-applied bifenthrin against mosquitoes. J. Vector Ecology, 22(2):146-149.

17.   Ansari MA. (1997). Trials with aerosol spray to control Aedes aegypti-biting during DF/DHF epidemic. Dengue Bulletin, 21:105-108.