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Am. J. Trop. Med. Hyg., 73(2), 2005, pp. 350-353
Copyright © 2005 by The American Society of Tropical Medicine and Hygiene

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FIELD EVALUATION OF SPATIAL REPELLENCY OF METOFLUTHRIN IMPREGNATED PLASTIC STRIPS AGAINST MOSQUITOES IN HAI PHONG CITY, VIETNAM

HITOSHI KAWADA*, NGUYEN THI YEN, NGUYEN THUY HOA, TRUONG MINH SANG, NGUYEN VAN DAN, AND MASAHIRO TAKAGI
Department of Vector Ecology and Environment, Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan; National Institute of Hygiene and Epidemiology, Hanoi, Vietnam; Hai Phong Preventive Medicine Center, Hai Phong, Vietnam; Do Son Health Center, Hai Phong, Vietnam


ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Spatial repellency of metofluthrin-impregnated polyethylene plastic strips against mosquitoes, Aedes aegypti and Culex quinquefasciatus, were studied in a residential area in Hai Phong city, Vietnam. Thirty houses were selected as trial sites; half of these were assigned as untreated control and the other half were assigned for treatment. Primarily, irrespective of the room size, one room was treated with one strip. The dominant species in the trial sites were Culex quinquefasciatus and Aedes aegypti. A rapid decrease in the mosquito index was observed immediately after the treatment with metofluthrin strips, and treatment was effective for at least 6 weeks.


INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Metofluthrin (2,3,5,6-tetrafluoro-4-methoxymethylbenzyl (E, Z)(1R, 3R)-2,2-dimethyl-3-(prop-1-enyl)cyclopropanecarboxylate) (S-1264) is a newly synthesized pyrethroid, produced by Sumitomo Chemical Co. Ltd. (Osaka, Japan).1 Metofluthrin belongs to the highly safe category of pyrethroids2 and has already been registered in several Asian countries such as Singapore, Indonesia, Myanmar, and Vietnam. The vapor pressure of metofluthrin is greater than 2-fold and 100-fold that of d-allethrin and permethrin, respectively, and it vaporizes at normal temperature without heating, whereas the other conventional pyrethroids require heating for evaporation. The high vapor pressure and insecticidal activity of metofluthrin may lead to the development of new mosquito controlling devices, which require no external energy for vaporization, are low cost, and provide long-term efficacy. Metofluthrin-impregnated multilayer paper strips have shown a promising spatial repellent effect against mosquitoes in the laboratory and in field conditions.3,4 Mosquitoes were repelled by airborne metofluthrin vapor for 4 weeks in simulated outdoor conditions.3 The field tests also suggest that metofluthrin may be useful for prevention of mosquito bites in shelters without walls (beruga), which people in Lombok Island, Indonesia, use for resting, praying, and evening conversations with neighbors, and in which there is a high risk of malaria transmission.4

The above preliminary investigations were conducted using the prototype paper devices, which were experimentally prepared in the laboratory. During another preliminary trial in Lombok, Indonesia, we observed that in outdoor conditions, the effective duration of the paper strips was shorter than what we had expected. To achieve a longer effectiveness, we manufactured the slow-release plastic formulation that was impregnated with 1,000 mg of metofluthrin. Using this formulation, we could achieve more than 14 weeks of activity in outdoor conditions in Lombok.5 In this study, we report the indoor residual spatial repellency of this new plastic formulation against mosquitoes in Hai Phong city, Vietnam.


MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Metofluthrin-impregnated plastic strips. Plastic strips (same devices as reported by Kawada and others5) were supplied by Sumitomo Chemical Co. Ltd. (Hyogo, Japan). The plastic material had a folded cylinder-shaped network structure (11.5 cm wide, 3–4 mesh) made of polyethylene impregnated with 5% (w/w) metofluthrin. The plastic material was cut into a piece of 20 g (11.5 cm x 37 cm) to make a strip (Figure 1Go).



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  		    FIGURE 1. Metofluthrin-impregnated polyethylene plastic strip used in the study.

 
Location of the field study. The field study was conducted at Do Son town, Hai Phong city, Vietnam (20°40'–20°45'N, 106°46'–106°49'E). Do Son town is located in a coastal area that includes mountains and plains and is recognized as a famous tourist landscape; the three sides of the town face toward the eastern seacoast. The total area of Do Son is 4 sq. km with a population of 31,000, and it is divided into four wards and one commune. The three main occupations of the residents are fishing, tourism, and agriculture. This area obviously reflects typical tropical climate with four seasons, while it is colder in winter and cooler in summer. During the period from 1976 to 1978, outbreaks of dengue posed major health problems (Annual Report on Infectious Diseases, Hai Phong Preventive Medicine Center, Hai Phong, Vietnam). Thus far, dengue cases have rarely been reported in this location. From January to October 2004, only three clinical diagnosed dengue cases were reported at commune health stations. People residing in the coastal area usually use traditional mosquito-killing measures such as burning dry leaves to repel mosquitoes, implementing environmental sanitation program, spraying insecticides, and impregnating bed nets.

Strip treatment and mosquito collection. Thirty houses were selected in the trial sites; half of these were assigned as untreated control and the other half were assigned for treatment. The treatment of the metofluthrin-impregnated plastic strips was conducted from May 25 to 27, 2004. Irrespective of the room size, one room was treated with one strip, as shown in Figure 2Go. A total of 60 strips were used for treating 15 houses. A preliminary survey for mosquito density was conducted for two consecutive weeks prior to the treatment. Mosquito collection in the houses was conducted 1 day, 1 week, and 2 weeks after the treatment. The same procedure was repeated at intervals of 2 weeks until 18 weeks after the treatment.



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  		    FIGURE 2. Treatment with a metofluthrin-impregnated plastic strip in a room, Do Son.

 
Mosquito collection was conducted by the conventional method, that is, using glass test tubes (20 cm in length, 1.5 cm in diameter) to catch mosquitoes resting on the walls, under the furniture, on clothes, and so forth. Mosquitoes were collected during 7 AM–11 AM. The total collection time in each house was 30 minutes; that is, 10 minutes by each of the three staffs. After collection, the mosquitoes were identified, the number of mosquitoes per house was recorded, and the mosquito density index was calculated as general (based on the total number of mosquitoes) and species specific.

Data analysis. The density index was calculated as the mean number of mosquitoes in a house. The density indices in treated and untreated houses were compared using repeated measure ANOVA.


RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Changes in the number of mosquitoes collected in the metofluthrin-treated and untreated houses are shown in Figure 3Go. The dominant species in the trial sites was Culex quinquefasciatus Say, followed by Aedes aegypti (L.). A small number of Culex tritaeniorhynchus Giles, Armigeres spp., and Anopheles spp. were also collected. The ambient temperature during the study ranged from 27°C to 39°C.



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  		    FIGURE 3. Changes in the number of mosquitoes collected in metofluthrin-treated and untreated houses. A, General density index based on the total number of mosquitoes collected. B, Ae. aegypti index. C, Cx. quinquefasciatus index. Bars indicate 95% confidence limits.

 
General density indices—based on the total number of mosquitoes collected—of both the treated and untreated sites were 3.7 and 3.3, respectively, at 2 weeks prior to treatment and 4.5 and 4.8, respectively, at 1 week prior to treatment. The index decreased rapidly in the treated houses 1 day after the treatment with metofluthrin strips, and it was maintained at significantly lower values than those in the untreated houses (Figure 3AGo; repeated measure ANOVA, F = 21.1, 1 df, P = 0.00018). Based on these observations, the effective duration of metofluthrin was shown to be less than 8 weeks.

The changes in the number of Ae. aegypti are shown in Figure 3BGo. Mosquito indices of both treated and untreated sites were 0.9 and 0.8, respectively, at 2 weeks prior to treatment and 0.6 and 0.4, respectively, at 1 week prior to treatment. After the treatment with metofluthrin strips, the values remained significantly lower than those in the untreated houses (repeated measure ANOVA, F = 9.9, 1 df, P = 0.0050). The effective duration of metofluthrin against Ae. aegypti was shown to be the same (less than 8 weeks) as that evaluated by the general index.

The difference in the density index between metofluthrin-treated and untreated houses was also significant (repeated measure ANOVA, F = 5.2, 1 df, P = 0.034) in Cx. quinquefasciatus (Figure 3CGo). However, the repellent activity of metofluthrin appeared to be lower for this mosquito species as compared with the other species, particularly during the first 5 weeks after treatment. This might indicate that Cx. quinquefasciatus has a lower susceptibility as compared with Ae. aegypti. The difference in the susceptibility between the two species, however, should not be merely attributed to insecticide resistance. Arugueta and others6 reported that LD50s of d-allethrin topically applied to several Asian strains of Cx. quinquefasciatus ranged from 0.013 to 0.063 µg per female, while LD50s of the same pyrethroid for Ae. aegypti were 0.0055 µg per female7 and 0.004 µg per female,8 indicating that Cx. quinquefasciatus appears to be less susceptible to pyrethroids than Ae. aegypti. Further investigation will be required for evaluating the difference in the susceptibility in both the mosquito species.

Kawada and others3,4 reported that mosquitoes were affected by airborne metofluthrin vapors and not by direct contact with the chemical, thereby resulting in the spatial repellency. The spatial repellency is considered to occur through two main modes of pyrethroid action; that is, knockdown activity and biting inhibition or disruption of orientation toward the host. Among these, the latter may be categorized as a sublethal effect that results from neural excitement, which appears to occur at an earlier stage of toxication or with a dosage that is lower than that required for knockdown or death.9 Winney10 reported that female Ae. aegypti exposed for a few minutes to the smoke of a pyrethroid-based mosquito coil did not bite. MacIver8 defined the "repellency" associated with pyrethroids as a reaction of mosquitoes at the threshold level when the neural activation and knockdown occur resulting in the loss of power to orient toward their hosts. Similarly, the spatial repellency of metofluthrin-impregnated paper strips is considered to be caused by the disruption of the orientation activity of mosquitoes. Using metofluthrin-impregnated plastic strips in indoor conditions, we could confirm the long-lasting spatial repellent efficacy of metofluthrin against mosquitoes. However, the effective duration (6 weeks) observed in the current result is considered to be insufficient for the practical application of the devices with regard to treatment cost. Long-term effectiveness might be achieved by further formulation studies, such as investigation for the optimum density of plastic polymer, to optimize the release rate of the active ingredient. Further improvements in methods for manufacturing plastic will enable the formulation of a more optimum recipe that results in a longer effective duration.

Received January 20, 2005. Accepted for publication April 9, 2005.

Acknowledgments: We express our deep gratitude to T. Igarashi, Sumika LifeTech Co., Ltd., Osaka, Japan, for providing the plastic samples. We also thank H. T. Long, the Director of the National Institute of Hygiene and Epidemiology, Hanoi, Vietnam, for supervising this study.

Financial support: This study is supported and sponsored by the Japan Society for the Promotion of Science (JSPS).

* Address correspondence to Hitoshi Kawada, Department of Vector Ecology and Environment, Institute of Tropical Medicine, Nagasaki University, Sakamoto 1-12-4, Nagasaki 852-8523, Japan. E-mail: vergiss{at}net.nagasaki-u.ac.jp Back

Authors’ addresses: Hitoshi Kawada and Masahiro Takagi, Department of Vector Ecology and Environment, Institute of Tropical Medicine, Nagasaki University, Nagasaki 852-8523, Japan. Nguyen Thi Yen and Nguyen Thuy Hoa, National Institute of Hygiene and Epidemiology, Hanoi, Vietnam. Truong Minh Sang, Hai Phong Preventive Medicine Center, Hai Phong, Vietnam. Nguyen Van Dan, Do Son Health Center, Hai Phong, Vietnam.

Reprint requests: Hitoshi Kawada, Department of Vector Ecology and Environment, Institute of Tropical Medicine, Nagasaki University, Sakamoto 1-12-4, Nagasaki, 852-8523, Japan, E-mail: vergiss{at}net.nagaski-u.ac.jp.


REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 

  1. Ujihara K, Mori T, Iwasaki T, Sugano M, Shono Y, Matsuo N, 2004. Metofluthrin: a potent new synthetic pyrethroid with high vapor activity against mosquitoes. Biosci Biotechnol Biochem 68: 170–174.[Medline]
  2. Shono Y, Kubota S, Sugano M, Yap HH, Tsushima K, 2004. Field evaluation of papers strip and mosquito coil formulation impregnated metofluthrin for mosquito control in Malaysia. The Abstract Book, 70th Annual Meeting of American Mosquito Control Association, 2004 February 21–26; Savannah, GA. Eatontown, NJ: American Mosquito Control Association. P40.
  3. Kawada H, Maekawa Y, Tsuda Y, Takagi M, 2004. Laboratory and field evaluation of spatial repellency with metofluthrin-impregnated paper strip against mosquitoes in Lombok Island, Indonesia. J Am Mosq Control Assoc 20: 292–298.[Web of Science][Medline]
  4. Kawada H, Maekawa Y, Tsuda Y, Takagi M, 2004. Trial of spatial repellency of metofluthrin-impregnated paper strip against Anopheles and Culex in shelters without walls in Lombok, Indonesia. J Am Mosq Control Assoc 20: 434–437.[Web of Science][Medline]
  5. Kawada H, Maekawa Y, Takagi M, 2005. Field trial of the spatial repellency of metofluthrin-impregnated plastic strip against mosquitoes in shelters without walls (Beruga) in Lombok, Indonesia. J Vector Ecol.
  6. Arugueta TBO, Kawada H, Sugano M, Kubota S, Shono Y, Tsushima K, Takagi M, 2004. Comparative insecticidal efficacy of a new pyrethroid, metofluthrin, against colonies of Asian Culex quinquefasciatus and Culex pipiens pallens. Med Entomol Zool 55: 289–294.
  7. Shinjo G, Yamaguchi T, Tsuda S, Yoshida K, Okuno Y, 1981. A study on the insecticidal activity of d-tetramethrin. Jpn J Sanit Zool 32: 221–228.
  8. MacIver DR, 1964. Mosquito coils Part II. Studies on the action of mosquito coil smoke on mosquitoes. Pyrethrum Post 7: 7–14.
  9. Birley MH, Mutero CM, Turner IF, Chadwick PR, 1987. The effectiveness of mosquito coils containing esbiothrin under laboratory and field conditions. Ann Trop Med Parasito. 81: 163–171.
  10. Winney R, 1975. Pyrethrins and pyrethroids in coils–a review. Pyrethrum Post 13: 17–22.



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