Research
The insecticide resistance status of malaria vectors in the Mekong region
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* Corresponding author: Wim Van Bortel wvbortel@itg.be
1 Institute of Tropical Medicine, Dept. Parasitology, Nationalestraat 155, B-2000 Antwerpen, Belgium
2 National Institute of Malariology, Parasitology and Entomology, Dept. Entomology, Luong The Vinh street, B.C. 10.200 Tu Liem, Hanoi, Vietnam
3 National Center for Malaria Control, Parasitology and Entomology, Dept. Entomology, 372 Monivong Boulevard, Phnom Penh, Cambodia
4 Mahidol University, Dept Biology, Rama IV road, Bangkok 10400, Thailand
5 Center of Malariology, Parasitology and Entomology, Dept. Entomology, Kualuang Road, Vientiane, Laos
6 Université Catholique de Louvain, Dept. Of Environmental Sciences and Land Use Planning, Croix du sud 2/16, B-1348 Louvain-la-Neuve, Belgium
7 Department of Biomedical Sciences, Faculty of Pharmaceutical, Veterinary and Biomedical Sciences, University of Antwerp, Universiteitsplein 1, B-2610 Antwerpen (Wilrijk), Belgium
Malaria Journal 2008, 7:102 doi:10.1186/1475-2875-7-102
Published: 5 June 2008Abstract
Background
Knowledge on insecticide resistance in target species is a basic requirement to guide insecticide use in malaria control programmes. Malaria transmission in the Mekong region is mainly concentrated in forested areas along the country borders, so that decisions on insecticide use should ideally be made at regional level. Consequently, cross-country monitoring of insecticide resistance is indispensable to acquire comparable baseline data on insecticide resistance.
Methods
A network for the monitoring of insecticide resistance, MALVECASIA, was set up in the Mekong region in order to assess the insecticide resistance status of the major malaria vectors in Cambodia, Laos, Thailand, and Vietnam. From 2003 till 2005, bioassays were performed on adult mosquitoes using the standard WHO susceptibility test with diagnostic concentrations of permethrin 0.75% and DDT 4%. Additional tests were done with pyrethroid insecticides applied by the different national malaria control programmes.
Results
Anopheles dirus s.s., the main vector in forested malaria foci, was susceptible to permethrin. However, in central Vietnam, it showed possible resistance to type II pyrethroids. In the Mekong delta, Anopheles epiroticus was highly resistant to all pyrethroid insecticides tested. It was susceptible to DDT, except near Ho Chi Minh City where it showed possible DDT resistance. In Vietnam, pyrethroid susceptible and tolerant Anopheles minimus s.l. populations were found, whereas An. minimus s.l. from Cambodia, Laos and Thailand were susceptible. Only two An. minimus s.l. populations showed DDT tolerance. Anopheles vagus was found resistant to DDT and to several pyrethroids in Vietnam and Cambodia.
Conclusion
This is the first large scale, cross-country survey of insecticide resistance in Anopheles species in the Mekong Region. A unique baseline data on insecticide resistance for the Mekong region is now available, which enables the follow-up of trends in susceptibility status in the region and which will serve as the basis for further resistance management. Large differences in insecticide resistance status were observed among species and countries. In Vietnam, insecticide resistance was mainly observed in low or transmission-free areas, hence an immediate change of malaria vector control strategy is not required. Though, resistance management is important because the risk of migration of mosquitoes carrying resistance genes from non-endemic to endemic areas. Moreover, trends in resistance status should be carefully monitored and the impact of existing vector control tools on resistant populations should be assessed.