Keywords:
Protozoal diseases
Malaria
Vectors
Anopheles dirus
Anopheles epiroticus
Anopheles minimus
Anopheles vagus
Insecticide resistance
DDT
Permethrin
Pyrethroids
Mekong region
Vietnam
Cambodia
Laos
Thailand
Asia, Southeast
Abstract:
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.