The insecticide resistance status of malaria vectors in the Mekong region

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.

Figure 1

The MALVECASIA study sites with their corresponding codes.

Figure 2

The insecticide resistance status as defined by the mortality rate obtained with bioassays of Anopheles dirus s.l. for five insecticides. The number of tested mosquitoes has been corrected for morphological misidentifications.

Figure 3

The insecticide resistance status as defined by the mortality rate obtained with bioassays of Anopheles epiroticus for five insecticides. The number of tested mosquitoes has been corrected for morphological misidentifications.

Figure 4

The insecticide resistance status as defined by the mortality rate obtained with bioassays of Anopheles minimus s.l. for five insecticides. The number of tested mosquitoes has been corrected for morphological misidentifications.

Figure 5

The insecticide resistance status as defined by the mortality rate obtained with bioassays of Anopheles vagus for five insecticides. The number of tested mosquitoes has been corrected for morphological misidentifications.

Figure 6

Overview of the bioassays mortality rates for Anopheles dirus s.s., An. epiroticus, An. minimus s.l. and An. vagus. Each point represents at least one study site. If more bioassays were done in the same site, only the highest value is plotted. Dotted lines indicate the limits of the WHO resistance classes, i.e. upper limit 98% and lower limit 80% mortality. Colour code black: Cambodia, blue: Laos, green: Thailand, red: Vietnam.

Figure 7

The time for 50% knockdown (KDT50) in function of the observed mortality for four different insecticides. Only tests were included when 80 or more mosquitoes were tested and when the morphological identification was reliable (90% of more correct identifications). The resistance cut off value of 80% mortality is indicated by a dotted line.

Figure 8

Insecticide resistance status of Anopheles minimus s.l. and the distribution of the two members of the An. minimus complex, An. minimus s.s. and An. harrisoni in northern Vietnam.