The insecticide resistance status of malaria vectors in the Mekong region.

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.

The status of malaria before and after distribution of ITNs from 1999 to 2006 in two districts of Khammouanne Province, Lao P.D.R

In 1999, an insecticide-treated net (ITN) distribution project was started in several malaria-endemic villages in Lao P.D.R., namely Vientiane, Bolikhamxay and Khammouanne Provinces. After the completion of the project, it was found that the ITNs were effective for malaria control based on the analysis of the slide positivity rate for malaria parasites between 1999 and 2000 [6]. We conducted malariometric, entomological and KAP surveys in 2005-06 to confirm the status of malaria and to determine the effectiveness of ITNs in three different socioepidemiological areas (<I>near city, rural and remote</I>), represented by the Xebangfay and Boualapha districts, Khammouanne Province.<br>A marked decrease in the annual malaria incidence and slide positivity rate was noted from 1999 to 2006 in the two districts. The malaria prevalence was significantly reduced in <I>near city</I> areas (5.6%-10.7% in 1999 to 0% in 2005-06) and <I>rural</I> areas (21.4%-50.9% in 1999 to 0%-1% in 2005-06). Twelve positive cases were recorded in <I>remote</I> areas (3.4 - 7.7% in 2006).<br>The illiteracy rate was significantly higher among the respondents in the <I>remote</I> (83.9%) than among those in the <I>near city</I> (32.7%) and <I>rural</I> (54%) areas. In all areas, more than 50% of the villagers indicated that they were not aware of malaria. Similarly, about 60% of the respondents were found to have incorrect knowledge concerning malaria transmission. In <I>remote</I>, 40.9% of the respondents were aware of the methods to prevent malaria infection and 49.5% used mosquito nets throughout the year, rates significantly lower than the corresponding rates in <I>near city</I> and <I>rural</I>. The density of persons per net in <I>remote</I> (3.1-5.9) was considerably higher than that in <I>near city</I> (1.8-2.1) and <I>rural</I> (1.2-2.7). Malaria vector mosquitoes, such as <I>An. minimus</I> and <I>An. nivipes</I> were collected by human-baited adult collection, and cow-baited and CDC light traps. The results of the analysis suggested that the lower the number of persons per net the more effective the reduction of malaria morbidity in these areas. It is imperative that ITN distribution and health education regarding malaria be strengthened, especially in <I>remote</I>, but also in other areas.

First report of mermithid parasitism (Nematoda: Mermithidae) in mosquitoes (Diptera: Culicidae) from Lao PDR

Unidentified mermithid nematodes were found in the abdominal cavity of three species of Anopheles mosquitoes from LAO PDR. This is a first record of mermithid nematodes in adult mosquitoes from Lao PDR. Two worms were found in each adult host and the infection was more prevalent in the northern region compared to the south.