Epidemiology of forest malaria in central Vietnam: a large scale cross-sectional survey

In Vietnam, a large proportion of all malaria cases and deaths occurs in the central mountainous and forested part of the country. Indeed, forest malaria, despite intensive control activities, is still a major problem which raises several questions about its dynamics. A large-scale malaria morbidity survey to measure malaria endemicity and identify important risk factors was carried out in 43 villages situated in a forested area of Ninh Thuan province, south central Vietnam. Four thousand three hundred and six randomly selected individuals, aged 10–60 years, participated in the survey. Rag Lays (86%), traditionally living in the forest and practising "slash and burn" cultivation represented the most common ethnic group. The overall parasite rate was 13.3% (range [0–42.3] while Plasmodium falciparum seroprevalence was 25.5% (range [2.1–75.6]). Mapping of these two variables showed a patchy distribution, suggesting that risk factors other than remoteness and forest proximity modulated the human-vector interactions. This was confirmed by the results of the multivariate-adjusted analysis, showing that forest work was a significant risk factor for malaria infection, further increased by staying in the forest overnight (OR= 2.86; 95%CI [1.62; 5.07]). Rag Lays had a higher risk of malaria infection, which inversely related to education level and socio-economic status. Women were less at risk than men (OR = 0.71; 95%CI [0.59; 0.86]), a possible consequence of different behaviour. This study confirms that malaria endemicity is still relatively high in this area and that the dynamics of transmission is constantly modulated by the behaviour of both humans and vectors. A well-targeted intervention reducing the "vector/forest worker" interaction, based on long-lasting insecticidal material, could be appropriate in this environment

The insecticide resistance status of malaria vectors in the Mekong region

<p>Abstract</p> <p>Background</p> <p>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.</p> <p>Methods</p> <p>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.</p> <p>Results</p> <p><it>Anopheles dirus s.s</it>., 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, <it>Anopheles epiroticus </it>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 <it>Anopheles minimus s.l</it>. populations were found, whereas <it>An. minimus s.l</it>. from Cambodia, Laos and Thailand were susceptible. Only two <it>An. minimus s.l</it>. populations showed DDT tolerance. <it>Anopheles vagus </it>was found resistant to DDT and to several pyrethroids in Vietnam and Cambodia.</p> <p>Conclusion</p> <p>This is the first large scale, cross-country survey of insecticide resistance in <it>Anopheles </it>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.</p

Predicted potential and current distribution area for <i>Anopheles dirus sensu lato.</i>

<p>The produced maps show in grey forested areas not suitable according to abiotic factor, in yellow the potential distribution based on abiotic factor but where forest is not present (potential niche) and the distribution as defined by favorable abiotic and biotic factors (“realized niche”). Performance tests for the model include test Gain (1.38), test AUC (0.90) and test extrinsic omission rate based on maximum test sensitivity plus specificity (6%).</p

Performance from univariate models.

<p>Gain or performance from univariate models: – (<0.25),–(0.25–0.5), *(above 0.5), **(above 0.75) and ***(above 1). Variables with training gain under 0.5 for all species are not presented and include: MMAXRAIN, QMAXRAIN, NMINRAIN, QMINRAIN, MMINCVRAIN, MMENSUN, SLOPE, FLOW. For each relevant variable, the suitability value for presence is defined. The letters refers to the text and to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0050475#pone-0050475-g004" target="_blank">Figure 4</a>.</p

Similarity dendrogram.

<p>Similarity dendrogram based on Ward clustering method and modified Hellinger distance.</p

Predicted potential and current distribution area for <i>Anopheles baimaii.</i>

<p>The produced maps show in grey forested areas not suitable according to abiotic factor, in yellow the potential distribution based on abiotic factor but where forest is not present (potential niche) and the distribution as defined by favorable abiotic and biotic factors (“realized” niche). Performance tests for the model include test Gain (1.7), test AUC (0.93) and test extrinsic omission rate based on maximum test sensitivity plus specificity (5.7%).</p

Probability of species occurrence.

<p>A) <i>Anopheles dirus sensu lato</i>, B) <i>Anopheles dirus sensu stricto</i> and C) <i>Anopheles baimaii</i>. Probability maps built using the mean of 100 replicates of the ECOOPT 1 km model based on 75% available samples. Inset maps present for each species presence/absence derived from the probability of species occurrence map based on 75% sample using as suitability threshold the value which maximizes sensitivity and specificity.</p

Environmental envelopes.

<p>A) The maps depict the maximum distribution range based on extreme limit values. A graded shade of grey depicts areas with unfavorable value for one or more variables, white being favorable areas for A) Dirus complex, B) <i>An. dirus s.s.</i> and C) <i>An. baimaii</i>. Some sites represented by a star symbol were selected in unfavorable areas. D) A small table analyze if the value is higher (+) or lower (−) to the actual range of value for each variables.</p

Predicted potential and current distribution area for <i>Anopheles dirus sensu strico</i>.

<p>The produced maps show in grey forested areas not suitable according to abiotic factor, in yellow the potential distribution based on abiotic factor but where forest is not present (potential niche) and the distribution as defined by favorable abiotic and biotic factors (“realized niche”). Performance tests for the model include test Gain (2.091), test AUC (0.9554) and test extrinsic omission rate based on maximum test sensitivity plus specificity (2.5%).</p