Human movement into insect vector and wildlife reservoir habitats determines zoonotic disease risks; however, few data are available to quantify the impact of land use on pathogen transmission. Here, we utilise GPS tracking devices and novel applications of ecological methods to develop fine-scale models of human space use relative to land cover to assess exposure to the zoonotic malaria Plasmodium knowlesi in Malaysian Borneo. Combining data with spatially explicit models of mosquito biting rates, we demonstrate the role of individual heterogeneities in local space use in disease exposure. At a community level, our data indicate that areas close to both secondary forest and houses have the highest probability of human P. knowlesi exposure, providing quantitative evidence for the importance of ecotones. Despite higher biting rates in forests, incorporating human movement and space use into exposure estimates illustrates the importance of intensified interactions between pathogens, insect vectors and people around habitat edges.
Mosquitoes are infectious vectors for a wide range of pathogens and parasites thereby transmitting several diseases including malaria, dengue, Zika, Japanese encephalitis and chikungunya which pose a major public health concern. Mostly synthetic insecticides are usually applied as a primary control strategy to manage vector-borne diseases. However excessive and non-judicious usage of such chemically derived insecticides has led to serious environmental and health issues owing to their biomagnification ability and increased toxicity towards non-target organisms. In this context, many such bioactive compounds originating from entomopathogenic microbes serve as an alternative strategy and environmentally benign tool for vector control. In the present paper, the entomopathogenic fungus, Lecanicillium lecanii (LL) was processed to make the granules. Developed 4% LL granules have been characterized using the technique of Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM). The developed formulation was also subjected to an accelerated temperature study at 40 °C and was found to be stable for 3 months. Further, GCMS of the L. lecanii was also performed to screen the potential biomolecules present. The developed formulation was found to be lethal against Anopheles culicifacies with an LC50 value of 11.836 µg/mL. The findings from SEM and histopathology also substantiated the mortality effects. Further, the SEM EDX (energy dispersive X-ray) studies revealed that the treated larvae have lower nitrogen content which is correlated to a lower level of chitin whereas the control ones has higher chitin content and healthy membranes. The developed LL granule formulation exhibited high toxicity against Anopheles mosquitoes. The granule formulations can be used as an effective biocontrol strategy against malaria-causing mosquitoes.
The filaria vector competence of Anopheles stephensi was compared with Brugia-susceptible Aedes aegypti Liverpool strain, An. gambiae Badagry Lagos strain and An. dirus Perlis Malaysia strain. An. stephensi ingested more Brugia pahangi microfilariae, had the highest infectivity rate and yielded more infective mosquitoes than the other two anopheline species. The overall vector competence of An. stephensi was 0.13 times that of Ae. aegypti, 0.62 times that of An. gambiae and 2.17 times that of An. dirus. However, heavy mortality among infected An. stephensi in the present investigation indicates that the filaria vectorial capacity of the mosquito might be limited epidemiologically. The relationship between filaria vector competence and mosquito foregut armature is discussed. It was observed that the relative vector competence of the three anopheline species tested was in the same order as their relative degrees of armature elaboration. The converse would be expected if foregut armatures really give partial protection to the mosquitoes against filarial infection. It is suggested that high host microfilariae density favours larval survival proportional to the degree of armature development in Anopheles (Cellia) species.
A longitudinal study was conducted on the prevalence of Anopheles in three malaria endemic villages in Sekong province, in the southern region of Lao PDR, from August 2000 to October 2001. All night, human landing collections took place in August and October 2000 and April and October 2001, and blood smears were taken for malaria parasites during the same period. Mosquitoes were tested for sporozoite antigen using enzyme-linked immunosorbent assay. In August 2000 (wet season) and April 2001 (dry season) the ovaries of the mosquitoes were examined for parity. A total of 16 species of Anopheles were caught in the study sites of which An. dirus A, An. maculatus sl and An. jeyporiensis were positive for sporozoites. The entomological inoculation rate (EIR) ranged from 0.06 to 0.25. There was a good correlation between EIR and vectorial capacity in the wet season, especially in Pai Mai where the prevalence of malaria was also high during the wet seasons (11.8 and 10.53). An. dirus A showed ambivalence in their choice of feeding as approximately 50% attacked man indoors and an equal proportion outdoors. An dirus A was the main vector in Pai Mai. The parous rate did not significantly differ between the wet and dry season, although it was higher in the dry season. In Takaio the parasite prevalence ranged from 8.7% (dry season) to 37.1% (wet season) and An. jeyporiensis was the vector, and the risk of infection was 0.85 in the dry season while 0.99 in the wet season. In Toumgno An. maculatus sl was the vector and infection was found only in August and October 2000. However, malaria prevalence ranged from 9.69 to 20.4% and was equally high in the dry season. Cattle were also present close to the houses in all the villages and this might be a contributory factor in the prevalence of malaria.
Residents in irrigated urban agricultural sites face numerous mosquito problems such as increased mosquito populations and reduced insecticides susceptibility due to the creation of mosquito breeding sites and agricultural use of insecticides and hence require effective protective products against them. In this study, the protection effectiveness of three pyrethroid formulated mosquito coils of Malaysian origin against Anopheles gambiae sensu lato from an irrigated urban agricultural site in Ghana were evaluated for their potential use. Sucrose fed An. gambiae s.l. were exposed to insecticide-containing coils in a 70 cm x 70 cm x 70 cm glass chamber to assess the insecticidal effect of the coils. The 0.005% metofluthrin coil caused the most rapid knockdown of 50% of the test mosquitoes. The mean lethal effect of the coils on An. gambiae s.l. were as follows; 0.005% metofluthrin (86%), 0.3% d-allethrin (74.33%), 0.15% d-trans allethrin (72%) and the 0.25% d-allethrin reference coil (69%). The 0.005% metofluthrin coil achieved the highest insecticidal effect on An. gambiae s.l. compared to the other coils and hence performed better than the others as an anti-mosquito product. All the three test coils were effective against An. gambaie s.l. from the irrigated agricultural site compared to the reference coil.
Chemical insecticides are still considered as important control agents for malaria vector control. However, prolonged use of these chemicals may select mosquito vectors for resistance. In this study, susceptibility status of adult Anopheles maculatus collected from 9 localities in peninsular Malaysia, viz., Jeli, Temerloh, Pos Banun, Senderut, Jeram Kedah, Segamat, Kota Tinggi, Kluang and Pos Lenjang were determined using the standard WHO bioassay method in which the adult mosquitoes were exposed to standard insecticide impregnated papers malathion, permethrin, DDT and deltamethrin--at pre-determined diagnostic dosage. Deltamethrin was most effective insecticide among the four insecticides tested, with the LT50 of 29.53 min, compared to malathion (31.67 min), DDT (47.76 min) and permethrin (48.01 min). The effect of all insecticides on the laboratory strain was greater (with all insecticides demonstrated LT50 < 1 hour) than the field strains (deltamethrin 32.7, malathion 53.0, permethrin 62.0, DDT 67.4 min). An. maculatus exhibited low degree of resistance to all test insecticides, indicating that these chemical insecticides are still effective in the control of malaria vector.
A 14-months survey was carried out to identify the species composition of Anopheles mosquitoes from Kampung Bongor, Grik, Perak. Adding to that, a preliminary one month mosquito population screening was done at Kampung Tepin, Serian, Sarawak. Consequently, the insecticide susceptibility status of a pyrethroid was tested against two selected species of Anopheles collected from these two locations in Malaysia. A total of 4,497 Anopheles from 11 species were identified from collections in Kampung Bongor, whereas 2,654 An. letifer were collected from Kampung Tepin. The An. maculatus of Kampung Bongor and An. letifer of Kampung Tepin were then selected and tested using WHO standard diagnostic test kits and impregnated papers with 0.75% permethrin. The response values of KT50 and KT95 for An. maculatus were recorded at 28.09 minutes and 62.98 minutes respectively. Anopheles letifer recorded much slower response values of KT50 and KT95, which was at 35.09 minutes and 73.03 minutes respectively. Both An. maculatus and An. letifer showed 100% mortality after 24 hours holding period. The results indicate that both species were still susceptible to the tested pyrethroid. For effective vector control and resistance management, accurate and periodic insecticide resistance monitoring should be undertaken especially in rural areas with agricultural usage of insecticides.
Plasmodium knowlesi in humans is life threatening, is on the increase and has been reported from most states in Malaysia. Anopheles latens and Anopheles cracens have been incriminated as vectors. Malaria is now a zoonoses and is occurring in malaria free areas of Malaysia. It is also a threat to eco-tourism. The importance of the vectors and possible control measures is reviewed here.
The adult population and species composition of mosquitoes collected in Ranau, Sabah are described. A total of 5956 mosquitoes representing 8 genera and 41 species were collected using human landing catch, indoor and outdoor. Anopheles maculatus was the most common species (15.6%) followed by Culex quinquefasciatus (12.8%), Culex pseudovishnui (12.1%), Anopheles balabacensis (11.1%), Culex vishnui (9.7%), Aedes vexans (9.6%), Culex tritaeniorhyncus (6.6%), Anopheles donaldi (5.6%) and others in very small percentage.
Use of mosquito coils for personal protection against malaria and mosquito nuisance is advocated under mosquito and malaria control programmes. We performed field studies of mosquito coils containing either metofluthrin or esbiothrin in experimental huts situated in Kamhororo village, Gokwe district, Zimbabwe. All tests were performed on 3-5 day old reared female Anopheles gambiae sensu lato mosquitoes. The burning times were 9hr 20min for mosquito coils containing metofluthrin and 8 hr for those containing esbiothrin and the results were significantly different (p = <0.001). The mean knock down rate for mosquito coils containing metofluthrin was 90% and that for esbiothrin was 73.3% and the results were significantly different (p = 0.00). Mosquito coils containing metofluthrin had a mean repellence of 92.7% as compared to 85.4% for esbiothrin and the results were not significantly different (p=0.27). The protection time as required by EPA (1999) was 6 hr for mosquito coils containing metofluthrin and 5 hr for those containing esbiothrin. The mean insecticidal effect of mosquito coils containing metofluthrin was 84% as compared to 83% for those containing esbiothrin and the results were not significantly different (p = 0.56). Both mosquito formulations could not be classified as having insecticidal effect since none of them met the 95% mortality rate criteria.
Studies profiling community and zonal malaria entomological risk indices are required to identify high risk areas where targeted control resources are most needed or likely to have the greatest impact on reducing risk of malaria infection. This study presents a first report on malaria vector risk indices in two vegetation zones within Adamawa state, Nigeria. Endophilic mosquitoes were collected for one year in selected communities in the Guinea and Sudan savanna zones within the State. Plasmodium falciparum Sporozoite and human blood meal ELISA assays were carried out on the female Anopheles mosquitoes collected. Sibling species composition of the An. gambiae complex were determined using PCR assays. Mean numbers of mosquitoes in the Guinea savanna communities were significantly (t = 7.73, DF = 11, p < 0.001) higher than the Sudan. Man-biting rates (F = 2.76, p = 0.13) of Anopheles mosquitoes were higher in the Guinea but not significantly different from Sudan savanna. Sporozoite rates of mosquitoes within the Guinea savanna were 2.7 times higher than the Sudan. The predominant Anopheles coluzzii species encountered in the state had higher overall human blood indices (0.63) and sporozoite rates (6.9%) compared to An. gambiae (0.39, 1.9%) and An. arabiensis (0.58, 2.3%) respectively. Overall annual human blood indices (0.59) of mosquitoes in Adamawa were lower compared to reports from other States. Prevalence and higher transmission risks indices of endophilic An. coluzzii mosquitoes reveal the need for LLIN and management of relatively permanent An. coluzzii breeding sites in the State. Widespread cattle rearing lifestyle and lower human blood indices of mosquitoes in the study area suggest the need to investigate cattle blood indices of the mosquitoes in the state. Higher entomological risk indices in the Guinea Savanna zone provide baseline information for prioritization of malaria vector control supplies within the State.
The assortment of paracentric chromosomal inversion 2La is associated with the maintenance of dieldrin resistance in laboratory colonies of the malaria vector Anopheles gambiae. This association has not been tested in field populations. The aim of this study was to test the association between inversion 2La and dieldrin resistance in a field population of An. coluzzii in Nigeria. Field collected immature stages of Anopheles were raised to adults and exposed to 4% dieldrin according to WHO criteria. Knockdown was recorded at 10 min intervals for 1 hour and final mortality was recorded 24 hours post exposure. Species and inversion 2La diagnostic PCR assays were conducted on the resistant and susceptible mosquitoes. The mosquitoes were highly resistant to 4% dieldrin (17.1% knock down and 25.7% final mortality; KDT50 and KDT95 calculated as 170 and 1, 514 minutes respectively). Frequencies of 2La in both the resistant and susceptible cohorts assorted within HardyWeinberg estimates (χ2=1.32, p=0.8 for dead/susceptible mosquitoes and χ2=2.54, p=0.5 for survivors or resistant mosquitoes). However, a higher number of heterozygous mosquitoes were observed in the resistant cohort compared to the susceptible, with significant variation in karyotype frequencies (χ2=11.08, DF=2, p<0.05) and a significantly higher frequency of the 2La inversion arrangement in the resistant cohort (Pearson's χ2 = 4.58, p = 0.03.). These data are the first to associate paracentric chromosome inversion 2La and dieldrin resistance in field population of An. coluzzii. Dieldrin resistance shows a weak but significant association with 2La whose assortment is affected by positive heterosis. Variation in the assortment of 2La inversion arrangements between resistant and susceptible cohorts of this An. coluzzii population suggests that dieldrin resistance is at least partially linked to inversion 2La which may explain the persistence of dieldrin resistance in this population despite a significant absence of selection for resistance to this insecticide.
Some species of the Anopheles dirus species complex are considered to be highly competent malaria vectors in Southeast Asia. Anopheles dirus is the primary vector of Plasmodium falciparum and P. vivax while An. cracens is the main vector of P. knowlesi. However, these two species are difficult to distinguish and identify based on morphological characters. Hence, the aim of this study was to investigate the potential use of antennal sensilla to distinguish them. Large sensilla coeloconica borne on the antennae of adult females were counted under a compound light microscope and the different types of antennal sensilla were examined in a scanning electron microscope. The antennae of both species bear five types of sensilla: ampullacea, basiconica, chaetica, coeloconica and trichodea. Observations revealed that the mean numbers of large sensilla coeloconica on antennal flagellomeres 2, 3, 7, 10 and 12 on both antennae of both species were significantly different. This study is the first to describe the types of antennal sensilla and to discover the usefulness of the large coeloconic sensilla for distinguishing the two species. The discovery provides a simple, reliable and inexpensive method for distinguishing them.
The Anopheles dirus mosquito is a primary malaria vector that transmits many species of Plasmodium parasites in Thailand and is widely spread across its geographic area. In the current study, the levels of expression of the suppressor of cytokine signaling (SOCS) gene in An. dirus mosquitoes infected with P. vivax were examined. The level of the gene's expression determined by mRNA extraction in An. dirus females (n=2,400) was studied at different times (0, 12, 24, 36, and 48 h after feeding), with different types of blood feeding (non-feeding, parasite-negative blood feeding, parasite-positive blood feeding) and in different parts of the body of mosquito samples (thorax and abdomen). The datasets were analyzed based on their relative expression ratio by the 2-ΔΔCT method and were tested for significant differences with ANOVA. The results showed that the An. dirus SOCS gene was stimulated in the abdomen 12 h and 24 h after blood feeding about three times more highly than in unfed females, with the difference being significant. At 24 h after P. vivax-infected blood feeding, the SOCS gene in the abdomen was expressed more highly than 24 h after parasite-negative blood feeding and expression was almost 36 times higher than in the control group who were not fed blood. However, in the thorax at all times after feeding and non-feeding, there was no expression of the SOCS gene. Therefore, the SOCS gene in An. dirus was most highly expressed 24 h post-feeding with a P. vivax-infected bloodmeal, which indicates that the SOCS gene in the major malaria vector in Thailand plays an important role in its immune system and its response to P. vivax infection.
Four human deaths caused by Plasmodium knowlesi, a simian malaria species, are stimulating a surge of public health interest and clinical vigilance in vulnerable areas of Southeast Asia. We, and other colleagues, emphasize that these cases, identified in Malaysia, are a clear warning that health facilities and clinicians must rethink the diagnosis and treatment of malaria cases presumed to be caused by a less virulent human malaria species, Plasmodium malariae.
Four species of malaria parasites are known to infect humans. A fifth species, Plasmodium knowlesi, has been reported to infect humans in Malaysian Borneo. Here we report for the first time the incrimination of Anopheles latens as the vector of P. knowlesi among humans and monkeys in Sarawak, Malaysia.
Surveys were conducted in malaria-endemic villages in the southern province of Attapeu, Lao PDR during various seasons over a 3-year period. All-night mosquito landing collections, blood surveys and a case-control study were conducted. Plasmodium falciparum was the predominant species, and slide positivity rates were higher during the transition/dry season compared with the wet season. Anopheles dirus A was found to be the primary vector, and sporozoite rates were highest during the transition/dry season. Anopheles dirus was found to be endophagic and endophilic. Not using insecticide-treated bed nets, houses close to breeding sites and sleeping away from home were risk factors associated with malaria.
Quantitative understanding of the transmission dynamics of lymphatic filarial parasites is essential for the rational planning of control strategies. One of the most important determinants of transmission dynamics is the relationship between parasite yield, the success rate of ingested microfilariae (mf) becoming infective larvae in a mosquito vector, and mf density in the source of the human blood meal. Three types of relationship have been recognized in human filaria/mosquito couples--limitation, facilitation and proportionality; facilitation has hitherto been observed only in the couple Wuchereria bancrofti/Anopheles gambiae in Burkina Faso, in experimental studies on a high density mf carrier. The present paper demonstrates facilitation in W. bancrofti/An. gambiae and W. bancrofti/An. arabiensis in lower mf density carriers in The Gambia and Tanzania, and in W. bancrofti/An. funestus in Tanzania. Facilitation was not found in An. melas in The Gambia nor in An. merus in Tanzania. Analysis of published data shows limitation at low level mf densities in W. bancrofti/Culex quinquefasciatus in Sri Lanka, and in the same couple in India. Limitation also occurs in Brugia malayi/Aedes togoi in experimental cats; proportionality occurs in B. malayi/Mansonia bonneae in Malaysia. The epidemiological significance of these host/parasite relationships is discussed, and supporting evidence for its validity is presented from the published results of large-scale control programmes.
Anopheles donaldi Reid, a member of the A. barbirostris species group, is a vector of human filariasis and probably malaria. The discovery of some old specimens of this species, collected in Kuala Lumpur town where it no longer occurs, together with evidence from the literature about past malaria in the town, suggest that donaldi may have played a part in transmitting that malaria.