Daily survival and human blood index of major malaria vectors associated with oil palm cultivation in Cameroon and their role in malaria transmission

Objective To describe the daily survival rate, biting habits, feeding behaviour, sporozoite and entomological inoculation rates of Anopheles species and their relative contribution to Plasmodium falciparum transmission in large‐scale oil palm–growing zones in Cameroon. Methods Entomological surveys by employing human‐landing catches, both indoors and outdoors, two nights per month for 12 months from October 2004. Results A total of 2485 Anopheles were collected and four species recorded: An. funestus, An. hancocki, An. gambiae s.s. and An. nili. There was substantial indoors to outdoors variation and seasonality in the distribution of the Anopheles population. All four species showed similar nocturnal biting patterns and were sympatrically involved in malaria transmission simultaneously or replacing each other seasonally. Their constant presence throughout the year and longevity suggest that transmission can occur at any time if parasite reservoirs are present in the area. The circumsporozoite protein rates were 5.8%, 6.3%, 8.4% and 4.1%; mean anthropophilic rates were 97.1%, 94.5%, 73.9% and 77.6%, respectively, for An. funestus, An. gambiae s.l, An. nili and An. hancocki. The annual entomological inoculation rate was 1077 infectious bites per person. Of those An. gambiae specimens testing positive for P. falciparum sporozoites, 69.01% were M form and 30.99% were S form. Conclusion Any vector control strategy intending to significantly reduce the malaria burden in the palm estate will have to take into account entomological heterogeneity in addition to ecological and socio‐economic multiplicity.     

Differential attractiveness of human foot odours to Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) and variation in their chemical composition

Counter flow geometry (CFG) traps (American Biophysics) baited with foot odours (adsorbed overnight on a combination of a nylon and a cotton socks) from 4 groups of 4 male volunteers were initially used in a screen-house to compare and grade the attractiveness of Anopheles gambiae Giles sensu stricto to these odours. Ten individuals were then selected from the 4 groups for further grading in which mosquito attractiveness to odours adsorbed on socks worn by each of the individuals was compared against a control (clean, un-worn cotton and nylon socks). A gradation of attractiveness was found, with the most attractive foot odour catching 8-fold more mosquitoes than the least attractive one (t-test, p = 0.001). For comparison of the chromatographic profiles of the foot odours and identification of their constituents, six adsorbents (tenax, chromsorb, porapak Q, activated charcoal, reverse phase octadecyl and octyl bonded silica) were evaluated for their suitability in trapping the volatiles in a static mode. Of these, porapak Q was found to be the most effective. Comparison of the gas-chromatographic (GC) and gas chromatography–linked mass spectrometry (GC–MS) profiles of blends collected from the most and least attractive feet on porapak Q adsorbent revealed both qualitative and quantitative differences. The implication of our finding on differential attraction of An. gambiae to their preferred feeding site of different human subjects is highlighted.     

Influence of host and larval habitat distribution on the abundance of African malaria vectors in western Kenya

The abundance of anopheline mosquitoes varies substantially among houses within the same villages. Anopheles gambiae sensu stricto is highly anthropophilic, and Anopheles arabiensis is zoophilic; thus, it is often hypothesized that the abundance of An. gambiae and An. arabiensis in a house is associated with the distribution of livestock and humans. In this paper we examined the influence of livestock and human host availability on the distribution and abundance of malaria vectors in the basin region of Lake Victoria in western Kenya. Larvae and adults of An. gambiae, An. arabiensis and Anopheles funestus were collected in the beginning and the end of the rainy season in 1999. Anopheles gambiae was the predominant species in both larval and adult samples. Multiple regression analyses found that the ratio of distance between houses and larval habitats to distance between cowsheds and larval habitats had a significant and negative association with the relative abundance of An. gambiae larvae for both sampling periods. The ratio of human density to cow density was positively correlated with the relative abundance of An. gambiae larvae in the late rainy period. For the adult samples, distance from a house to its nearest larval habitats was the only variable that showed a significant correlation with the An. gambiae density in houses in both sample periods. More than 90% of anopheline adults were found in the houses within 300 meters from the nearest larval habitats. Anopheline mosquito density was not correlated to the density of cows or humans, or the distance to cowsheds from houses. These results suggest that livestock and human host availability affect the relative abundance of An. gambiae larvae in aquatic habitats, but the distribution of anopheline adults in houses is determined by the distance from houses to larval habitats.     

Analysis of Anopheles arabiensis blood feeding behavior in southern Zambia during the two years after introduction of insecticide-treated bed nets

Anopheles arabiensis mosquitoes are the primary vector responsible for Plasmodium falciparum transmission in Macha, Zambia. Because insecticide-treated bed nets (ITNs) have the potential to alter host feeding behavior, the extent of the zoophilic and exophagic tendencies of the vector was evaluated during the two rainy seasons after ITN introduction. Centers for Disease Control light traps, paired indoor/outdoor human landing catches, and outdoor cattle-baited collections were used to assess potential changes in host preference. Results support the hypothesis that An. arabiensis mosquitoes in Macha remain highly anthropophilic despite high ITN use. Anopheles arabiensis mosquitoes in Macha appear to be relatively exophagic and have been caught biting outdoors immediately after sunset and before sunrise, potentially circumventing some of the protective effects of ITNs.     

La transmission du paludisme dans la zone de Niakhar, Sénégal

La bio-écologie des anophèles et la transmission du paludisme ont étéétudiées de janvier à décembre 1995 dans trois villages de la zone rurale sahélienne de Niakhar, au Sénégal. Cette zone de 29.000 personnes est depuis plusieurs décennies un observatoire régional de la population et de la santé. La collecte des moustiques a été pratiquées selon trois méthodes: la récolte des larves, la capture nocturne des moustiques femelles agressifs pour l'homme, et la capture au pyrèthre des moustiques au repos l'après-midi dans les maisons. Les anophèles capturés ont été par ordre d'importance numérique: Anopheles arabiensis, An. rufipes, An. gambiae, An. pharoensis, An. funestus et An. coustani. Au sein du complexe An. gambiae, An. arabiensis a représenté 97% des femelles capturées sur homme et 98% des femelles semigravides capturées au pyrèthre (différence non significative); la seule autre espèce de ce complexe a étéAn. gambiae. Ces deux espèces appartenant au complexe An. gambiae ont été responsable de la totalité de la transmission. L'indice d'anthropophilie, calculé sur les femelles An. gambiae s.l. semigravides capturées au pyrèthre, a été de 83%. Le taux annuel de piqûre d'An. gambiae s.l. a varié entre 512 et 1558 piqûres par homme, selon les villages. Des vecteurs ont été observés toute l'année, à une densité très faible en saison sèche. Une importante augmentation de la population anophélienne, liée aux pluies, a atteint un maximum de l'ordre d'une dizaine de piqûres par homme par nuit en septembre et au début d'octobre; en septembre le taux de piqûres représentait 48% du taux de piqûre annuel. L'indice sporozoïtique moyen d'An. gambiae s.l., déterminé par ELISA révélant la protéine circumsporozoïte, a été de 1,6% pour les femelles agressives et de 1,8% pour les femelles semigravides au repos (différence non significative). Plasmodium falciparum a été la seule espèce plasmodiale observée parmi les anophèles infectés. La transmission dans les deux villages représentatifs de la zone de Niakhar a été de 9 et 12 piqûres d'anophèles infectés par homme par an, survenant principalement entre août et octobre. Dans le troisième village, non représentatif de la zone par ses gîtes larvaires permanents, la transmission a été de 26 piqûres d'anophèles infectés par homme par an. L'ensemble de ces résultats a été discuté dans le contexte de la SénéGambie et du Sahel. The anopheline bioecology and the malaria transmission were studied from January to December 1995 in three villages of the sahelian rural area of Niakhar, Senegal. This area of 29 000 inhabitants, has been for several decades, a regional observatory for population and health. The three methods used for collecting mosquitoes were the collection at larval stages, the all night human biting collection, and the pyrethrum spray catch in houses during afternoons. The anophelines collected were, by numerical importance: Anopheles arabiensis, An. rufipes, An. gambiae, An. pharoensis, An. funestus and An. coustani. In the An. gambiae complex, An. arabiensis represented 97% of man biting females and 98% of half gravid resting females (difference not significant); the other reminding species of this complex was always An. gambiae. These two species belonging to the An. gambiae complex were responsible for the totality of the transmission. The anthropophilic index, obtained from half gravid indoor resting An. gambiae s.l., was 83%. The annual biting rate of An. gambiae s.l. varied from 512 to 1558 bites per man per night, depending on the villages. Vectors were observed all year long but their densities were low during the dry season. Vector population presented a notable increase due to the rains, with a maximum of about 10 bites per man per night in September or at the beginning of October; during September the biting rate represented 48% of the annual biting rate. The sporozoitic index of An. gambiae s.l., obtained by ELISA revealing the circumsporozoite protein, was 1.6% for human biting females and 1.8% for half-gravid resting females (difference not significant). Plasmodium falciparum was the only plasmodial species observed among infected anophelines. The annual transmission in the two villages representative of the Niakhar area were 9 and 12 bites of infected anophelines per man, occurring mainly from August to October. In the third village, not representative of the area regarding permanent breeding places, the transmission was 26 bites of infected anopheline per man per year. These results were discussed in the Senegambian and sahelian contexts.     

Anthropophilic mosquitoes and malaria transmission in the eastern foothills of the central highlands of Madagascar

Malaria remains a major public health problem in Madagascar, as it is the first cause of morbidity in health care facilities. Its transmission remains poorly documented. An entomological study was carried out over 1 year (October 2003-September 2004) in Saharevo, a village located at an altitude of 900 m on the eastern edge of the Malagasy central highlands. Mosquitoes were sampled weekly upon landing on human volunteers and in various resting-places. Out of 5515 mosquitoes collected on humans, 3219 (58.4%) were anophelines. Eleven anopheline species were represented, among which Anopheles funestus, Anopheles gambiae, Anopheles arabiensis and Anopheles mascarensis. Out of 677 mosquitoes collected in bedrooms by pyrethrum spray catches and in Muirhead-Thomson pits, 656 (96.9%) were anopheline belonging to these four latter species. The proportion of mosquitoes that fed on human varied according to the resting-places and the mosquito species: 86% of An. funestus resting in bedrooms fed on humans, whereas only 16% of An. funestus and 0% of An. mascarensis resting in pits fed on humans. The proportion of anopheline mosquitoes infected with human Plasmodium was measured by circumsporozoite protein-ELISA: 10/633 An. funestus (1.58%), 1/211 An. gambiae s.l. (0.48%) and 2/268 An. mascarensis (0.75%). The annual entomological inoculation rate (number of bites of infected anophelines per adult) was estimated at 2.78. The transmission was mainly due to An. funestus and only observed in the second half of the rainy season, from February to May. These results are discussed in the context of the current malaria vector control policy in Madagascar.     

Behavioural biting preference of Culex quinquefasciatus in human host in Lagos metropolis Nigeria

BACKGROUND & OBJECTIVES: Olfactory cues play an important role in the attraction of major disease vectors towards their host. Hence we intend to investigate the contribution of selected parts of the human host on the behavioural biting preference of Culex quinquefasciatus. METHODS: Two volunteers were trained to collect host seeking female mosquitoes landing to feed on different parts of the lower limbs. The Cx. quinquefasciatus were collected in paper cups with the aid of a flash-light and aspirator. Each paper cup was labeled to represent the selected parts where the mosquitoes were collected. RESULTS: The composition of Cx. quinquefasciatus from the total mosquitoes collected showed that Cx. quinquefasciatus was more predominant (90%) over other species present. The average minimum and maximum atmospheric temperature recorded during the night catches were 27 and 29.5 degrees C while the average relative humidity range was 78-81.6%. The behavioural biting preference results obtained showed that the density of Cx. quinquefasciatus mosquitoes preferring the foot region (298) was significantly higher (p > 0.05) when compared with other different parts of the human host such as the ankle, calf and the thigh. INTERPRETATION & CONCLUSION: The study indicates that the foot region of the human host has a stronger influence in orientating mosquitoes towards the human host. Hence, the exploitation of the characteristic human odour will add to the existing vector control strategies.     

Malaria vectors in the Republic of Benin: Distribution of species and molecular forms of the Anopheles gambiae complex

Members of the Anopheles gambiae complex are among the best malaria vectors in the world, but their vectorial capacities vary between species and populations. A large-scale sampling of An. gambiae sensu lato was carried out in 2006 and 2007 in various bioclimatic areas of Benin (West Africa). The objective of this study was to collate data on the relative frequencies of species and forms within the An. gambiae complex and to produce a map of their spatial distribution. Sampling took place at 30 sites and 2122 females were analyzed. Two species were identified through molecular methods. The overall collection showed a preponderance of An. gambiae s.s., but unexpectedly, An. arabiensis was reported in the coastal-Guinean bioclimatic area characterized by a mean annual rainfall of >1500 mm where only An. gambiae s.s. was reported previously. Our study of Benin indicates that An. arabiensis would be adapted not only to the urban areas but also to the rural humid regions. Among 1717 An. gambiae s.s., 26.5% were of the M form and 73.3% were S form. Few hybrid specimens between the M and S forms were observed (0.2%). Only the spatial distribution of the M form appears to be mainly a function of bioclimatic area.Factors that influence the distribution of these malaria vectors are discussed. This study underlines the need of further investigations of biological, ecological, and behavioral traits of these species and forms to better appreciate their vectorial capacities. Acquisition of entomological field data appears essential to better estimate the stratification of malaria risk and help improve malaria vector control interventions.     

Behaviour and population dynamics of the major anopheline vectors in a malaria endemic area in southern Nigeria

BACKGROUND & OBJECTIVES: Anopheline mosquitoes consist of a large number of species each of which differs from another in population, resting and feeding behaviour in relation to the prevailing conditions in the locality. A longitudinal study was carried out to investigate the population dynamics, resting and feeding behaviour of the major anopheline species found in a rain forest zone of Nigeria. METHODS: Mosquitoes resting and biting indoors were collected using WHO standard techniques and supplemented with outdoor-biting collections in the study areas between January and December 2004. Samples were sorted and identified microscopically for morphological features while molecular identification was carried out using polymerase chain reaction (PCR) techniques. RESULTS: PCR-based tests showed that both indoor and outdoor collections constitute three groups of Anopheles mosquitoes, An. gambiae s.l. Giles (68.6%), An. funestus Giles (30.7%) and An. moucheti Evans (0.7%). Of the 1342 female Anopheles mosquitoes collected indoors, 799 were caught resting and 543 were caught biting. The outdoor-biting population accounted for 28.8% of the total collections (n=1885). There was no significant difference (p > 0.05) in the biting activities (indoors and outdoors) of these species in four villages. However, An. arabiensis and An. moucheti were more exophagic with >60% of their biting occurring outdoors while An. gambiae and An. funestus were more endophagic with >55% of their biting occurring indoors. The human-vector contact with An. gambiae and An. funestus (indoors) was about 73.3 and 66.7%, respectively as against 27.7% in An. arabiensis and 25.3% in An. moucheti. This gave the corresponding man-biting rates (MBR) of 17.5 bites/man/night for An. gambiae, 14.6 bites/man/night for An. funestus, 6.7 bites/man/night for An. arabiensis and 4.3 bites/man/night for An. moucheti. Moreover, the number of An. gambiae and An. funestus caught resting indoors was significantly higher than the other two species (p < 0.05). The wet season collections showed thatAn. gambiae caught were more than 67% of the total catch while An. arabiensis was predominant in the dry season (chi2 = 75.44, df = 3, p < 0.01). INTERPRETATION & CONCLUSION: The present study highlights some aspects of the behaviour of anopheline mosquitoes in southern Nigeria which is an important component of epidemiological study of malaria. This information provides basis to the understanding of the role played by the identified anopheline species in malaria transmission and a baseline to formulate malaria control programme.     

Comparison of DNA probe and cytogenetic methods for identifying field collected Anopheles gambiae complex mosquitoes

A recently developed DNA probe method was compared with the standard cytogenetic method for identifying the species of individual mosquitoes in the Anopheles gambiae complex. The complex consists of 6 morphologically indistinguishable sibling species that include the major African malaria vectors. Half-gravid, field collected mosquitoes were split into 2 portions: the abdomen was preserved for ovarian nurse cell cytotaxonomy and the head/thorax portion was desiccated for DNA extraction. Cytogenetic examination of the Kenya specimens showed 88 An. gambiae and 108 An. arabiensis. The Zimbabwe specimens consisted of 6 An. gambiae and 55 An. Quadriannulatus. All samples of the 3 species were polymorphic for the major chromosomal inversions previously recorded in field specimens from eastern and southern Africa, indicating that the collections reflected natural levels of intraspecific variation in the field populations sampled. Approximately 97% of the cytologically identified mosquitoes were also identified to species by the DNA probe method, and in every case the DNA probe and cytogenetic methods of species identification produced concordant results.     

Knockdown resistance mutations (kdr) and insecticide susceptibility to DDT and pyrethroids in Anopheles gambiae from Equatorial Guinea

Objectives To determine the frequency of knockdown resistance (kdr) mutations in the malaria vector Anopheles gambiae s.s. from continental Equatorial Guinea; and to relate kdr genotypes with susceptibility to DDT and pyrethroid insecticides in this vector. Methods Female mosquitoes were collected in two villages, Miyobo and Ngonamanga, of mainland Equatorial Guinea. Insecticide susceptibility tests were performed following WHO procedures. Anopheles gambiae complex specimens were identified to species and molecular form by PCR. Genotyping of the kdr locus was performed by allele‐specific PCR and direct sequencing in a subset of samples. Results Both M and S molecular forms of A. gambiae were found in Ngonamanga whereas only the S‐form was identified in Miyobo. The two kdr mutations were detected in S‐form samples of both villages, with a higher frequency of the kdr‐e (Leu‐1014‐Ser) allele (Miyobo: 16%; Ngonamanga: 40%). The kdr‐w (Leu‐1014‐Phe) mutation was also detected in 3% of the M‐form. All individuals tested for pyrethroids were susceptible. A mortality rate of 86% was obtained for DDT. An overall kdr allele frequency (i.e. kdr‐e + kdr‐w) of 22% was detected in DDT resistant individuals, whereas susceptible individuals had a kdr frequency of 6%. Conclusion The co‐occurrence of both kdr mutations and reduced susceptibility to DDT found in A. gambiae highlights the importance of implementing efficient surveillance of insecticide resistance in Equatorial Guinea.     

Differential infectivities of o'nyong-nyong and chikungunya virus isolates in Anopheles gambiae and Aedes aegypti mosquitoes

O'nyong-nyong virus (ONNV) and chikungunya virus (CHIKV) are closely related alphaviruses that cause human disease in Africa and Asia. Like most alphaviruses, CHIKV is vectored by culicine mosquitoes. ONNV is considered unusual as it primarily infects anopheline mosquitoes; however, there are relatively few experimental data to support this. In this study, three strains of ONNV and one strain of CHIKV were evaluated in Anopheles gambiae and Aedes aegypti mosquitoes and in four cell lines. As predicted, CHIKV was not infectious to An. gambiae, and we observed strain-variability for ONNV with respect to the ability of the virus to infect An. gambiae and Ae. aegypti. The species specificity in vivo was reflected by in vitro experiments using culicine and anopheline-derived cell lines.     

Multiple insecticide resistance mechanisms in Anopheles gambiae and Culex quinquefasciatus from Benin, West Africa

Because free-insecticide treated net distribution is planned in Benin (West Africa) during the next few years, we investigated the type, frequency and distribution of insecticide resistance mechanisms in Anopheles gambiae and Culex quinquefasciatus mosquitoes in four localities selected on the basis of contrasting agricultural practices, use of insecticides and environment. Bioassays with WHO diagnostic test kits were carried out using pyrethroid, carbamate, organophosphate and organochlorine insecticides. An. gambiae mosquitoes were identified to species and to M or S molecular forms using PCR techniques. Molecular and biochemical assays were carried out to identify kdr and Ace.1 mutations in individual mosquitoes and to detect any increase in the activity of enzymes typically involved in insecticide metabolism (oxidase, esterase and glutathion-S-transférases). WHO diagnostic tests showed high frequency of resistance in An. gambiae and Cx. quinquefasciatus to permethrin and DDT in three areas. This was consistent with the presence of target site insensitivity due to kdr mutation and to increased metabolism through enzymatic activity. Kdr was expressed in both M and S forms. However, less than 1% of An. gambiae or Cx. quiqnuefasciatus showed the presence of the Ace.1^R mutation. Carbamate/OP resistance was present at higher frequency in Culex than in An. gambiae. Dieldrin resistance was present in both species at all four localities. A higher frequency of pyrethroid-resistance was found in An. gambiae mosquitoes collected in urban areas compared to those collected in rice growing areas. The expansion of vegetable growing within urban areas probably contributed to selection pressure on mosquitoes. The detection of multiple resistance mechanisms in both An. gambiae and Cx. quinquefasciatus in Benin may represent a threat for the efficacy of ITNs and other forms of vector control such as indoor residual spraying in the future.     

Trial of pyrethroid impregnated bednets in an area of Tanzania holoendemic for malaria. Part 2. Effects on the malaria vector population.

The malaria vector population consisted mainly of Anopheles gambiae s.s. with a small contribution from An. funestus and An. rivulorum. The mosquitoes coming to bite in bedrooms were monitored with light traps set beside untreated bednets. When impregnated bednets were provided for all the other beds in a village the Anopheles populations declined but the Culex quinquefasciatus populations were unaffected. The survival of An. gambiae (as measured by the mean number of ovarian dilatations) and the sporozoite rate declined following introduction of the nets and the estimated sporozoite inoculation rates into people not under their nets declined by more than 90%. The net introductions caused sharp declines in the number of mosquitoes resting indoors, but the evidence was inconclusive regarding diversion to outdoor resting, animal biting, earlier biting or outdoor biting. DDT spraying greatly reduced the Anopheles populations.     

Contribution of Anopheles funestus, An. gambiae and An. nili (Diptera: Culicidae) to the perennial malaria transmission in the southern and western forest areas of Côte d'Ivoire

The involvement of members of the Anopheles gambiae complex Giles and An. funestus Giles and An. nili Theobald groups in the transmission of Plasmodium falciparum was recently investigated in the villages of Gbatta and Kpéhiri, which lie, respectively, in forest areas in the west and south of C?te d'Ivoire. Adult female mosquitoes were collected, using human landing catches, inside and outside dwellings. After identification and dissection, the heads and thoraces of all the anopheline mosquitoes were tested, in an ELISA, for circumsporozoite protein (CSP). All the female anopheline mosquitoes collected and identified to species using PCR were found to be An. gambiae s.s., An. nili s.s. or An. funestus s.s., with An. gambiae s.s. and An. funestus s.s. predominant in Gbatta but An. nili s.s. the most common species in Kpéhiri. In Gbatta, 3·1% of the female An. gambiae collected, 5·0% of the female An. funestus and 1·8% of the female An. nili were found CSP-positive. The corresponding values in Kpéhiri were even higher, at 5·9%, 6·2% and 2·4%, respectively. The estimated entomological inoculation rates (EIR) were very high: 302 infected bites (139 from An. gambiae, 146 from An. funestus and 17 from An. nili)/person-year in Gbatta and 484 infected bites (204 from An. gambiae, 70 from An. funestus and 210 from An. nili)/person-year in Kpéhiri. In Gbatta, An. gambiae s.s. was responsible for most of the rainy-season transmission while An. funestus became the main malaria vector in the dry seasons. In Kpéhiri, however, An. nili appeared to be the main vector throughout the year, with An. gambiae of secondary importance and An. funestus only becoming a significant vector during the rainy season. Although, in both study sites, intense transmission was therefore occurring and the same three species of anopheline mosquito were present, the relative importance of each mosquito species in the epidemiology of the human malaria at each site differed markedly.     

The effect of oral anthelmintics on the survivorship and re-feeding frequency of anthropophilic mosquito disease vectors

In the Tropics, there is substantial temporal and spatial overlap of diseases propagated by anthropophilic mosquito vectors (such as malaria and dengue) and human helminth diseases (such as onchocerciasis and lymphatic filariasis) that are treated though mass drug administrations (MDA). This overlap will result in mosquito vectors imbibing significant quantities of these drugs when they blood feed on humans. Since many anthelmintic drugs have broad anti-invertebrate effects, the possibility of combined helminth control and mosquito-borne disease control through MDA is apparent. It has been previously shown that ivermectin can reduce mosquito survivorship when administered in a blood meal, but more detailed examinations are needed if MDA is to ever be developed into a tool for malaria or dengue control. We examined concentrations of drugs that follow human pharmacokinetics after MDA and that matched with mosquito feeding times, for effects against the anthropophilic mosquito vectors Anopheles gambiae s.s. and Aedes aegypti. Ivermectin was the only human-approved MDA drug we tested that affected mosquito survivorship, and only An. gambiae s.s. were affected at concentrations respecting human pharmacokinetics at indicated doses. Ivermectin also delayed An. gambiae s.s. re-feeding frequency and defecation rates, and two successive ivermectin-spiked blood meals following human pharmacokinetic concentrations compounded mortality effects compared to controls. These findings suggest that ivermectin MDA in Africa may be used to decrease malaria transmission if MDAs were administered more frequently. Such a strategy would broaden the current scope of polyparasitism control already afforded by MDAs, and which is needed in many African villages simultaneously burdened by many parasitic diseases.     

应用多重PCR法分析西藏察隅疟疾流行区按蚊吸血习性

目的用多重PCR法分析西藏疟疾流行区察隅县常见按蚊的吸血习性,为下一步研究传疟媒介提供参考。方法2011年7~8月选择察隅县不同生态环境的3个自然村(日玛村、塔玛村和京都村),每个村在人房和畜舍选择8个点,采用诱蚊灯全通宵(20∶00至次日08∶00)诱捕法捕捉按蚊,次日清晨收集诱捕的蚊虫,经形态学鉴定蚊种,分析按蚊组成。收集饱血按蚊,分别提取单只蚊胃血的DNA,采用基于不同动物mtDNA-cytb序列差异的多重PCR法鉴定各蚊胃血源,计算人血指数,分析按蚊的吸血习性。结果共捕获按蚊1 442只,经形态学鉴定,多斑按蚊种团占99.6%(1 436/1 442),带足按蚊和腹簇按蚊占0.4%(6/1442)。多斑按蚊种团中,伪威氏按蚊占85.5%(1228/1436),威氏按蚊占14.5%(208/1 436)。用多重PCR检测202只多斑按蚊种团(伪威氏按蚊188只和威氏按蚊14只)的饱血蚊胃血,结果显示,伪威氏按蚊兼吸牛/猪血和人血,人血指数为0.35,威氏按蚊吸食猪血和人血,人血指数为0.29。结论西藏察隅县2种常见按蚊(伪威氏按蚊和威氏按蚊)均兼吸人畜血,伪威氏按蚊的人血指数较高。     

Impact of urbanisation on bionomics and distribution of malaria vectors in Lagos, southwestern Nigeria

BACKGROUND & OBJECTIVES: The patterns of annual, rainfall as well as average daily temperature have not changed drastically in the study area since 1960 when detailed baseline entomological surveys were carried out. However, the increase in human population from 1.2 to 10 million has resulted in both expansion of land and tremendous ecological and environmental change. This has led to drastic changes in vectors' densities as well as species' composition while the preferred larval habitat of malaria vectors has shifted to water reservoirs. A longitudinal study was carried out to investigate the impact of urbanisation on bionomics and distribution of malaria vectors in Lagos, a mega city in Nigeria. METHODS: Mosquitoes were collected indoors and outdoors using WHO standard techniques in the selected areas between January and December 2000. Specimens were identified using the morphological keys and PCR assays. ELISA tests were used for Plasmodium falciparum sporozoite infection. RESULTS: The Anopheles gambiae species-specific PCR identified 56% (435/777) of the An. gambiae s.1. as An. gambiae s.s. and 44% (342/777) as An. arabiensis. The molecular M and S forms represented 35.3 and 64.7% of the An. gambiae s.s. population, respectively. The An. funestus species-specific PCR identified 60% (239/401) of the An. funestus group as An. funestus s.s. and 40% (162/401) as An. rivulorum. The biting activity of An. funestus s.s. both indoors and outdoors attained a peak at 0200 and 2200 hrs, respectively, with a significant increase in the "pre-bed time" collections indoors (Chi2 = 6.15, df = 1, p < 0.05) and outdoors (Chi2 = 6.28, df = 1, p < 0.05). The overall outdoor collection was significantly higher (Chi2 = 28.23, df =3, p < 0.05) than that recorded indoors. The overall sporozoite rates for An. gambiae s.s., An. arabiensis and An. funestus were significantly different in both localities (Chi2 = 0.58, df = 2, p < 0.01). Infection rates in both indoor and outdoor collections were also different statistically (Chi2 = 0.67, df = 2, p < 0.01). INTERPRETATION & CONCLUSION: Large number and species of anopheline mosquitoes collected in the study area may be associated with the availability of aquatic breeding sites. A phenomenon leading to an increase in man-vector contact and a high level of risk to the crowded urban population is observed.     

Ingestion of Plasmodium falciparum sporozoites during transmission by anopheline mosquitoes.

We investigated the process of sporozoite transmission during blood feeding for Anopheles gambiae and An. stephensi experimentally infected with Plasmodium falciparum. When infective mosquitoes were fed 22-25 days postinfection on an anesthetized rat, sporozoites were detected in the midgut of 96.5% of 57 An. gambiae (geometric mean [GM] = 32.5, range 3-374) and in 96.2% of 26 An. stephensi (GM = 19.5, range 1-345). There were no significant differences between species either in salivary gland sporozoite loads or in the number of ingested sporozoites. There was a significant linear relationship between sporozoite loads and the numbers of ingested sporozoites for both An. gambiae (r = 0.38) and An. stephensi (r = 0.69). Subsequently, An. gambiae were tested for sporozoite transmission by allowing them to feed individually on a suspended capillary tube containing 10 microliters of blood. A total of 83.3% of 18 infective mosquitoes transmitted a GM of 5.9 (range 1-36) sporozoites. The same mosquitoes contained a GM of 23.4 (range 2-165) ingested sporozoites. The number of ingested sporozoites was related to sporozoite loads (r = 0.42) but not to the number of sporozoites ejected into capillary tubes. Ingested sporozoites remained in the midgut up to 10 hr after feeding. The comparable numbers of sporozoites ingested by infective mosquitoes in both experiments indicates that the actual number of sporozoites transmitted to the vertebrate host during blood feeding is significantly reduced by the blood ingestion process.(ABSTRACT TRUNCATED AT 250 WORDS)