Vectorial Capacities Research Articles

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.

Multiple insecticide resistance mechanisms involving metabolic changes and insensitive target sites selected in anopheline vectors of malaria in Sri Lanka.

BackgroundThe current status of insecticide resistance and the underlying resistance mechanisms were studied in the major vector of malaria, Anopheles culicifacies, and the secondary vector, Anopheles subpictus in five districts (Anuradhapura, Kurunegala, Moneragala, Puttalam and Trincomalee) of Sri Lanka. Eight other anophelines, Anopheles annularis, Anopheles barbirostris, Anopheles jamesii, Anopheles nigerrimus, Anopheles peditaeniatus, Anopheles tessellatus, Anopheles vagus and Anopheles varuna from Anuradhapura district were also tested.MethodsAdult females were exposed to the WHO discriminating dosages of DDT, malathion, fenitrothion, propoxur, λ-cyhalothrin, cyfluthrin, cypermethrin, deltamethrin, permethrin and etofenprox. The presence of metabolic resistance by esterase, glutathione S-transferase (GST) and monooxygenase-based mechanisms, and the sensitivity of the acetylcholinesterase target site were assessed using synergists, and biochemical, and metabolic techniques.ResultsAll the anopheline species had high DDT resistance. All An. culicifacies and An. subpictus populations were resistant to malathion, except An. culicifacies from Kurunegala, where there was no malathion carboxylesterase activity. Kurunegala and Puttalam populations of An. culicifacies were susceptible to fenitrothion. All the An. culicifacies populations were susceptible to carbamates. Both species were susceptible to the discriminating dosages of cypermethrin and cyfluthrin, but had different levels of resistance to other pyrethroids. Of the 8 other anophelines, only An. nigerrimus and An. peditaeniatus were resistant to all the insecticides tested, probably due to their high exposure to the insecticides used in agriculture. An. vagus showed some resistance to permethrin.Esterases, GSTs and monooxygenases were elevated in both An. culicifacies and An. subpictus. AChE was most sensitive to insecticides in Kurunegala and Trincomalee An. culicifacies populations and highly insensitive in the Trincomalee An. subpictus population.ConclusionThe complexity of the resistance segregating in these field populations underlines the need for new molecular tools to identify the genomic diversity, differential upregulation and different binding specificities of resistance conferring genes, and the presence of different subspecies with different vectorial capacities.

Distribution of Anopheles in Vietnam, with particular attention to malaria vectors of the Anopheles minimus complex

BackgroundThe distribution of anopheline mosquitoes in Vietnam was examined, with a particular interest for the two sibling species of the Anopheles minimus complex (Cellia: Myzomyia), An. minimus and Anopheles harrisoni, respectively former species A and C. Because the morphological identification of both sibling species is difficult and may lead to misidentifications, accurate data on their respective distribution are missing. This is of fundamental importance since the two species seem to exhibit differential vectorial capacities for malaria transmission.MethodsLarge entomological surveys based on cattle collections and molecular identifications of An. minimus s.l. were carried out in 23 sites throughout northern, central and south-eastern regions of Vietnam.ResultsBased on previous molecular works and our data, the distribution of anopheline species and the relative densities of An. minimus and An. harrisoni were mapped. It is noteworthy that there was a high specific biodiversity at each study site. Anopheles minimus s.l. and Anopheles sinensis were the main anopheline species in the northern region, whereas Anopheles aconitus and Anopheles vagus were the most frequent ones in the central region. The southern limit of An. harrisoni was increased to the latitude of 11°N. Sympatry between both sibling species has been extended to new provinces.ConclusionMalaria transmission is still high in central Vietnam and along bordering countries. Therefore, it is important to know and map the precise distribution of the main and secondary malaria vectors in Vietnam for applying efficient vector control programmes. Moreover, these maps should be regularly updated and linked to environmental characteristics relative to disease epidemiology, and environmental and climatic changes occurring in southeast Asia.

Molecular homogeneity in diverse geographical populations of Phlebotomus papatasi (Diptera, Psychodidae) inferred from ND4 mtDNA and ITS2 rDNA: Epidemiological consequences

An intraspecific study on Phlebotomus papatasi, the main proven vector of Leishmania major among the members of the subgenus Phlebotomus, was performed. The internal transcribed spacer 2 (ITS 2) of rDNA and the ND4 gene of mt DNA were sequenced from 26 populations from 18 countries (Albania, Algeria, Cyprus, Egypt, Greece, India, Iran, Israel, Italy, Lebanon, Morocco, Saudi Arabia, Spain, Syria, Tunisia, Turkey, Yugoslavia and Yemen), and compared. Samples also included three other species belonging to the subgenus Phlebotomus: P. duboscqi, a proven vector of L. major in the south of Sahara (three populations from Burkina Faso, Kenya and Senegal), P. bergeroti, a suspected vector of L. major (three populations from Oman Sultanate, Iran and Egypt), and one population of P. salehi from Iran. A phylogenetic study was carried out on the subgenus Phlebotomus. Our results confirm the validity of the morphologically characterized taxa. The position of P. salehi is doubtful. Variability in P. papatasi contrasts with that observed within other species having a wide distribution like P. (Paraphlebotomus) sergenti in the Old World or Lutzomyia (Lutzomyia) longipalpis in the New World. Consequently, it could be hypothesized that all populations of P. papatasi over its distribution area have similar vectorial capacities. The limits of the distribution area of L. major are correlated with the distribution of common rodents acting as hosts of the parasites.

Influence of larval density or food variation on the geometry of the wing of Aedes (Stegomyia) aegypti

Variation in wing length among natural populations of Aedes (Stegomyia) aegypti (L.) (Diptera: Culicidae) is associated with different vectorial capacities. Geometric morphometrics allowed us to use a more powerful estimator of wing size ('centroid size'), as well as to visualize the variation of wing shape, to describe the effects of density or food variation at larval stage on 20 anatomical landmarks of the wing of A. aegypti. Almost perfect correlations between (centroid) size and larval density or size and larval food were observed in both sexes: a negative correlation with increasing density and a positive one with increasing amount of food. The allometric component of shape change was always highly significant, with stronger contribution of size to shape under food effects. Within each experiment, either food or density effects, and excluding extreme conditions, allometric trends were similar among replicates and sexes. However, they differed between the two experiments, suggesting different axes of wing growth. Aedes aegypti size is highly sensible to food concentration or population density acting at larval stages. As larger individuals could be better vectors, and because of the stronger effect of food concentration on size, vector control activities should pay more attention in eliminating containers with rich organic matter. Furthermore, as a simple reduction in larval density could significantly increase the size of the survivors, turning them into potentially better vectors, the control activities should try to obtain a complete elimination of the domestic populations.

RDNA-ITS2-PCR assay for grouping the cryptic species of Anopheles culicifacies complex (Diptera: Culicidae)

Anopheles culicifacies, a predominant vector of malaria in India exists as a complex of five sibling species A, B, C, D and E, of which, except species B, all the rest are vectors with varying vectorial capacities. With a combination of PCR assays, it is possible to identify all the five members of this species complex. These assays include amplification of the rDNA-ITS2 region followed by digestion of the ITS2 amplicon using restriction enzyme, Rsa I which groups the five members of the An. culicifacies complex into two categories: species A and D forming one category and species B, C and E forming another. The samples grouped thus are then subjected to two allele-specific PCR assays (AD-PCR and BCE-PCR), which has been designed using sequence differences in the mitochondrial cytochrome oxidase II (CO II) subunit. The AD-PCR assay distinguishes species A and D, whereas the BCE-PCR assay distinguishes species B, C and E. In the present study, the differences in the ITS2 region of the five species was used to design a PCR assay which groups the five members into the same two categories as obtained after digestion of the ITS2-PCR product. This assay uses a common forward primer based on the 5.8S region and two reverse primers, which is specific for the two categories. Amplification of a PCR product of size 253 bp indicates the presence of species A/D, while a product of size 409 bp indicates the presence of species B/C/E. By using this ITS2 PCR assay, the three-step procedure is reduced to two cutting down the time and cost involved. The ITS2 PCR assay has been validated on specimens collected from different regions of India and the results confirm to the earlier reports on the distribution of the members of the An. culicifacies complex.

Identification of sex pheromones of Lutzomyia longipalpis (Lutz & Neiva, 1912) populations from the state of São Paulo, Brazil

In Brazil, four populations of Lutzomyia longipalpis each producing different sex pheromones are recognised. It has been suggested that these chemotype populations represent true sibling species. In this study we present the results of an analysis, by coupled gas chromatography-mass spectrometry, of the pheromones of males L. longipalpis from two different municipalities of the state of São Paulo. Our study showed that L. longipalpis from these two municipalities produced different sex pheromones from each other. This coupled with the remarkable difference between the epidemiological situation in Araçatuba and Espírito Santo do Pinhal, suggests that the (S)-9-methylgermacrene-B and cembrene-1 populations may have different vectorial capacities.

Vecteurs de paludisme : du terrain à la génétique moléculaire Recherches en Afrique

Only about 60 Anopheline species transmit malaria among more than 3,000 mosquito species recorded in the world. In Africa, the major vectors are Anopheles gambiae,An. arabiensis, An. funestus, An. nili and An. moucheti. They all belong to species complexes or groups of closely related species that are very difficult to set apart on morphological grounds, but which may have highly variable behaviours and vectorial capacities. Understanding this complexity is of major importance in vector control programs or for implementing any public health intervention program such as drugs or vaccine trials. Among the seven species of the complex,Anopheles gambiaes.s. shows a huge chromosomal polymorphism related to adaptation to specific natural or anthropic environments, from equatorial forested Africa to dry sahelian areas. Recent studies conducted in West and Central Africa suggest an incipient speciation into 2 molecular forms provisionally called M and S. A similar evolutionary phenomenon is observed in An. funestus, in which sympatric populations carrying specific chromosomal paracentric inversions showed restricted gene flow. Distribution of species from An. nili group and An. moucheti complex is restricted to more humid regions of Africa. However in some areas these species play the major role in malaria transmission. Comprehensive knowledge of transmission cycles and of behavioural and underlying genetic heterogeneities that exist within and among natural vector populations will thus benefit the whole area of malaria control and epidemiology. Molecular and genetic studies, as well as in depth monitoring of vector biology, have been recently facilitated by advances in functional and comparative genomics, including recent publication of the nearly complete genome sequence of An. gambiae. Challenge for the next years is to answer to the very simple question: why is an insect a vector?

Distribution of the species of the Anopheles gambiae complex and first evidence of Anopheles merus as a malaria vector in Madagascar

BackgroundMembers of the Anopheles gambiae complex are amongst 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 various bioclimatic domains of Madagascar. Local abundance of an unexpected member of this complex raised questions regarding its role in malaria transmission.MethodsSampling took place at 38 sites and 2,067 females were collected. Species assessment was performed using a PCR targeting a sequence in the IGS of the rDNA. Analysis focused on the relative prevalence of the species per site, bioclimatic domain and altitude. Infectivity of Anopheles merus was assessed using an ELISA to detect the presence of malarial circumsporozoite protein in the head-thorax.ResultsThree species were identified: An. gambiae, Anopheles arabiensis and An. merus. The distribution of each species is mainly a function of bioclimatic domains and, to a lesser extent, altitude. An. arabiensis is present in all bioclimatic domains with highest prevalence in sub-humid, dry and sub-arid domains. An. gambiae has its highest prevalence in the humid domain, is in the minority in dry areas, rare in sub-humid and absent in sub-arid domains. An. merus is restricted to the coastal fringe in the south and west; it was in the majority in one southern village. The majority of sites were sympatric for at least two of the species (21/38) and two sites harboured all three species.The role of An. merus as malaria vector was confirmed in the case of two human-biting females, which were ELISA-positive for Plasmodium falciparum.ConclusionDespite the huge environmental (mainly man-made) changes in Madagascar, the distribution of An. gambiae and An. arabiensis appears unchanged for the past 35 years. The distribution of An. merus is wider than was previously known, and its effectiveness as a malaria vector has been shown for the first time; this species is now on the list of Malagasy malaria vectors.

ITS 2 sequences heterogeneity in Phlebotomus sergenti and Phlebotomus similis (Diptera, Psychodidae): possible consequences in their ability to transmit Leishmania tropica

An intraspecific study on Phlebotomus sergenti, the main and only proven vector of Leishmania tropica among the members of the subgenus Paraphlebotomus was performed. The internal transcribed spacer 2 (ITS2) sequences of 12 populations from 10 countries (Cyprus, Egypt, Italy, Lebanon, Morocco, Pakistan, Portugal, Spain, Syria, and Turkey) were compared. Samples also included three species closely related to P. sergenti: Phlebotomus similis (three populations from Greece and Malta), Phlebotomus jacusieli and Phlebotomus kazeruni. Our results confirm the validity of the taxa morphologically characterised, and imply the revision of their distribution areas, which are explained through biogeographical events. At the Miocene time, a migration route, north of the Paratethys sea would have been followed by P. similis to colonise the north of the Caucasus, Crimea, Balkans including Greece and its islands, and western Turkey. Phlebotomus sergenti would have followed an Asiatic dispersion as well as a western migration route south of the Tethys sea to colonise North Africa and western Europe. This hypothesis seems to be well supported by high degree of variation observed in the present study, which is not related to colonisation or to intra-populational variation. Two groups can be individualised, one oriental and one western in connection with ecology, host preferences and distribution of L. tropica. We hypothesise that they could be correlated with differences in vectorial capacities.

Variation of the vectorial capacity of some anophelines in western Venezuela.

The vectorial capacities of Anopheles nuneztovari, An. albitarsis s.l., and An. triannulatus to transmit Plasmodium vivax in western Venezuela were estimated in different seasons. In general, there was a decrease in the vectorial capacities of the three species during the dry season, which seems to be due mainly to a reduction in the mosquito biting populations. Comparing the observations made during the two wet seasons, there was a decrease in the vectorial capacities of the three species due to different factors for each species. In An. nuneztovari, the reduction in vectorial capacity seems to be due to a reduction in the human blood index (HBI); in An. albitarsis, it is probably due to a reduction in the HBI and a slight reduction in parity, whereas in An. triannulatus, the cause is probably a reduction in the mosquito biting population and in parity. The overall reduction in vectorial capacities of the three anopheline species in 1989 compared with the vectorial capacities in 1988 could partly explain the marked reduction in prevalence from 46.2 per 1,000 population in 1988 to 7.5 per 1,000 population in 1989.

Minimal efficacy requirements for malarial vaccines to significantly lower transmission in epidemic or seasonal malaria

The effect of vaccines against the pre-erythrocytic, the erythrocytic and the sexual stages of malaria on the transmission of malaria was examined using a simulation model of seasonal or epidemic transmission. The primary outcome of the model was the number of cases of malaria present at the end of the transmission season. A range of efficacy for the vaccine, or of coverage of the population and of vectorial capacities were tested for each vaccine type. For a similar level of efficacy or coverage under conditions simulating hypo or mesoendemic malaria, all types of vaccines gave similar decreases in the number of people infected. Although all vaccine types gave significant reductions in malaria incidence at efficacy or coverage levels considerably lower than that required to completely block transmission, this reduction in case numbers was highly dependant on the vectorial capacity. The simulation model is equally applicable to other transmission blocking measures such as insecticide treated bed nets.

Malaria transmission potential of Anopheles mosquitoes in the Mwea-Tebere irrigation scheme, Kenya

1. Anopheles arabiensis Patton and An. funestus Giles were identified as vectors of Plasmodium falciparum malaria in the Mwea-Tebere irrigation scheme, Kenya. An. arabiensis was the only member of the An. gambiae complex identified from chromosome characteristics. Other Anopheles species found included An. pharoensis Theobald, An. rufipes Gough and An. coustani Laveran. Survival rates per gonotrophic cycle for An. arabiensis averaged 0.37 during the short rains (October-November), 0.49 during the dry season (February) and 0.78 during the long rains (May-June). Vectorial capacities were correspondingly low due to low survival rates and a high degree of zoophily. The average duration of infective life for P. falciparum was 0.2 days for both An. arabiensis and An. funestus. In contrast, entomological inoculation rates were comparatively high: 6-8 infective bites/man/month. An. pharoensis averaged 110 bites/man/night during the short rains; 1/999 (0.1%) was positive by ELISA for P. falciparum circumsporozoite antigen, but the ELISA evidence is not conclusive for vector incrimination. In correspondence with clinical observations, the transmission of P. malariae and P. ovale is unlikely due to the low vector survival rates. The observed anomaly between low vectorial capacities and high entomological inoculation rates demonstrates the importance of accurately estimating vector sporozoite rates to monitor unstable malaria transmission in irrigated areas.

Biting prevalence and malaria transmission patterns in the Anopheles punctulatus complex (Diptera: Culicidae) in Papua New Guinea.

Seasonal prevalence and malaria transmission patterns of Anopheles farauti, An. punctulatus and An. koliensis were studied in two coastal and two inland villages in the Madang district of northern Papua New Guinea. The monthly manbiting rates (average biting density of vector mosquitoes per man per night) were observed over a two-year period to establish typical seasonal prevalence patterns. Observations on the frequency of feeding and the human blood index were combined to derive the manbiting habit. A daily survival rate was determined from the dissection of mosquito ovaries by Polovodova's age-grading method. The extrinsic cycle in the mosquito was not observed, and the value for this parameter was extracted from the literature. All parameters were combined to estimate the monthly variations in vectorial capacity of each of the vector species, and the vectorial capacities of the individual species were added together to give a total monthly vectorial capacity of the punctulatus complex in each locality. The results showed that malaria transmission can be perennial, with a period of more intense transmission between August and January in the southern area, and between September and February in the northern. A period of very low transmission can be expected between February/March and July/August.