Anopheles culicifacies Y-chromosome dimorphism indicates sibling species (B and E) with different malaria vector potential in Sri Lanka

In Sri Lanka, malaria is transmitted mainly by Anopheles culicifacies Giles sensu lato (Diptera: Culicidae). In India, this nominal taxon comprises sibling species A, B, C, D and E, distinguished by their chromosome morphology. Species B (identified by polytene chromosome sequence Xab, 2g1 + h1) is not such an efficient vector of malaria as other members of the An. culicifacies complex in India. All specimens of An. culicifacies s.l. examined from Sri Lanka possess Xab, 2g1 + h1 polytenes, previously interpreted as species B, despite their important vector status. Recently, species E was described from Rameshwaram Island (Tamil Nadu, India) between Sri Lanka and the Indian mainland, where both species B and E are sympatric. Species B and E share polytene sequence Xab, 2g1 + h1 but differ by the mitotic Y-chromosome being acrocentric in species B, submetacentric in species E, the latter implicated as vector of vivax malaria. From May 1999 to January 2000, we surveyed Y-chromosomes of male progeny from An. culicifacies Xab, 2g1 + h1 females collected from cattle bait in diverse malarious districts of Sri Lanka: Badulla, Monaragala, Puttalam and Trincomalee. Karyotypes of readable quality were obtained from 42/83 families examined, with overall proportions 24% acrocentric and 76% submetacentric Y-chromosome carriers, both types being sympatric in at least 3/4 localities sampled. By analogy with the situation on Rameshwaram Island, we interpret these observations to demonstrate widespread presence of two members of the An. culicifacies complex in Sri Lanka, their karyotypes being compatible with species B and E, the latter predominant and having greater vector potential.     

Responses of Anopheles culicifacies sibling species A and B to DDT and HCH in India: implications in malaria control

Differential responses of Anopheles culicifacies Giles sibling species A and B to DDT were evident from higher survival rate of species B in laboratory bioassays and greater proportions of species B in DDT-sprayed villages of northern India, compared with those under HCH pressure. Both species A and B have become almost completely resistant to HCH in this area due to regular house-spraying with HCH for about the last 10 years. Because species A predominates in northern India, where it has been incriminated as an important vector of malaria, and species A is more susceptible than species B to DDT, it is suggested that DDT would control malaria transmission more effectively than HCH in this situation. Monitoring of insecticide resistance in species A is therefore recommended as the basis for future choice of insecticides to be used by the National Malaria Eradication Programme.     

Multiple host feeding in field populations of Anopheles culicifacies and An. subpictus in Sri Lanka

A histological technique was used to detect the multiple blood-feeding of female mosquitoes, within the same gonotrophic cycle, in field populations of the malaria vectors Anopheles culicifacies and An. subpictus (Diptera: Culicidae) at a village in the low country of Sri Lanka, during 1994-96. Among 3306 An. culicifacies and 871 An. subpictus engorged females examined, respectively, 34.4% and 30.4% were multiple-fed. In these two species, double meals accounted for 92.7% and 89.5%, and triple meals for 7.3% and 10.5% of multiple meals, respectively. More females of An. culicifacies (53.7%) than An. subpictus (44.5%) multiple-fed on different nights. Multiple feeding rates in the two species were independent of sample size and field abundance. In An. culicifacies, but not An. subpictus, these rates were proportional to ambient temperature but not to humidity. Bloodmeal ELISA showed that 8.3% of 242 An. culicifacies were human-fed and 80% of these (i.e. 6.6%) were concurrently bovid-fed. Combining the histological and ELISA data, it was estimated that multiple feeding involving a human host had occurred in 7.2% of the blood-feeding An. culicifacies population at this study site. Only 1.2% of 406 An. subpictus were human-fed and all of these were concurrently bovid-fed. Multiple bloodfeeding within the same gonotrophic cycle was attributed to a local 'frequent feeding strategy' in these primarily zoophagic and endophilic malaria vectors. Unfortunately this strategy enhances their human-biting rate and hence their vectorial capacity.     

Abundance, biting behaviour and parous rate of anopheline mosquito species in relation to malaria incidence in gold-mining areas of southern Venezuela

Abstract A longitudinal entomological and epidemiological study was conducted in five localities of southern Venezuela between January 1999 and April 2000 to determine the abundance, biting behaviour and parity of anopheline mosquitoes (Diptera: Culicidae) in relation to climate variables and malaria incidence. A total of 3685 female anopheline mosquitoes, representing six species, were collected. The most abundant species were Anopheles marajoara Galvão & Damasceno (60.7%) and Anopheles darlingi Root (35.1%), which together represented 95.8% of the total anophelines collected. Abundance and species distribution varied by locality. Malaria prevalence varied from 12.5 to 21.4 cases per 1000 population. Transmission occurred throughout the year; the annual parasite index (API) for the study period was 813.0 cases per 1000 population, with a range of 71.6?2492 per 1000 population, depending on locality. Plasmodium vivax (Grassi & Feletti) (Coccidia: Plasmodiidae) accounted for 78.6% of cases, Plasmodium falciparum (Welch) for 21.4% and mixed infections (Pv+Pf) for < 0.1%. Anopheles marajoara and An. darlingi were more abundant during the rainy season (April–September). There was no significant correlation (P > 0.05) between mosquito abundance and rainfall. Correlations between malaria incidence by parasite species and mosquito abundance were not significant (P > 0.05). Monthly parous rates were similar for An. marajoara and An. darlingi throughout the year, with two peaks that coincided with the dry?rainy transition period and the period of less rain. Peaks in the incidence of malaria cases were observed 1 month after major peaks in biting rates of parous anophelines. Anopheles darlingi engages in biting activity throughout the night, with two minor peaks at 23.00–00.00 hours and 03.00–04.00 hours. Anopheles marajoara has a different pattern, with a biting peak at 19.00?21.00 hours and 76.6% of biting occurring before midnight. Although both vectors bite indoors and outdoors, they showed a highly significant (P < 0.01) degree of exophagic behaviour. The present study constitutes the first effort to characterize the bionomics of anophelines in malaria endemic foci in different ecological situations in relation to malaria transmission in southern Venezuela and to provide relevant information to be considered when planning and implementing vector control programmes.     

Presence of Anopheles culicifacies B in Cambodia established by the PCR-RFLP assay developed for the identification of Anopheles minimus species A and C and four related species

Abstract A polymerase chain reaction‐restriction fragment length polymorphism (PCR‐RFLP) assay developed for identification of five species of the Anopheles minimus Theobald group and a related mosquito species of the Myzomyia Series (Diptera: Culicidae) was applied to morphologically identified adult female specimens collected in Ratanakiri Province, north‐eastern Cambodia. In addition to finding An. aconitus Dönitz, An. minimus species A and An. pampanai Büttiker & Beales, some specimens showed a new restriction banding pattern. Siblings of specimens that exhibited this new PCR‐RFLP pattern were morphologically identified as An. culicifacies James sensu lato. Based on nucleotide sequences of the ribonuclear DNA internal transcribed spacer 2 region (ITS2) and the mitochondrial cytochrome oxidase I gene (COI), these specimens were recognized as An. culicifacies species B (sensu Green & Miles, 1980 ), the first confirmed record of the An. culicifacies complex from Cambodia. This study shows that the PCR‐RFLP assay can detect species not included in the initial set‐up and is capable of identifying at least seven species of the Myzomyia Series, allowing better definition of those malaria vector and non‐vector anophelines in South‐east Asia.     

PCR assay for identification of Anopheles quadriannulatus species B from Ethiopia and other sibling species of the Anopheles gambiae complex

Sibling species A and B of Anopheles quadriannulatus (Theobald) are recognized as allopatric members of the Anopheles gambiae Giles complex of Afrotropical mosquitoes (Diptera: Culicidae). Species A represents An. quadriannulatus sensu stricto, widespread in southern Africa, whereas An. quadriannulatus species B occurs in Ethiopia. Because of difficulty of identification, distribution of An. quadriannulatus sensu lato remains poorly known. Cytotaxonomy and the standard DNA polymerase chain reaction (PCR) assay do not distinguish between species A and B of An. quadriannulatus. By optimizing the standard PCR assay (Scott et al., 1993) for identification of members of the An. gambiae complex, we identified two discriminant fragments of 153 bp and 900 bp from DNA of An. quadriannulatus species B, whereas only the 153 bp fragment was amplified for species A from South Africa. This modified PCR assay can therefore be used to distinguish between species A and B of An. quadriannulatus s.l. as well as other members of the An. gambiae complex.     

Pyrethroid resistance/susceptibility and differential urban/rural distribution of Anopheles arabiensis and An. gambiae s.s. malaria vectors in Nigeria and Ghana

Resistance to pyrethroid insecticides and DDT caused by the kdr gene in the malaria vector Anopheles gambiae Giles s.s. (Diptera: Culicidae) has been reported in several West African countries. To test for pyrethroid resistance in two more countries, we sampled populations of the An. gambiae complex from south-western Ghana and from urban and rural localities in Ogun State, south-west Nigeria. Adult mosquitoes, reared from field-collected larvae, were exposed to the WHO-recommended discriminating dosage of exposure for 1 h to DDT 4%, deltamethrin 0.05% or permethrin 0.75% and mortality was recorded 24 h post-exposure. Susceptibility of An. gambiae s.l. to DDT was 94-100% in Ghana and 72-100% in Nigeria, indicating low levels of DDT resistance. Deltamethrin gave the highest mortality rates: 97-100% in Ghana, 95-100% in Nigeria. Ghanaian samples of An. gambiae s.l. were fully susceptible to permethrin, whereas some resistance to permethrin was detected at 4/5 Nigerian localities (percentage mortalities 75, 82, 88, 90 and 100%), with survivors including both An. arabiensis Patton and An. gambiae s.s. identified by PCR assay. Even so, the mean knockdown time was not significantly different from a susceptible reference strain, indicating absence or low frequency of kdr-type resistance. Such low levels of pyrethroid resistance are unlikely to impair the effectiveness of pyrethroid-impregnated bednets against malaria transmission. Among Nigerian samples of An. gambiae s.l., the majority from two urban localities were identified as An. arabiensis, whereas the majority from rural localities were An. gambiae s.s. These findings are consistent with those of M. Coluzzi et al. (1979). Differences of ecological distribution between molecular forms of An. gambiae s.s. were also found, with rural samples almost exclusively of the S-form, whereas the M-form predominated in urban samples. It is suggested that 'urban island' populations of An. arabiensis and of An. gambiae s.s. M-form in the rainforest belt of West Africa might be appropriate targets for elimination of these malaria vectors by the sterile insect technique.