Illustrated keys to the anopheline mosquitoes of Myanmar.

In the newly revised and illustrated keys to 4th instar larvae and adult female mosquitoes, the following 36 Anopheles species from Myanmar are included: Anopheles aconitus, An. aitkenii, An. annularis, An. argyropus, An. barbirostris, An. bengalensis, An. culicifacies, An. dirus, An. fluviatilis, An. gigas, An. insulaeflorum, An. jamesii, An. jeyporensis, An. karwari, An. kochi, An. kyondawensis, An. lindesayi, An. maculatus, An. majidi, An. minimus, An. nigerrimus, An. nitidus, An. pallidus, An. peditaeniatus, An. philippinensis, An. pseudojamesii, An. sinensis, An. splendidus, An. stephensi, An. subpictus, An. sundaicus, An. tessellatus, An. theobaldi, An. vagus, An. varuna, and An. willmori. The new keys presented in this paper will enable public health workers to rapidly identify mosquito vectors of malaria and to distinguish them from other species in the same genera.     

Effects of salinity on the larvae of some Afrotropical anopheline mosquitoes

ABSTRACT. Laboratory experiments showed that the larvae of some freshwater-breeding Afrotropical anopheline species exhibit various degrees of tolerance to 25% sea-water. At 24^oC, survival rates from egg to fourth instar larva were significantly less in salt-water than in freshwater for four species: 39.5% versus 55.4% for Anopheles tenebrosus Dönitz, 31.9% versus 86.2% for An. mousinhoi De Meillon & Pereira, 3.5% versus 67.9% for An. pharoensis Theobald and 3.9% versus 41.9% for An. quadriannulatus (Theobald). Conversely, An. merus Dönitz showed a significantly better survival rate of 46.4% in 25% sea-water compared with 15.5% in freshwater. Simulated winter conditions did not significantly change these differential survival rates of An. merus. Effects of salinity on the anal papillae of third instar larvae are discussed.     

Salivary gland apyrase determines probing time in anopheline mosquitoes

Salivary gland homogenates of adult female anopheline mosquitoes, of three different species, hydrolysed ATP and ADP, thereby demonstrating an apyrase activity. Total enzyme activity was greatest in the vector species A. freeborni (20.7 ± 2.4 mU/pair of glands) and least in the autogenous mosquito A. sp. nr. salbaii (3.0 ± 0.4 mU/pair of glands); another vector species, A. stephensi, produced intermediate levels of the enzyme (7.8 ± 0.7 mU/pair of glands). In all cases, the reaction was activated by divalent cations and maximal at pH 9.0 and in the presence of 2-mercaptoethanol. Apyrase activity in each salivary gland correlated with the degree of inhibition of ADP-induced platelet aggregation in vitro. Duration of probing correlated inversely with salivary apyrase content. We conclude that salivary apyrase largely determines a mosquito's ability to locate blood. Differential selective pressures for facility of blood location would have influenced the level of salivary apyrase in these mosquitoes.     

Use of an electrostatic sprayer for control of anopheline mosquitoes

The Electrodyn sprayer was compared with a compression sprayer (Hudson X-pert) for residual application of cypermethrin, a pyrethroid insecticide, to control the malaria vectors Anopheles arabiensis Patton and An. funestus Giles in experimental huts at Magugu in Tanzania. The time taken for hut spraying, 2-2.5 min per hut, was similar for both types of sprayer. Two or three huts were treated internally with cypermethrin at 40 or 80 mg a.i./m2 using Electrodyn formulation for comparison with 80 mg a.i./m2 using wettable powder formulation. Each of the twelve huts (including five untreated controls) was fitted with window exit traps and either louvre or verandah traps for mosquito sampling. The Electrodyn sprayer was fitted with a pair of elbowed deflectrodes to direct the positively charged spray droplets onto walls and ceiling. All treatments gave 94-100% mortality-rates of indoor-resting anophelines throughout the evaluation period of 11 weeks post-spray. Reductions of An.arabiensis and An.funestus females by 10-42% and 62-91%, respectively, in rooms and by 72% and 51% in exit traps indicated that cypermethrin deterred mosquitoes from entering the huts. Overall mortality-rates of mosquitoes were 66% of both species in huts treated with 40 mg/m2 Electrodyn, 43% An.funestus and 71% An.arabiensis due to 80 mg/m2 Electrodyn formulation and 49% An.funestus and 64% An.arabiensis due to 80 mg/m2 WP formulation (no significant differences). It is concluded that the Electrodyn sprayer with deflectrodes is a convenient and effective means of residual house-spraying with pyrethroid insecticide for malaria vector control.     

Fitness of anopheline mosquitoes expressing transgenes that inhibit Plasmodium development

One potential strategy for the control of malaria and other vector-borne diseases is the introduction into wild vector populations of genetic constructs that reduce vectorial capacity. An important caveat of this approach is that the genetic construct should have minimal fitness cost to the transformed vector. Previously, we produced transgenic Anopheles stephensi expressing either of two effector genes, a tetramer of the SM1 dodecapeptide or the phospholipase A2 gene (PLA2) from honeybee venom. Mosquitoes carrying either of these transgenes were impaired for Plasmodium berghei transmission. We have investigated the role of two effector genes for malaria parasite blockage in terms of the fitness imposed to the mosquito vector that expresses either molecule. By measuring mosquito survival, fecundity, fertility, and by running population cage experiments, we found that mosquitoes transformed with the SM1 construct showed no significant reduction in these fitness parameters relative to nontransgenic controls. The PLA2 transgenics, however, had reduced fitness that seemed to be independent of the insertion site of the transgene. We conclude that the fitness load imposed by refractory gene(s)-expressing mosquitoes depends on the effect of the transgenic protein produced in that mosquito. These results have important implications for implementation of malaria control via genetic modification of mosquitoes.     

An evolutionary view of the interactions between anopheline mosquitoes and malaria parasites

The transmission of malaria is governed by the mosquito vector's biting rate, its mortality, and the developmental period of the parasite within the mosquito. This review covers some data on the interactions among these parameters and describes possible evolutionary mechanisms underlying two aspects of the parasite's life cycle.     

Engorgement response of anopheline mosquitoes to blood fractions and artificial solutions

ABSTRACT. Anopheles stephensi Liston, Anopheles freeborni Aitken, Anopheles gambiae Giles and Anopheles dirus (Peyton & Harrison) fed equally well on whole blood, red blood cells, platelet-rich plasma and platelet-poor plasma. Similar feeding ability on 0.15 M NaCl containing 10^-2 M NaHCO₃ was shown by the first three species, but An. dirus required an addition of albumin. The need for ATP as a phagostimulant could not be demonstrated in any of these species.     

Mosquito immune responses and compatibility between Plasmodium parasites and anopheline mosquitoes

Background  

Functional screens based on dsRNA-mediated gene silencing identified several Anopheles gambiae genes that limit Plasmodium berghei infection. However, some of the genes identified in these screens have no effect on the human malaria parasite Plasmodium falciparum; raising the question of whether different mosquito effector genes mediate anti-parasitic responses to different Plasmodium species.     

Analysis of the sporogonic development of Plasmodium falciparum and Plasmodium berghei in anopheline mosquitoes

Basic knowledge of the sporogonic development of malarial parasites is crucial when evaluating the sporontocidal activity of antimalarial drugs or when determining why certain vectors are refractory to a particular parasite while others are competent vectors. We have developed a model which we have used to i) assess the sporogonic development of Plasmodium berghei ANKA in Anopheles stephensi and A. freeborni mosquitoes and ii) determine the effect of chloroquine on the sporogony of P. falciparum NF-54 in A. stephensi. Criteria used to assay sporogonic development include: i) number of oocysts present, ii) percentage of mosquitoes with oocysts, iii) time of release of sporozoites from the oocysts into the hemolymph, iv) time and degree of sporozoite invasion of salivary glands, and v) transmission (P. berghei) into vertebrate hosts. Parasite development in the mosquito is evaluated every other day, commencing on ca. day 7 post-feed (PF) and continuing until ca. day 22 PF. These detailed observations allow us to delineate the chronology of sporogonic development.     

Host choice of anopheline mosquitoes in a malaria endemic area of western Venezuela

Abstract. Bloodmeals of exophilic anopheline mosquitoes collected resting on vegetation in a malaria endemic area in western Venezuela were identified by ELISA. Using a TMB peroxidase substrate in the ELISA, human bloodmeals were readily identified up to 40 h after ingestion in all laboratory-fed mosquitoes tested. Assay sensitivity declined to 75% identifiable 44 h post-feeding.
The Human Blood Index and the Feeding Index of each species differed between the three villages studied. An.triannulatus was generally more anthropophilic than An.nuneztovari and An.oswaldoi. These contrasting results emphasize the difficulties of interpreting host choice data.     

Rio Meta strain of Plasmodium vivax in New World monkeys and anopheline mosquitoes

An archived strain of Plasmodium vivax, isolated from Rio Meta, northern Colombia, in 1972 was adapted to grow in splenectomized Aotus lemurinus griseimembra and A. nancymai monkeys. Anopheles freeborni, An. maculatus, An. dirus, An. culicifacies, and An. albimanus were shown to be susceptible to infection by feeding on infected monkeys. Infections were more readily obtained by feeding on A. L. griseimembra than on A. nancymai. Transmission through sporozoites was obtained in an A. l. griseimembra monkey after a prepatent period of 24 days.     

Production of polymorphic sperm by anopheline mosquitoes and their fate within the female genital tract

The males of two mosquito species within the Anopheles gambiae complex, An. gambiae s.s. and An. quadriannulatus, as well as males of An. darlingi, produced sperm of significantly varying lengths, while a sperm polymorphism was absent in Aedes aegypti and other anophelines not suspected of belonging to species complexes. The polymorphic distribution of these sperm lengths was not significantly different in smaller adult males that were reared on a low larval diet. The reproductive tract of the female was more likely to contain larger sperm, but overall sperm retention varied depending on the size of the female and the volume of the spermatheca she contained. The presence of a sperm polymorphism may be a factor that has promoted speciation, as well as providing an indication that females may mate multiply.     

Spatially explicit analyses of anopheline mosquitoes indoor resting density: implications for malaria control

Background

The question of sampling and spatial aggregation of malaria vectors is central to vector control efforts and estimates of transmission. Spatial patterns of anopheline populations are complex because mosquitoes'' habitats and behaviors are strongly heterogeneous. Analyses of spatially referenced counts provide a powerful approach to delineate complex distribution patterns, and contributions of these methods in the study and control of malaria vectors must be carefully evaluated.

Methodology/Principal Findings

We used correlograms, directional variograms, Local Indicators of Spatial Association (LISA) and the Spatial Analysis by Distance IndicEs (SADIE) to examine spatial patterns of Indoor Resting Densities (IRD) in two dominant malaria vectors sampled with a 5×5 km grid over a 2500 km² area in the forest domain of Cameroon. SADIE analyses revealed that the distribution of Anopheles gambiae was different from regular or random, whereas there was no evidence of spatial pattern in Anopheles funestus (Ia = 1.644, Pa<0.05 and Ia = 1.464, Pa>0.05, respectively). Correlograms and variograms showed significant spatial autocorrelations at small distance lags, and indicated the presence of large clusters of similar values of abundance in An. gambiae while An. funestus was characterized by smaller clusters. The examination of spatial patterns at a finer spatial scale with SADIE and LISA identified several patches of higher than average IRD (hot spots) and clusters of lower than average IRD (cold spots) for the two species. Significant changes occurred in the overall spatial pattern, spatial trends and clusters when IRDs were aggregated at the house level rather than the locality level. All spatial analyses unveiled scale-dependent patterns that could not be identified by traditional aggregation indices.

Conclusions/Significance

Our study illustrates the importance of spatial analyses in unraveling the complex spatial patterns of malaria vectors, and highlights the potential contributions of these methods in malaria control.     

Comparison of sampling anopheline mosquitoes by light-trap and human-bait collections indoors at Bagamoyo, Tanzania

Abstract. The mosquito sampling efficiency of CDC miniature light-traps, relative to night-biting collections, was evaluated indoors at two sites in coastal Tanzania. We found that the total number of anophelines captured overnight by light-traps (hung beside a bednet in use) was 1.23 times the number of anophelines captured by human-bait collections. This relationship was not affected significantly by changes in the mosquito density, order of trapping method, date of sampling, or number of household occupants. Malaria sporozoite rates were twice as high among mosquitoes captured by light-trap as compared to those captured by night-biting collection. This was attributed to the tendency of light-traps to capture a larger proportion of gravid mosquitoes, which also had high sporozoite rates. The differences in sporozoites rates according to abdominal stage indicates that unfed mosquitoes captured by light-traps may define more precisely the human-biting activity and sporozoite rates as seen by night-biting collections. Our study shows that light-traps, when used in combination with night-biting collections, can be an effective and sensitive means for measuring human-biting activity and the sporozoite rate.     

Malaria infection potential of anopheline mosquitoes sampled by light trapping indoors in coastal Tanzanian villages

Abstract. Anopheline mosquito populations were studied during 1992 in seven villages south of Bagamoyo, coastal Tanzania, prior to malaria control intervention using insecticide treated bednets. To collect mosquitoes, CDC light traps were used in ten houses per village fortnightly for 12 months. Anopheles females were identified and checked by ELISA for the presence of malaria sporozoite antigen and source of bloodmeal. An. funestus peaked in June-July after the long rains. Three members of the An. gambiae complex had different seasonality: An. arabiensis, An. gambiae and small numbers of An. merus were collected.
In most villages transmission was extremely high and perennial with the entomological inoculation rate reaching three to eleven infective bites per person per night in July and persisting at around 0.1 and 1 for most of the remainder of the year. Sporozoite infection rates within the An. gambiae complex ranged from 2% to 25%, with the peaks in January and July following the two rainy periods. An. funestus showed a similar pattern. The light traps were reliable, simple to operate, and proved to be satisfactory to study the mosquito vector population.     

Adaptation of the AMRU-1 strain of Plasmodium vivax to Aotus and Saimiri monkeys and to four species of anopheline mosquitoes.

A chloroquine-resistant strain of Plasmodium vivax (AMRU-1) from Papua New Guinea has been adapted to grow in 4 species of Aotus monkeys (Aotus lemurinus griseimembra, Aotus vaciferans, Aotus nancymai, and Aotus azarae boliviensis), hybrid Aotus monkeys, and Saimiri boliviensis monkeys. Whereas it was possible to infect Saimiri monkeys with this parasite by inoculation of parasitized erythrocytes, only 42% of Saimiri monkeys became infected, compared to 92% of Aotus monkeys attempted. Comparative mosquito feedings showed that only A. vociferans, A. l. griseimembra, and Saimiri boliviensis monkeys produced infections in mosquitoes. Oocysts were observed on the guts of the 4 species of mosquitoes used (Anopheles gambiae, Anopheles stephensi, Anopheles freeborni, and Anopheles dirus), but sporozoite transmission was effected only with the intravenous inoculation of sporozoites from An. dirus into an A. l. griseimembra monkey.