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It all began with Ronald Ross: 100 years of malaria research and control in Sierra Leone (1899-1999).

It was in Sierra Leone, 100 years ago in 1899, that human malarial parasites were first observed in wild-caught Anopheles gambiae and An. funestus, the principal vectors of malaria in Africa. In the same year, Ronald Ross initiated the first antilarval measures for malaria control. This paper reviews 100 years of malaria field research and control in Sierra Leone, which became known as the 'White Man's Grave' in the 19th century largely because of the high malaria-related mortality amongst Europeans living there. The establishment of a field laboratory for the Liverpool School of Tropical Medicine in Freetown in 1920 made Sierra Leone the centre for malaria field research in Africa up to and during the Second World War. Eminent malariologists including Ronald Ross, Samuel Christophers, George Macdonald, Leonard Bruce-Chwatt, Brian Maegraith, Ian Macgregor, Brian Greenwood and Michael Service visited Sierra Leone for malaria-related activities. This review highlights the tremendous efforts made towards defining the epidemiological picture of the disease and the most effective means of combatting it. Malaria control in Sierra Leone, as in many other parts of the world, used to be based largely on mosquito eradication. However, experience gained over the past 100 years has shown that mosquito control is often not cost-effective in areas where the interruption of transmission cannot be sustained. Emphasis should now be on early diagnosis, treatment with effective antimalarials, and the selective use of preventive measures including vector control and insecticide-treated materials where they can be sustained.

History, 19th Century↗

Malaria control at the district level in Africa: the case of the muheza district in northeastern Tanzania.

An assessment was done in Tanzania to determine the extent to which the primary health care services have contributed to reducing the burden of malaria since the system was initiated in the 1980s. Seven descriptive processes and outcome indicators of effectiveness were used: changes of malaria transmission and incidence over time; use of facility-based care services for malaria; patients' access to professional advice; the trend of treatment failure over time of sulfadoxine-pyrimethamine and chloroquine; survival rates of severe cases at the district hospital; a district malaria control strategy; number of malaria specific training for care providers; and the number of activities carried out on mosquito control measures. The data were collected from 1996 to 2003 in the Muheza district northeastern Tanzania. It covered household interviews with a stratified sample of 1,250 respondents, and in-depth interviews with all 175 health care providers in the 35 health facilities within the district. All six members of the district health management team were also interviewed. Additional data came from dispensary and hospital records, and published literature. The results show an unchanged malaria disease burden. The average number of clinical malaria episodes per child less than five years of age remained between 3 and 3.5 episodes per year in the district since the 1960s. The comparison of cases expected in the population less than five years old with those seen in the district health facilities shows a coverage rate of 33%. Furthermore, between 1990 and 2003, little training on malaria was provided to health staff. The findings imply a limited effectiveness of district health services on malaria control, suggesting a weak process of translating national malaria goals to activities at the district level.

Antimalarials↗

Cyt1A from Bacillus thuringiensis restores toxicity of Bacillus sphaericus against resistant Culex quinquefasciatus (Diptera: Culicidae).

The 2362 strain of Bacillus sphaericus, which produces a binary toxin highly active against Culex mosquitoes, has been developed recently as a commercial larvicide. It is being used currently in operational mosquito control programs in several countries including Brazil, France, India, and the United States. Laboratory studies have shown that mosquitoes can develop resistance to B. sphaericus, and low levels of resistance have already been reported in field populations in Brazil, France, and India. To develop tools for resistance management, the Cyt1A protein of Bacillus thuringiensis subsp. israelensis De Barjac was evaluated for its ability to suppress resistance to B. sphaericus in a highly resistant population of Culex quinquefasciatus Say. A combination of B. sphaericus 2362 in a 10:1 ratio with a strain of B. thuringiensis subsp. israelensis that only produces Cyt1A reduced resistance by >30,000-fold. Resistance was suppressed completely when B. sphaericus was combined with purified Cyt1A crystals in a 10:1 ratio. Synergism was observed between the Cyt1A toxin and B. sphaericus against the resistant mosquito population and accounted for the marked reduction in resistance. However, no synergism was observed between the toxins against a nonresistant mosquito population. These results indicate that Cyt1A could be useful for managing resistance to B. sphaericus 2362 in Culex populations, and also provide additional evidence that Cyt1A may synergize toxicity by enhancing the binding to and insertion of toxins into the mosquito microvillar membrane.

Animals↗

Engineering RNA interference-based resistance to dengue virus type 2 in genetically modified Aedes aegypti.

Mosquitoes (Aedes aegypti) were genetically modified to exhibit impaired vector competence for dengue type 2 viruses (DENV-2). We exploited the natural antiviral RNA interference (RNAi) pathway in the mosquito midgut by constructing an effector gene that expresses an inverted-repeat (IR) RNA derived from the premembrane protein coding region of the DENV-2 RNA genome. The A. aegypti carboxypeptidase A promoter was used to express the IR RNA in midgut epithelial cells after ingestion of a bloodmeal. The promoter and effector gene were inserted into the genome of a white-eye Puerto Rico Rexville D (Higgs' white eye) strain by using the nonautonomous mariner MosI transformation system. A transgenic family, Carb77, expressed IR RNA in the midgut after a bloodmeal. Carb77 mosquitoes ingesting an artificial bloodmeal containing DENV-2 exhibited marked reduction of viral envelope antigen in midguts and salivary glands after infection. DENV-2 titration of individual mosquitoes showed that most Carb77 mosquitoes poorly supported virus replication. Transmission in vitro of virus from the Carb77 line was significantly diminished when compared to control mosquitoes. The presence of DENV-2-derived siRNAs in RNA extracts from midguts of Carb77 and the loss of the resistance phenotype when the RNAi pathway was interrupted proved that DENV-2 resistance was caused by a RNAi response. Engineering of transgenic A. aegypti that show a high level of resistance against DENV-2 provides a powerful tool for developing population replacement strategies to control transmission of dengue viruses.

Aedes↗

Evaluation of methoprene (Altosid XR) sustained-release briquets for control of culex mosquitoes in urban catch basins.

A sustained-release, briquet formulation of methoprene (Altosid XR), applied at a rate of one briquet per catch basin in Saginaw, Michigan, provided ca. 70% reduction in emergence of Culex pipiens and Cx. restuans adults, compared with nontreated catch basins, during a period of 15 wk in the summer of 1990. In a parallel study using one briquet per 10.5 liter bucket, there was 99% reduction in adult emergence of these species for a period of 12 weeks. The difference between catch basins and buckets may be attributable to water movement through the catch basins with each rainfall, causing a dilution of methoprene through time. However, both studies indicated that the briquets released methoprene for 12-15 wk, suggesting that this formulation may offer season-long control of Culex mosquitoes from urban catch basins in Michigan, with a single treatment of insecticide.

Animals↗

An assessment of mosquito breeding and control in four surface flow wetlands in tropical-subtropical Australia.

In Queensland, Australia, the tropical-subtropical climate is ideal to promote macrophyte growth in surface flow wetlands; however, there have been concerns that constructed wetlands are potential breeding sites for disease-bearing mosquitoes. The aim of this study was to assess whether mosquitoes were breeding in these constructed wetlands, and if so, where they breed, and what parameters might influence breeding: e.g. water quality, vegetation, or macroinvertebrate communities. A study of four surface flow constructed wetlands located in different climatic regions was undertaken. Mosquito larvae were sampled using 240 ml dippers and macroinvertebrates using dip nets. The wetland with the greatest biodiversity of macrophytes and macroinvertebrates had the least number of mosquito larvae (< 1% of all dips). Samples with most mosquito larvae occurred amongst dense mats of Paspalum grass or dead Typha. Despite the presence of larvae in some parts of these wetlands very few late instars or pupae were found i.e. completion of the mosquito life cycle to adult mosquitoes was unsuccessful. This study has shown that the presence of mosquito larvae can be minimised by increasing macroinvertebrate biodiversity, by planting a variety of macrophyte types and species, excluding aggressive plant species, and maintaining at least 30% open water. Macroinvertebrates are probably a crucial factor in the control of mosquito larvae ensuring that predation of the early instars prevents or limits the development of pupae and the emergence of adults.

Animals↗

[Evaluation of the inhibiting activity of the diflubenzuron on the ecdysis of larvae of Aedes aegypti (Linnaeus, 1762) (Diptera, Culicidae)].

The inhibiting activity of diflubenzuron on the ecdysis of Aedes aegypti larvae was evaluated, with a view to using this product in mosquito control. This study also aimed to determine the interaction between this product, the type of artificial containers and the susceptibility of the mosquito. Bioassays were carried out in the backyard of a residence, using seven kinds of artificial habitats: tires, glass, concrete roofing, cans, plastic containers, cement and pottery. In each kind of artificial habitat, 20 Aedes aegypti larvae in the 4th instar were set. The same number of larvae was used as a control. Each test was repeated five times and the observation of mortality was done once every 24 hours, until 100% mortality was reached at 1 ppm. There was no significant difference between the main surveillance periods of the larvae, nor between the various kinds of artificial habitats. A significant difference was found between the instars, in that the 3rd instar was the most resistant to diflubenzuron inhibiting activity. It was also shown that concentrations did not interact with instars or material of the artificial habitats at the 5% significance level.

Aedes↗

Susceptibility of various mosquitoes of California to subperiodic Brugia malayi.

Laboratory experiments were conducted to determine the susceptibility of six species of mosquitoes, representing three genera, to subperiodic Brugia malayi. The black-eye, Liverpool strain of Aedes aegypti was the susceptible control. Mosquitoes were fed on microfilaremic jirds (Meriones unguiculatus). All mosquitoes, except wild caught Culex erythrothorax, were laboratory-reared and allowed to feed when 8 to 10 days old. Anopheles freeborni, Anopheles hermsi, and Culiseta inornata proved refractory. Both Anopheles species allowed invasion of flight muscle and development to the late first stage, after which larval growth ceased and melanization occurred. Culiseta inornata prevented any larval development. Culex tarsalis and Cx. erythrothorax proved highly susceptible to B. malayi infection. In all, 95.6% and 88.7% of the Cx. tarsalis harbored third-stage larvae after infective feedings of 15.7 and 81.8 mf/microliters of blood, respectively, while only 11.5% were found susceptible when microfilaremia was low (1.1 mf/microliters). Culex erythrothorax demonstrated a susceptibility rate of 82.3% with 17.0 mf/microliters. Both Culex species appear to be excellent experimental hosts for subperiodic B. malayi. This is the first conclusive evidence that mosquitoes of the genus Culex can naturally support the complete development of a stain of subperiodic B. malayi.

Animals↗

Detection and characterization of Wolbachia infections in Wuchereria bancrofti (Spirurida: Onchocercidae) var. pacifica and Aedes (Stegomyia) polynesiensis (Diptera: Culicidae).

Despite control programs based on mass drug administration (MDA) of microfilaricidal compounds, Bancroftian lymphatic filariasis remains a problem in French Polynesia. For an alternative strategy to MDA, we investigated the potential role of Wolbachia to control filarial transmission. Wolbachia are intracellular alpha-proteobacteria endosymbionts that infect a broad range of insects and nematodes. These bacteria have a suspected role in the pathogenesis of filariasis. They also may be useful in mosquito control through cytoplasmic incompatibility. To detect and characterize these bacteria in the filarial and mosquito-vectors in French Polynesia, a survey was conducted on field-collected mosquitoes and microfilariae from infected people. Samples were analyzed by a polymerase chain reaction and gene sequencing. The results indicate that these bacteria are widespread. Sequence analysis of the wsp and ftsZ genes positioned the Aedes polynesiensis Wolbachia in cluster A and Wuchereria bancrofti var. pacifica Wolbachia in cluster D. The implications for possible improved treatment and vector control are discussed.

Aedes↗

Expression of mosquito active toxin genes by a Colombian native strain of the gram-negative bacterium Asticcacaulis excentricus.

Mosquito control with biological insecticides, such as Bacillus sp. toxins, has been used widely in many countries. However, rapid sedimentation away from the mosquito larvae feeding zone causes a low residual effect. In order to overcome this problem, it has been proposed to clone the Bacillus toxin genes in aquatic bacteria which are able to live in the upper part of the water column. Two strains of Asticcacaulis excentricus were chosen to introduce the B. sphaericus binary toxin gene and B. thuringiensis subsp. medellin cry11Bb gene cloned in suitable vectors. In feeding experiments with these aquatic bacteria, it was shown that Culex quinquefasciatus, Aedes aegypti, and Anopheles albimanus larvae were able to survive on a diet based on this wild bacterium. A. excentricus recombinant strains were able to express both genes, but the recombinant strain expressing the B. sphaericus binary toxin was toxic to mosquito larvae. Crude protease A. excentricus extracts did not degrade the Cry11Bb toxin. The flotability studies indicated that the recombinant A. excentricus strains remained in the upper part of the water column longer than the wild type Bacillus strains.

Animals↗

Immune defense mechanisms of Culex quinquefasciatus (Diptera: Culicidae) against Candida albicans infection.

Mosquitoes have an efficient defense system against infection. The cellular immune defense mechanism initiated by the mosquito Culex quinquefasciatus infected with the fungus Candida albicans was investigated in this study. Differences in the hemocyte counts in hemolymph perfused from uninoculated, saline-inoculated, and C. albicans-infected mosquitoes were compared using a light microscope. Phagocytosis was also investigated using electron microscopy. Four types of hemocytes were identified in control mosquitoes: prohemocytes (9.8%), plasmatocytes (38.8%), granular cells (44.2%), and oenocytoids (7.3%). Between 3 and 18 h postinoculation the total hemocyte count was significantly higher in infected, compared to uninfected, mosquitoes. Differential hemocyte counts from infected mosquitoes at 3, 6, and 18 h after inoculation showed that the relative proportion of plasmatocytes (48.6, 50.7, 45%) was higher and, concomitantly, the proportion of granular cells was lower (38, 36.8, 35%, respectively). Yeast cells were phagocytosed and limited growth was observed within the plasmatocytes. Melanized nodules were found attached to different insect tissues at 24 to 72 h following infection. These results suggest that phagocytosis, followed by nodule formation, was capable of clearing the hemolymph of yeast cells.

Animals↗

Effects of sustained-release methoprene and a combined formulation of liquid methoprene and Bacillus thuringiensis israelensis on insects in salt marshes.

Aquatic insects are an important component of the food web in salt marshes, therefore it is necessary to test whether pesticides used to control mosquitoes in salt marshes are safe for nontarget insects. We tested the nontarget effects of a combined formulation (duplex) of Bacillus thuringiensis israelensis (B.t.i.) and liquid methoprene (an insect development regulator) or sustained-release methoprene pellets (Altosid(R) pellets) by applying these materials to replicated salt marsh ponds at maximum label rates. Untreated ponds served as controls. We measured effects of the pesticides by rearing immature mosquitoes (Aedes dorsalis) and water boatmen (Trichocorixa reticulata) in predator-exclusion cages and by monitoring uncaged populations of invertebrates using replicated sweep-net samples. Both pesticides killed caged mosquitoes, and the activity of the Altosid(R) pellets continued through 99 days. There were no detectable effects of either pesticide on the survival or maturation of T. reticulata, or on abundances of uncaged invertebrates. The long-term activity of the pellets could help minimize mosquito abatement activity in salt marshes where there are breeding birds or endangered species. However, other studies suggest that this advantage needs to be balanced against the risks that sustained-release formulations could lead to development of resistance in mosquitoes or that initially undetected nontarget effects could build over time.

Aedes↗

Efficacy of various Bacillus thuringiensis var. israelensis formulations against Psorophora columbiae larvae as assessed in small rice plots, 1984-88.

Granular, liquid and briquette formulations of Bacillus thuringiensis var. israelensis were tested in small rice plots during 1984-88 against Psorophora columbiae larvae. Vectobac AS, Skeetal G, Teknar HPD, ABG 6172, ABG 6188, ABG 6193, ABG 6197, ABG 6199, ABG 6138F and ABG 6221 provided excellent control at high dosages and good control (generally greater than 85%) at relatively low concentrations. ABG 6221 and ABG 6172 provided excellent control at low concentrations. Additional testing of these formulations will be required to ascertain specific lower limits such that more economical larval mosquito control can be obtained.

Agriculture↗

Status of Plasmodium vivax malaria in the Republic of Korea during 2001-2003.

The Republic of Korea experienced a re-emergence of Plasmodium vivax malaria in 1993. The incidence of this disease increased rapidly through 2000 with its geographic distribution expanding from the vicinity near the Demilitarized Zone to the adjacent outlying areas. However, the number of cases of P. vivax malaria since that time period has decreased. A total of 2,538 cases occurred in 2001, and this decreased to 1,761 cases and 1,164 cases in the two subsequent years. A total of 5,463 cases of P. vivax malaria were reported from 2001 through 2003; 25.26% (1,380) were reported among Republic of Korea military personnel, 27.48% (1,501) were among veterans who had been discharged from the military within two years, and 47.26% (2,582) were among the civilian population. Mosquito control activities by the North Korean and South Korean governments, chemoprophylaxis of Republic of Korea Army personnel, and the low level of Anopheles mosquitoes in 2001 may have been factors responsible for the decreasing number of malaria cases. However, local transmission might have taken place in urban regions of the malaria-risk areas that are within 30 km south of the Demilitarized Zone. Extensive intervention and continued surveillance are warranted to prevent the epidemic from re-expanding and to eliminate this disease in the Republic of Korea.

Animals↗

The use of bacterial larvicides in mosquito and black fly control programmes in Brazil.

Bacillus spp. based larvicides are increasingly replacing, with numerous advantages, chemical insecticides in programmes for controlling black fly and mosquito populations. Brazil was among the pioneers in adopting Bacillus thuringiensis israelensis (B.t.i) to control black flies. However, the major current mosquito control programme in Brazil, the Programme for Eradication of Aedes aegypti launched in 1997, only recently decided to replace temephos by B.t.i based larvicides, in the State of Rio de Janeiro. In the last decade, works developed by research groups in Brazilian institutions have generated a significant contribution to this subject through the isolation of B. sphaericus new strains, the development of new products and the implementation of field trials of Bacillus efficacy against mosquito species under different environmental conditions.

Animals↗

A new design of membrane feeder incorporating an electrical blood stirring device.

A selection of the literature illustrating the origin and development of membrane feeding devices is reviewed. A new membrane feeding device-incorporating an electrical stirrer is described and its mode of operation explained. The sedimentation of red blood cells within a membrane feeding device, and the consequent hazard to arthropod feeding, is prevented by the use of the redesigned feeding unit and electrical blood stirrer. The mosquito feeding rate and mean blood meal size (mg) were unaffected by the action of the stirrer when compared to those in an unstirred control. Mosquitoes, bugs, ticks and tsetse flies have been fed sucessfully using the apparatus.

Animals↗