Perspectives on the bacterial control of vectors in the tropics

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

Larvicides based on the bacteria Bacillus thuringiensis svar. israelensis (Bti) and Lysinibacillus sphaericus are effective and environmentally safe compounds for the control of dipteran insects of medical importance. They produce crystals that display specific and potent insecticidal activity against larvae. Bti crystals are composed of multiple protoxins: three from the three-domain Cry type family, which bind to different cell receptors in the midgut, and one cytolytic (Cyt1Aa) protoxin that can insert itself into the cell membrane and act as surrogate receptor of the Cry toxins. Together, those toxins display a complex mode of action that shows a low risk of resistance selection. L. sphaericus crystals contain one major binary toxin that display an outstanding persistence in field conditions, which is superior to Bti. However, the action of the Bin toxin based on its interaction with a single receptor is vulnerable for resistance selection in insects. In this review we present the most recent data on the mode of action and synergism of these toxins, resistance issues, and examples of their use worldwide. Data reported in recent years improved our understanding of the mechanism of action of these toxins, showed that their combined use can enhance their activity and counteract resistance, and reinforced their relevance for mosquito control programs in the future years.

Microbial control of mosquitoes: management of the upper rhine mosquito population as a model programme

For more than a decade, Bacillus thuringiensis israelensis (Bti) and B. sphaericus have been used successfully as biological control agents against mosquitoes in Germany. Over 1000 km(2) of breeding areas have been treated with Bti, resulting in a yearly reduction of the mosquito population by more than 90%, without evidence of any harmful impact on the environment. Here, Norbert Becker describes the control strategy and operational use of Bti and B. sphaericus in Germany.

Twenty-two years of blackfly control in the onchocerciasis control programme in West Africa

Twenty-two years after the launch of the Onchocerciasis Control Programme in West Africa (OCP), Jean-Marc Hougard and colleagues critically review the vector-control strategy adopted. They go on to identify the few hydrological basins where transmission of the infection remains difficult to control, to analyse the causes and to propose appropriate corrective measures on a case-by-case basis. Most of these measures, which are mainly based on ivermectin chemotherapy, will continue to be applied after the end of the OCP in 2002, under the control of the countries concerned.

Mosquitocidal toxins, genes and bacteria: the hit squad

Certain entomopathogenic species of bacilli and Clostridium produce one or more toxins that kill mosquito larvae even at concentrations in the picomolar range. Altogether, 19 distinct genes are known that encode mosquitocidal toxins, which vary in their potency, species specificity and mode of action. Unlike chemical insecticides, mosquitocidal bacilli used as larvicides are safe for animals and the environment, and do not affect non-pest insects. Mosquitocidal bacteria are effective to varying degrees against Culex, Anopheles and Aedes mosquito larvae, but their rapid sedimentation from the larval feeding zone, UV-light sensitivity and narrow host range have hampered their development. New genetic engineering approaches are being investigated that could overcome these limitations and allow stable expression of broad host range combinations of toxins in UV-resistant, buoyant recombinant bacteria, as discussed here by Alan Porter.

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.

Current status of malaria and potential for control

Malaria remains one of the world's worst health problems with 1.5 to 2.7 million deaths annually; these deaths are primarily among children under 5 years of age and pregnant women in sub-Saharan Africa. Of significance, more people are dying from malaria today than 30 years ago. This review considers the factors which have contributed to this gloomy picture, including those which relate to the vector, the female anopheline mosquito; to human activity such as creating new mosquito breeding sites, the impact of increased numbers of people, and how their migratory behavior can increase the incidence and spread of malaria; and the problems of drug resistance by the parasites to almost all currently available antimalarial drugs. In a selective manner, this review describes what is being done to ameliorate this situation both in terms of applying existing methods in a useful or even crucial role in control and prevention and in terms of new additions to the antimalarial armory that are being developed. Topics covered include biological control of mosquitoes, the use of insecticide-impregnated bed nets, transgenic mosquitoes manipulated for resistance to malaria parasites, old and new antimalarial drugs, drug resistance and how best to maintain the useful life of antimalarials, immunity to malaria and the search for antimalarial vaccines, and the malaria genome project and the potential benefits to accrue from it.

Resistance in a laboratory population of Culex quinquefasciatus (Diptera: Culicidae) to Bacillus sphaericus binary toxin is due to a change in the receptor on midgut brush-border membranes

Direct binding experiments with isolated brush border membrane fractions (BBMF) from larvae of a susceptible laboratory strain of Culex quinquefasciatus Say, indicated the presence of a single class of Bacillus sphaericus binary toxin receptors. The dissociation constant (Kd) was approximately 11 nM and the maximum binding capacity (Bmax) approximately 8 pmol/mg BBMF protein. Similar binding experiments with a field population of C. quinquefasciatus that had been selected in the laboratory to more than 100,000-fold resistance to B. sphaericus binary toxin failed to reveal the presence of any specific binding. Thus this resistant strain had lost the functional receptor for B. sphaericus toxin. The binding characteristics of BBMF from the F1 larval progeny (susceptible females x resistant males) were very close to those of the parental susceptible strain, consistent with the resistance being recessive.

Criteria for the selection of larvicides by the Onchocerciasis Control Programme in west Africa

One of the weekly decisions the Onchocerciasis Control Programme has to make, in its operations in 11 west African countries, is the selection of one insecticide out of the six used that is most appropriate to the river stretches to be treated. This decision depends on several criteria, linked not only to the compounds themselves but also the hydrological conditions and blackfly populations involved. Given the great number of breeding sites (gites) to be treated, in 23,000 km of rivers at the height of the rainy season, this paper identifies the parameters needed to facilitate the choice of insecticide and to optimize the larviciding in terms of cost effectiveness, management of resistance and minimizing the environmental impact.