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Wolbachia is a bacterium that
lives only within insect cells and is passed from one generation to the next
through the insect’s eggs. It is present in up to 70% of all the different
species of insects around us including some mosquitoes that bite people but not
the major mosquito species involved in the transmission of diseases such as
dengue and malaria.
For many years now scientists have been
studying Wolbachia
looking for ways to utilise the bacterium to potentially control the mosquitoes
that spread human diseases.
Wolbachia pipientis was first
observed in the ovaries and testes of the mosquito, Culex pipiens in the 1920’s. Early studies showed that it was not a
pathogen of mammals but instead a naturally occurring and harmless symbiotic bacterium of insects.
Since those early studies it has since been determined that it is extremely
common in insects, with estimates suggesting that 60-70% of all insect species
naturally carry different strains of Wolbachia.
Considering that there may be 2-5 million different insect species on the
planet Wolbachia
is a very successful and pervasive insect bacterium.
The diagram above explains Cytoplasmic Incompatibility and how by releasing a limited number of mosquitoes with Wolbachia to breed with wild type mosquitoes, over a small number of generations, will result in all the mosquitoes having Wolbachia.
a) When male mosquitoes with Wolbachia mate with female wild mosquitoes that don’t have Wolbachia those females will have eggs but they won’t hatch.
b) When male mosquitoes with Wolbachia mate with females that are already carrying Wolbachia the mating will be normal and the offspring will all have Wolbachia.
c) When female mosquitoes with Wolbachia mate with males without Wolbachia all her offspring will have Wolbachia.
Many
different strains of Wolbachia
Eliminate Dengue is currently field-testing two Wolbachia strains - wMel and wMelPop - with other Wolbachia strains in different stages of development.
Each strain has a slightly different effect on the mosquito’s fitness and consequently how easily it will establish in the wild mosquito population once it is released. Wolbachia strains also differ in terms of their ability to block dengue virus inside the mosquito. Over time we hope to select strains that have a strong blocking effect on dengue virus in the mosquito and therefore prevent dengue transmission and yet are easily introduced into mosquitoes in the field and thereby provide a low-cost, long-term solution to dengue control.
Bacillus thuringiensis israelensis (Bti) for the control of dengue vectors:
systematic literature review
Objective
To systematically review the literature on the
effectiveness of Bacillus thuringiensis israelensis (Bti),
when used as a single agent in the field, for the control of dengue vectors.
Method
Systematic literature search of the published and grey
literature was carried out using the following databases: MEDLINE, EMBASE,
Global Health, Web of Science, the Cochrane Library, WHOLIS, ELDIS, the New
York Academy of Medicine Gray Literature Report, Africa-Wide and Google. All
results were screened for duplicates and assessed for eligibility. Relevant
data were extracted, and a quality assessment was conducted using the CONSORT
2010 checklist.
Results
Fourteen studies satisfied the eligibility criteria,
incorporating a wide range of interventions and outcome measures. Six studies
were classified as effectiveness studies, and the remaining eight examined the
efficacy of Bti in more controlled settings. Twelve (all eight efficacy
studies and 4 of 6 effectiveness studies) reported reductions in entomological
indices with an average duration of control of 2–4 weeks. The two
effectiveness studies that did not report significant entomological reductions
were both cluster-randomised study designs that utilised basic interventions
such as environmental management or general education on environment control
practices in their respective control groups. Only one study described a
reduction in entomological indices together with epidemiological data,
reporting one dengue case in the treated area compared to 15 dengue cases in
the untreated area during the observed study period.
Conclusion
While Bti can be effective in reducing the number
of immature Aedes in treated containers in the short term, there is very
limited evidence that dengue morbidity can be reduced through the use of Bti
alone. There is currently insufficient evidence to recommend the use of Bti
as a single agent for the long-term control of dengue vectors and prevention of
dengue fever. Further studies examining the role of Bti in combination
with other strategies to control dengue vectors are warranted.
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