Scientists at the ICMR-Vector Control Research Centre (VCRC) are working on cross-breeding new generations of mosquitoes with a specific bacteria that will render them incapable of transmitting dengue among humans.
VCRC scientists have bred vectors injected with Wolbachia, an endo-symbiotic bacteria that naturally occurs in 60% of the insects and inhibits infection and transmission of dengue from the bacteria-hosting mosquitoes. The eggs of the vector aedes aegypti were injected with the bacterium supplied by Monash University in Australia.
The cross-bred local strains will be released into the field strictly under regulatory conditions set by the Government of India. Biosafety of this technology was being evaluated to secure the approval of regulatory bodies for this method.
“We have crossed 25 generations of local strains of mosquitoes from the progeny race and are moving towards getting regulatory approvals for the pilot release of Wolbachia harbouring vectors into the community next year,” Ashwani Kumar, VCRC Director, told The Hindu .
And the benefit of the experiment could be three-pronged as it would target three major killer diseases transmitted by the Aedes species, namely dengue, zika disease and chikungunya, he said. The VCRC has developed and maintains at its insectaria colonies of two strains of aegypti carrying Wolbachia — wMel Aedes aegypti (Pud) and wAlbB Aedes aegypti (Pud).
The colonies of these strains, which do not undergo any change in genetic codes while gaining the ability to inhibit the virus, would be released in controlled volumes from stations to mix and mate with regular aedes mosquitoes, and in the long run outnumber the native species.
“The colony proportion released on a weekly basis from release stations within a square kilometre would be determined by the vector population in the area,” Dr. Ashwani Kumar said.
At present, Wolbachia in Aedes aegypti has been shown to inhibit the replication and transmission of dengue virus.
The Wolbachia harbouring Aedes aegypti strains are being tested in different countries for their potency to establish in nature and control dengue.
The cross-bred carrying Aedes aegypti mosquitoes have been released in small-scale field trials in Australia, Brazil, Columbia, Indonesia and Vietnam. Largescale citywide field trials have been conducted in Yogyakarta city in Indonesia, Rio de Janeiro in Brazil, and Medellín in Colombia.
“The reduction of dengue incidence in some places, where the Wolbachia hosting mosquitoes overwhelmed native vectors, has been an astonishing 70%-95%,” Dr. Ashwani Kumar said. The WHO Vector Control Advisory Group recently recommended Wolbachia-based control of aedes -borne diseases as one of the biological control methods.
According to VCRC scientists, there is neither a specific drug for treating dengue nor an effective vaccine. Controlling the vector mosquito is the only option to prevent transmission of this disease. Use of chemical insecticides, biological control agents and prevention of mosquito breeding are the methods of vector control.
However, these measures have yielded only a limited success in reducing the number of dengue cases not only in India but also in other countries. Environmental sanitation – “source reduction” – is the best method, but is not always feasible in all situations, S. Sabesan, senior consultant at VCRC, pointed out.
“At present, there are too few studies on whether a similar strategy would work on the anopheles species, which are vectors of malaria,” the Director said.
Even if immediately successful in reducing incidence of dengue, the work does not end for VCRC scientists. “We need more long-term studies to see if the Wolbachia presence sustains or declines over a period of time before the project can qualify as a sustainable success,” Dr. Ashwani Kumar added.