Lure and kill

What is common to both chikungunya and dengue is the carrier of their causative viruses - the female of the mosquito type Aedes aegypti. Unlike other types, A. aegypti breeds in fresh water and is a day biter. More significantly, its eggs can survive even after the water dries up for a year and more. Increasing incidence of the two diseases, not only in urban and semi-urban areas but in rural areas as well, shows that conventional vector control methods are clearly proving to be inadequate and ineffective in combating A. aegypti.

The Defence Research and Development Establishment (DRDE), Gwalior, has developed a new insecticide for the mosquito based on a novel concept of "lure and kill". The principle is simple. Mosquitoes communicate through biological chemicals called pheromones and one such pheromone of larval origin attracts female mosquitoes to breeding sites, inducing them to lay more eggs. That is, larvae growing at a particular site signal that the place is safe for breeding. What DRDE scientists have done is to mix the pheromone, a chemical of biological origin, with an Insect Growth Regulator (IGR), a synthetic chemical, in a suitable formulation. The latter prevents hatching of the eggs and, even if some do, the larvae cannot attain the adult mosquito stage.

The key achievement is the development of a concoction in which the properties of one do not reduce the effectiveness of the other. The scientists have nicknamed this new vector control system "attracticide". "This has been demonstrated for the first time in the world," K. Sekhar, Director, DRDE, told Frontline.

Following detailed experiments during 2002-03, which clearly showed that the concept worked, the DRDE joined hands with the Municipal Corporation of Delhi (MCD) in 2004 to undertake field evaluation. In the first phase, such "ovitraps" were set up in 400 locations for controlled studies. These showed that there was a tremendous increase in the amount of eggs laid, as much as 10 times, Sekhar said.

This was followed by large area field trial this year. The Janakpuri locality in West Delhi, which had earlier witnessed a high incidence of dengue, was identified as the control area for the studies. Given the widespread use of `desert coolers' in summer in the urban areas of the North, these have become the main breeding sites for A. aegypti. So, 4,000 such coolers of Janakpuri became the "ovitraps" for this field trial in which the "attracticide" was added once a week to the water in the coolers. "There have been practically no cases of dengue from Janakpuri in the current outbreak," Sekhar points out. Apparently because of lack of response from the Health Ministries of the southern States, such large area trials there have not been possible. Limited trials by the Madurai unit of the Indian Council of Medical Research (ICMR) in Chennai and Thiruvananthapuram had shown good results, Sekhar said.

How about large-scale commercial production for use in vector control programmes? Publication of the findings after the laboratory studies in 2002-03 resulted in an American company evincing interest in the product and signing a confidentiality agreement with the DRDE for licensed production. "The patent for the product, for which the DRDE has applied, is awaited, after which the licence agreement will be signed," Sekhar said. As regards production in the country, while the IGR is a chemical already approved by the Central Insecticide Board (CIB), the pheromone that is being used needs to be cleared. The DRDE applied for the same a few months ago and its approval will be based on a couple of multi-centric trials, Sekhar said. "Once approved, the technology can be transferred to any Indian company for large scale production," he added. At present the formulation is in liquid form, but the DRDE is working on a tablet form.

"The toxicity is about 1,000 times less than conventional insecticides. It can be safely added even to drinking water," Sekhar said. Another interesting aspect is that the pheromone has been found to have "mosquito polymorphism"; that is, it has been found to be effective for the entire Aedes species, not A. aegypti alone. This was apparently demonstrated in Thiruvanathapuram where A. albopictus is also an important vector in transmission. As regards other species, different pheromones would probably be required, Sekhar said. One such has already been identified at the DRDE for the malaria vector Anopheles stephensi. "Studies are going on for developing a suitable product and we intend to apply for a patent for this too very soon," Sekhar added.

R. Ramachandran