Keeping mosquitoes at bay, the Vietnamese way
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THE EXPLOSION of chikungunya cases in India, with well over a million people across seven States falling prey to the disease, has given fresh impetus to the need to eliminate the Aedes aegypti mosquito.
Aedes aegypti not only spreads the chikungunya virus but also the virus that causes dengue. These are both diseases for which no vaccines are currently available. The only way to stop people becoming infected and of the disease spreading is to control the mosquito population. But this mosquito is an elusive foe that rests indoors and bites during the daytime. It breeds in a wide variety of water containers, from water tanks, cisterns, and jars found in houses to rubbish, discarded plastic containers, and abandoned vehicle tyres that trap little puddles of water. The difficulty in eradicating this mosquito has led to a dramatic spread of dengue in recent decades. The disease is now endemic in more than 100 countries and two-fifths of the world's population are at risk of getting the disease, according to the World Health Organisation. A novel effort carried out in several communes of Vietnam in recent years is, however, showing that a community-based approach using biological control, rather than insecticides, can be highly effective against this mosquito. In each commune, the Vietnamese project team and relevant provincial and district-level representatives would meet the commune leader and local health centre staff to discuss the mosquito control strategy and gain their consent, according to an account published in the prestigious medical journal The Lancet last year. Scientific survey Well-trained entomologists from the district and provincial health departments carry out a high-quality scientific survey to establish what containers produced the most mosquitoes and therefore needed to be targeted for control. The old dogma used to be that all types of water containers were equally important, Peter Ryan, head of the Mosquito Control Laboratory of the Queensland Institute of Medical Research in Australia, told The Hindu in the course of a telephonic interview. "We realise now that in many areas certain types of water-storage containers produce 90 per cent of the dengue mosquitoes." Tiny creatures known as Mesocyclops, which have a prodigious appetite for the mosquitoes and their larvae, were introduced into water containers. The Mesocyclops existed naturally in water tanks and containers in Vietnam. So it was merely a question of showing local communities how to effectively manage these natural control agents, says Dr. Ryan. Australian scientists have been helping to implement the control strategy in Vietnam. Several health volunteers were appointed in each commune to work part-time on mosquito control. They would inspect water-storage containers in every house once a month and make sure that the householder knew how to manage the Mesocyclops in those containers. Dr. Ryan emphasises that it was not simply a matter of the health volunteers eliminating the mosquitoes for the householders. The householders needed to be closely involved to ensure that the containers in their houses were not producing mosquitoes during the intervening weeks. "Aedes aegypti is fairly unique in that the flight range of this mosquito is quite short. So if someone gets dengue within a house, it is probably due to mosquitoes breeding at their own house or at their neighbour's house," he adds. School children and all groups in society were actively involved. Children carry home information about where mosquitoes breed and how to control them, participate in clean-up campaigns to remove discarded containers, and help old people with mosquito-control measures. It took about a year for these control measures to have a significant impact on mosquito densities, he says. This was therefore a preventive method that needed to be sustained over the long term. If the control programme was discontinued after the mosquitoes were eliminated, the mosquitoes would return and recolonise the water containers. The control strategy has already been implemented in 46 communes in Vietnam, according to Dr. Ryan. The mosquito had been completely eliminated in 40 of those communes and the remaining six communes showed very low densities of the mosquito. Expansion of the control strategy to more communes is planned. But the strategy could not be used in urban areas in Vietnam where the breeding sites of the mosquito were usually not suitable for deploying Mesocyclops, he says. It was the fear of dengue that had motivated people to participate in the mosquito control programme. "If people don't see dengue as a problem, it is very difficult to run these community-based programmes," remarks Dr. Ryan. Source: The Hindu |
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