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| Controlling the dengue mosquito | | |
N. Gopal Raj
Public health experts are unanimous that mosquito control has to be a continuous process, with the government and local communities working together.
ALL TOO briefly, when deaths occur, the glare of the media spotlight falls on the issue of mosquito-borne diseases. A flurry of activity then ensues to reduce mosquito numbers. These sporadic attempts at mosquito control, however, fail to have any lasting impact and the insects soon return — and so too do the diseases they carry.
Currently, dengue and chikungunya are both very much in the news. As there are currently no vaccines against either of these viral diseases, the only way to protect people is to target the mosquito that spreads them.
The culprit in the case of both diseases is Aedes aegypti, a mosquito that once dwelled in forests and has, like rats and cockroaches, adapted to live as an unwelcome visitor in human habitation. The unwilling human hosts also end up providing blood meals for the female mosquito, which may well repay them by passing on disease-causing germs present in its body.
Erratic or non-existent water supply that forces families to store water in their houses as well as poor sanitation and hygiene that allow garbage to accumulate provide plentiful breeding spots for the mosquito. The unplanned growth of towns and cities has exacerbated these problems.
Water containers of all sorts, be they water tanks or jars, are natural breeding grounds for the mosquito. Disposable cups and plastic containers that are carelessly thrown away can trap sufficient rainwater for mosquito eggs to develop, notes P.K. Das, Director of the Vector Control Research Centre at Puducherry. Old vehicle tyres are another favoured breeding site, he says. In Delhi, room coolers too are used by the mosquitoes to lay eggs.
The eggs of this mosquito are exceptionally hardy, says N. Arunachalam, deputy director of the Centre for Research in Medical Entomology in Madurai. While the eggs of other mosquitoes would rapidly die in the absence of water, those of Aedes aegypti can survive for a year. As a result, suspect water containers must not only be emptied but also thoroughly scrubbed to remove any eggs that may remain.
Globally, controlling the dengue-carrying mosquito has proved difficult. The prevalence of dengue has grown dramatically in recent decades, says the World Health Organisation (WHO). Before 1970, only nine countries had experienced epidemics of dengue haemorrhagic fever, a potentially lethal complication; that number increased more than four-fold by 1995. The spread of dengue could be attributed to the expanding geographical distribution of the viruses and of the mosquitoes that transmit them, it added.
An estimated 50 million dengue infections occur globally every year and one case of dengue haemorrhagic fever requiring hospitalisation occurs somewhere in the world every minute.
In 1986, a massive epidemic of dengue, principally in Delhi, led to more than 2 lakh people becoming infected. The number of dengue cases in the country during a year usually ranges from 7,000 to 16,000, according to information on the website of the Vector Control Research Centre.
But experts believe the true figures are likely to be substantially higher than the official statistics reveal as many people are likely to have been treated privately. Dengue haemorrhagic fever is already the leading cause of hospitalisation and death among children, according to the Vector Control Research Centre.
In recent decades, only Cuba and Singapore have successfully controlled Aedes aegypti, according to Duane Gubler, an internationally known expert on dengue and other vector-borne diseases who is currently Director of the Asia-Pacific Institute of Tropical Medicine and Infectious Diseases at Hawaii. Both countries had used a combination of government-run top-down initiatives with community-based bottom-up programmes.
Mosquito control had to be an ongoing programme, not just an emergency response, Dr. Gubler told The Hindu . "That is where most governments make the mistake: they don't do anything until there is an epidemic. Then it is too late."
In a WHO-sponsored guide published a few years back, he pointed out that, in the face of uncontrolled urbanisation, top-down approaches to mosquito control that had been successfully used in the past were no longer practical. "A city of 10 million people will have approximately 2 million households that would need to be visited and checked for mosquitoes weekly; this is simply not feasible."
Moreover, mosquito control had to be "an ongoing programme that never ends as long as the threat of epidemic dengue transmission exists." Such sustainability would come only through community participation in mosquito control programmes, he added.
It is worth recalling that the Malaria Eradication Programme carried out in India during the 1950s and 1960s was remarkably effective. As a result of the programme, the incidence of malaria plummeted from an estimated 75 million cases and eight lakh deaths in 1953 to one lakh cases and no deaths by 1965-66. (Since then, mosquito populations and malaria cases have rebounded although not to the old levels.)
High priority was given to mosquito control in the 1950s and 1960s, recalls V. Ramachandran, former Chief Secretary of Kerala and currently a member of the Second Administrative Reforms Commission set up by the Union Government. But these days mosquito control received very low priority at all levels of government. Besides, the old importance given to public health was no longer there and the health system was increasingly focussed on treatment, not prevention, he told The Hindu .
A paper published in the WHO's Dengue Bulletin of 2003 pointed to the heavy reliance often placed on space spraying of insecticide for adult mosquito control. "This method must be repeated at frequent intervals, its cost is high and its effectiveness is variable," the experts pointed out. Moreover, adult mosquito populations quickly rebound after spraying because larval habitats remained largely unaffected.
But as the paper observed, public trust in such measures was often high and the complacency it produced "only increases the challenge of explaining the need for community involvement in the control of larval habitats."
Water containers the culprits
Denying the mosquito access to water containers is an effective way to stop it from multiplying. Studies have found that the mosquito does not breed equally in all types of water containers. Rather, just a few types of containers are likely to produce most of the mosquitoes. A study published this year by the U.N.-supported Special Programme for Research and Training in Tropical Diseases found that in Venezuela a single type of water barrel helped breed between 25 and 55 per cent of all dengue mosquito larvae while bottles, although the most common containers, contributed less than five per cent. In Cuba, containers in use accounted for only 11 per cent of the adult mosquitoes; unused and abandoned containers were responsible for more than 50 per cent of the mosquitoes.
Local initiatives are needed to identify the types of containers that are most conducive to mosquito-breeding and to work out suitable control measures. Community involvement is essential for making sure such containers are properly disposed of or adequately protected from mosquitoes.
Under Indian conditions, it is impractical to ask people to throw away water they have stored, points out Dr. Das. There are, however, simple ways to protect these water containers from mosquitoes.
Simple netting with a rubber band can prevent mosquitoes from laying eggs in pots that hold water. Syntex tanks are mosquito-proof and other sorts of overhead water tanks can be safeguarded with nettings, he points. These measures should go hand-in-hand with steps to improve sanitation and environmental hygiene, including stopping littering and ensuring efficient garbage disposal.
Every district needed a body to oversee mosquito control, argues Dr. Das. One possibility was to set up district disease surveillance committees as suggested in the Eighth Five Year Plan. These committees should be separate from the health care services and be able to set public health priorities for their districts, covering both communicable and non-communicable diseases, he says.
Greater community participation must not, however, lead to government disengagement. In the past 20 years, dengue mosquito control programmes failed after governments washed their hands of them and turned them over to the community, without providing the resources and expertise needed, says Dr. Gubler.
Mosquito control "has to be a partnership between the people who live in the houses where transmission occurs and the government. Neither can do it alone," Dr. Gubler
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