Controlling the dengue mosquito

Public health experts are unanimous that mosquito control has to be a
continuous process, with the government and local communities working
together.Public health experts are unanimous that mosquito control has to be
a continuous process, with the government and local communities working
together.
N. Gopal Raj

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 . "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 .

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 told .


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