Mosquito Hub ~Update 2

Sept 11, 2005
Three theories on why dengue is out of control

# Fumigation not effective as it does not kill enough mosquitoes
# Foreign workers may have introduced a few strain here
# Because of the successful drive in the 1970s, people now lack immunity

By Jeremy Au Yong

FOR the fourth week in a row, dengue cases have surged to record-breaking levels.

This week, more than 600 cases were reported in just five days, from Tuesday to Saturday. It surpasses last week's 546, 493 of the week before and 414 from before that.

With these mounting figures, the total number of dengue cases so far this year has gone beyond 9,000, including eight dead. This is more than double that of last year for the same period.

Many of the measures deployed to eradicate the 1973 epidemic in which 26 people died - extensive fogging, strict law enforcement and diligent inspection of premises - are still practised today. So, why are they not working now?

Experts give The Sunday Times three theories on how the war on dengue has changed.

Fumigation does not work

THAT'S because it does not kill enough of the Aedes mosquitoes. This point was made back in 1998 by Dr Paul Reiter, who was then chief entomologist of the dengue branch in the United States-based Centre for Disease Control. He said, after reviewing Singapore's Aedes control programme, that fogging killed only a small number of mosquitoes, took too much effort and money, and gave people the impression there was a quick fix.

Epidemiologist Duane Gubler - deemed by his peers as 'the grandfather of dengue' for his pioneering work in dengue prevention - agrees.

'Fumigation as it is now practised, is not effective in controlling, let alone preventing dengue transmission,' he said on the phone from Hawaii where he is the director of the Asia-Pacific Institute for Tropical Medicine and Infectious Diseases.

Dr Gubler's research in dengue control - done in the 1980s - forms the basis for the World Health Organisation's (WHO) current global strategy against dengue.

'The mosquito you are targeting is an indoor species that rests in closets where the insecticide rarely penetrates,' he said.

He added that fogging had worked in the Americas in the 1950s because DDT was used - an insecticide that was residual so it continued to work even after the fogging. The poison is banned in Singapore.

Dr Chusak Prasittisuk, WHO's regional adviser for vector-borne diseases in South-east Asia, said: 'Fogging works only in certain situations - if you do it at the right time, at the right place. The best way is still to go after and destroy the mosquito-breeding sites.'

New dengue virus strain

FOUR strains of dengue are currently known and Dr Gubler believes it is likely a new strain has entered Singapore. The new strain could have been introduced via foreign workers who were infected in their home countries. There are over 500,000 foreigners here on work permits and many come from dengue-prone countries such as Malaysia, India, Indonesia and the Philippines.

Low immunity

THIS is the paradox of the fight against dengue: Win the fight and you will be more vulnerable next time. Because Singapore did such a good job at eradicating the disease in the 1970s, experts say locals lack immunity against it.

Said Dr Chusak, based in New Delhi: 'The mosquitoes now have a higher probability of biting a non-immune human in their lifetime. As a result, fewer mosquitoes are required to spread an epidemic.'

A study in 2001 by epidemiologist Ooi Eng Eong showed that immunity levels here were low. Dr Ooi found that only 7 per cent of children aged between six and 15 have dengue antibodies. In contrast, a similar study in Thailand found the antibodies in over 70 per cent of school children.

The experts stressed that the best way to fight dengue is to target mosquito-breeding areas - any spot for even a small amount of water to collect.

Singaporeans are urged to change the water in vases daily, remove water that collects in plant pot plates every other day and to add granular insecticide into containers, vases and roof gutters.

When leaving for a trip overseas, they should cover the toilet bowls and overturn pails and watering cans.

Associate Professor Mary Ng, a microbiologist, who will be chairing a regional dengue conference at the end of the month, said: 'Until we can get a vaccine, everyone must do his best to go after the mosquitoes. That is the only way to beat the disease.'

jeremyau@sph.com.sg

Update

# Number of dengue cases reported for Sept 10: 125

# Number of dengue cases admitted to public hospitals on Sept 9: 85

# Number of beds taken up by dengue on Sept 9: 225

Prevention tip: Protect yourself with insect repellent during the day, when the Aedes mosquito is active.