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Online 13 Oct 05 Their eggs can survive 6 months without water Ansley Ng ansley@newstoday.com.sg Channel NewsAsia 12 Oct 05 Dengue-carrying mosquito evolves super resistance: report KUALA LUMPUR - The Aedes mosquito in Malaysia which carries the dengue virus has evolved into an extremely hardy insect, with its eggs surviving for up to six months without water, a report said Wednesday. An entomologist specialising in mosquitoes from the Universiti Sains Malaysia, Abu Hassan Ahmad, said research showed that water was needed for the insect's eggs to be laid but they could then survive for months in dry conditions, the Star daily reported. "Our lab tests have shown that the eggs can survive for four months and we are sure they can even last up to six months without a drop of water," Abu Hassan was quoted as saying by the newspaper. Malaysia is moving out of the dry southwest monsoon period into a wetter season and the scientist said this would see eggs being hatched. "We have had a very long dry spell and in the last month we've had rain. With the onset of rain, the eggs will hatch and the larvae will find their way into water-logged areas," he said. Abu Hassan said "fogging" -- the process of spraying insecticide to kill the mosquitoes -- killed the adult insects but not the eggs. "The eggs are laid in tree crevices, drains and containers when water collects there. It is impossible to destroy the eggs totally," he said. Separate studies by health and environment officials in eastern Sarawak state showed continuous fogging with the same chemical would result in the Aedes mosquito becoming resistant to the poison, said the newspaper. Malaysia has been waging a campaign to stamp out the mosquito, with the latest reports saying 76 people have died from dengue so far this year, compared to 73 over the same period in 2004. The country recorded a 17 percent increase in the number of suspected cases last week, with 1,201 reported compared to the 1,023 the week before. The total number of suspected cases so far this year has spiked to 29,820, in comparison to 28,290 last year, when the highest-ever death toll of 102 was recorded. Neighbouring Singapore is suffering its worst dengue fever outbreak on record, with the death toll reaching 13 this year and total cases approaching the 12,000 mark. - AFP/ir Today Online 13 Oct 05 Their eggs can survive 6 months without water Ansley Ng ansley@newstoday.com.sg Aedes mosquito a tough opponent but the pest can be beaten, though it won't be easy, say experts IT MAY have been a known fact among dengue experts. But a Malaysian academic yesterday reminded the world how vulnerable humans are to the Aedes mosquito, which carries the deadly dengue virus. The eggs of the Aedes mosquito, which is laid and hatched in stagnant water, can survive up to six months without water, said Professor Abu Hassan Ahmad of the Universiti Sains Malaysia in Penang. This was probably how the deadly mosquito invaded the United States almost two decades ago, Prof Abu Hassan told Today "it was likely that their eggs had remained dormant in threads of tyres that were exported from dengue-infected countries. One area where eggs could be hidden while waiting for rainfall would be in crevices in the trunks of trees, he said. "Holes and crevices in trees in the wild can collect water, and these become ideal places in which Aedes mosquitoes can lay their eggs," said the academic, who spent the last two decades studying mosquitoes and who almost died of dengue fever in 1997. After the rain comes, once enough water is collected in the crevices, the eggs need just 24 hours to hatch, he said. The ideal weather for mosquitoes to reproduce would be a situation of intermittent rain. Said the professor. "When it rains, then stops, it produces the best conditions to lay their eggs--when the water is stagnant." Although fogging kills adult mosquitoes, it does nothing to the eggs because of the "protective layer of the egg shell", said Prof Abu Hassan. "Fogging kills the mosquitoes but not the eggs. They are not destroyed because the shell protects the poison from getting to the would-be larvae," he said. But despite stepping up fogging, the Aedes mosquito could have "evolved" and cultivated "insecticide resistance", especially if fogging is done frequently over an extended period, he added. For research, Prof Abu Hassan keeps eggs of the mosquito in his laboratory, hatches them and conducts experiments on Aedes mosquitoes to help develop new insecticides. "The Aedes mosquitoes can 'detox' and become resistant to existing insecticide. We need a good supply of eggs as we are doing continuous research to find new types of insecticide," he said, though he added that the "evolution theory" has not been proven. So, is there a way to tackle the dengue scourge? Said the professor: "It's not the end of the world. Don't worry. We can keep trying to get rid of their breeding sites. Fogging will kill the adults. Without them, there will be no eggs. "We have to try to end their cycle, though you can't guarantee a 100-per-cent result." To be safe, Prof Abu Hassan said homeowners should not only change the water in vases regularly, but scrub them to remove any mosquito eggs. When contacted last night, a spokesman for the National Environment Agency said: "The Aedes mosquito is highly adaptable and hardy--that's why it is important to do source reduction. "Fogging, if need be, will kill the adult mosquitoes and supplement source reduction." In a 24-hour period from 3pm on Tuesday to 3pm yesterday, there were 88 reported cases of dengue, a rise of 21 cases from Tuesday. PEST CONTROL THE National Environment Agency is looking for Environmental Health Officers to help curb mosquito breeding. Officers will be involved in pest control, performing duties like including the inspection and control of pests in residential areas and outdoor fields. A walk-in interview will be conducted today at Level 5, Toa Payoh Central Community Club, 93 Toa Payoh Central from 2pm to 5pm. Call the Central Singapore CDC on 6370 9470 for more information. links Related articles on Singapore: dengue issues, policies, discussions |
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