Lethal weapon vs dengue mosquitoes

Tuesday, July 19, 2011

"DENGUE is very hard to control," admits Dr. Teogenes F. Baluma, the Davao regional director of the Department of Health. The reason: there are four types of dengue viruses and each can cause serious disease.

Doctors said that infection by one dengue virus and survive from it does not guarantee protection against the other three. "Dengue viruses of multiple types are now endemic in most countries in the tropics," wrote Dr. Abram S. Benenson, editor of Control of Communicable Diseases in Man.

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Any of the four viruses are carried and transmitted by Aedes aegypti mosquito, which is sometimes called as tiger mosquito because of its peculiar white stripes. Like other mosquito species, only the female Aedes bites. The adult female mates and takes it first blood meal about 48 hours after coming out of its pupa stage. It bites during the day time throughout the day. However, highest biting intensity is about two hours after sunrise and before sunset.

To lay eggs, it has to be engorged with blood, which may take two to five days. Each Aedes female can lay up to four batches of eggs during its life cycle. The female Aedes survives an average of 30 days while the male species only 20 days.

"An infected Aedes mosquito is capable of transmitting the virus to susceptible individuals for the rest of its life -- about three weeks during probing and blood feeding," explains Dr. Simon Ng, a pediatrician at the Mount Elizabeth Medical Center in Singapore.

Once the virus enters the body, it multiplies slowly in blood cells. "A person bitten by a dengue-carrying mosquito does not immediately show symptoms," says Professor Zulkifli Ismail, a Malaysian pediatrician. "It takes from five to seven days for the virus to incubate."

Until now, there is still no vaccine available against dengue. The best way to prevent the spread of dengue is to keep one's environment clean, especially the areas that can become Aedes breeding places. Favored places for breeding are barrels, drums, jars, pots, buckets, flower vases, plant saucers, tanks, discarded bottles, tins, tires, and water coolers, among others.

To prevent the mosquitoes from multiplying, the World Health Organization recommends draining out the water from those favored places for breeding mentioned earlier. It also urges to remove all objects containing water (like flower vases) from the house. Discarded containers in which water may accumulate must also be collected and destroyed.

But there is a simple and organic device that slows down the spread of the disease. The Department of Science and Technology (DOST), which developed the lethal weapon against the deadly dengue, called it ovicidal-larvicidal trap (oltrap).

The oltrap creates optimum conditions to attract the Aedes mosquito that lays its eggs in clear and still water in dark and damp areas. The trap, filled with ordinary water mixed with pellets that contain plant extracts deadly to the mosquito egg and larva, is placed in dark areas inside and outside the house and in dry places protected from rain and sun.

Actually, the trap is a plastic cup (about the size of a tin can) painted black, the most attractive color to mosquitoes. A small strip of lawanit (measuring one inch by five inches) is placed inside it with the rough surface facing up.

"The scent of the larvicide solution invites female mosquitoes to the trap where they lay eggs on the stick and on the solution itself," the DOST explains. "The stick, moistened by the solution through capillary action, is highly attractive for mosquitoes to lay eggs on."

"As the eggs and the hatched larvae (also called "wrigglers") get exposed to the solution, they die. The oltrap's ovicidal (harmful to the mosquito eggs) and larvicidal (killing the larvae) effects prevent the next generation of mosquitoes from reaching adulthood, thus curbing the Aedes mosquito population."

Aside from killing eggs and larvae of the Aedes mosquito, the oltrap also monitors mosquito density and acts as an early indicator of dengue outbreaks, says Dr. Lilian de las Llagas, a professor from the University of the Philippines who specializes in the study of mosquitoes.

Although several trap devices has been developed already in other countries, the Philippine innovation is the first plant-based system against the dengue-carrying mosquito. In Singapore, for instance, they use a sticky strip of wood. Malaysia, on the other hand, has a funnel-shaped device that only traps mosquitoes.

Laboratory tests indicate that the Philippine technology attracts about 70 percent of the dengue-carrying mosquito.

"The technology is simple and effective, safe, cheap and makes use of a natural plant material," points out Dr. Nuna E. Almanzor, head of the DOST Industrial Technology Development Institute.

More importantly, the pellets are not poisonous to humans and pets.