Perils from Gossamer Wings

Rewrites of Jottings: Perils from Gossamer Wings

Mindanao Post, 31 July 1996

A recent news item fro Australia warned that the bloodsucking Stomoxys fly which capable of carrying the HIV virus is encroaching on the Western Australian City of Perth. The fly can carry all sorts of diseases including salmonellosis and shigellosis.

Filipinos can dismiss this threat as a full continent away, but the news spotlights the insidious peril from life-threatening diseases borne by flying insects like the fly and mosquito belonging to the animal order Diptera. Although the great majority of these winged insects are justannoying pests, the diseases some of them carry are lethal. Noah’s failure to swat the pests in the Ark has led to their delisting as endangered species, and man’s attempts to exterminate them since have been dismal failures.

AS HARMLESS AS A FLY

The housefly is the most commonly known of the order Diptera. Modern knowledge of sanitation has disproved the expression “as harmless as a fly” as we now know that flies are a source of contagion. They carry disease germs from their filthy breeding places to human food upon which they alight and thus cause the spread of typhoid fever, tuberculosis and infant disorders.

The insect appears to rely on two abilities for survival. First is its ability to multiply; a new generation can be produced in ten days. Second is its agility, as any would-be swatter can confirm. The pests swoop down in clusters up to a dozen, landing almostsimultaneously. Swat one and the rest scamper to thumb their noses at their adversary. Exasperated swatters claim that a fly can dodge a swat coming down from any direction except from above, their blind spot.

The Department of Health (DOH) alerted by several outbreaks of dengue fever cases and deaths in different areas of the country, declared war on mosquitoes and devised a two-prong strategy: the public to clean up their homes, and local government units to conduct fumigation ofsuspectareas. The arsenal seems a bit puny pitted against such a durable foe. Can such a frail creature triumph over man, the most brutal killer on earth? Handily.

Aedes Egypti, known as the yellow-fever mosquito, can carry more different diseases, including dengue, than any other species.

Anopheles (Philippine species flavirostis) carries malaria, a disease caused by protozoan parasite, genus Plasmodium.

Aedes Triseriatus causes elephantiasis.

Only the female mosquito bites, actually a thrust piercing the skin with her needle beak, (the male preferring nectar) needing the protein in blood to nourish her eggs which she deposits in watery places.. Just about any container that can collect water will do --- upturned bottle caps, flower vases. Cleaning household rubbish does help reduce breeding opportunities but is inadequate for mosquito control. The environs offer much more water pools: tree holes, banana leaf stems, rain puddles, discarded tires (those left unburned by rallyist mobs.)

Fumigation and fogging with insecticides do kill some adult mosquitoes but spares the larvae and pupae which soon mature into adult bloodsuckers. The chemicals used in the fumigation poison the environment and wipe out beneficial insects.

To control the mosquito population (not exterminate as this is impossible), a much wider array of weapons is needed, some old, some new. The simplest form used for centuries is draining or filling up watery breeding places. Another is stocking rice paddies with fish to gobble the larvae. Biological methods require DOH to spend scarce funds, a practice officials are disinclined to do, such as infecting mosquito larvae with parasites, bacteria and fungi, sterilizing male mosquitoes with radiation then turning them loose to mate, resulting in female mosquitoes laying sterile eggs. A novel method is the use of an Israeli discovery bacterium which produces a toxin effective against many species of mosquito larvae. Another creates genetically engineered bacteria that reproduce rapidly and produces spores lethal to larvae tat eat them.

In general, lore on mosquito behavior may help to avoid being bitten. A rising level of carbon dioxide initiates mosquito excitation and stimulation to fly. Convection currents generated by a warm object and the water evaporated from this object appears to be the clue that enables a mosquito to distinguish between a lifeless object and a warm living animal. The mosquito flight is initially random but when it encounters a wet and warm convection current it moves steadily forward until it passes out of the current into cooler and drier air. It then turns, although not always in the right direction. Back in the convection current the mosquito finds its prey most of the time. This explains the finding that human sweat is a strong attraction.

Dengue mosquitoes normally fly in search of food during the day when they can see. Human odor is the primary factor that attracts the mosquito from a distance. In contrast, malaria mosquitoes fly at night when vision is less useful.

Some chemicals effectively repel one species of mosquito but are ineffective against other species. Mosquito repellents work by confusing the mosquito which uses its antenna receptors to detect warmth and moisture currents. It seems that repellent vapor shuts off the moisture sensors and the mosquito flies right thru the current and away from the prey. A leading commercial brand of repellent contains an amide which is effective against dengue mosquitoes but not against malaria mosquitoes, the repellent being a halogen compound. Repellents are applied to the skin and are effective for a few hours.

Finally, the dreaded question: Can AIDS, which is transmittable by infected needles, be transmitted by a species of mosquito?