Gossamer Wings Strike Again

Rewrite of Jottings: Gossamer Wings Strike Again

(A sequel of Perils from Gossamer Wings, Mindanao Post, 31 July 1996)

Published Mindanao Post 26 Oct 1998

Now subsiding, the 1998 nationwide Dengue Fever outbreak peaked in September as expected. The frenzy of activity stirred by the outbreak followed a familiar pattern ─ frightened and anxious parents scrambling to bring their sick children to hospital, and health functionaries, normally unflappable, scurrying to cope with the medical emergency. The outbreak’s toll was not unusual for this country: of the thousands of reported dengue cases, about one percent was fatal.

As usual, there was the palpable absence of a firm agenda aimed at eliminating the scourge, a disease that has no known vaccine to provide immunity. The abatement of the outbreak is somewhat puzzling in the light of the haphazard general response to the affliction. Health authorities were less than resolute in tactics to vanquish the vector or to contain the spread of the disease. Media and the education community were certainly supportive in the campaign to inform the public about the transmission and prevention of the disease. Pharma firms happily helped the public, and themselves, with their ads promoting products and such remedies as insect repellents and “katol” insecticides. A number of households, urged more by self-preservation motives than by cleanliness, were seen actually observing the “four o’clock habit” ploy and shooing their mosquitoes with smoke, thus driving the insects toward a neighbor. Even City Hall joined the fray, but found their foggers inoperable.

The disease has somewhat subsided but may linger through out the seasonal dry period of January to April which normally diminishes mosquito swarms. If weather forecasters are to be believed, the La Niña rains will subdue the dry season and the dengue mosquito will, like a resurgent PhilippineAirLines, continue to fly. And produce more bloodsuckers.

Is there any hope for a reduced dengue threat in the foreseeable future? Can we honestly expect government to marshal sufficient resources to combat dengue? There may be a glimmer of hope for a vaccine to be developed, but in faraway Thailand.

The Virus - Dengue disease is caused by an arbovirus (arthropod borne virus) and is primarily an infection of vertebrates other than man, and of arthropods, but can be transmitted to man. (Arthropods are members of a large phylum of invertebrate animals characterized by jointed legs, chitinous exoskeletons and segmented body parts, including insects, spiders and crabs.)

Vertebrate hosts of the 480 or so known arboviruses, particularly the maintenance host, are essential for continued existence of the virus. They usually live symbiotically with the viruses without actual disease. Some of the recognized or incriminated hosts include birds, rodents, insectivores, rabbits, cattle, deer and monkeys. Monkeys are the suspected link to dengue. (Man is usually considered an incidental host, often, but not always, a dead end in the chain of transmission.)

The invertebrate host of dengue is the mosquito. After this vector has imbibed virus from a vertebrate host, the virus undergoes an incubation period (about 10 days) within the mosquito which then becomes infective for life without any ill-effects to itself. The mosquito infectivity increases with its biting frequency.

First detected in a 1953 outbreak in Manila, the virus reached Thailand (about 200,000 cases) and then Vietnam in 1963. The disease has spread and is endemic in the tropics and subtropics. There are four identified Philippine strains of the virus.

The Victim - Apart from yellow fever, dengue has caused more deaths than any other arbovirus disease, and may now have surpassed yellow fever as a killer after the discovery of the Yellow Fever Vaccine, although malaria is catching up fast. The dengue victim is often a child although adults are also vulnerable. The symptoms are fatigue, abdominal pain, loss of appetite, vomiting, headache joint and muscle pains and fever lasting about 5 days. Nose bleed and rashes may occur. In severe cases, a victim’s vomit and stool will show traces of blood and could be the beginning of dengue hemorrhage fever which could be fatal.

The Vector - The primary vector of dengue is a species of mosquito named Aedes Aegypti. This bloodsucker flying insect with gossamer (thin and flimsy) wings is found throughout a vast span of territory ranging from 40º North to 40º south of the equator. (To the meticulous, the vector is classified as: Phylum Anthropoda, Class Insecta, Order Diptera, Family Culicidae, Genus Aedes, Species Aegypti.) It is a domestic species rarely breeding more than 90 meters from houses , and can be recognized by its yellow parallel lines in the middle and curved silvery line on each side of the thorax, and a bassally banded abdomen.

The eggs are laid in water inside small, dark receptacles such as tins, pots, tree-rot holes, coco shells, cut bamboo, sagging or blocked eaves, banana leaf stems, pineapple tops, sisal leaves, bromeliads, vases in cemeteries, beer bottles, plastic cups and car tyres (a favorite). When newly deposited, the eggs are white, but darken after a few hours. The eggs can resist drying for as long as six months and could even be dormant during a drought.

The larvae emerge when moistened, usually after 2 or 3 days and feed on the organic matter in the water. The larval stage is usually 10 days, but could span a period of 6 days to several weeks, then turn into pupae for 2-4 days and finally into an adult winged mosquito. The adult mosquito with suitable food can live for several months.

Only the female of the species bites needing the protein in blood for its eggs. Biting activity is diurnal, peaking at dawn and dusk, with a foraging range maximum of 200 meters, usually hovering about 50 meters from the breeding site.

A secondary vector, the Aedes Albopictushas similar habits as Ae. Aegypti. It is known to transmit dengue virus in Japan and is now suspected to have migrated here. Members of the Aedes Scutellaria group are the main vectors of the Pacific filaria and are also vectors of the dengue virus.

U.S. researchers working on an encephalitis virus discovered that the vector, Aedes Trisariatus, if infected, could lay virus-carrying eggs that in turn produced infected and infectious offspring. This process of a mosquito inheriting the virus is known as transovarial transmission. This discovery could be extremely important to dengue research and eradication locally.

The Victor Obviously, the strategy to control dengue would have to be two-pronged: 1) to develop a vaccine, and 2) to contain the vector. The vaccine-search approach is complex and would require trained virologists, loads and loads of research funds and emphatic legislators, all of which are virtually non-existent in this poor country. So, only the vector-eradication avenue is feasible.

Mosquito eradication programs, mostly targeting larvae, have been tried by many countries with varying measures of success, although re-infestation also occurred when surveillance slackened. Old methods include draining watery breeding places, stocking fish in rice paddies and applying oil film on stagnant puddles. The use of fungus and bacteria to attack larvae are two of the newer methods. The discovery of a certain fungus found to be a parasite of larva was introduced into some Pacific islands and in Zambia, while an Israeli entomologist discovered a bacterial strain that is deadly to mosquito larva.

Holding the most promise as a comprehensive program is a research by the U.S. National Aeronautics Administration (NASA, the space people) who, in the early ‘90s conducted a pilot project using earth-orbiting satellites aimed at malaria control. The electronic remote sensors carried aboard monitors environmental conditions such as rainfall and surface water that trigger the breeding of malaria mosquitoes. The project goal is to develop a system that any nation can use, needing only a computer and a rooftop antenna to collect data for helping predict where malaria outbreaks might occur so that ground crews can take intervention measures. The technique would be applied to many kinds of insect-borne diseases.

Entomologists now seem to disfavor the insecticide method to zap the mosquito. Fogging is deemed an illusory method, killing the adult but sparing the larvae and pupae, and thus lulls people into a false sense of security. Fumigation with smoke makes no sense as it would do more harm than good. The “smoker” may drive the pests away from his yard but transfers the menace to adjacent areas, a belligerent act contradicting the love-thy-neighbor dictum, and could endanger his own family by exposing them to the toxic carbon monoxide produced by the smoke(incomplete combustion).

The clean-your-premises slogan is perhaps gratuitous to the neat households routinely performing this rite habitually. In contrast, the slothful family will do the chore only for as long as the dengue scare exists. Yet, chances are, the “litter” family would be the source of the index host in the next outbreak.