Strategies to combat the dengue menace

By Ranjit Seneviratne

One of the concerns in Sri Lanka these days is the rise of cases of dengue. Various efforts are being made by the authorities, and the Colombo Municipality personnel have been going around clearing plants and insisting people who grow useful food plants like banana, aloe vera etc. should remove them – as well as flowering plants, so useful to encourage bees and butterflies, already so absent in the city.

This appears to be a sort of knee-jerk reaction, because if it were plants that harboured/allowed breeding of the dengue-carrying mosquito, then dengue should be rampant in villages with so much vegetation around them. The reason for this is perhaps because plants are known to exude oils and bio-chemicals from their surfaces (much like humans exude salty sweat from the skin’s surface) and the dengue mosquito normally does not breed in such polluted water. It normally prefers clean water, but has now taken to breeding in polluted water too, according to some reports.

Dengue is rampant not in villages surrounded by trees, shrubs, and vegetation, but in cities that are densely populated, and near schools. In other words, where there are people and children who throw plastic yogurt and ice cream cups and plastic bags all over the place, as well as thambili and coconut shells.

Therefore, is it not wiser for the authorities to stop asking people to uproot their garden plants and instead insist that ice cream, yogurt, etc. should only be sold in paper cups that could be easily torn to prevent water collection? Thambili sellers should collect their shells in gunny bags – and be fined if they leave them around.

Another source of clean water favoured by the Aedes aegypti mosquito (implicated in the spread of dengue) is water collected in plastic bags, including the so-called bio-degradable bags that are now in use, and strewn around city roads, which are now a free dumping ground for garbage. Therefore it is in everyone›s interest if the authorities take serious and implemented steps to eliminate plastic bags and insist that only paper or cloth, jute-hessian or woven leaf (dumbara mat “pang” and palmyrah leaf) bags be used. This is actually done at the Saturday Good Market held in Colombo. If they can do it, so can all shops and supermarkets, especially if we the people also co-operate. 

This will, in addition, help our rural people and those in the North, where palmyrah is plentiful, to have gainful employment and help reduce poverty. 

In order to make people more aware of their responsibility towards children and the environment, and a practical patriotism, it is suggested that some organisation produces cloth bags with the logo “NO PLASTIC” and below that “I LOVE SRI LANKA.”

Another breeding place for the dengue mosquitos are gutters, and some of the authorities concerned want to impose a “No Gutters” policy for new buildings. Again, there is a very simple way to stop water collecting in gutters of both old and new buildings. It needs someone to go up to the roof, pour water into each of the gutter lengths, and to note where it collects (usually at the centre of the gutter length, as plastic gutters sag with time). Next drill a small hole (1mm diameter) at each of these lowest points in the gutters. Then to prevent the dripping water splashing on the walls, wrap a wire (of non-rusting material) or Nylon fishing line (thangus) round a small brass screw and thread the wire/nylon line through the hole in the gutter, and lead it to the ground by wrapping the other end round another brass screw, or even a stone, and burying it a few inches into the ground. Water therefore cannot collect in gutters, as it will quickly leak off into the ground without making an unsightly hole, or splashing mud on the walls. If the holes get blocked by dirt and/or slime – just jiggle the wire up and down to clear the block.

Perhaps the manufacturers of plastic gutters could make gutters with 1mm holes at points every meter along the gutter length, as their contribution towards dengue mosquito control.

 Other more public places where water collects are roads, because in Sri Lanka, people allow water from their property to flow on to the road, which causes the road to break up, and water collects in small pot-holes. 

Among other mosquito breeding territories in the city are public drains, because we, the uncaring public, carelessly throw rubbish into drains, blocking water flow. The present practice is to spray toxic chemicals into these drains. As the drains are not water-tight, these harmful chemicals leak into ground water and could poison us, the people. The effect is not immediate, but these chemicals are known to gradually accumulate, especially in the fat cells, and over time could cause cancer and other non-communicable diseases (NCDs).

A simple and inexpensive solution is to spray used engine oil (most of the harmful additives in the oil are “burnt up” in used oil). The oil forms a film on the surface, and the larvae of the mosquito during the pupal stage (when it is shaped like a “comma”) is an air breather, and comes to the surface to breathe. The oil chokes the breathing tube, and kills the pupal stage mosquito larvae.

However, a better solution to prevent people from using drains as rubbish dumps is to convert the drains into ‘pebble banks’ by filling them with round pebbles from the rivers, or even just small granite stones. In order to drain the water that flows into the drains, «leak-off» holes (4 to 6 inch diameter) would need to be made at the bottom of the drains at intervals of say, a meter, before filling the drains with pebbles. These holes would allow the drain water to leak directly into the soil, where soil bacteria would convert biological waste to soil nutrients. Any excess water would continue to flow through the pebbles, and any waste trapped by the pebbles would also be converted to soil nutrients by the same soil bacteria. Such an arrangement would also drain flood water faster, due to having so many direct leak-offs into the ground.

Making a freshwater pond and putting in guppy fish (as they eat mosquito larvae) is another strategy that could be adopted by the public to prevent dengue. Such ponds will encourage dragonflies, which eat flying mosquitoes, and dragonfly larvae, which also breed in ponds and feed on larvae. The pond would attract frogs too, and they feed on flying insects at night. This way we can use nature to fight the dengue mosquito.

Planting holy basil (maduruthala/tholasi) is a great way to discourage mosquitoes. In addition, regularly eating 4 or 5 raw leaves of holy basil leaves increases the body’s resistance to viral diseases such as dengue, Influenza and chikengunya.

Using mosquito repellents like lemongrass oil and crushed betel leaf also deters the dengue mosquito, as they do not like the smell, so rub crushed leaves on your skin or place crushed leaves in a paper plate near where you sit. There are also mosquito-repelling Incense sticks available in the market. 

Ranjith Seneviratne is an advisor/educator at Earth Life Water Knowledge Trails and a marine engineer by profession. He was Project Operations Officer, FAO, Rome and one of his projects dubbed “Blue Revolution” by the locals, resulted in the Government of Bangladesh winning the first Souma Award, given by FAO to a government that best developed an FAO project. His FAO Project in Eritrea was shortlisted for the Second Souma Award and two of his Project Managers won B.R. Sen Awards for excellence. He is currently a soil healing consultant who maintains his Colombo home garden as a laboratory to develop various techniques of bio diversity conservation that includes rainwater harvesting.