Who is responsible for Dengue?
Posted on June 29th, 2009

By Dr. Mrs. Mareena Thaha Reffai Vanderwert Place, Dehiwela.

The government seems to be taking genuine strong measures to curb the dengue menace by checking backyards house to house, imposing fines, imprisoning culprits of negligence and what not.

However, at least in the city of Colombo and suburbs, the mosquitoes must be laughing all the way to the next victim; for while the backyards are being inspected, cleared, fumigated and people being fined and put behind bars the mosquitoes are having a rollicking time laying the eggs on the heaps of garbage on the main roads.

The other day from the Dehiwela junction to the supermarket alone I counted 5 heaps of garbage. We must realize that these are wonderful breeding grounds for the mosquitoes and no amount of clearing the backyards of the houses is going to help in eradicating the mosquitoes until these garbage dumps are got rid of.

Similarly right along the lanes, most of the drains are clogged up and overtime there is a shower the water stagnates creating wonderful breeding medium.

Therefore what is really needed at present is acquiring the wholehearted participation of the citizens which must be harnessed using much more vigorous advertisements and persuasion of proper disposal of garbage and prevention of water collection everywhere.

Without these we are fighting a loosing battle and what a shame if after conquering the 30 year terrorist war, we cannot conquer this tiny mosquito. Doing a little search on Dengue I came across the following interesting facts about this minute but deadly enemy. .

ƒÆ’-¡ƒ”š‚ Aedes aegypti mosquito, which carries the virus causing dengue, is small in comparison to others, usually between three to four millimeters in length discounting leg length. It is totally black apart from white ‘spots’ on the body and head regions and white rings on the legs. The thorax is decorated with a white ‘Lyre’ shape of which the ‘chords’ are two dull yellow lines.

Many people believe mosquitoes only live two or three days, but in actual fact, left unmolested they can live for months. The males of all species of mosquitoes do not bite humans or animals of any species, they live on fruit. Only the female bites for blood which she needs to mature her eggs. The eggs of most species are laid together in a raft form, but Aedes lays her eggs separately thus allowing them to spread over large surfaces of water if conditions permit, this way the eggs stand a better chance of survival.

Most species lay their eggs in any type of water, mainly dirty or even polluted. Not Aedes, she only lays her eggs in clean water which contains no other living species.

The mosquito’s preferred breeding areas are in areas of stagnant water, such as flower vases, uncovered barrels, buckets, and discarded tires, but the most dangerous areas are wet shower floors and toilet tanks, as they allow the mosquitoes to breed right in the residence.

Research has shown that certain chemicals emanating from bacteria in water containers stimulate the female mosquitoes to lay their eggs.

They are particularly motivated to lay eggs in water containers that have just the right amounts of specific fatty acids associated with bacteria involved in the degradation of leaves and other organic matter in water. The chemicals associated with the microbial stew are far more stimulating to discerning female mosquitoes than plain water, for example, or filtered water in which the bacteria once lived.[9] Aedes does well even in chlorinated water.

When freshly laid the eggs are white but soon turn black in colour.

After a few weeks or even shorter in the summer, the larvae reach the pupa stage; The young larvae feed on bacteria in the water; this stage is usually very short and the pupae rise to the surface of the water where the top of the pupal case opens like the lid on a can and out emerges the new adult. This life cycle can be complete in 1.5-3 weeks.

After the mosquito feeds on a virus-carrier, the virus starts to replicate in the mosquito. After eight to 12 days incubation period the mosquito can transmit the virus on subsequent feeding attempts (several times per day is not uncommon) depending on the availability of the host. Feeding generally occurs at one to two hour intervals.

The virus remains in the salivary glands of the mosquito, and when she bites for food, she injects saliva into the wound where the anti-coagulants contained in her saliva facilitate feeding, without knowing it, she also injects the virus into the host.

The eggs can survive for very long periods in a dry state, often for more than a year. Since the virus can be passed from adult to egg then the virus too is guaranteed survival until the next summer and heavy rains.

Aedes aegypti, unlike other species is very intelligent, if one could say that mosquitoes are intelligent. Living near man for so long she has become totally dependant on him and has learned a lot from him.

For instance, she has greatly reduced the `humming’ sound she makes with her wings so man cannot hear it, unlike other species whose humming is extremely irritating and awakens the deepest sleeper. She never lives more than ninety meters from dwellings thus guaranteeing her meals. She attacks from below or behind, usually from underneath desks or chairs and mainly at the feet and ankles. The insect is very fast in flight unless gorged with blood. Other types of mosquito even fly into your face and can be easily caught or killed, not Aedes, she’s too smart.

There are no intermediate animal vectors for the virus, it seems the system is contained in man/mosquito/man relationships. Aedes is very domesticated, as much as your pet dog or cat, most mosquitoes can live in forested areas a long way from humans and live on animal blood, not Aedes, she relies on man and will only bite animals in his total absence, although the female does actually take juices from various types of fruit at times.

There are actually four virus types. The first infection is `mild’ in comparison to the second and following infections, this is referred to as ‘Classical Dengue Fever,’ very high fever (40C) and heavy flu’- type symptoms with extreme pains in the joints and eye orbits. The second infection can and usually results in hemorrhagic Dengue, where bleeding occurs from the eyes, nose and other more remote parts of the body.



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