Dengue- What the community should know and could do

Dr. K. D. P. Jayatilaka

Aedes aegypti mosquitoes generally known as a container breeder normally breed in pure water which is not very deep. Sometimes it could breed even in slightly polluted or slightly brackish water. It takes 7-9 days for the mosquito to develop from the egg to the flying adult. There are about 950 species of Aedes mosquitoes in the world of which about 180 species exist in the South East Asian countries and in Sri Lanka about 18 species are present.

Life cycle of the mosquito

However, Ae. aegypti and the related variety Ae.albapictus are the known important vectors of diseases like dengue,. chikungunea, yellow fever and other viral diseases which are very dangerous. These mosquitoes in addition to their capacity of being vectors of fatal or incapacitating diseases which cause epidemics are a serious biting nuisance to humans as well as animals.

Blood meal

Adult mosquitoes live up to about 60 days under natural environment in the wild. The female mosquito mates only once, very often immediately after coming out of the breeding place, but produce eggs at intervals throughout its life. Only the female mosquitoes take a blood meal either from humans or animals for its survival and maturation of eggs which she lays in suitable a place each 2-5 days depending on the surrounding temperature and humidity.

After the blood meal these gravid females rest on soft, dark, humid places like bush and the brush or under fallen branches and tree holes etc. outdoors or under surfaces of furniture, hanging cloths, curtains, and hangers etc. inside houses and out-houses till the eggs mature.

Before sunset

After laying the eggs in a suitable place, they go in search of another blood meal. Ae aegypti prefers human blood to animal blood and the peak biting hours are about 2 hours before sunset and about 2 hours after sunrise.

This does not mean that they strictly adhere to these meal times as we frequently hear during various community education sessions these days, but a hungry mosquito will bite for a blood meal at any time during day or night.

Aedes mosquito is usually characterized as a very nervous feeder, i.e. it gets disturbed and flies away at the slightest movement of the victim (pray) but comes back within a very short time either to the same or to a different person to feed till it gets the full blood meal.

This is why; one infective mosquito trapped inside a house could infect a number of people or the whole family in the house during a feeding session in the day or the night.

Injects virus

The mosquito picks up the dengue virus from a dengue patient during feeding and the mosquito gets infected.

The infected mosquito will have to live for a period of about 7 days for the virus inside the mosquito to go through a process of maturation/changes and the mosquito becomes infective. This period 4-12 days average 7 days from the time it gets infected to the time it becomes infective is known as the extrinsic incubation period of the dengue virus.

After this, each time the infective Aedes mosquito bites a person to get the blood meal it could inject some virus in to the victim.

Appearance of signs

If the victim is a human who is not exposed to the same serotype of dengue virus he will become a dengue patient after another 5-12 days (average 6 days).

This time interval between the infective bite and the appearance of signs and symptoms of dengue in the patient is referred to as the incubation period of the disease. There are 4 serotypes of dengue virus D1, D2, D3 and D4 and all these four types are present in Sri Lanka today.

The proportion of infective mosquitoes in the mosquito population at any time depends on (a) Density of Aedes mosquitoes (b) Number of dengue patients in the area and (c) Longevity or the survival rate of the infected mosquito. Often the mosquito gets killed during the process of feeding and waiting period for the feed.

Life Cycle of Aedes Mosquito

A female Aedes mosquito lays eggs in batches of 20-60 at a time on the wet surface above the water level and in contact with water these eggs float in singles till the first instar larva emerges in 2-3 days. The mosquito larva goes through 4 stages of development and feeds on various organic matter like bacteria, protozoa, algae etc. present in water till it turns to the non-feeding stage of the pupa which lasts for a day or two to finally come out as the adult mosquito.

The whole process from egg to adult takes 7-11 days depending on the outside temperature. Usually in a batch of eggs there are about 50% male and 50% female mosquitoes.

Could withstand

So, any collection of water which lasts for more then 7 days is a possible breeding place for Aedes mosquito. The eggs of Aedes mosquitoes have the property of sticking to the surface of the container which holds the water or the ground water pool and could withstand even desiccation up to several months.

It is important to know this property of the Aedes eggs in the control program as these desiccated eggs lying dormant especially in dried up ground pools could become viable in coming into contact with water again. Hence, it is important to scrub and clean the sides of the containers, vases, drums, tanks, bird-baths etc. when emptying and refiling with water. It is also known that infective female Aedes mosquito could pass the dengue virus through the eggs to the next generation. However, this does not have much of an influence in causing or/and continuing a dengue epidemic.

Breeding sites

The common breeding places of the Aedes mosquito are so varied and many.

We normally speak about discarded plastic cups, coconut shells, empty bottles, containers used as ant traps in houses, discarded tyres, tree holes, drums and containers used for collection and store water, temporary man made pools at building sites etc. sheaths of some plants like bromeliad and bananas etc.

Broken places of roadside drains where water stagnates, in large masses of water like ponds especially the edges and small rock pools which form during the rainy season are favourite places for the female mosquito to lay eggs.

Fallen leaves like those in teak plantations and even the pieces of bottles fixed to parapet walls to prevent burglars climbing can be temporary breeding places during the rainy season.

There are also so many unsuspected places in our garden where water remains for more than 7 days and these could be Aedes mosquito breeding place.

Dengue control

We in Sri Lanka are fortunate in not having many major problems like rain water collection receptacles for drinking, except in few areas, extensive ground water - Aedes breeding places, inaccessible areas during rain and floods etc., faced by many other countries where dengue and other mosquito borne diseases are a problem.

However, we have few problems which are manageable, sometimes with difficulty but not impossible. The unfortunate thing is that a moderate Dengue Epidemic (caused mainly with D2 and D3) has occurred these days causing many deaths (about 250) and sensitizing many (20,000-50,000) to those dengue serotype viruses.

The problem will arise if another serotype (D1 or D4) becomes predominant with the next epidemic. The only way to prevent this unfortunate situation is to have a sustainable, community friendly, community helpful dengue control program with the full participation and cooperation of the community.

The assistance, participation and cooperation of other parties mentioned below are very essential to apply all possible and appropriate control measures.

Dengue control should be built into the people’s daily life like the grow more food ‘Api Wawamu Rata Nagamu’ program which is functioning at different degrees practically in every house in the country today.

Dengue control is not synonymous with proper garbage management, but proper garbage management helps tremendously to a successful dengue control program.

The basic principles in a Dengue Control Program should be :-

Reduce the breeding places to prevent build up of mosquito (vector) densities.

Reduce the survival rate or the longevity of the mosquito.

Reduce or prevent man vector (mosquito) contact.

Identify and manage cases early.

Dengue control is vector control. To prevent Aedes mosquitoes breeding at different localities and different situations different methods have to be applied. Some of these cannot be carried out by the community alone and neither can it be done by the Ministry of Health alone, even though it has the responsibility of preventing dengue epidemics.

Some methods or procedures could easily be carried out by individual house holders and some by the community in groups. Some need the assistance of a central organization like the Dengue Control Unit of the Ministry of Health and even that will have to get the cooperation. assistance and guidance from other Ministries like Education, Agriculture, Environment, Local Government, Irrigation, Roads and Highways, private Organizations like Builders, Architects etc. and even private entrepreneurs.

In short it is a team work build into the life of the community.

Any person irrespective of the social status in Sri Lanka could become a dengue patient and even die of the disease.

The first steps in the dengue control activity begins at home and that is proper collection/disposal of household waste and reduction/elimination of other possible Aedes breeding/resting places. Teaching school children the basic ideas of dengue control is very important and will go a long way. Motivate the children at school and let them take the subject to their homes.

They are the best resource to change the knowledge, attitudes and practices of elders at home. Or, when the children become adults and start running their own homes, they will practise what they learned at school. Mosquito breeding and people getting mosquito borne diseases is a natural thing in any tropical part of the world.

Control Measures available are:-

* Source reduction and cleaning up operations,

* Biological control,

* Insecticide spraying (larvicides and adulticides)

* Environment modification and manipulation

* Personal and family protection etc.

* Get immediate medical assistance in case of suspicion of dengue complications.

To be continued