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Dengue haemorrhagic fever is a killer disease which has been spreading fast throughout in Sri Lanka. Following is a discussion held with Dr. Shirani Gunawardene, Entomologist, Medical Research Institute, Colombo.

From an article first published in the Daily News earlier when there was an epidemic of dengue fever in Sri Lanka. It is re-published with a few minor changes.

Q: Is there a vaccine to prevent dengue fever?

A: Not at present. They are still trying to develop one.

Q: It is common knowledge that dengue is spread by a mosquito. How would you identify the mosquito?

A: All mosquitoes do not spread dengue. Aedes mosquitoes who belong to subgenus stegomyia contains the most important vectors of dengue viruses. The mosquitoes are Aedes aegypti and Aedes albopictus. Of them Aedes aegypti is considered as the primary vector of dengue fever.

They are small in size when compared with other mosquitoes. They are only 2-4mm in size. Only female mosquitoes suck blood. Aedes mosquitoes have contrasting black and white rings on their legs and Aedes aegypti has a lyre shaped pattern of silvery markings on the upper surface of the thorax. Aedes albopictus has a single longitudinal line on the upper surface of the thorax. They bite during day time. Aedes aegypti mainly bites indoors.

Q: What are their breeding habits?

A: Ae. aegypti mainly breeds in the domestic environment. Its preferred habitats are discarded tyres with water, water storage tanks and jars inside and outside houses, roof gutters, tins, cans, coconut shells with water, plant pots, bottles etc. All these habitats typically contain relatively clean water.

A Albopictus breeds in temporary containers but prefers natural sites in gardens such as tree holes, leaf axils, and breeds more often in gardens and less frequently indoors in artificial containers.

Only the female mosquitoes suck blood and hence are able to carry dengue virus. The average lifespan for adult female Ae. aegypti is four weeks and for males about three weeks.

The eggs are black in colour and are laid singly on damp surfaces just above or near the water line in temporary pools and other habitats where the water level rises and falls. They can withstand desiccation for many months and hatch only when flooded with water.

Normally the eggs hatch out two days after laying and the larvae show S-shaped movements and are visible to the naked eye.

The period from egg to adult mosquito is roughly six weeks but it depends on environment factors like temperature, humidity and food availability. Two main factors that regulate (Ae. Aegypti population in a town is the climate and the availability of artificial breeding sites).

Q: What are the major dengue areas in Sri Lanka?

A: Major cities in the western province - Colombo and suburbs. Other major cities are Kandy, Kurunegala, Matara and Gampaha.

Q: What are the key breeding sites?

A:Major breeding sites are tyres with water and plastic containers with water. Heavy Aedes breeding is found at CTB depots, the repair shops, garages, construction sites, dump yards etc.

Q: What should be the approach to eliminate this problem?

A: These places should be visited and inspected for Aedes larvae. It is not only the government but the householders, business personnel who are also responsible for cleaning their gardens and business premises regularly. This is called a bottom up campaign. There should be a system to fine the householders and commercial owners who are positive for Aedes breeding, on the spot.

Q: If mosquito coils are used to expel the dengue mosquitoes, is it practicable and what coils should be used?

A: Mosquito coil burning does not contribute much to Aedes control since Aedes mosquitoes are day biters. Any company before introducing their products (coils) to the market should register it under Registrar of Pesticides. Hence the public should see the packet for SLSI certificate.

Q: What is the sequence of events inside the body of an infectious Aedes for dengue?

A: Once a mosquito becomes capable of transmitting the dengue virus it retains that capacity for life. For a few days after the ingestion of an infectious blood meal, viral replication is limited to cells of the posterior midgut of the mosquito; next, the virus is found in the proventriculus fat body, ovarial sheath, haemocytes and nervous system.

Virus is present in large amounts in the brain, thoracic and abdominal ganglia before it appears in the salivary glands.

Q: Do female Aedes mosquitoes fly far?

A: Spontaneous dispersal of adults is usually reported as limited, averaging about 50-100 metres a day for females, which means that a female rarely visits more than two or three houses during her life time. However, some authors have stated that the flight distance of females may be related to the availability of a place for egg laying sites and so might be much longer.

Q: So dengue is transmitted to a person when an Aedes mosquito who has the dengue virus in its saliva bites a person. Tell me more about the dengue virus.

A: Dengue is caused by RNA viruses, designated DEN-1, DEN-2, DEN-3 and DEN-4 of the family, Flaviridae..Flavi viruses replicate in a variety of cultured cells of both vertebrate and arthropod origin.

Q: What is the incubation period (i.e. the interval between the time of being bitten by the mosquito and the onset of symptoms)?

A: It is about 5-8 days.

Q: What is the clinical course of the illness.

A: Dengue virus infections cause a spectrum of illnesses ranging from symptom-free, mild, undifferentiated fever to classical dengue fever (DF) and dengue fever with haemorrhagic manifestations (DHF) and dengue shock syndrome (DSS). Dengue fever in its classic form is a non fatal, febrile illness of 5-7 days duration, of older children and adults.

The illness usually begins abruptly with high fever accompanied by flushing of the face, redness of the skin, headache and muscle pain. Mild redness of the eyes is occasionally noted while an infected pharynx (showing as sore-throat) is common. Loss of appetite, vomiting and abdominal pain are common.

Dengue virus infections may be confused clinically with influenza, measles, typhoid, leptospirosis or any non-specific viral syndrome. A definitive diagnosis of dengue fever there fore can only be made by specific laboratory tests. Physician in endemic areas are encouraged to seek collaboration with a diagnostic laboratory that can provide this service.

Dengue haemorrhagic fever (DHF) is the most serious form of dengue infection with bleeding tendency and a tendency to develop shock and it is called dengue shock syndrome. The major facts which determine disease severity and distinguish DHF from dengue fever (DF) are plasma leakage and abnormalities in blood clotting mechanism.

Q. Should dengue patients essentially be hospitalized?

A. Dengue without complications can be managed at home.

1. Give paracetamol for fever.

2. Bed rest.

3. Fluids to drink with normal diet.

Meanwhile blood is tested like in any fever. In dengue, platelet or thrombocyte count goes down indicating thrombocytopenia. Thrombocytopenia is defined by WHO as a platelet count less than 100,000 mm-3. Bleeding is associated with a platelet count below 50,000 mm-3. Bleeding often occurs with a decrease in platelet count but thrombocytopenia without bleeding is not uncommon.

If uncomplicated, a dengue patient recovers after 7-10 days. Dengue is associated with haemorrhage in 5-30% cases. Variable degrees of bleeding may occur at any site, most commonly petechiae (spots of bleeding under skin), purpura (bleeding patches under skin), bleeding from nose gums or vagina. Passage of black coloured stools containing altered blood (melaena) and vomiting or coughing of blood can be present.

Typically DHF is characterized by four major clinical manifestations.

1. High continued fever of 2-7 days.

2. Bleeding tendency.

3. Enlargement of the liver.

4. Circulatory disturbances.

Q. How is a dengue haemorrhagic patient managed in hospital?

A. Regular platelet counts are done. Intravenous fluids-plasma infusions are given.

Q. After recovery from dengue how long should the patient rest?

A. Depending on the severity of the DHF, two weeks to one month.

Q. Is it possible for a dengue patient to fall ill with dengue again immediately after recovery?

A. There are four strains of viral sero-types. So it is possible for a person to get infected with another sero-type even immediately after infection with one sero-type.

Q. On entering the body of a person what happens to the virus?

A. It replicates at the site of inoculation (i.e. just under the skin) and multiplies in reticuloendothelial system, fibroblasts or both. Then they are transferred to regional lymph nodes and viraemia of blood is caused. This causes signs and symptoms of dengue. It multiplies in the liver as well. Platelet count is reduced because platelet destruction occurs. Bleeding could occur in the brain, lungs or gut. Even kidney failure could occur.

Q. Do patients recover from shock and haemorrhagic (bleeding)?

A. They recover but mortality rate is high.

Q. What other conditions cause thrombocytopenia?

A. They recover but mortality rate is high.

Q. What other conditions cause thrombocytopenia?

A. Bone marrow dysfunction, aplastic anaemias, infiltration of bone marrow by cancers, immune thrombocytopenia purpura, and drugs.

Q. What causes death in dengue? A. Shock and Multiple organ failure.

Q. A dengue epidemic was in Sri Lanka two years ago. Now again dengue is raising its head. How would you advise the people to prevent such occurrences in future?

A. I feel I should stress on the importance of every person being aware of keeping his or her house, garden, business premises or any other place within his or her reach devoid of mosquito breeding places. They should help themselves rather than wait for help given by the government. That would be a more effective way of eradicating the disease.

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