This is the commonest arthropod-borne viral infection in humans: over 100 million cases occur every year in the tropics, with over 10 000 deaths from dengue haemorrhagic fever. Dengue is caused by a flavivirus and is found mainly in Asia, South America and Africa, although it has been reported from the USA. Four different antigenic varieties of dengue virus are recognized and all are transmitted by the daytime-biting A. aegypti which breed in standing water in refuse dumps in inner cities. A. albopictus is a less common transmitter. Humans are infective during the first 3 days of the illness (the viraemic stage). Mosquitoes become infective about 2 weeks after feeding on an infected individual, and remain so for the rest of their lives. The disease is usually endemic. Immunity after the illness is partial.

Clinical features

The incubation period is 5-6 days following the mosquito bite. Asymptomatic or mild infections are common. Two clinical forms are recognized.

Classic dengue fever

Classic dengue fever is characterized by the abrupt onset of fever, malaise, headache, facial flushing, retrobulbar pain which worsens on eye movements, conjunctival suffusion and severe backache, which is a prominent symptom. Lymphadenopathy, petechiae on the soft palate and skin rashes may also occur. The rash is transient and morbilliform. It appears on the limbs and then spreads to involve the trunk. Desquamation occurs subsequently. Cough is uncommon. The fever subsides after 3-4 days, the temperature returns to normal for a couple of days, and then the fever returns, together with the features already mentioned, but milder. This biphasic or 'saddleback' pattern is considered characteristic. Severe fatigue, a feeling of being unwell and depression are common for several weeks after the fever has subsided.

Dengue haemorrhagic fever (DHF)

Dengue haemorrhagic fever is a severe form of dengue fever and is believed to be the result of two or more sequential infections with different dengue serotypes. It is a disease of children and has been described almost exclusively in South East Asia. The disease has a mild start, often with symptoms of an upper respiratory tract infection. This is then followed by the abrupt onset of shock and haemorrhage into the skin and ear, epistaxis, haematemesis and melaena known as the dengue shock syndrome. Serum complement levels are depressed and there is laboratory evidence of a consumptive coagulopathy.

Diagnosis and treatment

Isolation of dengue virus by tissue culture in sera obtained during the first few days of illness is diagnostic. Demonstration of rising antibody titres by neutralization (most specific), haemagglutination inhibition 'ELISA' or complement-fixing antibodies in sequential serum samples is evidence of dengue virus infection. Blood tests show leucopenia and thrombocytopenia.

Treatment is supportive with analgesics and adequate fluid replacement; in DHF, blood transfusion may be necessary.

Prevention

Travellers should be advised to sleep under impregnated nets but this is not very effective as the mosquito bites in daytime. Topical insect repellents should be used. Adult mosquitoes should be destroyed by sprays, and breeding sites should be eradicated.