Dengue is produced by a group B arborvirus, indistinguishable in appearance from the Yellow Fever virus. It is caused by one of four related but antigenically distinct serotypes within the genus flavovirus. It is transmitted by Aedes aegypti mosquitoes and is endemic in many tropical regions of Asia, the Pacific, Central America and West Africa, with epidemics after severe rainy seasons. The first epidemics were reported in 1779, and a global pandemic began after World War II, particularly in South East Asia. In temperate countries, it is seen only in travellers. Nowadays, it is primarily an urban disease of the tropics, with humans as the primary reservoir. It is the second most important tropical infection after malaria. Two forms of illness are seen.

• Dengue fever (DF) is a mild to moderate non-fatal illness which follows an incubation period of 5—7 days. There is fever, severe headache, myalgia, backache, bone pain, facial flush and profound weakness. A morbilliform rash involves the trunk and extremities and may desquamate. Neutropenia is typical. The illness subsides in 5—7 days, but it may be followed by prolonged asthenia.

• Dengue haemorrhagic fever (DHF) is a much more serious condition, usually confined to South East Asia and only 1/200th as common as DF. It is additionally associated with thrombocytopenia, petechiae, multiple haemorrhages and shock. The platelet count is <100X 109/L and there is haemoconcentration. It is possible that this form of illness may arise from a more virulent strain of organism.

The diagnosis is made from serology or viral isolation. The differential diagnosis, particularly in the returned traveller with shock, includes:

• various other infections, such as

— viral haemorrhagic fever;

— rickettsial disease;

— toxic shock syndrome;

• non-infectious conditions, such as

— drug-induced Stevens—Johnson syndrome.

Treatment is symptomatic with analgesics, fluids and electrolytes. Clearly, avoidance of mosquito exposure is an important prophylactic measure. There is as yet no publically available vaccine.

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