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A flavivirus with 4 distinct subgroups.

Aëdes mosquitoes.

Dengue is a mosquito-borne infection which in recent years has become a major international public health concern. A more lethal complication, dengue haemorrhagic fever (DHF), was first recognized during the 1950s and is today a leading cause of childhood deaths in many countries.

Dengue viruses are transmitted to humans through the bite of infected Aedes mosquitos. Once infected, a mosquito remains infective for life, transmitting the virus to susceptible individuals during probing and blood feeding. Infective female mosquitos may also transmit the virus to the next generation of mosquitos by transovarial transmission. Humans are the main amplifying host of the virus, although studies have shown that in some parts of the world monkeys may become infected and perhaps serve as a source of virus for uninfected mosquitos.

The spread of dengue is attributed to expanding geographic distribution of the four dengue viruses and of their mosquito vectors, the most important of which is Aedes aegypti. A rapid rise in urban populations is bringing ever greater numbers of people into contact with the vector mosquito, while poor sanitation in many urban centers and lack of adequate resources for water storage and waste disposal provides more opportunities for mosquito breeding.

Dengue fever is a severe, flu-like illness that affects infants, young children and adults but rarely causes death. The clinical features of dengue fever vary according to the age of the patient. Infants and young children may have an undifferentiated febrile disease with rash. Older children and adults may have either a mild febrile syndrome or the classical incapacitating disease with abrupt onset and high fever, severe headache, pain behind the eyes, muscle and joint pains, and rash.

Dengue haemorrhagic fever is a potentially deadly complication that is characterized by high fever, haemorrhagic phenomena -- often with enlargement of the liver -- and in severe cases, circulatory failure. The illness commonly begins with a sudden rise in temperature accompanied by facial flush and other nonspecific constitutional symptoms of dengue fever. The fever usually continues for 2-7 days and can be as high as 40-41C, possibly with febrile convulsions and haemorrhagic phenomena. In moderate DHF cases, all signs and symptoms abate after the fever subsides. In severe cases, the patient's condition may suddenly deteriorate after a few days of fever; the temperature drops, followed by signs of circulatory failure, and the patient may rapidly go into a critical state of shock and die within 12-24 hours, or quickly recover following appropriate volume replacement therapy.

The global prevalence of dengue has grown dramatically in recent decades. The disease is now endemic in more than 100 countries in Africa, the Americas, the Eastern Mediterranean, South-East Asia and the Western Pacific. Before 1970 only nine countries had experienced DHF epidemics, a number which by 1995 had grown more than four-fold to at least 41.

WHO currently estimates there may be 50 million cases of dengue infection worldwide every year. Some 2500 million people -- two fifths of the world's population -- are now at risk from dengue. Without proper treatment, DHF fatality rates reach 15% or more.

There is no specific treatment for dengue fever. However, careful clinical management by experienced physicians and nurses frequently save the lives of DHF patients.

At present, the only method of controlling or preventing dengue and DHF is to combat the vector mosquito. In Asia and the Americas, Aedes aegypti breeds primarily in man-made containers like bottles, cans, used tyres and other items that retain water. In Africa it breeds both in artificial containers as well as in natural habitats like tree holes and leaf axils.

Vector control is implemented through community-based clean-up campaigns to remove the vector breeding sites, and through the application of insecticides. Emergency control measures are based primarily on insecticide applications, and it is essential to monitor periodically the vector's susceptibility to the insecticides that are most widely used.

Vaccine development for dengue and DHF is difficult because any of four different viruses may cause disease, and because protection against only one or two dengue viruses could actually increase the risk of more serious disease. Nonetheless, progress is being made in the development of vaccines that may protect against all four dengue viruses. Such products could be commercially available within several years.

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