Not only is the number of cases increasing as the disease is spreading to new areas, but explosive outbreaks are occurring. In Brazil nearly 475,000 cases were reported between January and October 1998 more than were reported from the entire continent in previous years. 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 the predominantly urban species Aedes aegypti. A rapid rise in urban population is bringing ever greater numbers of people into contact with this vector, especially in areas which are favourable for mosquito breeding e.g., where household water storage is common and where solid waste disposal services are inadequate. Transmission Dengue viruses are transmitted to humans through the bites of infective female Aedes mosquitoes.
Mosquitoes generally acquire the virus while feeding on the blood of an infected person.
Once infective a mosquito is capable of transmitting the virus to susceptible individuals for the rest of its life, during probing and blood feeding. Infected femalemosquitoes may also transmit the virus to the next generation of mosquitoes by transovarial transmission i.e. via its eggs, but the role of this in sustaining transmission of virus to humans has not yet been delineated. 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 mosquitoes. Characteristics 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.