00000000Although DHF may affect persons of all ages in dengue endemic areas, most DHF cases occur in children less than 15 years of age. Since 1964, the trend in Bangkok has been towards progressively lower attack rates (constant hospital admission rates despite an increasing population), with the modal age of hospitalized children being 6 - 7 years throughout Thailand. Surveillance data from some areas have suggested a slight excess of infected girls over boys, while other areas have shown an almost even distribution.

00000000 A retrospective evaluation of the impact of DHF during an outbreak in Bangkok/Thon Buri in May - November 1962 indicated that in a population of 870 000 children under 15 years of age, an estimated 150 000 - 200 000 minor febrile illnesses were caused by dengue and occasionally by chidungunya viruses; 4187 patients were hospitalized with DHF, and 4000 additional patients were treated in private clinics or at home. Moreover, shock occurred in about one-third of the hospitalized DHF patients. In the more recent large epidemic in Thailand in 1987, the attack rate of DHF/DSS was 320 cases per 100 000 population for all ages. In southern Viet Nam between 1975 and 1992, the attack rate of DHF/DSS ranged from 30 to 380 per 100 000 population, with mortality rates from 0.39 to 6.42 per 100 000 population, while the incidence of DHF in Indonesia for 1991 and 1992 was 11.56 and 9.45 per 100 000, respectively.

Transmission of dengue viruses

00000000 Dengue viruses are transmitted to humans through the bite of infected Aedes mosquitos, principally Aedes aegyti, and are therefore considered to be arboviruses (arthropod-borne viruses9) Once infected, a mosquito remains infected for life, transmitting the virus to susceptible individuals during probing and feeding. Infected female mosquitos may also pass the virus to the next generation of mosquitos by transovarian transmission, but this occurs infrequently and probably does not contribute significantly to human transmission. Humans are the main amplifying host of the virus, although studies have shown that monkeys in some parts of the world may become infected and perhaps serve as a source of virus for feeding mosquitos. The virus circulates in the blood of infected humans at approximately the time that they have fever, and uninfected mosquitos may acquire the virus if they feed on an individual when he or she is viraemic. The virus then develops in the mosquito for a period of 8 - 10 days before it can be transmitted to other humans during subsequent probing or feeding. The length of time required for this extrinsic incubation depends in part on environmental conditions, especially ambient temperature.