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 (see Table 1). South-East Asia and the Western Pacific are most seriously affected. Before 1970 only nine countries had experienced DHF epidemics, a number which had increased more than four-fold by 1995. Some 2500 million people - two fifths of the world's population - are now at risk from dengue. WHO currently estimates there may be 50 million cases of dengue infection worldwide every year. In 1998 alone, there were more than 616,000 cases of dengue in the Americas, of which 11,000 cases were DHF. This is greater than double the number of dengue cases which were recorded in the same region in 1995. 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.
Some other statistics:
· During epidemics of dengue, attack rates among susceptibles are often 40 50%, but may reach 80 90%.
· An estimated 500 000 cases of DHF require hospitalisation each year, of whom a very large proportion are children and roughly 5% die.
· Without proper treatment, DHF case fatality rates can exceed 20%. With modern intensive supportive therapy, can be reduced to less than 1%.
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 populations 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.