In many parts of the tropics and subtropics, dengue is endemic, that is, it occurs every year, usually during a season when Aedes mosquito populations are high, often when rainfall is optimal for breeding. These areas are, however, additionally at periodic risk for epidemic dengue, when large numbers of people become infected during a short period. Dengue epidemics require a coincidence of large numbers of vector mosquitoes, large numbers of people with no immunity to one of the four virus types (DENV 1, DENV 2, DENV 3, DENV 4), and the opportunity for contact between the two. Although Aedes are common in the southern U. S., dengue is endemic in northern Mexico, and the U.S. population has no immunity, the lack of dengue transmission in the continental U.S. is primarily because contact between people and the vectors is too infrequent to sustain transmission.
Dengue or dengue-like epidemics were reported throughout the nineteenth and early twentieth centuries in America, Southern Europe, North Africa, the east Mediterranean, Asia, Australia, and on various islands in the Indian Ocean, the south and Central Pacific, and the Caribbean. DHF has increased both in incidence and distribution over the past 40 years, and, in 1996, 2.5-3.0 billion people lived in areas potentially at risk for dengue virus transmission. It is estimated that there are 20 million cases of dengue infection annually, resulting in around 24,000 deaths. The geographic distribution of dengue viruses and their mosquito vectors has expanded, and DHF has emerged in the Pacific region and the Americas. In Southeast Asia, epidemic DHF first appeared in the 1950s, but by 1975 it had become a leading cause of hospitalization and death among children in many countries in that region.[ In Europe, the last dengue epidemic dates from 1927-1928 in Greece, with high mortality. However, there continues to be imported cases of DF in travelers returning to Europe from endemic areas.
In the 1980s, DHF began a second expansion into Asia when Sri Lanka, India, and the Maldives Islands had their first major DHF epidemics; Pakistan first reported an epidemic of DF in 1994. The recent epidemics in Sri Lanka and India were associated with multiple dengue virus serotypes. After an absence of 35 years, epidemic DF occurred in both Taiwan and the People's Republic of China in the 1980s. The People's Republic of China had a series of epidemics caused by all 4 serotypes, and its first major epidemic of DHF, caused by DENV-2, was reported on Hainan Island in 1985. Singapore also had a resurgence of DF/DHF from 1990 to 1994 after a successful control program had prevented significant transmission for over 20 years. In other countries in Asia where DHF is endemic, the epidemics have become progressively larger in the last 15 years.
A recent outbreak of DF in Karachi occurred in 2005 when Aga Khan University reported 30 positive cases out of 100. A recent trend of DF in southeastern countries is that it has become endemic, causing cyclical epidemics every 2-3 years.
A major challenge for public health officials in all tropical areas of the world is the development and implementation of sustainable prevention and control programs that will reverse the trend of emergent DHF. Environmental controls, including solid waste management, decreasing vector breeding sites by eliminating standing water, improvement in public awareness by media, and the use of household insecticides and mosquito repellants can help prevent the spread of dengue virus. Active case surveillance is important for early detection and implementation of control programs in the setting of acute epidemics. Unfortunately, there is no commercially available vaccine to prevent dengue. Tetravalent vaccines are currently being studied.