Dengue is an old and the most important arthropod-borne viral disease affecting humans in terms of morbidity. There are reports in the medical literature about epidemics caused by an illness comparable to dengue that reach back to the late 17th century. The Chinese, however, described similar symptoms even earlier, more precisely during the Chin Dynasty (265 to 420 A.D.) (Gubler, 1998).
Sporadic and widespread outbreaks of dengue caused a major burden on public health between the 17th and the 20th century (Gubler, 2004). Nowadays, over 2.5 billion people live in risk areas (Figure 1.1) and 50 to 100 million people suffer from dengue fever (DF) every year. The World Health Organization (WHO) estimates that 500,000 cases of Dengue Hemorrhagic Fever / Dengue Shock Syndrome (DHF/DSS) and more than 20,000 deaths occur per year (WHO, 2002). In the last 25 years of the 20th century, dengue has emerged as a major public health problem and epidemics have a tremendous impact on social as well as economic structures of society especially in developing countries of the tropics (Gubler, 2002).
Figure 1.1: Countries and areas at risk of dengue transmission. Yellow color indicates dengue infested areas and countries that experienced new dengue epidemics between 2000-2006 are colored in red (WHO, 2006).
1.1.1.1 Situation in the Americas
In the Americas, major epidemics started to occur periodically in the early 17th century and the USA was confronted with a last major dengue outbreak in 1945 (Ehrenkranz et al., 1971; Gubler, 2004). Epidemics normally had their origin in one country subsequently spreading all over the region and were caused by only one serotype that disappeared after several months. Infections affected thousands of people and were characterised by self-limited classical DF (Gubler, 2004). In the 1900s, implementation of preventive measures and of control programs for subduing Aedes Aegypti (Ae. Aegypti) mosquitoes as the vector of yellow fever virus also had a highly
acceptable effect on combating dengue fever. It resulted in the declining or even disappearance of DF throughout the region (Graham et al., 1999; Gubler, 2004).
1.1.1.2 Situation in the Asia/Pacific
However, in the Asia/Pacific region, DF was a common occurrence in the first 50 years of the 20th century. There was an epidemic every 10 to 40 years depending on the introduction of a new virus (Gubler, 2004). Various reports from this time show that dengue virus was endemic during this period but the exact distribution of all four serotypes was unknown. The isolation of all four serotypes in the 1940s (DENV-1 and DENV-2) and 1950’s (DENV-3 and DENV-4) finally suggested that dengue virus was already present earlier and was maintained by a monkey-mosquito-monkey cycle (Mackenzie et al., 2004; Weaver and Barrett, 2004). But after and during World War II, the epidemiology of dengue dramatically changed, causing major epidemics and the spread of dengue virus into new geographic areas. The reasons for this change are not completely understood but it is thought that the insertion of hundreds of thousands soldiers, increased population density, troop movement and shipment of war material played a crucial role in causing Asia to become hyperendemic with the co-circulation of multiple dengue virus serotypes (Gubler, 2004). This resulted in an increased transmission of multiple serotypes and in the sequential emergence of dengue hemorrhagic fever (DHF) in the 1950s with a first outbreak in Manila, Philippines.
Singapore experienced its first DHF epidemic in the early 1960s (Gubler, 2004).
1.1.1.3 Reasons for the Global Emergence of Dengue
The fledgling stages of the pandemic emergence of dengue originated in the social as well as economic disruptions caused by World War II, that created ideal conditions for mosquito-borne diseases (Gubler, 1998; Gubler, 2002; Gubler, 2004; Mackenzie et al., 2004). Due to the hyperendemic situation in the Asia/Pacific, a newly described disease emerged in the form of DHF in 1960. It spread throughout South-East Asia and by the mid 1970s, DHF had became a major burden among children in this region (Gubler, 2004). On the other hand, the Americas faced dramatic epidemiological changes after discontinuing Ae. Aegypti eradication programs in the early 1970s. By the end of the 1990s, Ae. Aegypti mosquitoes had nearly regained the geographic distribution leading to major dengue outbreaks in countries that were known to be nonendemic or hypoendemic. From 1981 and 1997, the first DHF cases were reported in the Americas suggesting the same emergence of DHF as it happened 25 years before in the Asia/Pacific (Gubler, 2004).
In summary, we can specify five factors that have been playing a crucial role in the emergence of dengue. Unprecedented population growth combined with unplanned and uncontrolled urbanization led to an increased transmission of arboviral diseases in tropical countries. Additionally, the lack of mosquito control and modern transportation introduced new virus strains and serotypes into new geographic regions that have been regained by the mosquito vector. The last factor of equal importance is represented by the decay of public health infrastructures and the changes in public health policies causing inappropriate outbreak and disease management (Gubler, 2002;
Gubler, 2004; Mackenzie et al., 2004).