**5. The paradoxical absence of yellow fever from Asian countries**

YF has never been reported from Asia, but, should it be accidentally imported, the potential for outbreaks, as the appropriate mosquito vector is present over there [21]. The lack of YFV in Asia is not clearly understood, although a number of hypotheses have been put forward [24]. The mosquito vector *Ae. aegypti* is prevalent in Asia and Pacific countries and has been important in the rapid emergence of dengue as a major public health problem in the twenti‐ eth century [25]. Laboratory studies indicate that Asian strains of *Ae. aegypti* can transmit YFV but are less competent than strains from the Americas. Demographic factors, including the remote location of sylvatic YF transmission and the cross-protective immunity provided by prior exposure to dengue and other flaviviruses, likely play a role in the lack of YF in Asia [26].

**Year Countries Name of City Cases Deaths**

**1942** Brazil NA NA NA **1981-1982** Bolivia NA NA NA

towns

Yellow Fever Encephalitis: An Emerging and Resurging Global Public Health Threat in a Changing Environment

Guajira

In the 18th and 19th centuries, YF was a huge public health problem until mosquito control measures and production of an effective vaccine brought the epidemics under control in the 20th century. Yet as we enter the 21st century this virus is once again a significant public health problem [15,26,28] and is classified as a reemerging disease. Urban YF has not been reported from the Americas since 1954, but jungle yellow fever transmitted by Haemagogus vectors increasingly affects forest dwellers in Bolivia, Brazil, Columbia, Ecuador, and Peru, and periodically causes small outbreaks [15, 29,30]. The reinvasion of South America by *Ae. aegypti* after relaxation of the eradication programme in the 1970s, and presence of *Ae. aegyp‐ ti* in cities near areas in which sylvatic yellow fever is endemic, poses a threat of urbanisa‐ tion of yellow-fever transmission [25,29]. Following several decades of relative calmness, YF reappeared in Africa in the 1980s, endangering populations not only in the so-called endem‐ ic countries but in the rest of the world too [31]. The resurgence of YF is also closely connect‐ ed with changes in the modern world and with the interaction of various economic, climatic,

**8. Compounding factors for emergence and resurgence of YF**

YF has been subjected to partial control for decades, but there are signs that case numbers are now increasing globally, with the risk of local epidemic outbreaks [33]. The agent of YF, yellow fever virus, can cause devastating epidemics of potentially fatal, hemorrhagic dis‐ ease. We rely on mass vaccination campaigns to prevent and control these outbreaks. How‐ ever, the risk of major YF epidemics, especially in densely populated, poor urban settings, both in Africa and South America, has greatly increased due to: (1) reinvasion of urban set‐ tings by the mosquito vector of YF, *Ae. aegypti*; (2) rapid urbanization, particularly in parts of Africa, with populations shifting from rural to predominantly urban; and (3) waning immu‐ nization coverage. Consequently, YF is considered an emerging, or reemerging disease of

Magdalena and La

NA 20000

http://dx.doi.org/10.5772/46041

213

28 11

**1878** United States America Over 100 American

**2003** Colombia States of Cesar,

**7. Recent emergence and resurgence of YF**

social and political factors [32].

considerable importance [22].

**Table 1.** The history of Yellow fever outbreaks in subtropical regions of America
