The Absence of Yellow Fever in Asia: History, Hypotheses, Vector Dispersal, Possibility of YF in Asia, and Other Enigmas

Abstract

Since the recent epidemics of yellow fever in Angola and Brazil as well as the importation of cases to China in 2016, there has been an increased interest in the century-old enigma, absence of yellow fever in Asia. Although this topic has been repeatedly reviewed before, the history of human intervention has never been considered a critical factor. A two-stage literature search online for this review, however, yielded a rich history indispensable for the debate over this medical enigma. As we combat the pandemic of COVID-19 coronavirus worldwide today, we can learn invaluable lessons from the historical events in Asia. In this review, I explore the history first and then critically examine in depth major hypotheses proposed in light of accumulated data, global dispersal of the principal vector, patterns of YF transmission, persistence of urban transmission, and the possibility of YF in Asia. Through this process of re-examination of the current knowledge, the subjects for research that should be conducted are identified. This review also reveals the importance of holistic approach incorporating ecological and human factors for many unresolved subjects, such as the enigma of YF absence in Asia, vector competence, vector dispersal, spillback, viral persistence and transmission mechanisms.

Keywords: Aedes aegypti; Asia; hypothesis; international sanitary convention; medical enigma; spillback; transmission cycle; vector dispersal; yellow fever.

Figure 1

Geographic distributions of yellow fever (A) and of Ae. aegypti (B) (Source: Boyce, 1911; with permission).

Figure 2

Possible routes of Ae. aegypti dispersal to tropical Asia from Africa. (Red: As proposed in [119] by Powell et al.; Blue: East Africa route; Green: From the Americas to the Pacific; Orange: Inland dispersal to the Middle East).

Figure 3

Patterns of YF transmission in Africa and South America (Vertical transmission in the vectors is not shown because it is infrequent).