How Could Global Control of Dengue Hemorrhagic Fever Achieve Success - Taiwan's Valuable Experiences from Epidemiological Findings

in International Symposium (Abstract oral presentation), 國際研討會(摘要口頭發表)
標題How Could Global Control of Dengue Hemorrhagic Fever Achieve Success - Taiwan's Valuable Experiences from Epidemiological Findings
AuthorsNiann Tai Chang, 張念台
出版日期Nov 25 2010 12:0

Epidemics of dengue/DHF have been increasing at an alarming rate worldwide in the absence of a vaccine and as a result of dengue hemorrhagic fever’s (DHF) unclear pathogenesis, particularly in Asia and South America. An important global health challenge will be to identify the epidemiological conditions that have resulted in the rapid increase in DHF cases (the most severe form of dengue), as well as the spread of dengue virus to different parts of the world during the last two decades. Because dengue epidemics often occur but have not been endemic to Taiwan, we have had a unique opportunity to investigate epidemiological factors to promote DHF cases that can improve the prevention and control of dengue/DHF epidemics globally.
In Taiwan, dengue epidemics occur most frequently in the South, where Aedes aegypti are prevalent. These epidemics are usually triggered mostly by imported cases from dengue-endemic countries in South East Asia. DENV-1 has been the most common serotype and DENV-4 has been the least common of Taiwan’s imported cases from 1989 to 2010. This pattern is consistent with global trends. Among indigenous dengue cases, DENV-1 has been the dominant serotype for most years between 1987 and 1997 and in 2007 in the city of Tainan, reemerged as the dominant serotype in the largest epidemic of dengue in Taiwan since World War II. DENV-2 caused outbreaks in both 1995 and 1998, and became more prevalent in Taiwan in 2001-02. DENV-3 caused Tainan’s 1998 epidemic and Kaohsiung’s 2006 and 2009-2010 epidemics.
Increases in Taiwan’s local prevalence rates for DENV-3 since 1998 and DENV-2 since 2002 have paralleled global pandemic patterns. Virological and serological surveillance are able to predict which DENV serotype will be the source of the next epidemic or identify which serotype has been silently transmitting after a large epidemic. Co-circulating multiple serotypes of DENV that have been identified to be associated with increasing DHF cases in dengue-endemic/hyper-endemic countries were, in fact, not factors in Taiwan’s past epidemics involved DHF cases.
Our epidemiological investigations on dengue cases in Taiwan have found the following four conditions for occurring dengue hemorrhagic fever (DHF) that will be helpful for global dengue control. First, After a careful review of recent dengue epidemics involving DHF in 1998, 2001-2003, and 2006 in southern Taiwan, we found that DHF cases tended to occur in areas and families where dengue cases mostly clustering during the peak of dengue epidemics. Second, DHF cases appeared after the mild form dengue fever (DF) cases. Third, DHF cases have higher viral loads and are more likely to occur in the later stages of epidemics and this phenomenon generally leads to the severe epidemics become irreversible. Fourth, once DHF cases happen, dengue cases spread very quickly and were identified even though the total dengue case numbers decreased during winter seasons. Using geographical information systems (GIS), we identified that the 2002 dengue/DHF epidemic in Kaohsiung had both neighborhood spread and relocation diffusion patterns. Once relocation diffusion occurred, containment became further complicated and difficult. The longer the mean duration per wave and/or the greater the intensity of transmission at local sites, the higher percentage of DHF cases were found among total dengue cases at the epidemic foci. These findings imply that mosquito control cannot target only on focal areas after the appearance of DHF cases.
To the best of our knowledge, we are the first group to examine the tempo-spatial conditions between dengue fever and DHF. Our efforts to assess the potential severity of dengue epidemics have prompted us to identify risk patterns for the occurrence of DHF. Such information will be helpful to public health officials attempting to minimize the severity of future DHF epidemics globally.
At present, mosquito surveillance has several barriers, including (1) low infection rate of DENV, (2) Breteau index cannot serve as early warning signal for dengue epidemic, and (3) mosquito sampling lack geographical representativeness. Therefore, integrated environmental entomological virological and serological surveillance plus community-based behavior information is necessary to monitor for dengue virus activities among vulnerable populations in high-risk areas. We believe that severe DHF epidemics worldwide can be prevented or minimized through improved surveillance, implementation of integrated, community-based prevention and control programs, utilization of GIS to monitor tempo-spatial trend analyses of dengue clusters, and the development of a dengue vaccine.

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