Overview of Emerging Arboviruses

Ann M. Powers

Disclosures

Future Virology. 2009;4(4):391-401. 

In This Article

Abstract and Introduction

Abstract

Numerous arboviral outbreaks during the past decade have demonstrated that arthropod-borne pathogens continue to be significant public and animal health threats. These outbreaks have occurred globally and have not been limited to tropical or developing countries, as people and goods can be moved anywhere in the world within days. Several examples of recent outbreaks have been described, including how they were identified, tracked and the resulting outcomes from these events. Fortunately, scientific research, including advances in rapid detection of this diverse group of pathogens, has also been progressing. While arboviruses are likely to continually emerge and re-emerge, improved scientific technologies and approaches will hopefully make each future epidemic less likely to occur.

Introduction

Since the initial discovery that mosquitoes could serve as vehicles for transmitting viral diseases, scientists have strived to understand the relationships between arthropod-borne (arbo)viruses and their invertebrate vectors. Arboviruses have a complex life cycle involving replication in both their invertebrate vectors, as well as their vertebrate hosts, where disease manifestations may or may not be present. This need to replicate efficiently in two completely disparate systems affects not only the viral properties and evolutionary patterns, but also the scientific efforts to control these pathogens. In addition, arboviruses, while having many characteristics in common with each other, are an exceptionally diverse group of viruses belonging to numerous distinct families, existing in virtually every known habitat and transmitted by multiple arthropods as different as mosquitoes, ticks, fleas and culicoides (Table 1).[1] Scientists are continually learning more about the approximately 500 unique arboviruses,[1] with the ultimate goal of finding approaches for controlling their transmission.

However, even with these advances, arboviruses continue to emerge and re-emerge, causing massive and widespread epidemics of human and animal disease (Table 2). These emergence events can manifest in different formats, making them even more unpredictable. Some arboviruses cause sporadic outbreaks in areas where they have previously been documented; however, activity is not continuously recorded. Recent examples of this re-emergence phenomenon include yellow fever virus (YFV) epidemics in South America, as well as outbreaks of Rift Valley fever in parts of Africa. Other outbreaks follow the pattern of the ongoing chikungunya virus (CHIKV) epidemic in India and Southeast Asia or the multiple bluetongue viruses that are spreading through Europe. These viruses have caused periodic outbreaks in specific geographic regions, but have now expanded their range and may establish themselves in these novel ecological niches. Another type of emergence occurs when a virus not only expands to novel geographic regions, but also changes its pattern of illness as it moves. A very specific and well-known example of this scenario involves West Nile virus (WNV). WNV is now considered enzootic in North America, causing seasonal human outbreaks of neuroinvasive disease after being introduced 10 years ago; disease in both humans and equines is also documented in the original geographic range of Africa and Europe, where prior to the 1990s, the pathogen was considered to be a minor threat. Finally, there are the completely unexpected outbreaks where a virus that is virtually unknown causes an epidemic in a completely unexpected location; the 2007 emergence of Zika virus (ZIKV) in the Federated States of Micronesia is an example. This article will describe an example of each type of these recent outbreaks and offer likely explanations for the factors affecting their emergence. It is worth noting that there are also arboviruses, such as the dengue viruses, that continuously cause extensive human disease and may be considered by some to be re-emerging. While these agents are certainly of tremendous importance, their continual circulation and sheer magnitude puts them in a distinct category that is outside the scope of this review.

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