Overview of Emerging Arboviruses

Ann M. Powers

Disclosures

Future Virology. 2009;4(4):391-401. 

In This Article

Factors Affecting Arboviral Emergence

Finding the answer to the simple question of what causes an arbovirus to 'emerge' is anything but straightforward, as there are undoubtedly a number of factors involved. The reason for any arboviral epidemic is most probably that a combination of multiple factors combines in precisely the right manner to allow the outbreak to occur. A few of these factors are outlined below. Understanding the interactions involved in the biology of the virus, hosts and ecology will hopefully lead to effective control and prevention strategies.

Since most arboviruses are zoonotic pathogens, the first logical place to look at in the emergence of a vector-borne epidemic is the ecological and environmental origin of the outbreak. For example, consider the environmental conditions that were present in Kenya leading up to the initiation of the CHIKV outbreak in 2004. A significant drought had occurred along the coast of Kenya, and some of the driest conditions since 1998 for parts of the coastal region were recorded.[95] Given that mosquitoes require an aquatic environment during their larval and pupal phases, these hot, dry conditions are counter intuitive for the emergence of a mosquito-borne epidemic. However, it may have been these exact features that led to the initiation of the outbreak. Owing to the drought conditions, water in storage vessels was kept in close association with human dwellings, and these vessels were replenished less frequently than normal. Therefore, the mosquito populations were functionally forced into close associations with people, perhaps establishing a dynamic for transmission that would otherwise not have been present. At the very least, this information has shown that there is a need for safe water storage in all conditions to limit the likelihood of generating large vector mosquito populations. Furthermore, it is important to consider that many of the epidemic vectors are peridomestic, naturally existing in close association with human hosts. The vectors of CHIKV, YFV and ZIKV all utilize human habitat to maintain their populations;[96–99] thus, general living conditions can contribute to providing one component necessary for arboviral transmission.

A second factor that certainly plays a role in the generation of arboviral outbreaks is the host status and herd immunity of the vertebrate hosts in affected areas. In two of the epidemic situations described previously, CHIKV in islands of the Indian Ocean, as well as ZIKV in Yap, there were functionally completely naive populations involved, as these outbreaks were the first ever recorded for those areas.[100] In island and other tropical populations where air conditioning and well-screened dwellings are not particularly common, there would be no functional barriers to infection for a significant number of inhabitants. This is reflected in the extremely high attack rates that were documented in Lamu, Comoros and Yap.[8,10] Even in areas where epidemics had previously occurred, such as India for CHIKV or Argentina for YFV, infrequent epidemics with significant interepidemic periods render entire young generations susceptible to arboviral infection.[16,101] By contrast, having control options such as highly effective vaccines readily available could rapidly alter the immune status of a population, thus modulating the magnitude of an outbreak.[102,103] The administration of over 12.4 million doses of YFV vaccine throughout affected areas of South American immunized a significant percentage of the population at risk, and most likely prevented outbreaks of much greater degree.

Finally, the third component of the arboviral transmission cycle, the virus itself, must be considered when trying to determine the cause of emergence of an arboviral epidemic. As the arboviruses are virtually all RNA viruses lacking proofreading functions, genetic mutations would certainly be a possible reason for changes in virulence, epidemiology or vector competence. As noted previously, the finding of a point mutation in the CHIKV genome, which implicated adaptation to a secondary vector species of mosquitoes, may have contributed to the scope of the outbreak in La Reunion. These genetic laboratory studies of the virus are highly important; when mutations are found, they can be used not only to track the movement of viruses, but also to predict possible changes in the epidemiological patterns of an outbreak. For example, knowing whether viral strains containing a valine at a particular residue in the envelope glycoprotein are present in an area (such as Italy), where the only mosquitoes present are the 'alternate' vectors, is critical public health information.

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