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Human viruses: discovery and emergence
CONCLUSION
The lines of evidence described earlier combine to suggest the following tentative model of the emergence process for novel human viruses. First, humans are constantly exposed to a huge diversity of viruses, though those of others mammals (and perhaps birds) are of greatest importance. Moreover, these viruses are very genetically diverse and new genotypes, strains and species evolve rapidly (over periods of years or decades). A fraction of these viruses (both existing and newly evolved) are capable of infecting humans. It is not clear whether some of these human-infective viruses will already be capable of reaching higher levels of the pathogen pyramid—so-called ‘off-the-shelf’ pathogens—or whether subsequent evolution of their ability to infect and transmit from humans is usually required—‘tailor-made’. The distinction is potentially important as it implies different determinants of the rate of emergence of viruses with epidemic or pandemic potential: for off-the-shelf pathogens this rate is largely driven by the rate of human contact with a diversity of virus genotypes (possibly rare genotypes) within the non-human reservoir (i.e. ecology); for tailor-made viruses, the key variable is likely to be the rate of genetic adaptation within the new human host (i.e. evolution).
Whichever of these two models is correct (perhaps both), there is a clear implication that the emergence of new human viruses is a long-standing and ongoing biological process. Whether this process will eventually slow down or stop (if the bulk of new virus species constitute extant diversity) or whether it will continue indefinitely (if a significant proportion of newly discovered virus species are newly evolved) remains unclear, although this makes little difference to immediate expectations. There is a hint, from the slower accumulation of new virus families found in humans, that virus diversity may be bounded, but that does not preclude there being a much larger number of virus species ‘out there’ than we are currently aware of. If anthropogenic drivers of this process are important then it is possible that we are in the midst of a period of particularly rapid virus emergence and, in any case, with the advent of new virus detection technologies, we are very likely to be entering a period of accelerated virus discovery. The unavoidable conclusion is that we must anticipate the emergence and/or discovery of more new human viruses in the coming years and decades. By no means all of these will pose a serious risk to public health but, if the recent past is a reliable guide to the immediate future, it is very likely that some will.
The first line of defence against emerging viruses is effective surveillance. This topic has been widely discussed in recent years, but we will re-iterate a few key points here. Firstly, emerging viruses are everyone's problem: the ease with which viruses can disperse, potentially worldwide within days, coupled with the very wide geographical distribution of emergence events, means that a coordinated, global surveillance network is essential if we are to ensure rapid detection of novel viruses. This immediately highlights the enormous national and regional differences in detection capacity, with the vast majority of suitable facilities located in Europe or North America. Secondly, reporting of unusual disease events is patchy, even once detected, reflecting both governance issues and lack of incentives. Thirdly, we need to consider extending the surveillance effort to other mammal populations as well as humans, because these are the most likely source of new human viruses.
Improving the situation will require both political will and considerable investment in infrastructure, human capacity and new tools. However, the benefits are potentially enormous. It is possible to forestall an emerging disease event, as experience with SARS has shown. However, our ability to achieve this is closely linked to our ability to detect such an event, and deliver effective interventions, as rapidly as possible. A better understanding of the emergence of new human viruses as a biological and ecological process will allow us to refine our currently very crude notions of the kinds of pathogens, or the kinds of circumstances, we should be most concerned about, and so direct our efforts at detection and prevention more efficiently.
This work was partly supported by the USAID