Emerging zoonotic viruses : what characterizes them and what are the risk factors for their emergence?

Abstract: Emerging infectious diseases poses a great future threat, not only to humans but also to domestic animals and wildlife. Even though the majority of these infections only cause minor health problems, the relatively recent emergence of HIV clearly illustrates that the next major human pandemic may surface at any time. Regardless of what the name indicates, most emerging human pathogens are not believed to be recently evolved, but to have existed previously in the natural environment. Most emerging pathogens are zoonotic, i.e. able to infect animals besides humans. Viruses are significantly overrepresented, which is thought to be due to the general difficulties in treating and controlling viral diseases, as well as the substantial amount of genetic diversity that viruses exhibits. Even though there is no clear association between the classification of a virus as zoonotic and its’ risk of emergence, many emerging viruses have been shown to be zoonotic or have a zoonotic history. Therefore it is probably accurate to conclude that zoonotic viruses constitute a significant risk for future emergence events. Since the majority of emerging pathogens are zoonotic, the process of cross-species transmission of a microbe from its reservoir to humans is likely to play a central role. It is probable that humans are exposed to unfamiliar pathogens regularly, but that only a few of them will possess the required genetic features to successfully cause human infection. A fraction of these pathogens may go on to acquire the ability to transmit between humans, whereof an additional few may evolve to transmit at a rate that enables major epidemics. When emergence occurs, it appears to disproportionately often happen in areas that are affected by anthropogenic activities. Some of these activities are believed to act by augmenting exposure of humans to novel pathogens, reservoirs or vectors, others by enhancing disease transmission. In order to be prepared for future emergence events, there is a great need for additional research on numerous aspects of the emergence process. Also, given the complexity and multifactorial nature of disease emergence, a multidisciplinary and internationally conjunctive approach is often advised.