Emergent virus
An emergent virus (or emerging virus) is a virus that is either newly appeared, notably increasing in incidence/geographic range or has the potential to increase in the near future.[1] Emergent viruses are a leading cause of emerging infectious diseases and raise public health challenges globally, given their potential to cause outbreaks of disease which can lead to epidemics and pandemics.[2] As well as causing disease, emergent viruses can also have severe economic implications.[3] Recent examples include the SARS-related coronaviruses, which have caused the 2002–2004 outbreak of SARS (SARS-CoV-1) and the 2019–2023 pandemic of COVID-19 (SARS-CoV-2).[4][5] Other examples include the human immunodeficiency virus, which causes HIV/AIDS; the viruses responsible for Ebola;[6] the H5N1 influenza virus responsible for avian influenza;[7] and H1N1/09, which caused the 2009 swine flu pandemic[8] (an earlier emergent strain of H1N1 caused the 1918 Spanish flu pandemic).[9] Viral emergence in humans is often a consequence of zoonosis, which involves a cross-species jump of a viral disease into humans from other animals. As zoonotic viruses exist in animal reservoirs, they are much more difficult to eradicate and can therefore establish persistent infections in human populations.[10]
Emergent viruses should not be confused with re-emerging viruses or newly detected viruses. A re-emerging virus is generally considered to be a previously appeared virus that is experiencing a resurgence,[1][11] for example measles.[12] A newly detected virus is a previously unrecognized virus that had been circulating in the species as endemic or epidemic infections.[13] Newly detected viruses may have escaped classification because they left no distinctive clues and/or could not be isolated or propagated in cell culture.[14] Examples include human rhinovirus (a leading cause of common colds which was first identified in 1956),[15] hepatitis C (eventually identified in 1989),[16] and human metapneumovirus (first described in 2001, but thought to have been circulating since the 19th century).[17] As the detection of such viruses is technology driven, the number reported is likely to expand.
Prevention[edit]
Control and prevention of zoonotic diseases depends on appropriate global surveillance at various levels, including identification of novel pathogens, public health surveillance (including serological surveys), and analysis of the risks of transmission.[37] The complexity of zoonotic events around the world predicates a multidisciplinary approach to prevention.[37] The One Health Model has been proposed as a global strategy to help prevent the emergence of zoonotic diseases in humans, including novel viral diseases.[37] The One Health concept aims to promote the health of animals, humans, and the environment, both locally and globally, by fostering understanding and collaboration between practitioners of different interrelated disciplines, including wildlife biology, veterinary science, medicine, agriculture, ecology, microbiology, epidemiology, and biomedical engineering.[37][38]
Virulence of emergent viruses[edit]
As hosts are immunologically naïve to pathogens they have not encountered before, emergent viruses are often extremely virulent in terms of their capacity to cause disease. Their high virulence is also due to a lack of adaptation to the new host; viruses normally exert strong selection pressure on the immune systems of their natural hosts, which in turn exerts a strong selection pressure on viruses.[39] This coevolution means that the natural host is able to manage infection. However, when the virus jumps to a new host (e.g., humans), the new host is unable to deal with infection due to a lack of coevolution, which results in mismatch between host immunoeffectors and virus immunomodulators.
Additionally, in order to maximize transmission, viruses often naturally undergo attenuation (i.e., virulence is reduced) so that infected animals can survive long enough to infect other animals more efficiently.[40] However, as attenuation takes time to achieve, new host populations will not initially benefit from this phenomenon. Moreover, as zoonotic viruses also naturally exist in animal reservoirs,[10] their survival is not dependent on transmission between new hosts; this means that emergent viruses are even more unlikely to attenuate for the purpose of maximal transmission, and they remain virulent.
Although emergent viruses are frequently highly virulent, they are limited by several host factors including: innate immunity, natural antibodies, and receptor specificity. If the host has previously been infected by a pathogen that is similar to the emergent virus, the host may also benefit from cross-protective immunity.