Current Opinion in Virology 2016, 16:v–vi
For a complete overview see the Issue
http://dx.doi.org/10.1016/j.coviro.2016.03.005
1879-6257/Published by Elsevier B.V.
Population growth and the ongoing rapid industrialization and globalization of human activity in the 21st century has significantly affected the life styles of humans, their interactions with wild and domestic animals and the impact they are having on the environment. These developments have altered the ecology of animal viruses and the nature and frequency of their transmissions to humans. The SARS outbreak in 2003 was this century's first major cautionary tale of animal viruses and the problems they can cause for humans. Increasing contacts between humans and wildlife at live-animal markets led to the acquisition of an animal virus by humans, human-to-human transmission of the virus and the emergence of a global outbreak of severe disease. Fortunately, most transfers of animal viruses to humans are naturally constrained, as subsequent human-to-human transmission is not possible or inefficient. However, when a transferred animal virus can easily be transmitted amongst humans, extensive international travel can rapidly lead to a pandemic, as was the case with the ‘Swine Flu’ virus in 2009.
In this issue of Current Opinion in Virology several experts in the field discuss aspects of emerging viruses and their capacity for interspecies transmission, particularly to humans. Bats feature as a primary host for three of the emerging virus discussed in this issue; with two of these viruses, MERS [Reusken et al.] and the Hendra virus [Field], reaching humans via intermediate hosts of dromedary camels and horses, respectively. Whether the influenza viruses now found in bats pose a zoonotic threat is not yet known [Brunotte et al.]. The role of vectors in facilitating the emergence or greater spread of zoonotic viruses is shown in the emergence of Huaiyangshan or Severe Fever with Thrombocytopenia Syndrome virus across East Asia [Zhang and Xu] and the continuing reemergence and spread of Chikungunya virus [Tsetsarkin et al.]. Pybus and Thézé raise the possibility that a vector, tabanids, could link the cross species transmissions of Hepaciviruses.
Zhu et al. describe the pathway that led to the emergence of an H7N9 avian influenza virus in humans with wild birds introducing viruses to domestic ducks, which then interact with chickens at farms or live poultry markets introducing new viruses or segments to terrestrial poultry. Reassortment of viruses in chickens led to the H7N9 virus, which was then transmitted to humans, mainly at markets. H7N9 viruses have continued to reassort with H9N2 viruses in chickens, leading to distinct regional genotypes within China. They point out that the pattern seen in the emergence of H7N9 viruses is not unique and results from the extensive interactions between wildlife, agricultural systems and humans driven by the pressures caused by population growth and development.
Zhang and Xu summarize the infection prevalence of Huaiyangshan virus (HYSV), also known as severe fever with thrombocytopenia syndrome virus (SFTSV), among different domestic and wild animals, as well as its occurrence in different tick species. By combining the infection prevalence and the host and vector ecology, the authors propose a model for the local transmission and long-range dissemination of the SFTSV, giving clues to which animal and vector populations should be monitored to reduce the risk of human infections.
Brunotte et al. present the recent discovery of influenza A viruses in bats. They describe the evidence from the phylogenetic, structural and functional analyses of the viral protein that provide insights into the virus origin — whether the ancestor or the recent descendant of the classical avian influenza A virus, as well as the risk for their emergence in humans. They also highlight that a more confident answers to these issues requires better understanding of complete diversity of the influenza virus in different bat species, that could only be done through more surveillance in the exotic bat populations.
Claes et al. describe the recent emergence of a new clade of H5 avian influenza virus that contains several subtypes of neuraminidase. The virus, which is originated in China, has now spread to more than 20 Asian, European and North American countries. They summarize the evidence for roles of poultry trading and wild bird migration in geographic dissemination of the virus.
Global dissemination on an even larger, more complex scale can be observed in arboviral infections. Tsetsarkin et al. describe the history and recent epidemiology chikungunya virus (CHIKV), with highlights of the recent virus adaptation in other vector species, leading to the new spread in new urban regions. They summarize current knowledge of CHIKV transmission into a model that describes the transmission cycles and vectors between human and non-human primates.
Field summarizes the existing knowledge of transmission routes and prevalence of Hendra virus in different hosts including various bats species, horses, dogs and humans. This leads to a model of Hendra virus ecology in which flying foxes serve as the natural reservoir that occasionally transmit the virus to horses from which further to humans.
An eco-evolutionary scenario similar to that raised for Hendra virus has been suspected for long time in connection with measles. Human infection from cattle by a common ancestor to what today is measles and rinderpest virus has been considered as an explanation for the primordial human acquisition of measles. More recent research has identified morbillivirus precursors in bats, and a large number of less closely related viruses, the so-called unclassified morbilli-related viruses, in bats and rodents. Nambulli et al. provide a summary of old theories and new data on morbillivirus evolution and ecology, along with fascinating insights into the biogeography of a group of highly contagious infectious diseases.
Bats and rodents do also seem to have played a role in the ecology and evolution of hepaciviruses, including the hepatitis C virus of humans. Yet, a number of other animals including horses and dogs seem to carry viruses even closer related to the human hepatitis C virus than those viruses carried by small mammals. Pybus et al. address the question whether hepacivirus evolution may have involved these animals, and whether yet unrealized ways of transmission, e.g., by mechanical transfer of virus via insects, might play a role.
Another virus that is now thought to have originated in bats but was transferred to human via a major livestock species is the MERS (Middle East respiratory syndrome) coronavirus. Here, the intermediary host is dromedary camels. Reusken et al. summarize our present concept of the reservoir ecology and host barriers for MERS.
Next to their role as immediate virus reservoirs, bats are more and more being implicated as mediators of the evolution of human pathogens. In their review on hepadnaviruses, Rasche et al. focus on the complicated and controversial scenarios for the origin of human hepatitis B. Recent new insights regarding the deeper lineage evolution of mammalian hepadnaviruses, specifically bat-associated viruses, now root the tree of primate hepadnaviruses clearly to the New World.
In addition to the hygiene-keeping measures in the community, early detection and monitoring of the emerging viruses are essential for disease control at the human-animal interface. Virus surveillance is still facilitated by conventional RT-PCR technology, enabling most of the virus discoveries described in various articles in this issue. The advent of high throughput sequencing (HTS) has enabled the decoding the complete genomes of these novel virus and high-resolution genomic study of those recently re-emerging viruses such as chikungunya virus (Tsertsakin), H5Nx highly pathogenic avian influenza viruses (X), for mutations that co-incide with the renewed expansion of disease. The real challenge in the future will be to harvest HTS data on highly equidistant organisms that are yet too dissimilar to any other virus to be discovered as ‘viral’. Studying the biodiversity of viruses will open new chapters in the book of virus–host association and cross-host transmission.
Biographies
Guan's virology program leads the field of research on emerging viral diseases. He identified the zoonotic source of the SARS coronavirus and the major precursors and evolutionary paths of the H5N1, 2009 pandemic, and H7N9 influenza viruses, discerning measures to avert and control these major human outbreaks.
Christian Drosten (43) is a physician by training. He started his career in a diagnostic virology context but expanded his research interest to viral evolution and ecology after he co-discovered the SARS-coronavirus. He has since worked on the ecology of RNA viruses in mammals and insects, while maintaining a focus on epidemiological and molecular virology of emerging coronaviruses. He has co-authored more then 280 peer-reviewed papers. He currently heads the Institute of Virology at University of Bonn Medical Centre.