Emerging And Re-Emerging Viruses: Origins And Drivers*

Ab Osterhaus and Leslie Reperant, ‘Artemis One Health’ Utrecht, The Netherlands and ‘RIZ’, Hannover, Germany

Complex relationships between the human and animal species have never ceased to evolve since the emergence of the human species and have resulted in a human-animal interface that has promoted the cross-species transmission, emergence and eventual evolution of a plethora of infectious pathogens. Remarkably, most of the characteristics of the human-animal interface -as we know it today- have been established long before the end of our species pre-historical development took place, to be relentlessly shaped throughout the history of our species. More recently, changes affecting the modern human population worldwide as well as their dramatic impact on the global environment have taken domestication, agriculture, urbanization, industrialization, and colonization to unprecedented levels. This has created a unique global multi-faceted human-animal interface, associated with a major epidemiological transition that is accompanied by an unexpected rise of new and emerging infectious diseases.

Until the beginning of the last century, infectious diseases were the major cause of mortality of humankind. Around 1900 infectious diseases caused an estimated fifty percent of all deaths in the western world. In the following decades, this percentage decreased to a few percent. This was largely due to the implementation of public health measures such as the installment of sewage and clean drinking water systems, but also to the development of vaccines and antimicrobial compounds. A major success in this regard was the eradication of smallpox through a worldwide vaccination campaign orchestrated by the World Health Organization (WHO). Stimulated by these successes certain policymakers and scientists predicted that all infectious diseases of humankind would be brought under control. Paradoxically the following decades confronted the world with an ever-increasing number of emerging or re-emerging infectious diseases, some causing true pandemics. A complex mix of predisposing factors in our globalizing world, linked to major changes in our social environment, technology and global ecology, collectively created opportunities for viruses to infect new hosts. Subsequent adaptation to the newly invaded species then paved the way for an unprecedented spread with dramatic consequences for public health, animal health, animal welfare, food supply, economies, and biodiversity. Striking examples were the emergence of AIDS, Avian flu, SARS, MERS, Ebola and most recently Zika. Viruses spilling over from animal reservoirs have all caused these disease outbreaks.

HIV, the causative agent of the AIDS pandemic that started about thirty years ago, with changes in bush meat consumption, behaviour, demography, human mobility, medical practises and rapidly adapting viruses as main drivers, has now infected more than 55 million people of whom more than 20 million have died. The identification of HIV as the causative agent took more than two years after the recognition of AIDS as a new disease entity. Since then virus discovery techniques have evolved drastically with the advent of an ever-increasing range of new generation molecular techniques. This allowed us to rapidly identify dozens of new viruses of animals and humans, some of which were indeed newly emerging viruses, while others were viruses that had just not been discovered before due to technical limitations. Avian influenza viruses were first shown to sporadically infect humans in Hong Kong in the late 90ies of the last century, without subsequent efficient human-to-human transmission. However more recently it was shown that not more than a handful of mutations would allow such avian influenza viruses to become transmissible among mammals, thus creating a pandemic threat. In the light of the four influenza pandemics that have occurred in the last century, and together have cost the lives of more than 50 million people, this is an alarming observation. The emergence of severe acute respiratory syndrome (SARS) in China at the beginning of the 21st century, proved to be caused by a newly discovered coronavirus that most probably spilled over from a bat reservoir, an was transmitted via carnivores to humans. SARS coronavirus started to spread efficiently among humans, rapidly creating a pandemic threat. Through an international WHO-coordinated collaborative pathogen discovery and intervention network, this virus was identified and characterized within a month after the start of this collaboration and the emerging pandemic was subsequently rapidly controlled. A decade later yet another coronavirus of probable bat origin spilled over to humans in the Middle East, causing Middle East respiratory syndrome (MERS). The identification and characterization of MERS coronavirus was performed in a matter of weeks, whereas the dromedary camel proved to be the intermediate species that transferred the virus to humans. Different outbreaks in the Middle East and sporadic cases and outbreaks elsewhere, have indicated predominantly nosocomial human-to-human transmission. It is not clear at present whether this virus has the potential to cause a pandemic. Most recently the emergence of Zika virus, a flavivirus that had been discovered in Africa in the middle of the last century, emerged outside Africa and Asia in the last decade. Zika virus that is transmitted by Aedes mosquitos, was initially shown to cause a mild and self-limiting disease, and was until recently considered to be a relatively innocuous pathogen. However an increased incidence of microcephaly in unborn babies that coincided since last year with the emergence of Zika virus in South America, has created an ongoing public health emergency.

Importantly, the unprecedented emergence of these and other viruses is largely paralleled by medical, technological, and scientific progress, continuously spurred by our never-ending combat against pathogens. Investment in a better understanding of the human-animal interface will therefore offer humankind a future head start in the never-ending battle against emerging infectious diseases.

*Reperant LA, Cornaglia G, Osterhaus AD. Curr Top Microbiol Immunol. 2013