How do human viruses come to be?

Sylvia Rothenberger, project manager at the CHUV Institute of Microbiology, explains.

We use the word “emergence” to refer to the appearance or rapid spread of a new virus within a species. Are we to expect this type of event to occur more frequently in the years to come? Sixty percent of infectious diseases in humans brought about in this way are of animal origin, and 70% of those animals are wild. A multitude of factors are involved, but this emergence is primarily the consequence of ecosystems altered by humans and changes in human lifestyle.

Researcher Sylvia Rothenberger, points specifically to “deforestation, development of agriculture and live animal markets, which increase contact between humans and animals.” Viruses are generally adapted to a particular species, so they must cross a barrier to infect another species. But more frequent contact with animals that can carry viruses, especially rodents and bats, makes it easier for these microbes to pass to humans.

Wars also provide favourable conditions for the emergence of new viruses in humans. Soldiers and people move about. Health systems collapse. Poor hygiene is rampant. All these factors increase the transmission of viruses. During the Korean War (1950-1953), for example, thousands of soldiers were infected with the Hantaan virus causing haemorrhagic fever. The microbe was transmitted via the excrement of rodents prevalent in rice fields. More recently, years of war in Syria have prevented effective vaccination against the polio virus, which is believed to have temporarily re-emerged.

“We should probably expect increased outbreaks due to globalisation and population growth,” Sylvia Rothenberger says. “Global warming will also impact the spread of disease, as animal species could migrate to milder climates.” Examples include mosquitoes that transmit dengue and Zika, ticks carrying encephalitis, and rodents that are vectors for hantavirus.

RNA viruses mutate faster than DNA viruses, and some of these mutations make it easier for them to cross over to new hosts. Viruses go through different stages in adapting to a new host, with at one end of the spectrum viruses found exclusively in animals and at the other those exclusively associated with humans. In between are viruses that can infect humans in specific ways, such as Andes virus in South America. “It is rodent-borne with mortality rates equivalent to the Ebola virus. But the difference between the two is that there is no effective human-to-human transmission for Andes, because it has not yet adapted to humans.” This is in sharp contrast to SARS-CoV-2. After emerging, the virus spread rapidly from one human host to another, creating the current pandemic.

To fight emerging viruses, Sylvia Rothenberger emphasises that they and their hosts must be studied more closely. This is the basis for the One Health concept introduced in the early 2000s. One Health is a holistic and interdisciplinary approach to health that recognises the interconnection between humans, animals, plants and their environment. This approach promotes collaboration, communication and cooperation among professional partners, such as biologists, physicians, veterinarians, epidemiologists, agricultural workers and politicians. The concept could therefore play an important role in preventing, monitoring and controlling outbreaks of infectious diseases. /