Species hopping: when an animal virus leads to the emergence of a human disease

As we all know, the world's population is currently being shaken by a new pandemic virus: SARS-CoV-2. Capable of efficient transmission, this virus has rapidly saturated healthcare systems unprepared for such a threat. Because of its sudden appearance, this new virus has been described as "emerging".

This article was written by students in the Master 1 Microorganism-Host-Environment Interactions program at the University of Montpellier (class of 2019-2020), supervised by Jean-Christophe Avarre, Institut de recherche pour le développement (IRD) and Anne-Sophie Gosselin-Grenet, University of Montpellier.

In virology, an emerging virus is an agent newly observed in a given population. Of animal origin, this virus has triggered an epidemic in humans, as is the case with other viruses (influenza and Ebola viruses, for example). This phenomenon, which enables an animal virus to infect and multiply in humans, is linked to a mechanism known as "species jumping".

What is a virus? How does it work?

Viruses are microscopic biological entities widely distributed in the environment, playing an essential role in the evolution and regulation of the populations of organisms they infect.

A virus consists of a genome (its genetic information), a protein shell called the capsid, which protects this genome, and sometimes an envelope. A virus is not capable of multiplying on its own; it must necessarily infect a cell to divert the raw materials and machinery it needs to manufacture its own components.

Tropism and species specificity: major viral characteristics

Infection begins with the encounter between a virus and a cell. This encounter is initiated by proteins on the surface of the virus, which recognize a specific cell molecule, called a receptor, exposed on the cell surface. This recognition, which depends on the quantity and type of receptors present on the cell, defines a cell's sensitivity to a virus. It is essential for the virus to bind to and penetrate the cell. Virus multiplication then depends on the cell's permissiveness, i.e. its capacity to allow the production of new viral particles.

The two parameters, sensitivity and permissiveness, thus define the virus's cellular tropism, i.e. its ability to penetrate and multiply preferentially in a particular cell type.

The cells of an organism have their own sensitivity and permissiveness to a virus, which also differs from one species to another. The virus's cellular tropism therefore also plays a part in its host spectrum, i.e. the specificity of the species it is capable of infecting and in which it can multiply. Some viruses have a broad host spectrum, while others are only capable of infecting a single host species.

Species specificity therefore implies a species barrier that prevents the passage of viruses (and pathogens in general) from one species to another, and thus the inter-species transmission of associated viral diseases. This barrier is multifactorial: physico-chemical, molecular, metabolic and immunological.

Crossing a species barrier leading to viral emergence

Viral emergence can manifest itself in a number of ways: emergence in a new territory, linked to a change in the distribution range of the virus or its host, or emergence of a disease in a new host species, linked to a structural modification of the virus enabling it to infect it.

A large number of viral emergences are the result of the transmission of viruses from animals to humans: these are known as zoonotic diseases or zoonoses, as in the case of AIDS resulting from the passage of monkey viruses to humans, or SARS in 2003, following the transmission of a coronavirus from bats to humans. Inter-species transmission, or species hopping, implies that the virus is capable of crossing the species barrier.

This species jump requires close contact between a virus-infected animal species, known as the reservoir, and man. The virus is generally non-pathogenic for the reservoir, and they have been living together for a long time. Virus multiplication is thus maintained in the reservoir, and numerous viral particles can be produced without harming it.

Transmission of the virus from the reservoir to humans occurs either directly, notably by ingestion of contaminated raw food or by bite, or indirectly via vectors. The latter are often arthropods, such as mosquitoes, which carry the virus between different hosts during their blood meals.

Insofar as the cohabitation between the virus and the new host species is recent, species jumping can be at the origin of the emergence of viral diseases, as is currently the case with Covid-19.

How can species change occur?

For species hopping to be successful, the virus must complete 4 steps: come into contact with the new host species (in this case, man), infect and multiply in its cells, evade the host's defenses and transmit itself to the new host's population.

Contact is encouraged by the increased promiscuity between humans and animals, due in particular to the expansion of metropolises, the destruction of ecosystems (deforestation) and the illegal trade in wild species.

As proximity isn't everything, the virus needs to adopt a few changes in order to survive inside the new host. Indeed, the virus must be able to adhere to receptors on the surface of its new host's cells in order to penetrate them, and multiply by hijacking the cellular machinery. This ability to adapt is partly made possible by their very high mutation rate. The multiplication mechanism of viral genetic material makes many errors that are not repaired by the "proofreading" systems common to living beings. These mutations can lead to structural modifications of the virus' surface proteins, enabling it to adhere to new cell types, and thus altering its cellular tropism. These mutations may also enable the virus to multiply in the cells of the new host species.

The virus, exposed to the host's defenses (its immune system), must also develop escape strategies. Some viruses, like HIV, attack host defense cells directly, while others "hide" by infecting cells not accessible to the immune system, or scramble danger signals between host cells.

Finally, to disseminate in the new host population, the virus must be transmitted between individuals, via respiratory droplets, blood, sexual contact or simple direct contact between individuals. The strategy and efficiency of the mode of transmission will then define the virus's ability to disseminate and maintain itself in the new species. Various factors linked to the infected host can also influence transmission efficiency. For example, once installed in the new host, the virus can take advantage of its movements to spread. Through human population movements linked to globalization, trade and travel, the virus can infect individuals in another region, thereby extending its range. If dissemination remains localized, we speak of an epidemic, but if it spreads worldwide, we speak of a pandemic. If transmission is not possible between different individuals of the species, we speak of transmission leading to an epidemiological impasse.

Through its considerable impact on the environment - deforestation, poaching or intensive livestock farming - coupled with ever-increasing globalization, mankind has become a major player in the emergence of viral diseases, encouraging what are normally accidental jumps in species.

Jean-Christophe Avarre, Researcher in viral ecology, Institute of Research for Development (IRD) and Anne-Sophie Gosselin-Grenet, Senior Lecturer in Virology, UMR Diversité, Génomes et Interactions Microorganismes-Insectes (DGIMI), University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.