Viral shedding

Means

Shedding from a cell into extracellular space

Budding (through cell envelope)

"Budding" through the cell envelope into extracellular space is most effective for viruses that require their own envelope. In effect, the viral envelope is built from a part of the host cell membrane. Examples for viruses that shed through budding include HIV, HSV, SARS, and smallpox. When beginning the budding process, the viral nucleocapsid interacts with a certain region of the host cell membrane. During this interaction, the glycosylated viral envelope protein inserts itself into the cell membrane. In order to successfully bud from the host cell, the nucleocapsid of the virus must form a connection with the cytoplasmic tails of envelope proteins. Though budding does not immediately destroy the host cell, this process will slowly use up the cell membrane and eventually lead to the cell's demise. This is also how antiviral responses are able to detect virus-infected cells. Budding has been most extensively studied for viruses of eukaryotes. However, it has been demonstrated that viruses infecting prokaryotes of the domain Archaea also employ this mechanism of virion release.

Apoptosis (cell destruction)

Animal cells are programmed to self-destruct when they are under viral attack or damaged in some other way. By forcing the cell to undergo apoptosis or cell suicide, release of progeny into the extracellular space is possible. However, apoptosis does not necessarily result in the cell simply popping open and spilling its contents into the extracellular space. Rather, apoptosis is usually controlled and results in the cell's genome being chopped up, before apoptotic bodies of dead cell material clump off the cell to be absorbed by macrophages. This is a good way for a virus to get into macrophages either to infect them or simply travel to other tissues in the body.

Although this process is primarily used by non-enveloped viruses, enveloped viruses may also use this. HIV is an example of an enveloped virus that exploits this process for the infection of macrophages.

Exocytosis (cell release)

Viruses that have envelopes that come from nuclear or endosomal membranes can leave the cell via exocytosis, in which the host cell is not destroyed. Viral progeny are synthesized within the cell, and the host cell's transport system is used to enclose them in vesicles; the vesicles of virus progeny are carried to the cell membrane and then released into the extracellular space. This is used primarily by non-enveloped viruses, although enveloped viruses display this too. An example is the use of recycling viral particle receptors in the enveloped varicella-zoster virus.

Contagiousness

A human with a viral disease can be contagious if they are shedding virus particles, even if they are unaware of doing so. Some viruses such as HSV-2 (which produces genital herpes) can cause asymptomatic shedding and therefore spread undetected from person to person, as no fever or other hints reveal the contagious nature of the host.

References

References

1. "8 things you may not know about rabies – but should".
2. (October 1979). "Viral shedding patterns of children with influenza B infection". The Journal of Infectious Diseases.
3. (2017). "Virus Interactions With the Cell". Viruses.
4. (December 2002). "Mechanisms of enveloped RNA virus budding". Trends in Cell Biology.
5. (September 2016). "Eukaryotic-Like Virus Budding in Archaea". mBio.
6. (April 2000). "Human immunodeficiency virus type 1 vpr induces apoptosis through caspase activation". Journal of Virology.
7. (2017). "Virus Interactions With the Cell". Viruses.
8. (May 1997). "Endocytosis and recycling of varicella-zoster virus Fc receptor glycoprotein gE: internalization mediated by a YXXL motif in the cytoplasmic tail". Journal of Virology.
9. "Genital Herpes' Silent Spread".