| Influenza virus life cycle |
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Viral shedding is the expulsion and release ofvirus progeny following successful reproduction during ahostcell infection. Once replication has been completed and the host cell is exhausted of all resources in making viral progeny, the viruses may begin to leave the cell byseveral methods.[1]
The term is variously used to refer to viral particles shedding from a single cell, from one part of the body into another,[2] and from a body into the environment, where the virus may infect another.[3]
Vaccine shedding is a form of viral shedding which can occur in instances of infection caused by someattenuated (or "live virus") vaccines.
"Budding" through thecell envelope—in effect, borrowing from thecell membrane to create the virus' ownviral envelope— intoextracellular space is most effective for viruses that require their own envelope. These include such viruses asHIV,HSV,SARS orsmallpox. When beginning the budding process, the viralnucleocapsid cooperates 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.[4] 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.[5] Budding has been most extensively studied for viruses ofeukaryotes. However, it has been demonstrated that viruses infectingprokaryotes of the domainArchaea also employ this mechanism of virion release.[6]
Animal cells are programmed to self-destruct when they are under viral attack or damaged in some other way. By forcing the cell to undergoapoptosis or cell suicide, release of progeny into theextracellular 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'sgenome being chopped up, beforeapoptotic bodies of dead cell material clump off the cell to be absorbed bymacrophages. This is a good way for a virus to get intomacrophages 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.[7]
Viruses that have envelopes that come from nuclear or endosomal membranes can leave the cell viaexocytosis, in which the host cell is not destroyed.[8] Viral progeny are synthesized within the cell, and the host cell's transport system is used to enclose them invesicles; 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 envelopedvaricella-zoster virus.[9]
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A human with a viral disease can becontagious if they are shedding virus particles, even if they are unaware of doing so. Some viruses such asHSV-2 (which producesgenital herpes) can causeasymptomatic shedding and therefore spread undetected from person to person, as no fever or other hints reveal the contagious nature of the host.[10]
After an animal is exposed to rabies and the virus has spread to its salivary glands, the animal may be able to shed (or excrete) the rabies virus in its saliva; this means that the animal is infectious.
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| Viral life cycle | ||
| Genetics | ||
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