Viruses by Dorothy H. Crawford

Author:Dorothy H. Crawford
Language: eng
Format: epub
ISBN: 9780192539724
Publisher: OUP Oxford
Published: 2018-01-08T16:00:00+00:00

Chapter 7

Persistent viruses

Viruses fight a constant battle against host immunity, and for most there is just a small window of opportunity in which to reproduce and make a hasty exit before being wiped out by the formidable array of host defences. But some viruses have evolved strategies for overcoming these immune mechanisms and survive inside their host for prolonged periods, even for a lifetime. Although the detailed mechanisms involved in these evasion strategies are very complex and varied, overall they encompass three basic manoeuvres: finding a niche in which to hide from immune attack, manipulating immune processes to benefit the virus, and outwitting immune defences by mutating rapidly.

Most persistent viruses have evolved to cause mild or even asymptomatic infections, since a life-threatening disease would not only be detrimental to the host but also deprive the virus of its home. Indeed, some viruses apparently cause no ill effects at all, and have been discovered only by chance. One example is TTV, a tiny DNA virus found in 1997 during the search for the cause of hepatitis and named after the initials (TT) of the patient from whom it was first isolated. We now know that TTV, and its relative TTV-like mini virus, represent a whole spectrum of similar viruses that are carried by almost all humans, non-human primates, and a variety of other vertebrates, but so far they have not been associated with any disease. With modern, highly sensitive molecular techniques for identifying non-pathogenic viruses, we can expect to find more of these silent passengers in the future.

The frequency with which viruses succeed in persisting in their hosts varies, with herpesviruses virtually always establishing a lifelong relationship that usually does no harm to the host. Retroviruses also generally infect for life, but they may, like HIV, cause a disease in those they infect after a prolonged silent period. Other viruses, such as hepatitis B virus, struggle to evade the immune response, and many hosts eventually manage to clear the virus. Further, there are a few viruses that are usually cleared after primary infection but on rare occasions may stay put. Measles virus, for example, for unknown reasons persists after the acute infection in around 1 in 10,000 cases causing a fatal brain disease called subacute sclerosing pan encephalitis (SSPE).

Because of the lifelong presence of foreign (viral) genes inside a host cell, a persistent virus can sometimes drive the cell it lodges in into uncontrolled growth, that is, to become cancerous. These include human T lymphotropic virus, hepatitis B and C viruses, Epstein–Barr virus, Kaposi sarcoma-associated virus, and the papilloma viruses. The mechanisms involved in the evolution of these cancers are dealt with in Chapter 8.

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