Latent viruses are those viruses that can incorporate their genetic material into the genetic material of the infected host cell. Because the viral genetic material can then be replicated along with the host material, the virus becomes effectively "silent" with respect to detection by the host. Latent viruses usually contain the information necessary to reverse the latent state. The viral genetic material can leave the host genome to begin the manufacture of new virus particles.
The molecular process by which a virus becomes latent has been explored most fully in the bacteriophage designated lambda. The lysogenic process is complex and involves the interplay between several proteins that influence the transcription of genes that either maintain the latent state or begin the so-called lytic process, where the manufacture of new virus begins.
Bacteriophage lambda is not associated with disease. However, other viruses that can establish a latent relationship with the host are capable of causing disease. Examples of viruses include the Herpes Simplex Virus 1 (also dubbed HSV 1) and retroviruses. The latter group of viruses includes the Human Immunodeficiency Viruses (HIVs) that are the most likely cause of acquired immunodeficiency syndrome (AIDS).
In the case of HSV 1, the virus can become latent early in life, when many people are infected with the virus. The virus infects the mucous membranes located around the mouth. From this location the virus spreads to a region of certain nerve cells called the ganglion. It is here that the viral genetic material (deoxyribonucleic acid, or DNA) integrates into the host genetic material. The period of latency can span decades. Then, if the host is stressed such that the survival of the infected cells is in peril, the viral DNA is activated. The new virus particles migrate back to the mucous membranes of the mouth, where they erupt as "cold sores". A form of the reactivated herpes virus that is known as Herpes Zoster causes the malady of shingles. The painful sores associated with shingles can appear all over the body.
The re-emergence of HSV 1 later in life does qualify as a disease. However, it has been argued that the near universal prevalence of the latent form of the viral DNA in people worldwide qualifies HSV as being part of the normal microbial makeup of humans. Others argue that even the latent HSV state qualifies as an infection, albeit an infection that displays no symptoms and is essentially harmless to the host.
Other examples of a latent virus include the HIVs. The latent form of HIV is particularly insidious from the point of view of treatment, because the drugs traditionally given to treat AIDS are effective only against the actively replicating form of the virus. In the absence of detectable virus, drug therapy may be discontinued. Then, if the virus is stimulated to leave the latent state and begin another round of infection, that infection will be uncontrolled. Indeed, it has been shown that even the near continuous administration of anti-HIV drugs does not completely eliminate the pool of latent virus in the immune system.
A hallmark of latent viral infections is that the immune system is not stimulated to respond. Indeed, with little or no viral products or new virus produced, the immune system has no target. This complicates the development of vaccines to infections such as HSV 1 and AIDS, because the nature of the vaccine effect is the stimulation of the immune system.
One experimental approach that is being explored with latent viral infections is to establish whether there is some aspect of the host cell that predisposes the cells to infection with a virus capable of becoming latent. Identification of such host factors could help in the design of therapeutic strategies to target these cells against viral infection.
See also Lysogeny; Virology; Viral genetics
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