Retroviral disease

Retroviruses are not known to cause disease in plants, but they can produce a wide range of diseases in animals.In some retroviral infections, symptoms appear soon after an infection.

In other infections the retrovirus genes become incorporated into a host cell's DNA and enter a latent (dormant) state until some unknown agent or event triggers them to churn out new viruses.

In these cases, symptoms may not become apparent for months or even years after the initial infection.

Once installed, a retrovirus can damage host cells directly, or it can trigger cancer by disabling the systems that normally prevent cells from multiplying out of control.

Scientists first linked retroviruses to cancer in 1911, when American researcher Francis Peyton Rous investigated a form of tumor that occurs in chickens.

Rous passed a solution containing cells from these chicken tumors through a fine filter.The filter was intended to capture infectious agents, such as bacteria and protozoans.

Rous discovered that the filtered fluid from these tumors was still infectious, indicating the presence of an unknown infectious agent.He correctly concluded that a virus, which is so small that it passes through filters, was responsible for the chicken tumor.

Called Rous sarcoma virus, it was discovered many years later to be a member of the retrovirus family. Retroviruses also cause cancer in a range of mammals:

Cats and rodents are two well-studied examples.

In 1965, while studying the Rous sarcoma virus, American virologist Howard Temin made the surprising discovery that the virus’s RNA inserted its own genes into the DNA of the host cell.

In 1970 Temin and American molecular biologist David Baltimore, working independently, identified an RNA viral enzyme that copies genetic information to the host cell's DNA.

The enzyme later became known as reverse transcriptase.The discovery of reverse transcriptase shed new light on how a retrovirus changes a normal cell to a cancer cell.

The first human retrovirus was discovered in 1980. Known as human T-cell leukemia virus (HTLV), it exists in two forms, HTLV-I and HTLV-II, and appears to cause certain types of lymphoma (cancer of the lymphatic system) and leukemia.

In 1983 a third and quite different human retrovirus was discovered in patients suffering from a new immune deficiency disease (an illness that damages the immune system).

Initially labeled HTLV-III, it was renamed HIV in 1986 and has since gained worldwide notoriety as the cause of AIDS.HIV attacks the immune system of its host, undermining the very defenses that keep most viruses in check.

One group of immune system T cells, called CD4 cells, are particularly vulnerable to HIV and become primary targets soon after an infection begins.Initially, the amount of virus in the blood, known as the viral load, quickly climbs as newly replicated viruses are produced and released from host cells.

Following this stage, known as acute retroviral syndrome, the viral load drops, and it looks as though the immune system has brought the infection under control.Despite these promising signs, the virus is never completely eliminated.From its hiding place inside T cells themselves, HIV continues its attack, destroying T cells.

As T cells die, the immune system loses its ability to fight back.

Once the T-cell count drops below a critical level, microorganisms that are normally kept in check by a healthy immune system can reproduce at a rapid rate.

Without effective treatment, these opportunistic infections, rather than HIV itself, often have fatal results.

Retroviruses that cause immune deficiency diseases affect animals other than humans.

This group of retroviruses includes feline immunodeficiency virus (FIV), which infects cats, and simian immunodeficiency virus (SIV), which attacks monkeys and apes.

SIV is of particular interest in medicine as the origin of HIV. Scientists believe that SIV from a chimpanzee likely infected humans and underwent mutations to form HIV.