Retroviruses are spherical viruses that contain ribonucleic acid (RNA) as their genetic material. In contrast, most other organisms, including humans, store their genetic information in the form of deoxyribonucleic acid (DNA). Retroviruses are of concern to humans because of their disease causing ability. Examples of retroviruses include human T cell leukemia virus, which causes cancer in humans, and the several types of human immunodeficiency virus (HIV), which is widely acknowledged to be the cause of acquired immunodeficiency syndrome (AIDS).
In 1911, Peyton Rous successfully isolated the agent that caused tumors in chickens. This agent, later called Rous sarcoma virus, was the first retrovirus to be discovered. In the 1960, Howard Temin proposed that retroviruses accomplished the replication of their genetic material by going from RNA through DNA to RNA. This concept, which was called reverse transcription, garnered Temin and David Baltimore a Nobel Prize in medicine or physiology in 1975. The human T cell leukemia virus (HTLV; now two types are known), the first diseases causing retrovirus of humans, was discovered in 1981, followed two years later by the discovery of HIV.
The known retroviruses are classified into three families: Retroviridae, Metaviridae, and Pseudoviridae. The HIV and HTLV types are members of the Retroviridae. Members of the family Metaviridae infect fungi and insects. Finally, members of the family Pseudoviridae infect yeast and insects. From the human perspective, the Retroviridae are the most immediate concern.
Retroviruses produce new virus particles inside of host cells that they have infected. The infection process begins when the virus binds to a specific molecule (called a receptor) on the surface of the host cell. The host receptor has not been produced to specifically encourage the binding of retroviruses. Rather, the retroviruses evolved to exploit the surface molecule as a target.
Once inside the host cell, the viral RNA is freed from the viral particle, and is reverse transcribed into DNA. The viral DNA can then become part of the host's DNA, in a process called integration. When the host's DNA is used to make new RNA, the viral DNA produces new viral RNA. The RNA can become packaged in new viral particles, which are released from the cell (a process called budding). The replication cycle can be repeated over and over again with other host cells.
Some of the host cells that can be targeted include cells that are important for the functioning of the immune system. With these cells not operating properly, the host is at risk for infections. Indeed, many AIDS patients die from infections and maladies like cancer than from the HIV infection.
As of 2002, no cure exists for the leukemia caused by HTLV or for AIDS (the cause of HIV.) Several candidate HIV vaccines have not so far provided adequate protection.
Prevention is the only way to avoid these retroviral diseases. Because the retroviruses responsible may be transmitted during sexual contact, using condoms and avoiding unsafe sexual practices in which blood, semen, or vaginal fluids are exchanged has been shown to be highly effective in preventing retrovirus transmission. Avoiding the injection of drugs or the sharing of needles is another way to prevent transmission.
During the 1970s, the blood collected in Canada and the United States was subject to viral contamination, even with HIV. However, stringent control practices have once again made the blood supplies generally safe. HTLV is not as large a threat in the United States as it is in other areas of the world that are endemic for the virus. Estimates are that HTLV-infected blood donors constitute about 0.025% of all U.S. blood donors.
Coffin, J.M., S.H. Hughes, and H.E. Varmus. Retroviruses. Cold Spring Harbor, NY: Cold Spring Harbor Press, 1997.
Brooks, J.I., E.W. Rud, R.G. Pilon, et al. "Cross-Species Retroviral Transmission from Macaques to Human Beings." Lancet 360 (August 2002): 387–388.
Gallo, R.C., A.S. Liski, and F. Wong-Staal. "Origin of the T cell Leukemia-Lymphoma Virus." Lancet (ii) (1983): 962–963.
Weiss, R., "Getting to Know HIV." Tropical Medicine and International Health. 5 (July 2000): A10–15.
Zhang, Z-Q., T. Schuler, M. Zupancic, et al. "Sexual Transmission and Propogation of SIV and HIV in Resting and Activated CD4+ T Cells." Science 286 (November 1999): 1353–1357.