Virus relationships

How are visna, HIV, HTLV, SIV, etc. related?

Much of the motivation for various theories depends on how close HIV is to visna and other viruses. To summarize:

Back around 1986, when the various visna theories arose, the relationships were rather confused. Gallo thought HIV was closest to HTLV-I and HTLV-II, so he gave the name "HTLV-III" to HIV; he was wrong. Others discovered that visna was the closest known virus to HIV. At this point, the conspiracy theories arose that HIV was formed somehow from visna and maybe HTLV.

Over the next few years, however, things became much clearer. More viruses, such as SIV, were discovered. The genetic sequences of the viruses were obtained, showing how they were related. At this point, attention turned away from visna, since it was found to be not especially close to HIV compared to other viruses.

To summarize: The viruses described above are all retroviruses. Retroviruses have three subfamilies: Oncoviruses, Lentiviruses, and Spumaviruses. HTLV is a oncovirus, while the remainder are lentiviruses. The analysis of genetic sequences gives strong evidence for the evolution of lentiviruses. They apparently branched into the primate lentiviruses (HIV-1, HIV-2, and SIV), and the nonprimate lentiviruses (visna, BLV, EIAV, FIV, CAEV, etc.) Thus, HIV and visna have many similarities since they are both lentiviruses, but HIV and SIV are much more similar.

See Fields Virology for more information on retrovirus classification and "The Emergence of Simian Human Immunodeficiency Viruses", Myers et al, AIDS Research and Human Retroviruses, 8(3), 1992 373-386 for more information on lentivirus evolution.

Click here for a diagram of the relationships among retroviruses.

Click here for an explanation of how gene sequencing reveals these relationships.

Ken Shirriff:
This page: Copyright 2000 Ken Shirriff.