MadSci Network: Virology

Re: What is the difference between SIV & HIV?

Date: Thu Jun 24 12:31:35 1999
Posted By: Brian Foley, Post-doc/Fellow Molecular Genetics
Area of science: Virology
ID: 928330731.Vi

The short answer is that HIV is found in humans and SIV is found in non-human simians.

The long answer is that there are currently two very different immunodeficiency viruses which infect humans and cause AIDS. The most common is HIV-1, which is believed to have come from chimpanzees. Three groups of HIV-1, the M, N and O goups probably represent three different chimpanzee to human transmission events. Any of these viruses is called SIV-CPZ if it is isolated from a chimpanzee, and HIV-1 if it is isolated from a human. The second, much less common worldwide, is HIV-2, which came from sooty mangabeys. HIV-2 also has several different groups which are likely to have been derived from seperate sooty mangabey to human transmissions.

There are many other simian immunodeficiency viruses which are not yet known to infect humans. The African green monkeys are infected with AIV-AGM, the mandrils are infected with SIV-MND, sun-tailed monkeys are infected with SIV-SUN, etc...

Many other mammals also carry realted lentiviruses. Wild pumas (mountain lions) in the USA carry a FIV-PUMA. Housecats all over the world carry FIV that is somewhat related to FIV-PUMA, but is just called FIV. Wild buffalo in Asia carry a BIV-JMB (Also called Jembrana disease virus). Horses carry equine infection anemia virus. etc...

Brian Foley
HIV Database

Admin note:
Mike Conrad adds the following:

That's a good question and the simple answer is "not much". HIV and SIV are very similar except that one infects humans and the other infects monkeys. Both viruses are in the retrovirus family meaning they use RNA as their genetic material which, after the virus enters the cell, must be copied back to DNA for the virus to become active. Both viruses are in the genus Lentivirus which means they are "slow viruses". And both viruses infect the T cells of the host's immune system.

The species HIV, Human Immunodeficiency Virus, infects and causes acquired Immunodeficiency disease in humans. There are two strains of HIV; they are HIV-1 and HIV-2.

SIV infects simians (monkeys and apes). There are 11 strains (at least) of SIV, each infecting a species of monkeys, including African green monkeys, chimps, mandrills, macaques, sooty mangabeys, etc.

In general, SIV does not cause any symptoms in its natural host monkey species. Hosts and microbes have coexisted for thousands of years and it is not in the microbe's interest to kill the host. Disease can be thought of as something "out of balance" which neither organism wants. But when a microbe crosses its species barrier, the normal controls on its life cycle will be absent and disease can happen. For example, most SIV strains get macaques sick. There is also a case of a chimp developing AIDS and dying about ten years after being infected with HIV.

The earliest known HIV sample, ZR59, was found in Africa and it dates to 1959. It resembles the basic consensus for the HIV-1 virus. Compared with SIVs, it is most similar to SIV chimp. It's possible that a few (10 or 15) years before 1959, someone picked up SIV, possibly from butchering a chimp. After that, the human infection began.

Remember that these viruses don't copy themselves very accurately when they replicate. In other words they mutate readily. Therefore an infected individual will develop a range of slightly different HIVs as the infection progresses. The HIV viral protein that I do research on (the reverse transcriptase) is usually 95% (or more) identical between different HIV-1 isolates. And it's about 85% identical to the same protein found in SIV chimp (still pretty close as far as proteins go). And it varies about 50% from HIV-2 and also about 50% from SIV sooty mangabey. It is thought that the strain HIV-2 was picked up from sooty mangabeys. What do I mean by identity between proteins? Proteins are long chains containing hundreds of amino acids. And there are 20 amino acids to choose from to put in at each position. When comparing proteins, amino acid by amino acid, from end to end, if there are five differences for every hundred amino acids, then those proteins have 95% sequence identity.

There is also an FIV, feline immunodeficiency virus, which infects cats. However there has been no known case of FIV infecting humans, its just too different. There is also a BIV, bovine immunodeficiency virus, which infects cows.

An excellent site on SIV

To read more about FIV

A technical but still quite readable discussion of the origin of HIV and its relationship to SIV written by Simon Wain-Hobson can be found in the journal Nature, February 5, 1998, page 531 (Vol 391).

Mike Conrad

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