Introduction

Viruses are a kind of infectious agent (pathogen) containing genetic material enclosed inside a protein coat. In order to reproduce, they require living cells. So, our immune systems have multiple defence mechanisms against viruses to stop the spread of disease. 

Simian immunodeficiency virus (SIV) is a zoonosis. This means that the virus can spread from infected animals to humans. Understanding SIV has allowed the medical community to learn more about the evolution, causes, and immunology of diseases like human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS).¹

What is SIV?

SIV is a virus that attacks the immune systems of non-human primates, such as sooty mangabeys, chimpanzees, and African green monkeys. These apes can contract the virus through infected bodily fluids, via sexual transmission.²

SIV is classed as a lentivirus. The lentivirus family is capable of causing chronic infection, due to its variable incubation periods within the host. HIV is an example of a lentivirus that shares similarities to SIV.

Our immune systems are composed of multiple cells, chemical messengers, and organs. A family of cells, called T cells, play a central role in carrying out an immune response to pathogens. 

HIV and SIV cause the number of helper T cells to decrease. Without these, the body finds it difficult to manage pathogen attacks, leading to recurrent infections. 

Subsequently, this affects the balance of killer and helper T cells, which is measured when testing for HIV. Neurological abnormalities can also occur as a result of SIV and HIV.³

Types and species affected

There are 36 different strains of SIV which affect various primate species. These can be categorised into upwards of seven lineages:

• SIVsm from sooty mangabeys, including HIV-2
• SIVagm from African green monkeys
• SIV from guenon monkeys
• SIVcpz, from two chimpanzee species and SIVgor from gorillas
• SIVlho from L’hoest monkeys and SIVsun from sun-tailed monkeys
• SIVcol from black and white colobus monkeys
• SIVrcm from red-capped monkeys, SIVmnd and SIVdrl from mandrills and drill monkeys

Though these strains share similar genetic code for their enzymes and structure, only a few strains, such as SIVcpz (the chimpanzee strain) and SIVgsn (from the greater spot-nosed monkey), contain the vpu gene. This is unique to HIV-1, reflecting the evolutionary basis of HIV from SIV.⁴

Some primate species, such as chimpanzees, are more resistant to SIV, but some can develop symptoms that resemble AIDS. It is thought that this is due to a wider variance in the helper T cell proteins, which has been an evolution against SIV infections for thousands of years.⁵

How SIV works

As previously discussed, SIV attacks immune cells like helper T cells within tissues lined with mucus (the gut and reproductive mucosal tissues), but they also go after other white blood cells, known as monocytes and macrophages.⁶ 

Though lentiviruses are described as slow, the rapid reproduction of viruses inside helper T cells leads to a massive decrease in their numbers. This hinders the immune system’s ability to recognise and fight off pathogens that it might have encountered before. The infection can eventually spread to parts of the nervous system where it can hide for an extended period of time.⁷ 

Eventually, the immune system weakens due to its constant activation. Unlike chimpanzees, humans have not evolved ways to block infections. It is thought that the virus’s ability to adapt to the host environment helps it to successfully replicate, leading to the progression of the disease.⁸

SIV, and the evolution of HIV

Humans most likely contract SIV through contaminated bushmeat (i.e., wild animals eaten and hunted by humans). Either type 1 or type 2 HIV can lead to AIDS.

The transmission event leading to the emergence of HIV-1 groups M, N and O was caused by the SIVcpz (chimpanzee strain) in the southeastern region of Cameroon, in central Africa. HIV-1 was the primary cause of the AIDS pandemic.⁹

For HIV-2 groups A-H, the sooty mangabey strain (SIVsm) is thought to have given rise to the disease. Out of all groups, only HIV-2 A and B, as well as all of HIV-1 can be transmitted between humans.¹⁰

Impact of SIV in primates

Primates experience varied effects after becoming infected with SIV. It is noted that natural hosts to SIV, such as African green monkeys and sooty mangabeys are able to withstand large numbers of viruses in their bloodstreams with minimal symptoms. 

Though the reason for this remains unknown, the research aims to focus on B cells, another type of blood cell which neutralises pathogens through the production of proteins (antibodies).¹¹

Asian macaques are nonhuman primates that have reduced immune capacity when infected with SIV, with manifestations similar to AIDS.¹²

Natural hosts may possess evolutionary adaptations that can make the virus non-pathogenic. This means that the virus is unable to enter its body, or some of its function is reduced or destroyed. As a result, some of these species can live with SIV without going on to develop AIDS.¹³

Species that have not previously encountered SIV in their evolution experience more severe symptoms as they lack the defence mechanisms needed to protect themselves against the infection. 

Research significance of SIV

SIV studies are integral to our understanding of HIV and AIDS, providing a model for scientists to explore disease pathology, evolution, and treatment. 

Insights from SIV research have allowed us to find new targets for cures, which lie within immune components and our genes. Though no vaccines have been developed yet, we can observe how natural hosts are able to control their infections in order to gain information on prevention and prevention.¹⁴

Summary

SIV is a disease that crosses over from apes to humans, leading to diseases such as HIV and AIDS. It produces symptoms similar to HIV and AIDS when it infects primates that are not natural hosts of the disease. The evolutionary basis of SIV and HIV have provided significant value to scientists who want to study the disease and are crucial for advancement in the prevention and treatment of HIV/AIDS.