Tuesday December 2 2014
Natural selection favours viruses that don’t kill their hosts
“HIV is evolving to become less deadly and less infectious,” BBC News reports.
A new study showed that HIV adapts to a person’s immune system, and that some of these adaptations may reduce the virulence of the virus.
The research team looked specifically at HIV in Botswana and South Africa. It found that over time, human immune system proteins, in addition to the use of HIV drugs, may have forced the virus to change into less virulent forms.
This is consistent with the theory that viruses get less virulent over time. The optimal evolutionary strategy for a virus is to be infectious (so it creates more copies of itself) but non-lethal (so its host population doesn’t die out). The "poster boy" for successful long-living viruses is, arguably, the family of viruses that cause the common cold, which has existed for thousands of years.
The authors warn that HIV, even at the reduced virulence, can still trigger the onset of AIDS.
Similarly, this study does not show that HIV virulence in the UK is decreasing, and that the virus remains life-threatening.
The simplest way to protect yourself against HIV infection in the UK is to use a condom during sex and never share needles if you are an injecting drug user.
Where did the story come from?
The study was led by researchers from Oxford and several institutes in Canada, the US, South Africa, Botswana and Japan. It was funded by grants from the National Institutes of Health (US), the Wellcome Trust (UK), the Medical Research Council UK and the Canadian Institutes of Health.
The study was published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), a peer-reviewed science journal. It is an open-access study, meaning that anyone can read it online or download it for free.
The UK media reported the story accurately. It was important that they included a warning that, despite a small decrease in the virus’ potency in some areas of Africa, HIV still causes AIDS. This can significantly shorten life, due to impairment of the immune system if the correct treatment is not followed and made available soon after infection.
What kind of research was this?
This was a laboratory study investigating whether HIV virulence has changed over time, and what might be influencing it.
The term virulence means the ability of the virus to cause disease. This is generally described in terms of:
- how likely the virus is to be passed on to a different person (transmissibility)
- how much of the virus the person carries in their bloodstream (viral load)
- how quickly the virus replicates itself (viral replicative capacity)
Individuals can have slightly different immune responses to HIV infection. This (and other factors) can affect how virulent HIV is in the person’s body and how long it takes HIV infection to cause AIDS. Understanding this variation is important in the effort to minimise the suffering and deaths from the disease.
After HIV infection, some people develop AIDS quicker than others. This natural variation is partly caused by changes in human leukocyte antigen (HLA) genes, a group of genes that encode HLA proteins involved in the immune response. This study wanted to find out how HIV evolution has been shaped by specific HLA proteins that are known to protect against disease progression. They also wanted to know whether HIV drugs (antiretroviral therapy) had influenced the evolution of the virus.
What did the research involve?
This study looked at the genetics and virulence of HIV from epidemic regions in Botswana and South Africa, two countries severely affected by HIV infection. They compared many measures of virulence in the two areas and looked at whether HIV genetics had adapted to HLA proteins known to be protective against disease progression.
In characterising virulence they looked at:
- prevalence of the virus in adults in the two countries
- how much virus people carried in their bloodstream (viral load)
- how quickly the virus replicated itself (viral replicative capacity)
- CD4 count (CD4 cells are key to a person’s immune system, but HIV infects and destroys them; once CD4 cell level falls below a certain point, the person has AIDS)
The study also contained data from Japan, where HIV prevalence has remained low, and never exceeded 0.1% of the adult population.
What were the basic results?
The epidemic in Botswana started earlier than in South Africa. As such, the adult prevalence of HIV infection in Botswana was consistently and significantly higher than South Africa over the last 20 years. So too was the use of antiretroviral therapy to prevent disease progress to AIDS.
Despite the higher prevalence, viral load and viral replicative capacity of HIV in Botswana was significantly lower than HIV in South Africa. This meant that the virus was slightly less virulent. This appeared to be due to both an adaptation to different HLA proteins that forced the virus into a less virulent form and the use of antiretroviral therapy.
How did the researchers interpret the results?
The research team concluded that “HIV evolution is progressing rapidly” and that “The contrasts between Botswana and South Africa, in the degree of adaptation of HIV to prevailing HLA molecules in the populations and in the protective impact of protective alleles such as HLA-B*57 and HLA-B*58:01, coincide with the substantial differences in duration and magnitude of the epidemic in these two localities”.
This study shows that HIV adapts to a person’s immune system, and that some of these adaptations may reduce the virulence of the virus. It appears that over time, the virulence of HIV in Botswana has decreased compared with HIV in South Africa, because of such adaptations and the use of HIV drugs. Specific HLA proteins present in adults in Botswana have, over time, forced the virus to change into less virulent forms, allowing it to survive, replicate and spread.
This is consistent with a broader theory that epidemic viruses get less virulent over time, due to natural selection. The most severe viruses kill their hosts too early to be passed on. Hence, eventually, the very severe strains die out or mutate into milder forms.
This study furthers our understanding of the evolution of HIV in Botswana and South Africa. However, we cannot be complacent. HIV, even at the reduced virulence in Botswana, does cause pain, suffering and death. While HIV can be managed over the long term to push back the development of AIDS, this is dependent on quick and appropriate access to HIV drugs. This may not be the case for everyone.
Similarly, this study does not show that HIV virulence in the UK has decreased or is decreasing, so it is important not to be complacent or to diminish the serious and life-threatening risk of HIV infection.
The simplest way to protect yourself against HIV infection is to use a condom during sex; 95% of cases in the UK in 2011 were as a result of unprotected sexual contact. You should also never share needles if you are an injecting drug user. Some NHS trusts and local authorities run needle exchange programmes – this link can provide information about drug support services in your local area.
Read more about HIV prevention.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.