The pressure to adapt to our immune systems is gradually forcing HIV to become less aggressive in parts of Africa, a new study has found. This, combined with the use of antiviral drugs, could be slowly causing HIV to transition from a deadly virus to a milder form, meaning that infected individuals may be less likely to progress to AIDS in the future.

You might think that the most successful pathogens are those that are the most aggressive, or virulent, but this is not always the case. The risk of transmitting HIV to another individual, for example, is much higher if the infected person has lots of the virus in their body, so efficient viral replication mechanisms could be advantageous. However, these more virulent strains are also more likely to kill off the host before they have a chance to pass on the virus to someone else, meaning a trade-off exists. It’s therefore generally believed that over time, pathogens will evolve to become less deadly because natural selection favors those that cause minimal disease and are thus more likely to be transmitted.

To investigate whether this is happening to HIV, scientists from the University of Oxford examined more than 2,000 infected women in Botswana and South Africa. Although both countries are severely affected by HIV, the epidemic began around 10 years earlier in Botswana, meaning that the virus has had more time to evolve.

As described in Proceedings of the National Academy of Sciences, more patients in Botswana harbored viruses with mutations which helped them escape an immune system protein that is known to help immune cells recognize and target bits of the virus. Only a fraction of the population possess the gene for this protein, but those that do progress more slowly than usual to AIDS.

These viral adaptations meant that in Botswana, this protein no longer bestowed patients with a protective effect against HIV. Interestingly, however, it seems that this adaptation comes at a cost. When they compared the viruses as cells in a dish, they found that those isolated from patients in Botswana replicated much more slowly than those from South Africa. This means that these viruses probably take a lot longer to cause disease and are thus less virulent.

“Twenty years ago the time to AIDS was 10 years,” study author Professor Philip Goulder told the BBC, “but in the last 10 years in Botswana that might have increased to 12.5 years, a sort of incremental change, but in the big picture that is a rapid change.”

So why is HIV evolving so fast? The researchers think it could have to do with antiviral drugs. Highly virulent strains will make patients sick sooner and thus more likely to receive treatment. The drugs provided reduce viral replication to such an extent that the virus is virtually absent from bodily fluids, meaning it’s unlikely to be passed on. This means that the most aggressive strains are less likely to be transmitted than weaker viruses.

It’s important to note that while this study is encouraging, only two countries were investigated, so the same processes may not be at play in other countries. Furthermore, the results do not mean that HIV is going to become harmless any time soon.

[Via PNAS, Wellcome Trust, BBC News, New Scientist and Motherboard]