America's Dominant Strain of HIV is Getting More Infectious as a Result of 'Natural Selection', Study Warns

The dominant strain of HIV in the U.S. is becoming "more infectious" and "virulent" via the process of "natural selection," researchers have found. This suggests that while HIV might be on the decline thanks to public health measures, the virus that remains appears to be more transmissible today than it was in the past.

Researchers from the University of California San Diego (UCSD), working alongside scientists at the U.S. Centers for Disease Control and Prevention (CDC) were looking at the sub-type B strain of HIV that is found across the U.S. They published their findings in Nature Communications in December.

"The remarkable thing about evolution by natural selection is its inevitability," co-author Joel Wertheim, an assistant professor at the UCSD, told Newsweek. "Viruses, like all organisms, adapt and evolve. And our study suggests that HIV circulating today is slightly more transmissible than the HIV from a decade ago."

This is not a cause for "panic," he added—"tools available to public health officials and doctors mean that we can still effectively treat people infected with HIV and prevent infection in people exposed to HIV."

There are a number of factors that might increase the rate of HIV transmission. Most significantly, the actions of people carrying HIV—for example, do they take antiviral treatment as prescribed? Do they engage in unsafe sex?

Another factor is viral load. This is the concentration of HIV in the bloodstream. The higher the load, the more infectious the virus and the quicker the progression of the disease. This means that someone with a higher load—a trait inherited from the original host—will develop faster than someone with a lower load, if the virus is left untreated.

But while a higher viral load might increase the risk of HIV transmission, it might not increase the rate of HIV transmission. This is because it speeds up the disease progression process, giving the virus less time to spread before the person infected dies.

As the success of the less virulent, less infectious HIV-1 subtype A shows, a slower disease progression can aid the spread of the virus.

However, the new research suggests that in the U.S., more virulent versions of the HIV virus are spreading because of a process of natural selection.

Illustration of HIV
An illustration of HIV. New research suggests America's dominant strain of HIV is getting 'more infectious' and 'virulent' thanks to natural selection. Artem_Egorov/iStock

Using information collated from the U.S. National HIV Surveillance System—and with data from more than 40,000 people with an HIV positive diagnosis—researchers identified clusters of people with an elevated rate of HIV transmission.

The team used previous research that looked at the role of mutations related to drug resistance on transmission risk. They looked at "molecular clusters"—defined by the CDC as "groups of people with closely related strains of HIV."

Clusters of genetically similar strains suggest higher rates of transmission. This link enables scientists to find associations between viral load and transmission rate, as well as risk.

The results show that clustered individuals displayed higher viral loads than non-clustered individuals. Exactly how much higher depended on the stage of the disease at diagnosis, with the greatest disparities noticed between people diagnosed in the earliest stages of the virus.

"The study doesn't directly suggest that HIV is evolving to have a higher viral load," Jonathan Ball, a professor of molecular virology at the University of Nottingham in the U.K. who was not involved in the study, told Newsweek.

"But what it does suggest is that HIV infections that result in a higher viral load are more likely to be associated with onward transmission, and therefore any virus that carries mutations that allow it to produce a higher viral load will inevitably be seen more frequently in an infected population—although viral fitness and transmission is a very complex affair."

The changes in viral load reported in the study were relatively modest. There was around a one percent increase in transmission risk among clustered individuals compared to non-clustered individuals, Wertheim told Newsweek. The authors warn these effects could accumulate over time.

This is backed up by previous research that has found viral load increases over time.

"If a virus has a genetic underpinning for differences in its transmission rate, then natural selection will act," said Wertheim. "And this process is what we see happening with HIV in the United States.

"The evolutionary change we observed won't necessarily increase the number of new HIV infections every year, because public health measures are being even more successful at decreasing the rate of new infections."

Some have argued that the adoption of policies that offer suppression treatments to everyone with an HIV positive diagnosis could increase the success of strains with higher viral loads because treatment pauses disease progression, keeping the patient healthy. However, the researchers say the study does not provide enough info to support or undermine the idea that universal treatment encourages the proliferation of strains with higher viral loads.

Ball says it provides additional support for the need to start treatment early to reduce viral load. "It's been known for a while that reducing viral load in a patient means that you see less onward transmission."

The research also adds support for the use of PrEP, a pill that can protect patients from contracting HIV in the first place. "This has proven to be a very effective means to limit HIV sexual transmission in those at risk of HIV," said Ball.

The research specifically targeted people with wild-type sub-type B strains of HIV. Wild-type is the naturally-occurring, non-mutated version of the virus. Subtype B is one of nine genetically-distinct subtypes and the most common in the Americans, Western Europe and Australasia. According to international HIV and AIDS charity Avert, it only represents 12 percent of HIV infections worldwide.