Different subtypes of HIV are seen around the world

US researchers studied people in Uganda, and found HIV was more likely to progress rapidly in people with subtype D than in those with subtype A.

Most people in the UK and other western countries have type B.

Experts said the Johns Hopkins University study highlighted the need for people to come forward as soon as possible for HIV treatment.

The research was presented to the 2006 Conference on Retroviruses and Opportunistic Infections.

Traditional HIV tests monitor the progression of HIV by keeping track of viral load.

If there are fewer than 50 viral particles per cubic millilitre of blood, the disease is considered to be suppressed - while a viral load of more than 75,000 particles per cubic millilitre of blood means the disease will progress more rapidly.

Virulence

Researchers are following a group of 12,000 people in Uganda to see how HIV spreads throughout the country.

In this part of the research, more than 300 newly infected men and women were examined between 1995 and 2001.

Through annual blood tests, which were part of the study, the researcher knew when each person became infected.

Once the diagnosis was confirmed, researchers used DNA tests to determine the HIV subtype, A and D being the most common in Uganda.

Fifty-three people were found to have become infected with subtype A, and 203 infected with subtype D.

Another 70 were infected with a recombinant version, a combination of both subtypes.

Even though the quantity of virus infecting these individuals was roughly the same for each subtype, average years of survival for each subtype differed widely: 8.8 years for A, 6.9 years for D and 5.8 years for AD.

Ten percent of those infected with subtype D died within three years, while none with subtype A died.

Viral load ranged from 20,000 particles per cubic millilitre of blood to 100,000 particles per cubic millilitre of blood in those with both subtypes, and was not found to be an accurate predictor of rapid death within the same timeframe.

The researchers suggest that subtype D is more virulent than subtype A because D has the ability to bind to key receptors on immune cells, allowing it to kill more quickly.

Additional blood analysis showed that subtype A bound only to one kind of receptor, CCR5, to infect the cell.

But 25% of the subtype D virus bound to both CCR5 and another receptor, CXCR4. And two-thirds of those infected with a CXCR4-binding virus died within three years.

Early diagnosis 'key'

Oliver Laeyendecker, who led the research, said: "Knowing a person's HIV subtype is important for the management of the infection because the disease can progress more rapidly in those infected with subtype D and recombinant virus incorporating subtype D than in those with other subtypes."

Michael Carter, of Aidsmap, told the BBC News website: "This research is from a very important cohort study."

He added: "Subtype tests could be useful if they are cheaper than viral load tests.

"And, in the UK, there is an issue that people who get their diagnosis latest are people who are recent migrants from Africa.

"This study is more evidence of the importance to be diagnosed as early as possible, so that people can begin treatment."