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The number of New Zealand gay and bisexual men with diagnosed HIV more than doubled between 1999 and 2009, University of Otago research shows.
The study by Otago University researchers based in Dunedin and Auckland found an estimated 1313 gay and bisexual men were aware they had HIV in 2009 - which was an increase of 137% from 1999. This compares to an increase of 79% between 1989 and 1999.
Overall, 1936 gay and bisexual men were identified with HIV over the past 25 years, with 720 diagnoses and 552 deaths.
The study's lead author Dr Peter Saxton, of Otago University's Department of Preventive and Social Medicine's Aids epidemiology group, said the more rapid rate increase in the decade up to 2009 was largely due to the introduction of "very effective" antiretroviral therapies from 1997, which improved the life expectancy of people with HIV.
This meant fewer people who had HIV were dying and this combined with new infections every year meant the number of people with HIV was increasing at a greater rate, Dr Saxton said.
There had also been an "escalation" in the number of infections among gay men since 2003, due in part to the growth of online dating sites, which made it easier for people to find sexual partners.
"That includes sites like NZ Dating and ... also smartphone apps like Grinder, which are unique to gay men," he said.
This increased the likelihood of gay men contracting HIV and inadvertently transmitting it to a greater number of people before they knew they had it.
The other reason was that anal sex was a much more "efficient mechanism" for transmitting HIV than other types of sex.
"I think those two are the main explanations why we are seeing quite a different trajectory of the HIV epidemic amongst gay men, not just in New Zealand, but internationally," he said.
There were some encouraging signs that infection rates were decreasing. The number of new transmissions dropped in 2011 and condom use among gay men was not decreasing, Dr Saxton said.
The research was published in the International Journal of STD and Aids earlier this year.