* Nine percent of IV drug users in study infected
* HIV testing also down; progress may be short-lived
* Many of those infected don't know they have HIV (Adds background)
CHICAGO, March 1 (Reuters) - HIV infections among intravenous drug users in the United States have fallen by half in the past decade, but HIV testing is also down and risky behaviors such as needle-sharing persist, U.S. health experts said on Thursday.
A study by researchers at the U.S. Centers for Disease Control (CDC) and Prevention based on a 2009 survey of 10,000 people from 20 urban areas found that 9 percent of IV drug users were infected with the human immunodeficiency virus or HIV, the virus that causes AIDS.
That compares with a rate of 18 percent in the 1990s.
"Despite the fact that we've seen declines in new HIV infections, a substantial number of IDUs (injection drug users) in major US cities are HIV-infected and their risk behavior remains fairly high," said Dr. Cyprian Wejnert, an epidemiologist at the CDC, whose study appears in the CDC's weekly report on death and disease.
"We found 9 percent of IDUs were HIV-positive and nearly half of those were unaware of their infection," Wejnert said in a telephone interview.
After falling significantly since the peak of the epidemic, HIV rates in the United States have been leveling out, but pockets of infection persist, especially in high-risk groups such as young people and men who have sex with men.
The survey tested individuals for HIV and asked questions about their risk behaviors and use of HIV prevention services.
It found about a third of intravenous drug users in the survey said they shared syringes, most said they had unprotected sex in the past year and more than half said they had more than one sexual partner.
The study also found that rates of HIV testing in this at-risk population were falling.
"While CDC recommends that individuals are tested for HIV at least annually, only 49 percent ... reported being tested in the last 12 months," Wejnert said. That represents a significant drop from a survey done in 2005-2006, he said.
Dr. Amy Lansky, deputy director in the Division of HIV/AIDS Prevention at CDC, said the findings will be used as CDC focuses on prevention efforts on high-risk populations.
"It's a really important part of understanding the leading edge of the epidemic," she said.
"What the data from this report shows is we really do need to continue our efforts to expand HIV testing and improve testing," she said, adding that the CDC also needs to focus prevention efforts on reaching more drug users.
Effective prevention efforts include offering condoms and substance abuse treatment. But the CDC cannot distribute clean needles because U.S. lawmakers in December reinstated a ban on the use of federal funds for such programs.