* Doctors failing to screen for HIV in older patients
* HIV progresses faster to AIDS in those aged 50-plus
* Older people more likely to risk unprotected sex
By Laura MacInnis GENEVA (Reuters) - Doctors are failing to diagnose HIV in older patients, who are exposed to greater risk of infection as erectile dysfunction drugs extend their sex lives, a study published by the World Health Organization said on Tuesday.
The report in the WHO Bulletin found that increasing numbers of sexually active people aged 50 and upwards -- who are more likely to risk unprotected sex than younger people -- are contracting the AIDS virus.
And although many people are having sex into their twilight years, HIV is still rarely considered as a cause of illness in older individuals. "Screening is less common for older adults, who are assumed not to be at risk," the study found.
"HIV prevalence and incidence in the over-50-year-olds seem surprisingly high and the risk factors are totally unexplored," the authors from the WHO and Minnesota's St. Olaf College said.
Patients over the age of 50 make up roughly 8 percent of new HIV diagnoses in Europe and 11 percent in the United States, where rising numbers of older people are infected with the virus that spreads through sex, transfusions, and needle-sharing.
"These individuals have a shorter time from diagnosis to the onset of AIDS, reflecting both age-related faster progression to AIDS and doctors' failure to consider HIV as a diagnosis," the report said.
Among the aged, the study said sexual activity is the most likely mode of HIV transmission. One reason for the increase in frequency is the use of impotence treatments that have allowed both men and women to have more sexual partners.
"Since 1998, erectile dysfunction drugs have been extending the sex life of many older individuals and, at the same time, may be extending the HIV epidemic into older age groups," the study said.
"Older individuals are less likely than their younger counterparts to practice safer sex," it said.
While the sexual activity of older people in poor countries has not been studied, and there is no data about the prevalence of HIV among the aged in the developing world, the WHO Bulletin study said similar trends were likely occurring there.
"While erectile dysfunction is common and erectile dysfunction drugs are widely available in developing countries, no study has been done of their possible impact on the HIV epidemic, although their use in industrialized countries has been associated with risky safety practices," the report said.
Older women appear to be at higher risk of contracting HIV if they have sex without a condom because of thinning with age of the vaginal mucous membrane, which gives natural lubrication.
Antiretroviral drugs have dramatically extended the life expectancy of people who catch HIV in rich economies, such as NBA basketball star Magic Johnson who announced his infection more than 16 years ago.
But for those infected with HIV after the age of 65, the study said life expectancy is only four years. "Waning immunity with age may be the reason," it said. About 33 million people worldwide are living with HIV, mostly in Africa.
Editing by Richard Balmforth