WASHINGTON, March 5 (Reuters) - People may be most likely to transmit the AIDS virus when they are first infected -- before they start showing symptoms and even before many screening tests detect the virus, Canadian researchers reported on Monday.

This may help explain why the HIV epidemic moves so quickly, they report in the Journal of Infectious Diseases.

Their genetic analysis of HIV-infected patients in Quebec showed that nearly half of all transmissions occurred when patients were in the early stages of the disease.

"The early infection stage can be entirely asymptomatic," said Dr. Mark Wainberg, of the McGill AIDS Center in Montreal, who led the study.

"This is why people who are recently infected may not know it, and will probably often test negative by conventional antibody screening," Wainberg said in a statement.

Early on, the body has not yet mounted an immune defense against the virus. It does not produce many antibodies -- which is what most quick screening tests look for.

"Hence, we must do a much better job of identifying recently infected people if we are to be able to counsel them to modify high-risk sexual behavior and desist from transmitting the virus," Wainberg added.

Wainberg and colleagues from several hospitals and health clinics in Canada studied HIV transmission through phylogenetic analysis, which is a way to draw the family tree of the virus.

Viruses mutate when they live inside a person's body, and the mutations can be steadily clocked to estimate the first date of infection.

The study found that 49 percent of early infections appeared to cluster in a way that would suggest that most new infections were transmitted by people who themselves were in the early stages of infection, before the virus had time to mutate much.

This is in part because people have many copies of the virus in their blood when they first become infected. The more virus there is -- a measure called viral load -- the more people are likely to infect someone else.

The AIDS virus infects 39 million people globally and has killed more than 25 million since it was first identified in the early 1980s. There is no cure, although drug treatment can keep symptoms at a minimum and extend life.

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