Thai AIDS Virus May Pose Threat in U.S.

Strain spreads more readily by heterosexual intercourse, AIDS researchers say

By William J. Cromie

Gazette Staff

Harvard researchers have discovered that a strain of the AIDS virus in Thailand spreads more readily by heterosexual intercourse than the prevalent strain in the United States and Europe.

Called subtype E, the Thai virus may greatly increase the number of AIDS infections among heterosexuals in other countries because it grows much faster in the vaginal walls.

"It makes sense to expect that the subtypes in Asia and Africa will make their way to the U.S.," said Max Essex, director of the Harvard AIDS Institute. "Subtype E, in fact, has already been detected in this country. If E or another subtype takes hold here, we may be facing a much more significant heterosexual epidemic."

An AIDS epidemic, now raging among heterosexuals in Thailand, is largely due to the E strain of HIV, the virus that causes AIDS. The virus spreads chiefly among prostitutes and their customers, who then infect their wives.

In Thailand and other nations in Asia and Africa, 90 percent of HIV transmission occurs via vaginal intercourse. In the United States and western Europe, by contrast, only 10 percent of transmission is heterosexual, and subtype B is the predominant strain for all forms of transmission -- heterosexual, homosexual, and via contaminated needles.

Scientists have detected nine subtypes of the virus in the U.S., Europe, and Asia.

In laboratory experiments, AIDS Institute researchers put samples of the E and B subtypes into contact with cells that line the vagina and found that E grew much faster. This result led them to conclude that the Thai strain spreads easier than its U.S. relatives.

The B strain, also present in Thailand, infects drug users there but has not spread to more than about 100,000 people. An estimated 1 million Thais have E-type infections.

The same number of people have been infected with all strains of HIV in the United States, according to the U.S. Centers for Disease Control and Prevention. These infections have caused about 500,000 cases of AIDS.

Essex notes that the finding about E strain's faster growth will impact the development of an AIDS vaccine. "To date," he says, "most vaccines only target subtype B. Yet to be fully effective worldwide, such vaccines must protect against all subtypes and all modes of transmission."

Essex and his colleagues described their work in the most recent issue of the journal Science.