the AIDS virus that is beginning to appear in the United States.

Only a few US blood centres are testing for HIV-2, a second separate virus known to cause AIDS. The strain has been detected in at least 37 countries.

Dr. Keith Cunningham, pathologist at King Edward VII Memorial Hospital, said current testing techniques could identify both HIV-1 and HIV-2 -- but only in people who have already developed AIDS.

"We hope we'll have this test available here before we start seeing cases on this side of the Atlantic,'' he said.

"We can't test for it now. If someone has HIV infection then we can distinguish as to whether it's HIV-1 or HIV-2 but we cannot do it in the early stages. We need a specific screen for HIV-2.'' More than 50,000 people in Europe, Africa, India, the former Soviet Union, and North and South America are infected with human immunodeficiency virus Type 2, according to the World Health Organisation.

The mutation is widespread in Africa, where it primarily is spread heterosexually and has had an incubation period of up to 19 years. At least 43 cases of HIV-2 have been identified in the US and Canada. Though rare, many experts say there is reason to believe HIV-2 will spread.

"HIV-2 could be a bigger heterosexual transmission threat'' than HIV-1, the first AIDS virus discovered, said Dr. Michael Wandell, an epidemiologist and director of regulatory and clinical affairs for Genetic Systems, the only federally licensed maker of HIV-2 tests.

The US Food and Drug Agency is evaluating a September recommendation by its blood products advisory committee that mandatory universal HIV-2 testing be adopted nationwide by June 1.

Many HIV-1 tests now used are cross-reactive with HIV-2, and the committee's recommendation is being considered "purely for precautionary reasons,'' FDA spokesman Mr. Brad Stone said.

"There's no reason to believe...that HIV-2 represents a significant risk to the blood supply.'' The American Red Cross, the federal Centres for Disease Control and the FDA in 1990 said HIV-2 was too rare to spend time and money on testing. The CDC estimated testing every blood donor for HIV-2 would cost $60 million annually.

"In view of the extremely low prevalence of HIV-2 infections in the US population, there appears to be no public health need at this time to screen donors of blood or source plasma for antibodies to HIV-2 with this test,'' the FDA told blood centres.

"The blood supply today in America is the safest in the world and the safest it's ever been,'' American Red Cross president Elizabeth Dole said. "We keep a constant eye on the possible threat of new diseases, like HIV-2.'' The FDA licensed a Genetic Systems test for HIV-2 in 1990, and a Genetic Systems combination test for both strains in September.

Fewer than 20 of the nation's 2,400 blood banks and plasma centres -- representing five percent, by volume, of all the blood collected -- test for HIV-2, according to Mrs. Donna DeLong, Genetic Systems' marketing and business development director.

"I think it is very possible that HIV-2-infected people are donating today in the US and that blood has the potential to slip through and be transfused into an unsuspecting person,'' Dr. Wandell said.

The National Haemophilia Foundation says any additional cost from HIV-2 testing would be "minuscule'' compared with the cost of caring for people who become infected with HIV. The foundation is calling for immediate and mandatory universal HIV-2 screening.

The US is the principal supplier of Europe's blood products, and some US labs voluntarily test for HIV-2 when exporting blood products to Europe.

A person with either strain of HIV may not show detectable signs of infection for up to six months. The CDC found that half of the reported HIV-2 cases in the United States in 1989 tested negative for HIV-1.

A 1991 study by Mr. Richard T. Schumacher, of Boston Biomedica Inc., with Portugal's National Health Institute revealed FDA-licensed HIV-1 tests detected HIV-2 in only eight-62 percent of specimens.

"We're deeply concerned about the potential for HIV-2 (infection of the blood supply),'' Dr. S. Gerald Sandler, medical director of the American Red Cross' National Reference Laboratories, said in Washington, DC. The Red Cross collects six million units of blood annually and supplies half of the nation's blood supply.