Scientists are declaring war against a leukemia-causing virus that has infected millions

The discovery of the first human retrovirus in 1980 was a minor scientific sensation. Researchers knew retroviruses—which transcribe their RNA genome into DNA and integrate it into a host cell's genome—existed in animals. But until Robert Gallo, then at the National Cancer Institute in Bethesda, Maryland, found the human T-cell leukemia virus-1 (HTLV-1), some doubted that retroviruses infect humans. HTLV-1 was soon eclipsed, however, by another retrovirus that would go on to kill more than 35 million people and keep generations of scientists busy: HIV. "If you were working on retroviruses you switched to HIV," says epidemiologist Antoine Gessain of the Pasteur Institute in Paris.

But in a 10 May open letter to World Health Organization (WHO) Director-General Tedros Adhanom Ghebreyesus, Gallo and 59 other virologists, epidemiologists, and patient advocates call for a global effort to eradicate HTLV-1, which infects millions and causes cancer and several other diseases. Just like HIV, HTLV-1 spreads through blood, semen, and breast milk, and the letter argues that the testing and prevention strategies used against HIV should be employed to stop HTLV-1.

HTLV-1 is nowhere to be found among the many dozens of diseases on fact sheets on WHO's website; it's not recognized as a neglected tropical disease and isn't on WHO's list of sexually transmitted diseases either. "I still scratch my head about how we managed to stay on the margins of HIV and of sexual health," says Graham Taylor, who studies HTLV-1 at Imperial College London.

The virus probably jumped from monkeys to humans somewhere in Africa tens of thousands of years ago; the slave trade helped spread it around the world. Today, scientists estimate that 5 million to 10 million people are infected, but the real number is probably much higher, Taylor says. One hot spot is southern Japan, where up to 6% of pregnant women have been found to carry the virus. But poor countries are most affected, which partly explains the neglect, Gallo says. In Jamaica and other Caribbean islands, about 6% of the entire population may carry the virus. Parts of Brazil have high rates, millions are infected in Africa, and in some aboriginal communities in Australia, almost half the people over age 50 are infected, a recent study found. Europe and the United States can't rest easy, Taylor says: "If we are not careful, this could become more prevalent in our communities without us being aware. … As other infections like HIV come under control, people's behavior may change."

Most people infected with HTLV-1 never have symptoms, but about 3% to 5% develop adult T-cell leukemia, a cancer that kills patients after 8 to 10 months on average and for which treatment has not improved in the past quarter-century. Another 4% develop tropical spastic paraparesis, a disease similar to multiple sclerosis. Other inflammatory diseases and immune deficiency have also been reported, and an Australian study published in March showed that those infected with HTLV-1 are more likely to develop bronchiectasis, a disease in which parts of the airways are enlarged, and to die from it.

Yet HTLV-1's modest death toll, and the decades that often go by between infection and disease, have kept it out of the limelight. In response to the letter, a WHO spokesperson pointed out that another virus, hepatitis B, causes more than 400,000 cancers each year worldwide. In contrast, the International Agency for Research on Cancer estimates that only 3000 cancer diagnoses annually are directly linked to HTLV-1. Taylor thinks the real number is higher, but he says, "I really don't think this should turn into a ‘my disease is more important than your disease’ competition."

Routine testing for HTLV-1 should be available in sexual health clinics everywhere, the researchers write in the letter, and mothers in endemic regions should routinely be screened for HTLV-1 and advised not to breastfeed if they are infected. Since antenatal testing was introduced in Japan's Nagasaki region in 1987, the infection rate in the population has dropped from 7.2% to 1%. The letter also calls for universal HTLV-1 testing of blood and organ donors, because transplants have transmitted the virus, albeit rarely. "I honestly believe that nobody should have a transplant where the donor has not been tested for HTLV-1," Taylor says. (Until 2009, the United States did screen organ donors, but the practice was stopped because false positive results were disqualifying too many healthy organs.)

Gallo believes renewed attention to HTLV-1 could have broader benefits. In the early 1980s, the virus sped up the discovery of HIV, he says, because it alerted scientists to the possibility that a retrovirus might be causing the mysterious new syndrome called AIDS. Today, a better understanding of HTLV-1's powerful carcinogenic properties may lead researchers to new insights about cancer, Gallo says.

*Update, 28 May, 11:30 a.m.: Responding to the call for more attention to HTLV-1, the Australian government on 25 May announced it would set up a task force to "investigate the drivers behind the emerging prevalence of HTLV-1 in remote communities, in close collaboration with Aboriginal communities, and develop a roadmap to respond to this issue.” Health Minister Greg Hunt also announced that AU$8 million (about $6 million) would be provided to combat HTLV-1 and other emerging communicable diseases in remote communities. "Funding will also be available for detailed prevalence studies and international collaboration on treatment options—should the taskforce request it," he said, according to a press release.

Fabiola Martin of the University of York in the United Kingdom, one of the authors of the letter to WHO, called the decision a "monumental day for all people living with HTLV" in a tweet. Gallo says it's "very good news" as well, adding that "even if only a small amount, it is a beginning."