The Middle East respiratory syndrome (MERS) first made world headlines when it was discovered in 2012. But new research shows that the responsible virus has probably been lurking in camels in Saudi Arabia since 1992 or even longer—and that it is very common among the animals today. The study suggests that undiscovered human cases may also have occurred the past 2 decades.

MERS is caused by a coronavirus, a class of pathogens high on scientists' watch list because another member, the severe acute respiratory syndrome (SARS) virus, spread around the globe in 2003, killing more than 700 people. The syndrome has sickened 182 and killed 80 people so far, but it does not seem to transmit from one person to the next as easily as SARS did. That suggests most patients acquired the virus from animals. SARS probably originated in bats and spread to humans via civets. But scientists investigating MERS are finding more and more evidence pointing to camels as the source.

Last summer, researchers reported finding antibodies against the virus—signaling either a past or a current infection—in the blood of retired racing camels from Oman, but not in cows, goats, or sheep. Since then, studies have also reported such antibodies in camels from Qatar, Jordan, and Egypt. One study found antibodies against MERS in camel blood samples kept in cold storage in the United Arab Emirates since 2003. At least one team also found antibodies in recent samples from camels in Saudi Arabia, the country that has seen the majority of human cases. But many scientists are frustrated by how little information has emerged from that crucial country so far.

Studying the MERS virus has been fraught with other difficulties as well. For instance, importing samples from camels and other animals into the United States is hampered by fears of foot-and-mouth disease, a highly infectious disease that infects cloven-hoofed animals such as cattle and pigs; meanwhile, research resources in the affected countries in the Middle East are limited. To address both problems, Columbia University virologist Ian Lipkin built a mobile lab and sent it to Saudi Arabia late last year. "Two thermal cyclers, a robotic nucleic acid extractor, tools for serology, all this equipment went over there in six Pelican cases," he says.

Together with Saudi colleagues, the researchers used the equipment to test 203 camels from different parts of Saudi Arabia. They found antibodies against the MERS virus in 150 of them. They also discovered the virus's RNA in rectal or nasal swabs from 51 animals, a sign that they were harboring the virus at the time of testing. Viral sequences closely matched those from human patients. Because the nasal swabs yielded the most virus, the most likely scenario is airborne transmission of the pathogen, the authors write in mBio today.

The researchers also looked at more than 100 camel serum samples from an archive going back to 1992 and found antibodies against MERS in almost all of them. "This virus has clearly been in camels since at least 1992," Lipkin says.

The paper adds to the growing evidence on the animal reservoir of MERS, says Marion Koopmans, an infectious disease researcher at the National Institute of Public Health and the Environment in Bilthoven, the Netherlands. "It is now undisputable that these viruses circulate among dromedary camels," she writes in an e-mail.

Bart Haagmans, a virologist at Erasmus MC in Rotterdam, the Netherlands, says he is surprised that viral RNA could be found in so many of the animals. "You don't expect to find a virus for a respiratory infection in almost one in four animals," he says. It suggests that the virus knows how to maintain itself in the camel population very well. It's not clear whether the virus actually makes camels sick, but it seems to infect many camels at a young age, Haagmans says. Most scientists agree that studies looking at infected camels over time are needed to find out how long they can harbor the virus.

Finding antibodies in 22-year-old samples raises the question of why the first known human patients date from 2012, Koopmans says. "Have human cases been missed before, or has something changed in the camel viruses that makes them transmit to humans?" To resolve that question, scientists would have to look at more old samples from Middle Eastern patients with respiratory illness—and those aren't available, she writes.

Whether MERS has been infecting humans for 2 decades or 2 years, there is still a real danger that it will start spreading between people more rapidly and trigger a pandemic, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, Twin Cities. "We're now at the second level of concern," he says, because MERS is not just a dangerous virus in animals but is spilling over into the human population again and again. "The third level could happen tomorrow."