The virus that causes Middle East respiratory syndrome (MERS) has been found in camel milk. Scientists don’t know whether infected milk can sicken people, but experts say the results are reason enough to warn against drinking raw camel milk, a widespread tradition in the Middle East. The Qatari government has already issued new guidelines recommending that milk be boiled before consumption.

The new findings come from a group of researchers at Qatar's Supreme Council of Health; the country's Ministry of Environment; Erasmus MC in Rotterdam, the Netherlands; and the Dutch National Institute for Public Health and the Environment. They were announced at a press conference in Doha on Wednesday, and a paper about them was submitted to the journal Eurosurveillance today, says Erasmus MC virologist Chantal Reusken, the first author.

The researchers also discovered that almost one in 10 people who come in contact with camels on the job have antibodies against MERS, a sign that they were infected with the virus at some point—although none of them got very sick from it.

Almost 2 years after MERS emerged, it's still unclear how many of the patients become infected. Direct transmission between people occurs, and many of the more than 500 new MERS cases that Saudi Arabia has reported in the past 3 months appear to have occurred in hospitals as a result of inadequate infection control measures. But there isn't evidence yet of widespread human-to-human transmission outside hospitals. And researchers have uncovered more and more evidence that contact with dromedary camels may be a risk factor for getting MERS. Camels in nine countries in the Middle East and Africa have been shown to be infected with MERS, and the virus appears to jump from camels to people, researchers have reported. But where and how it crosses the species barrier is still very unclear.

The international team working in Qatar has collected thousands of samples from animals, people, and the environment and tested them for evidence of the virus or antibodies against it; their samples included milking camels at two locations in Qatar. Among the animals that were shedding the virus from their nose or in their feces—signaling an active infection—more than half also had virus RNA in their milk.

The milk was obtained using traditional methods, in which udders aren't routinely cleaned before milking, and a calf is allowed to suckle to get the milk flow started. As a result, the researchers can't tell whether the infected camels secrete the virus directly into the milk; it's possible that the milk becomes contaminated through the calves' saliva, traces of feces, or the milkers' hands. It's also not clear whether humans can get sick from drinking unpasteurized milk, says Marion Koopmans, who heads the Dutch group.

In another recent study, researchers at the U.S. National Institute of Allergy and Infectious Diseases showed that live MERS virus added to unpasteurized camel milk could survive for 3 days.

The World Health Organization (WHO) will soon issue revised guidelines that, like the Qatari government, recommend against drinking raw milk, says Peter Ben Embarek, a specialist in foodborne diseases at WHO's headquarters in Geneva, Switzerland; he says that's never a good idea.

Camel meat might be another transmission route; the scientists found that 13% of lymph node samples taken at a slaughterhouse contained the virus. But they can't say whether this means that meat is actually contaminated, or how much risk that would pose to consumers.

The team also discovered that 8.7% of workers at camel farms and a camel slaughterhouse had antibodies to the virus. The fact that none of them reported a serious illness suggests that MERS may be more common than scientists knew so far and cause mild disease or even none at all in more people. "We always suspected that the cases we knew about were the tip of the iceberg," Ben Embarek says. The study gives scientists a better understanding of how widespread the virus really is, he says.

In the guidelines issued this week, the Qatari government also indicated how to protect camel workers. Among other things, they should wash their hands frequently, wear protective facemasks—although temperatures of up to 50°C make this almost impossible in Qatar—and wear protective clothing and gloves that should be washed daily.

Given the paucity of data so far on MERS transmission, scientists have praised the Dutch-Qatari team for their work. The collaboration is a "showcase of what should happen in the region," Ben Embarek says. Ian Mackay, a virologist at the University of Queensland, St. Lucia, in Australia who follows MERS closely on his blog, called the study "great stuff" in a tweet today and said Qatar was "really to be congratulated" for embarking on it.