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Camels are almost certainly the source of the MERS virus that is on the upswing again across the Middle East, researchers reported on Tuesday.

A countrywide survey of camels shows many, if not most, are infected with a strain genetically almost identical to the strain that’s infecting people, a team at Columbia University, King Saud University, and the EcoHealth Alliance reported.

The World Health Organization has expressed alarm about the increase in reports of Middle East Respiratory Syndrome (MERS). WHO reports more than 250 confirmed cases and 93 deaths since the virus was identified in 2012. But Saudi Arabia reported more cases over the weekend, taking the reported total to more than 300, with more than 100 deaths.

The researchers ran genetic tests on virus taken from nasal swabs of the animals, who seem untroubled by it.

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“Given these new data, we are now investigating potential routes for human infection through exposure to camel milk or meat products,” says Abdulaziz Alagaili of King Saud University, who worked on the study, published in the journal mBio.

Last week, WHO predicted a springtime surge in MERS cases. The virus has infected people in Jordan, Kuwait, Oman, Qatar, Saudi Arabia, Egypt, the United Arab Emirate, France, Germany, Greece, Italy, Britain, Tunisia, Malaysia and the Philippines.

“The number of cases sharply increased since mid‐March 2014,” WHO says in its latest advisory. But it also notes that most people are infected by other people, not directly by camels.

“More individuals are likely to be infected until the mode of transmission is determined and preventive measures implemented to break transmission from the source to humans,” WHO added.

People are only infected with a few different genetic types of MERS, while camels seem to carry many different types, says Thomas Briese, associate director of the Center for Infection and Immunity at Columbia. “The narrow range of MERS viruses in humans and a very broad range in camels may explain in part why the human disease is uncommon: because only a few genotypes are capable of cross-species transmission," he said.

“Although there is no evidence that MERS is becoming more transmissible, the recent increase in reported cases is a cause for concern,” added Columbia’s Dr. Ian Lipkin, who’s been directing the work.

MERS first showed up in 2012, when it killed an elderly Saudi man. It worries health experts because it’s related to severe acute respiratory syndrome, or SARS, which swept around the world in 2003, infecting around 8,000 people and killing close to 800 before it was stopped. Both conditions are caused by coronaviruses, members of a family of viruses that usually cause common cold symptoms and that infect a wide range of mammals.

The virus can survive on surfaces, and might spread when people touch something contaminated. SARS appears to have spread that way, and many other viruses do, too.