Ever since people in the Middle East started dying of a mysterious new infection  last year, scientists have been trying to pinpoint the source of the outbreak. Now they may finally have found a clue in an unlikely population: retired racing camels.
The Middle East respiratory syndrome (MERS) virus has sickened 94 people so far and killed 46 of them. While some patients have clearly been infected by others, there are also cases who have been nowhere near a known patient. So scientists suspect that one or more animal species harbor the virus and are transmitting it to people.
Like severe acute respiratory syndrome (SARS), MERS belongs to the coronaviruses, a group of viruses thought to originate in bats. Indeed, researchers investigating the new pathogen have found a closely related virus in bats in South Africa , and they have been able to grow the MERS virus in various bat cell lines. But direct infection by bats is unlikely because humans have very little interaction with them. Instead, researchers suspect that there is an intermediate host.
Now, an international team of scientists has tested the blood of various livestock species, including cattle, sheep, goats, and camels from the Netherlands, Spain, Chile, and Oman. They also tried to get samples from countries that have had human MERS cases, such as Saudi Arabia and Qatar, says Marion Koopmans, an infectious disease researcher at the National Institute of Public Health and the Environment in the Netherlands and one of the authors of the paper. But none of those countries cooperated.
To find out if the animals had been exposed to the MERS virus, the team screened the blood for antibodies against part of an important protein called spike on the viral surface. In order to ensure that the antibodies were specific to MERS, the scientists also tested the spike proteins of SARS and OC43, a human coronavirus closely resembling one circulating in cows, sheep, goats, and camels.
As expected, several animals had antibodies against OC43 in their blood, and none carried antibodies against SARS. But 50 dromedary camels from Oman that were tested all had antibodies against the MERS virus , the scientists report today in The Lancet Infectious Diseases. (This link will be live after 6:30 pm U.S. Eastern time.) "There is something circulating in dromedary camels that looks very much like MERS coronavirus," Koopmans says. The camels are all female retired racing camels used for breeding, but they belong to different owners in separate locations.
"That's a really important paper," says coronavirus researcher Matthew Frieman of the University of Maryland School of Medicine in Baltimore. "The fact that 100% of the Omanian camels are positive means that the result is highly significant and likely very real." Ian Lipkin, a virologist at Columbia University who's also searching for MERS in animal samples, says that the paper provides "compelling evidence that camelids [a group that includes camels, llamas, and alpacas] may be implicated." But the search for other hosts needs to continue, he adds.
Countries like Saudi Arabia and the United Arab Emirates produce and consume large amounts of camel meat. The authors of the paper point out that huge numbers of camels are imported to the Middle East from African countries as well as from Australia, where the animals were introduced in the 19th century and which now has an estimated 1 million feral camels. (Australia started exporting camels to Saudi Arabia for meat production in 2002.) That raises the possibility that African or Australian bats harbor the virus and camels carried it to the Middle East.
That hypothesis needs to be followed up by investigating importation routes, Koopmans says. One possibility would be to study blood samples of imported camels that are taken in some Middle Eastern countries. "Qatar has that kind of collection, I'm told," Koopmans says. Such samples would be ideal material to find out whether the animals come to the Middle East already infected or only come into contact with the virus once there.
More information is also needed on the exposure that infected people had to animals, Koopmans says. A MERS patient from Abu Dhabi who died in a hospital in Germany in March owned racing camels and reported that he had close contact with a sick camel before falling ill , but the affected Middle Eastern countries have shared little information about other patients. A paper about an H7N9 influenza outbreak in China in this week's issue of BMJ  is a good example of the epidemiological data needed, Koopmans says. "Where do they live? Where are the markets? What kind of exposure did they have? That is the kind of information we need to get from the MERS patients as well."
The researchers also found MERS antibodies in 15 of 105 camels from the Canary Islands. Their antibody levels were lower, however, and the finding should not be overinterpreted, Koopmans says. "It may be due to exposure long ago," she says. Only three of the animals were imported—from Morocco, more than 18 years ago.
The researchers didn't try to isolate the actual virus from the samples, so they can't be 100% sure the camels were infected with MERS. They had only blood serum available, which is not a good starting material, Koopmans says, and besides, once an animal has high levels of antibodies, "you don't expect much luck in finding the virus."
But now researchers have a better idea of where to look. One possibility would be to find herds where only some of the animals have antibodies; within these groups, the ones that have not yet developed an immune defense may harbor the virus, Koopmans says. And scientists should also try to get stool or respiratory samples, Koopmans says—"though it may be a challenge to do a nose swab in a camel."