The work, published Tuesday in the journal mBio, reports on the finding of antibodies that recognize the MERS coronavirus in stored blood samples from camels dating back as far as 1992.
Until now the earliest evidence of the virus in camels dated back to 2003, in the United Arab Emirates.
A leading coronavirus expert suggests the additional evidence of MERS in dromedaries underscores that the way to protect people from this virus is to develop a vaccine for camels.
Dr. Christian Drosten of the University of Bonn, Germany, says it has become clear that the virus is prevalent among camels in the Middle East and that it will continue to jump from them to people who have contact with the animals.
To date there have been roughly 186 detected human cases of MERS and around 80 deaths.
"You have to vaccinate the virus out of the camels. We need a vaccine very urgently now," said Drosten, who heads his university's institute of virology.
Several other papers have reported finding antibodies to the virus in camels in the Middle East — in Qatar, Oman, Egypt, Dubai and Saudi Arabia. One study even reported finding evidence of infection among camels in the Canary Islands off northwestern Africa.
The new study was the result of a collaboration of Saudi and U.S. scientists led by Dr. Ian Lipkin of Columbia University's Center for Infection and Immunity.
The researchers analyzed both freshly drawn specimens (blood as well as nasal and rectal swabs) from camels from around Saudi Arabia, and looked for antibodies to the MERS virus in archived blood samples from camels that were drawn over four years in the early and mid-1990s and in 2004, 2009 and 2010.
Among the new samples, they found 74 per cent of the tested camels had antibodies that reacted to the MERS virus, which suggests they were infected either with it or with something very similar.
Older camels — those over two years of age — were more likely to be antibody-positive than younger camels. The percentages of positive animals were 95 per cent for the older animals and 55 per cent for the juvenile camels.
The team also found traces of viral RNA on some of the swabs, though in this case the rates of positives were higher among the younger camels. Traces of virus are signs of active infection, whereas antibodies are evidence of infection in the past.
The scientists also tested goats and sheep, looking to see if those animals can be infected with the MERS virus as well. But none of these animals showed signs of active or prior infection.
Lipkin, who sent a team equipped with a special mobile laboratory to Saudi Arabia to process the specimens, said this is the first paper that reports finding MERS RNA in a camel where the camel was the clear source of the virus.
An earlier scientific article reported isolating virus from camels on a farm in Qatar where two human cases were also discovered. But the scientists said analysis of the viruses from that outbreak left them unclear if the camels had infected the people or the people had infected the camels.
Lipkin said the findings suggest young camels are probably the ones passing this virus to people — or at least they are one source of the virus.
"I'm not saying that the cause of the majority of human infections is camels, directly or indirectly," he said in an interview. "I don't know that. The one thing we do know, given the number of camels that are infected, is that they do play a role in cross-species transmission."
"To the extent that we can implicate camels, it's going to be young camels."
Drosten agreed. He said it appears this virus is like a childhood disease for camels, something that they encounter early in life. Because of that, he is expecting more cases soon.
Camels are typically born during a season, and in recent weeks many new animals would have been born, Drosten said. That means in coming weeks they will become exposed to MERS and will become infected.
"There will be again a wave of infections in the young camels. And most likely the humans will get their share. There will be new (human) cases," he said.
Another MERS expert, Dutch virologist Marion Koopmans said it has become clear that camels are a reservoir for this virus, though like the others, she is not ready to say dromedaries are the only animals passing the virus to people.
Koopmans, who is chief of virology at the National Institute of Public Health for the Netherlands, said she would like to know why a virus that has apparently been infecting camels for more than 20 years has only recently been seen to infect people.
Some suggest human infections may have been occurring all along, but have been missed among the myriad respiratory illnesses that show up in hospitals. In many cases, a cause is never identified.
But Koopmans believes something may have happened to the virus to allow it to transmit more easily to people.
Lipkin and his co-authors said they would like to be able to test archived human respiratory tract samples from the region to look for evidence of MERS virus. But to date, they haven't been able to find a bank of such samples.
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