Since the virus was first identified last September, there have been 94 illnesses, including 46 deaths, from MERS, or Middle East respiratory syndrome, mostly in Saudi Arabia. Aside from several clusters where the virus has likely spread between people, experts have largely been stumped as to how patients got infected.
In a preliminary study published on Friday, European scientists found traces of antibodies against the MERS virus in dromedary, or one-humped, camels, but not the virus itself. Finding antibodies means the camels were at one point infected with MERS or a similar virus before fighting off the infection.
The antibodies were found in all 50 camel blood samples from Oman, compared to 15 of 105 samples from Spanish camels. Animals from Spain, the Netherlands and Chile were tested for comparison to those from Oman. No MERS antibodies were found in tests done in cows, sheep or goats.
"Finding the (MERS) virus is like finding a needle in a haystack, but finding the antibodies at least gives you an indication of where to look," said Marion Koopmans, chief of virology at the Netherlands' National Institute for Public Health and the Environment, the study's senior author. "What this tells us is that there's something circulating in camels that looks darned similar to MERS."
The study was published in the Lancet Infectious Diseases.
Koopmans expected results would be similar for other camel populations across the Middle East.
"We can't say this proves camels are a reservoir for MERS but it does show there is something going on with camels that may be relevant for people," Koopmans said.
Across the Middle East, camel products including milk are popular and the animals are often kept for racing and other purposes.
MERS is part of a family of coronaviruses that can cause the common cold as well as SARS, which sparked a global outbreak in 2003. Saudi Arabia health officials, in a letter this week to the New England Journal of Medicine, documented seven new infections of MERS in health workers, including some mild cases.
MERS is most closely related to a bat virus, leading some scientists to think bats are the natural source. Some experts think bats might be infecting other animals like camels with MERS before passing it to humans. MERS can cause symptoms including fever, cough, breathing problems, pneumonia and kidney failure. There is no known treatment or cure.
Some experts said more testing of other animals in the Middle East was needed.
"Camels may be involved in (MERS) transmission but there could also be cows, goats, or something else involved," said Vincent Munster, a virologist at the National Institutes of Health, who co-wrote an accompanying commentary.
There are ongoing tests on camel samples from elsewhere in the Middle East, as well as tests on dates, which some scientists think may be infected by bat excrement before being eaten by people.
The World Health Organization called the camel findings "an important development" but pointed out that many MERS patients had no known contact with animals.
"There must be some other step somewhere that results in human infections," WHO spokesman Gregory Hartl said.
He said it was still unclear what kind of animal contact might be needed for MERS to spread.
"Even if we know where the virus is, we don't know exactly how it's jumping into humans," Hartl said. "This is another piece of the puzzle, but there are still a lot of holes that need to be filled in."