The finding is the first confirmation of MERS infection in a species other than humans, though it does not prove that the animals are the source of infections in people, the authors were quick to say.
"I think it shows that something — either MERS or something that looks very similar to it — has been going around in camels, and that that really needs to be looked at as a possible source. That's as far as we can go, I would say," senior author Dr. Marion Koopmans said in an interview about the study, published in this week's issue of Lancet Infectious Diseases.
Koopmans and colleagues found antibodies to MERS or a closely related coronavirus in the blood of camels from Oman on the Arabian Peninsula and also on the Canary Islands, an archipelago off northwestern Africa which is part of Spain.
The fact that camels may be a source of the virus adds urgency to the task of finding out how people are getting infected and trying to prevent future infections.
That's because the animals are among a number slaughtered for sacrifice as part of the Hajj, the massive annual pilgrimage that draws several million Muslims to the holy city of Mecca in Saudi Arabia. All Muslims who are able are required by their faith to make the pilgrimage at least once in their lifetime.
To complete the responsibilities of the Hajj, each pilgrim must participate in the sacrifice of an animal. That participation often takes the form of contributing to the cost of buying and slaughtering the animal and then distributing the cooked meat to family members and the poor.
The animal sacrifices take place during Eid al Adha — the feast of sacrifice — that concludes the Hajj. This year the pilgrimage will occur in early to mid-October.
"Given that animal sacrifice is an essential and a mandatory component of the Hajj of the pilgrims ... finding an answer as quickly as possible would be quite important," said Dr. Kamran Khan, an infectious diseases physician at Toronto's St. Michael's Hospital who specializes in using airline traffic data to predict the global movement of diseases.
"I do think what this really does is helps guide some of the further investigations now. But there is certainly some time sensitivity around it."
The World Health Organization's lead expert on the MERS virus agreed getting answers sooner rather than later would be a good idea in light of the fast-approaching Hajj.
"If this virus is actually transmitting and circulating in camels and-or other animals, that could be a time of exposure," said Dr. Anthony Mounts. "It does mean that we need to do the investigations to find out the routes of transmission."
A number of scientific teams have been looking for the source of the virus, which is assumed to originate in bats. Given that humans don't interact often or readily with bats, the working hypothesis has been that a bat virus moved into one or several animal species, and those infected animals are somehow exposing humans to the virus. That is what happened with the SARS virus, which is also a coronavirus.
Koopmans, a veterinarian and virologist who heads the virology lab of the infectious disease centre of the Dutch National Institute of Public Health, worked with colleagues to look for the virus in blood samples from a variety of animals, both from within the Middle East and further afield. The samples had been taken for other reasons — testing for other animal diseases — and were made available to her team for the MERS study.
She said her group is in talks with officials of Qatar and Jordan — which have also had MERS cases — to do animal testing there.
In addition to the camels, the scientists looked for MERS antibodies in blood samples from cattle, sheep, goats and other camelids — animals in the same family as camels such as alpacas and llamas. These other animals were from Spain, the Netherlands and Chile.
None of the non-camels tested positive for MERS antibodies. But of 50 camels from Oman, all tested positive. And about 14 per cent of 104 camels from the Canary Islands also tested positive for the virus.
Oman is a neighbour of Saudi Arabia, which has reported the lion's share of MERS cases. Oman has not reported human cases of MERS, but Mounts said the country has a good surveillance system and has been looking for the infection.
Koopmans said finding evidence of prior infection in camels doesn't rule out the possibility that other animals may also be susceptible to the virus or may be the source or sources of human exposure.
Both she and Mounts said it would be important to test other animals from MERS-affected countries to try to get an idea of the range of animals that might be susceptible to the virus and which may play a part in spread of the virus.
The study says the team also looked for live MERS virus in the camel blood samples as well as in dung samples, but did not find any. That isn't surprising given that antibodies signal prior, not ongoing, infection.
Still, recovering live virus from an animal would have allowed the scientists to compare its genetic sequence to the viruses isolated from human cases to see if the same virus was causing infections in both species.
Koopmans admitted to being worried the findings might trigger anti-camel sentiment in the region. In the early days of the 2009 H1N1 swine flu pandemic, pork sales plummeted in some places and Egypt even ordered the mass slaughter of pigs in Cairo.
But Mounts said he has another worry. Camels are so important in the Middle East — they provide meat and milk, they are transport and beasts of burden, some are raced like horses and some are kept as pets — that Mounts is concerned some people won't want to believe they may be playing a role in the spread of the virus.
"For the racing industry and the food industry and all of these other things I have a bigger concern that people just won't want to accept the results," he said. "And I do think the results are enough that it means that we really should actively pursue this lead."