Saudi Arabian doctors say they've identified camels as one source of MERS infections in humans.
The scientists report they matched genetic samples from the virus that killed a Saudi man last November to virus samples present in one of nine camels that he owned.
They said the finding, published in the New England Journal of Medicine proves that camels are a source – but perhaps not the only source – of the Middle East Respiratory Syndrome (MERS) virus. The illness is contagious, potentially lethal and has been largely confined to the Middle East so far.
There have been two confirmed cases of MERS in the United States. Both involved health care workers who are believed to have been infected in Saudi Arabia before coming to the United States.
Read: Saudi Arabia reports 10 more deaths from MERS
The health risk from MERS to the American public is low, US officials said, because the virus is only passed through close contact.
"This [the Saudi study] confirms what we have been suspecting," said Dr Marc Siegel, an infectious disease expert and an associate professor of medicine at NYU Langone Medical Centre in New York City. "It's believed that the source of MERS is the camel, but it has really not been proven," until now.
"Camels are the Typhoid Mary of MERS," he said. "It is clear, however, that camel-to-human transmission started this thing."
The MERS virus first surfaced in 2012 in the Middle East, where most of the cases have occurred. As of June 4, there have been 681 laboratory-confirmed cases of MERS infection and 204 deaths. Most of those cases have been confined to Saudi Arabia, according to the World Health Organization.
MERS symptoms typically include shortness of breath, coughing and fever. The illness kills about one-quarter of the people who contract the virus, according to US Centres for Disease Control and Prevention.
One-fifth of all MERS cases have occurred among health care workers, CDC officials have said.
The new findings don't mean that camels are the only way a person can contract MERS, Siegel said. "And the new study has no implication in terms of human-to-human transmission," he added.
The 44-year-old Saudi man was admitted last November to King Abdulaziz University Hospital in Jeddah, Saudi Arabia, with what turned out to be MERS, according to the report.
In the days before he became ill, he had been in close contact with his nine camels. Tests of the man and one of his camels were positive for MERS, and further genetic testing confirmed that both the patient and camel had identical strains of the virus.
Read: MERS virus found in camels
In the days after being hospitalised, the patient's 18-year-old daughter also developed MERS. In her case, the symptoms disappeared after three days without complications.
The patient himself died of his illness on 18 November, according to the report.
Dr Tariq Madani, who's with the department of medicine at King Abdulaziz University, said this report confirms that camels are a reservoir for MERS and can pass the disease to humans. Camels themselves don't die from the virus.
Siegel said there are many viruses present in species of animals that don't infect humans or aren't easily passed from person-to person.
The original source of the MERS remains unknown, however.
Testing indicates that bats in Saudi Arabia are infected with several strains of MERS-like viruses, and virus from one bat was identical to the virus from a MERS patient. Bats, therefore, might also play a role in human infection, according to the CDC.
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