Researchers say they have found the first direct evidence that the potentially deadly Middle East respiratory virus, or MERS, jumps directly from camels to humans.
The virus is considered a deadlier, but less transmissible cousin of the SARS virus.
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MERS has hit Saudi Arabia the hardest, killing 282 people out of 688 infected, according to the Saudi health ministry's latest figures.
It has been found elsewhere in the Middle East and in Europe, Asia and North America, taken there by travellers who were infected in Saudi Arabia or the United Arab Emirates.
The latest findings in the New England Journal of Medicine are based on a 44-year-old Saudi man who kept a herd of nine camels and who died of MERS in November 2013. His friends said they witnessed him applying a topical medicine to the nose of one of his ill camels - four of them were reportedly sick with nasal discharge - seven days before he himself became stricken with MERS.
Researchers sequenced the virus found in one of the sick camels and the virus that killed the man, and found that their genomes were identical.
"These data suggest that this fatal case of human MERS-CoV infection was transmitted through close contact with an infected camel," said the study led by Tariq Madani at the department of medicine, King Abdulaziz University in Jeddah.
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Previous research has suggested that the virus has been quite common in camels for at least the past 20 years, and was likely making the jump into humans.
"Although others have shown that dromedary camels may be important reservoirs for MERS coronavirus, this paper from Azhar and colleagues provides the first unequivocal evidence of camel to human transmission of the virus," said W. Ian Lipkin, director of the Center for infection and Immunity at Columbia University in New York.
"The challenge now is to determine the extent to which camels or other animals contribute to outbreaks of human disease," said Lipkin, who was not involved in the New England Journal of Medicine study.
SARS first appeared in Asia in 2003 and infected 8273 people, nine per cent of whom died.
Like SARS, MERS appears to cause a lung infection, with patients suffering coughing, breathing difficulties and a temperature. But MERS differs in that it can cause rapid kidney failure.