In the clearest evidence yet that the MERS (Middle East Respiratory Syndrome) virus can be passed directly from camels to humans, genetic sequencing has shown that a 44-year-old Saudi man who died of it in November had a strain virtually identical to that in one of the camels in his herd.
Three of the man's friends accompanied him as he visited his nine camels daily before falling ill and saw him put medicine in the nose of a calf with a runny nose, scientists from King Abdulaziz University in Jeddah reported on Wednesday in The New England Journal of Medicine.
Samples taken by a veterinarian after the man was hospitalised, and again after he died, showed that the calf had the same virus. It recovered, and some of the other camels had antibodies to the virus, indicating they had also been infected and had recovered.
Neither the man's friends nor his daughter, who suffered cold symptoms after he fell ill, tested positive for the virus.
The news comes at a time when Saudi Arabia has made a dramatic upward revision of the number of people killed by the MERS the country.
Experts in global health and infectious diseases say transparency with data is critical to learning more about the virus, which until two years ago had never been seen in humans but has now killed more than 300 people worldwide.
But an announcement by Riyadh on Tuesday that a review of the outbreak had revealed 113 previously unreported cases, including 92 deaths, suggested greater openness.
Tariq Madani, head of the scientific advisory board in the Saudi Health Ministry's command and control centre, said he did not believe the under-reporting had been deliberate, and was due to a range of factors.
"We don't think this was intentionally done, intentionally under-reported. This can happen anywhere in the world, that 20 per cent of patients may not be reported. This is within the limit. It's actually less than 20 per cent," he said.
Early symptoms of MERS can be fever, coughing, shortness of breath and pneumonia. While it is thought to originate from camels, scientists say human-to-human spread is also taking place.
The Saudi agriculture minister was reported on Thursday as saying the kingdom's camels would be tested for MERS.