The novel coronavirus found in the Middle East earlier this year probably came to humans from bats, though whether it travelled through another species before infecting people is unclear, a new report suggests.
The scientists who first identified the new virus — which comes from the same family as SARS — reported Wednesday on the case that brought the virus to light, the fatal infection of a 60-year-old man from Saudi Arabia.
In their report, published in the New England Journal of Medicine, they predict pipistrellus bats may be the source of the virus.
Senior author Ron Fouchier, the virologist whose laboratory first spotted that this was a never-before-seen coronavirus, said bats and coronaviruses have co-evolved over millenniums.
Because of that fact, one can generally predict which bat is host to which coronavirus, Fouchier said in an interview. Study of the virus cannot reveal if there was an animal go-between, though it is a possibility, he said.
"It's hard to say anything definitive, but humans do not come in contact with bats a whole lot. And so the chance that two humans get into contact with bats is even less likely, with the same species of bats carrying the same virus," said Fouchier, a senior scientist at Erasmus Medical Centre in Rotterdam, the Netherlands.
"So it is most likely that those two humans either got it from a different animal species or another human."
To date only two infections with the new virus have been spotted. The first, the man from Saudi Arabia, occurred in June. The second was in a man from Qatar, who first sought medical assistance in early September. The Qatari man was sent by air ambulance to London, where he is still in hospital.
While Fouchier mentioned the possibility of spread from person to person, at this point the suspicion is that the men were probably infected by animals. Both are reported to have had contact with some animals, including sheep and camels in the case of the man from Qatar.
Public health authorities have been investigating whether friends and family of the cases or health-care workers who came in contact with them developed symptoms, but so far it appears that the virus has not spread from those two cases.
In fact, in the journal report the authors reveal that the microbiologist in Saudi Arabia who was trying to puzzle out the source of the man's infection has developed a test that can detect antibodies to the virus in blood samples.
Dr. Ali Mohamed Zaki, of Dr. Soliman Fakeeh Hospital in Jeddah, tested 2,400 blood samples from people who came to the hospital for treatment from 2010 to 2012. None of them contained antibodies to the virus. Zaki is one of the authors of the New England Journal paper.
His data support the idea that this may be an animal virus that is occasionally spilling over into people, not a human virus that just hasn't been spotted before, Fouchier said.
Whether there have been or will be more cases of the virus is a question without an answer at this point — though Fouchier suggested chances are good there will be.
"Personally I do think that we are going to find a few more cases. Because these two cases were three months apart and I do not think that our diagnostic capacity is such that we would detect two out of two cases in the world," he said.
"I'm guessing that we are going to find a few more cases and I hope it's going to be just a few."
Still, at this point the risk the new virus poses doesn't appear to be broadly based, an editorial in the journal suggests.
Dr. Larry Anderson, an infectious diseases expert at Emory-Children's Centre in Atlanta, Ga., said at present the risk that exists is likely within the local community or the region. Until the source of the virus is discovered it will be hard to assess what that means geographically.
"What we have right now . . . is a virus that doesn't appear to transmit person to person. So it's not a broader global risk," said Anderson, who during the SARS outbreak in 2003 was with the U.S. Centers for Disease Control, leading their laboratory efforts in the response to the outbreak.
"Now, it may still be a risk locally, depending on the types of exposure that lead to infection — which we don't really know about. The fact that they've only had a couple of cases would argue that it's not a frequent introduction. It may just periodically introduce."
But both Fouchier and Anderson admitted that this could be the early steps in a more complicated dance between this virus and humans.
It is now known that before SARS adapted to be able to spread reasonably efficiently from person to person the virus had a stuttering start, probably jumping from its animal source to people a number of times before taking off.
Fouchier said it's possible this virus is also on a path to adapting to people. But because it has been spotted at this early stage, it could be easier to control, he said. "(But) it's critical that we find out where it comes from."
Fouchier said samples from suspected cases are still coming into his laboratory.
In fact, he was waiting Wednesday for samples from Lebanon. Given the fact that it's currently the start of the season for respiratory illnesses in the Middle East, his lab may find a lot of influenza in the samples, he admitted.
Interestingly, the report notes that the Saudi man who died also had a mix of bacterial infections over the course of his illness. Acinetobacter and Staphylococcus aureus were isolated from his airways, as was Klebsiella pneumoniae, which can cause serious respiratory illness.
Could a bacterial co-infection explain the severity of the man's illness? Fouchier said it is possible, but the bacteria were susceptible to antibiotics and the man was treated aggressively with anti-bacterial drugs.
"We certainly cannot exclude that these bacteria have played a role in the disease, but what is really unusual that you find this coronavirus in the lower airways," he said.
"So it is quite likely that this lower airway coronavirus is a key in this disease. But it's absolutely true that bacterial co-infections or super-infections can explain this clinical picture quite well as well."