The origin of MERS

IN APRIL 2012, two Jordanians died of a mysterious respiratory illness. Shortly afterwards another man with similar symptoms died in Saudi Arabia. For a moment, though they did not make too much of a fuss at the time, the world’s public-health apparatchiks held their breath—for the cause was a coronavirus, a relative of the pathogen that causes Severe Acute Respiratory Syndrome (SARS).

That disease, unknown in 2000, broke out of its redoubt in southern China to infect, over the course of nine months during 2001 and 2002, more than 8,000 people in nearly 30 countries. It killed 10% of them. It also brought air travel, if not to a screeching halt, then at least to a slower pace. The fear was that something similar might happen with Middle East Respiratory Syndrome (MERS), as it has come to be known.

So far, disaster has not materialised. America’s Centres for Disease Control and Prevention, which monitors new diseases around the world, has recorded only 103 cases, in eight countries. Forty-nine of these cases were fatal, though, and even allowing for the non-reporting of some milder instances of MERS, that is a scary death rate. It would therefore be good to know where MERS came from. This has yet to be discovered, but the world’s biological-detective force is on the case.

The search for the source of MERS was started by Ali Zaki of the Dr Soliman Fakeeh Hospital in Jeddah, Saudi Arabia, and Ron Fouchier of Erasmus University Medical Centre in Rotterdam, in the Netherlands. (Dr Fouchier is also one of those who has been looking at which genes let influenza viruses pass easily between people.) Together, Dr Zaki and Dr Fouchier sequenced the MERS virus’s genome. With that information they were able to construct an evolutionary tree which showed it is related to two previously identified viruses, both of which infect bats.

Armed with this knowledge, Ziad Memish of Alfaisal University in Riyadh (who, besides being a researcher, is also the deputy health minister responsible for public health) and Ian Lipkin of Columbia University in New York collected samples of bat faeces from near the home of the first Saudi victim. As they reported in this month’s Emerging Infectious Diseases, one of these samples yielded a partial genetic sequence that was a 100% match to the virus isolated from the patient.

This did not, though, solve the case. People do not commonly come into contact with bats, so an intermediate host is probably involved. That would not be unprecedented. SARS itself is believed to have been originally a bat virus, which was passed to civets, possibly in the wild, and thence to people from civets being sold for food in street markets. (Civets have other uses too; see article.) Bats also transmit Nipah virus, which infects both the lungs and the brain, to pigs—which then sometimes pass it to people.

Civets are not native to Saudi Arabia and pigs are all but unknown in that rigorously Islamic country. But camels are common, and in another paper published this month, in Lancet Infectious Diseases, Chantal Reusken of the National Institute for Public Health and the Environment in Bilthoven, in the Netherlands, and Bart Haagmans, also of Erasmus University, report that blood-serum samples from 50 healthy camels in Saudi Arabia’s neighbour Oman all contained antibodies to the MERS virus.

That indicates these camels had once been exposed to the virus. It also suggests the infection is common in camels—and that they shake it off easily. It thus points an accusing finger. And camels are plausible intermediaries. Besides being beasts of burden, they are sources of meat and milk, so there are plenty of ways for them to pass MERS to people. But this does not prove they did.

To provide that proof, someone would have to isolate MERS virus from a camel and show it was the same strain as that found in those who have caught the disease. If it were not, the hunt would have to continue. Sheep or goats, for example, might be to blame.

If camels do turn out to be the missing link, then another group of biological detectives would try to establish by what route (meat, milk or mere proximity) the virus jumps to humans. If they could do that, they could recommend how to stop it happening—before a version emerges that actually does start an epidemic. /economist