The public health response to MERS-CoV has been modelled on SARS (severe acute respiratory syndrome), a related virus which caused a pandemic more than ten years ago.
But the features of MERS-Cov are very different, as I outline in a new study, published today in the journal Environment Systems and Decisions.
And while camels have been attributed as the source of the outbreak, the peculiar patterns of the virus suggest we need to consider another possible source: deliberate release.
Many of the features are paradoxical and cannot be explained by known principles of disease transmission.
MERS-CoV caused outbreaks in hospitals, for example, with some human-to-human transmission. Yet MERS-CoV was present in the Kingdom of Saudi Arabia during several mass gatherings such as the Hajj pilgrimage, and did not result in an epidemic.
We have shown that, under current assumptions about the virus, the probability of no epidemic arising after the Hajj in two consecutive years is extremely low. This shows the current assumptions about MERS-CoV could be wrong.
SARS was estimated to be more infectious than MERS-CoV, caused a classic epidemic of over 8,000 cases globally and was eliminated within eight months. In contrast, MERS-CoV is less infectious, but has persisted for more than three times the duration of SARS. This is not the expected behaviour of a virus with low infectivity.
Further, SARS caused satellite epidemics in countries to which it spread, whereas MERS-CoV has remained mainly in the Middle East, without sparking epidemics in other countries where cases have occurred. Cases which have occurred in other countries are mainly in travellers to the Middle East.
Outbreaks of SARS in hospitals were caused by a single strain, but more than one strain of the virus has been identified in some outbreaks of MERS-CoV. This means that affected people were exposed to several different viruses at around the same time. Where did these viruses come from? What was the source of infection to humans?
It remains a mystery why this virus, which has low infectiousness has persisted for so long when a consistent source of infections has not been identified.
There is evidence that camels are the source of MERS-CoV. But while a few infected people have had contact with camels, many have no known contact with camels or other animals; and some have no contact with infected humans or animals.
The finding of the virus in camels does not exclude bioterrorism as a cause. A virus that has a trophism for particular animals, when present in an environment, will tend to infect those animals. It’s a case of which came first: the chicken or the egg.
The other postulated explanation for the paradoxes of MERS-CoV is that there are many mild or asymptomatic cases which have gone undetected. But attempts to look for such cases has not turned up high numbers of mild or asymptomatic cases. Whether this is the explanation will become clearer as better screening methods are developed.
The features of MERS-CoV do not suggest an epidemic disease, but rather, a sporadic pattern. This could be an animal source or deliberate release. My paper shows there is slightly more weight to deliberate release than an animal source, although both are possible.
In the case of bioterrorism, if it is not considered at all, it can never be detected, unless it involves an eradicated pathogen such as smallpox. Public health does not have a good track record of correctly interpreting aberrant patterns.
Take the 1984 Rajneesh cult salmonella attack in the United States, for example. Public health authorities did not consider the possibility of bioterrorism, despite a local politician arguing the case that it might be bioterrorism and despite the facts not fitting with ordinary food poisoning.
If Rajneesh had not confessed later, the attack would have never been recognised, and even when he did confess, he wasn’t believed. This case illustrates normal human tendency to force available facts into common, comfortable explanations, rather than to view the facts objectively.
Getting to the bottom of it
The patterns of MERS-CoV are unusual, and while an animal reservoir or undetected mild human cases as the source of ongoing human infections are both possible, bioterrorism is also possible.
In the case of deliberate release, simulating nature would be difficult with a sporadic infection, and this may explain the observed discrepancies in the epidemiology.
Whatever the source of persistent MERS-CoV infections, using a framework based on SARS to tackle this infection is a mistake, because it is very different to SARS.