The World Health Organization and other experts are looking for answers to explain how Saudi Arabia missed or failed to report a substantial number of MERS cases and deaths over the past year.

Several experts praised the kingdom's health ministry for making what must have been a difficult disclosure, revealing on Tuesday that the country had not previously reported an additional 113 cases and 92 deaths caused by the Middle East respiratory syndrome virus.

Saudi Arabia announced Tuesday that that a review of the illness led authorities to sharply revise upward the number of confirmed infections and deaths from the disease. (Hasan Jamali/Associated Press)

But they suggested until there is an explanation of how the cases failed to come to light until now, doubts will continue to dog Saudi Arabia's response to the MERS outbreak.

"I think you have to get an explanation in order for this to become credible," said Marion Koopmans, a Dutch virologist whose work provided the first tangible clue that the MERS virus infects camels.

"The question is: What exactly happened? How were these cases found? Were they in the record and not reported? How sure can we be that this a is now a problem solved and from now we will get the real numbers?"

Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota, agreed that Saudi Arabia should be forthcoming on why these cases and deaths were not previously reported.

The health ministry statement referred to strengthening systems for catching cases in future, but did not explain if these were missed or were deliberately not disclosed. Under the International Health Regulations, each case of a disease like MERS must be reported to the WHO within 24 hours of discovery.

"I give them great credit for this new sense of transparency. I can only hope that this continues going forward," Osterholm said.

He noted that in the chaos of a surging outbreak such as Saudi Arabia has been experiencing this spring, occasional cases will be missed. But the new cases represent a 20 per cent increase over the previously declared total for the country; the additional deaths are a 48 per cent increase.

Those numbers are hard to explain away, Osterholm suggested. "I think whatever the explanation was for adding these new cases it is not good. It's either a lack of a competent surveillance system or an intentional effort to report fewer cases. It has to be one of the two."

Saudi Arabia has been the epicentre of the MERS outbreak, reporting over 85 per cent of the now roughly 800 cases diagnosed since 2012 when the world first realized a new animal virus was infecting people.

As cases have mounted, the kingdom has come under increasing criticism for failing to conduct some basic research needed to understand how what seems to be a camel virus is infecting people.

Following a sizable surge in cases beginning in mid-March, the Saudi government fired the health minister in late April. On the eve of the disclosure of the previously unreported cases, the deputy health minister — University of Ottawa graduate Dr. Ziad Memish — was also fired, without explanation. Memish had been the point person for the Saudi MERS response.

Cases in files?

With the newly disclosed cases, plus one new one reported Tuesday, the Saudi MERS total now stands at 689. The additional deaths, plus a new one recorded Tuesday, bring the country's death toll to 283.

The health ministry suggested the unreported cases had occurred over a period of months. A graphic released by the department showed occasional unreported cases occurred from May 2013 onward.

In March of this year, however, there was a sharp spike in these newly discovered cases. In fact, in the third week of March, there were more undisclosed cases than reported ones. In April, when MERS cases soared in Saudi Arabia, previously undisclosed cases appeared to make up about a quarter or a fifth of all cases in two different weeks.

The WHO's technical lead for the MERS outbreak said a team from the global health agency is in Saudi Arabia and is going through the data on each of the newly disclosed cases and deaths.

Dr. Stephane Hugonnet said while the WHO takes it as a positive sign that Saudi Arabia conducted a review to see if it had missed cases, the agency is currently uncertain whether all the newly identified cases will fit the WHO's MERS case definition.

"We don't know yet exactly how those cases have been confirmed. Probably some meet the WHO definition … but we don't know yet if all of them meet this definition," he said from Geneva.

The agency is still looking into why the cases were not previously reported, but Hugonnet said it does not appear to be a

case where stored patient samples were freshly retested.

"I don't think they went to the fridge, got out all the samples and tested them again recently. I think the tests had been done and they went back to the files and picked [up] cases," he said.

A Canadian infectious diseases expert said one possible explanation for the newly uncovered cases may relate to the way tests are read.

Dr. Allison McGeer, who helped investigate a MERS outbreak in a Saudi hospital last year, explained the polymerase chain reaction or PCR tests used to diagnose the infections don't give a positive or negative response, but shows a reading that requires some interpretation.

Some readings make it clear that a case is a true case. But towards the end of the positive range, there is a chance false positive tests can occur. McGeer, who is head of infection control at Toronto's Mount Sinai Hospital, said some people might use a lower cut-off point to reduce the risk of false positives while others would include more cases to be sure not to miss any true positives.