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A case control study is one of the first tools disease detectives reach for when they need to solve a new mystery. These studies lay the groundwork for understanding a new malady and identifying what is making people sick when outbreaks occur.

And yet, inexplicably, there has never been a case control study published about Middle East respiratory syndrome, or MERS, a disease that first cropped up more than three years ago.

Until now, that is. The first MERS case control study has finally been published.


Researchers from Saudi Arabia, aided by scientists from the Centers for Disease Control and Prevention, reported this week that people who had direct and indirect contact with camels were more likely to develop the often-fatal lung disease than people who didn’t have contact with dromedaries.

Milking a camel appeared to increase a person’s risk of getting sick, according to the study, which will be published in the January issue of the CDC journal Emerging Infectious Diseases. The CDC posted it online Tuesday.


But drinking unpasteurized camel milk or camel urine — believed to have medicinal qualities by some in the Middle East — did not seem to increase the risk of catching MERS, at least for people in this study. The study, however, was small and in all likelihood not the final word on MERS risks.

The researchers studied 30 people they were confident were “primary” cases — people who had not contracted MERS from another person but caught it instead from an animal or environmental source. The goal was to determine the source.

Zoonoses are diseases or infections that transmit from animals to humans. Here are some examples. Alex Hogan/STAT

Here’s how it was done: Researchers asked people who caught MERS scads of questions — what they did, where they went, what they ate, what kind of animal contact they had. The same questions were put to “controls” — people of similar ages, health-status and who lived near the cases, but who did not contract MERS.

If cases and controls ate dates, dates weren’t a problem. But if cases kept camels as pets and controls did not, that’s a clue.

Of the 30 cases in the study, 17 were either dead or too sick to speak, so the researchers interviewed a family member instead. The researchers acknowledge that might affect the reliability of the information they were able to collect on more than half of their cases.

When case control studies are conducted at the start of an outbreak, they generally illuminate facts that have not yet come into view. In the case of this study, most of the clues it provides have already been unearthed through other means.

Why the study wasn’t conducted earlier is not clear. The World Health Organization and others urged, pleaded, cajoled, and eventually criticized MERS-affected countries — especially Saudi Arabia, which has reported more than 85 percent of MERS cases — for not doing this fundamental research.

International scientists offered assistance. The WHO provided a template Saudi researchers could follow. And the community of public health experts concerned about MERS waited. And waited.

This likely isn’t the study they would have hoped for. A larger study involving cases from Qatar, Jordan, and other countries that record MERS cases would have been more helpful, said Peter Ben Embarek, the WHO’s technical lead for MERS. Likewise, at this point, more specific questions about camel interactions would have been useful, he suggested. “We need a better understanding,” he said.

Still, it’s a start. And Ben Embarek said it may help quiet the chorus of camel defenders who refuse to believe these animals — much beloved on the Arabian Peninsula — are playing a role in the spread of MERS.