The bottom of the iceberg: MERS transmission to Illinois man underscores that many asymptomatic infections are going undetected

March 18th, 2015

An Illinois man who had close contact with the first Indiana U.S. MERS case during two business meetings was apparently mildly infected with the coronavirus as evidenced by antibodies in his bloodstream, the Centers for Disease Control and Prevention reported May 17.

One meeting lasted 40 minutes and the other was “shorter,” with the only known physical contact a handshake. However, the two men sat face to face within six feet of each other, raising the possibility of droplet or airborne transmission. While there is some question if the Illinois man actually had MERS symptoms, the Indiana man was beginning to feel sick but had not yet gone to a hospital.

“The Indiana patient was reported to have already been having fever and myalgia and not feeling well,” David Swerdlow, MD, one of the CDC’s leading MERS investigators, said at a press conference Saturday. “We don't really know exactly when people transmit this virus. We do know that the Indiana patient was having symptoms at the time of the interaction. Prior to being admitted to the hospital, the Indiana patient had extended face-to-face contact on April 25th with a business associate in Illinois. The two had another brief contact on April 26th.”

The previously reported Indiana MERS patient is a U.S. resident who had traveled from Saudi Arabia and was admitted to an Indiana hospital on April 28. The patient was confirmed to have MERS on May 2, and has since been released from the hospital. The secondary case was apparently only mildly symptomatic and “did not seek care or require medical care,” the CDC reported.

In the Illinois resident, laboratory test results showing “apparent past MERS infection” were determined by the presence of antibodies that indicate a successful immune response to an invading pathogen. (The miracle that is the human immune system creates antibodies to attack invading pathogens, neutralizing them directly or tagging them for subsequent attack by mobilized defenders.)

The CDC did a blood test to find the MERS antibodies, which could probably be detected in hundreds of Saudi citizens and other Middle Easterners who have successfully fought off the virus following an exposure. These suspected – but largely untested – mild and asymptomatic cases are sort of the “bottom of the iceberg,” with the known cases (with a mortality rate in the 30% range) the part of the outbreak that has primarily been identified. Serologic testing for MERS is not being routinely done in Saudi Arabia and the World Health Organization case count only includes people actively infected with the coronavirus. While the failure to look for MERS antibodies certainly leaves the investigation in the Middle East blind to a vital piece of information, there does appear to be improved detection of mild cases of active infection. Indeed, one explanation for the recent upsurge of MERS in the region is that more mild cases are being detected and included in the case count.

“We're certainly learning now that there can be a much broader spectrum of illness with MERS,” Swerdlow says. “We know just from reports from Saudi Arabia and the United Arab Emirates and other countries that at least 20% of the [known] patients that they're reporting have not had any symptoms. It's not surprising that we would find a person within the U.S. without symptoms either. There's a much broader range of illness than we recognize with this virus because mainly people who had very severe illnesses were being tested.”

The CDC has tested more than 50 people who were identified during the contact investigation of the Indiana MERS patient, but only the Illinois man showed evidence of past infection. Similar contact tracing of the hospitalized second U.S. patient in Orlando is underway, but thus far no transmission has been reported.

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