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MERS-CoV Asymptomatic Cases Raise Testing Concerns
Recombinomics Commentary 15:15
June 4, 2013

 The question raised by the presenting physician as to whether they are identifying a changed virus or whether they are identifying a different virus from the virus that has been associated with an apparently high case fatality rate is a good question. Another curious observation is that these positive specimens were obtained from throat swabs, and not lower respiratory tract specimens.

The above comments from the ProMED update on the MERS-CoV confirmed asymptomatic cases in Florence, Italy fail to mention the most obvious difference between the testing of the recent cases in Italy (see map), and testing by other countries, is the timing of the sample collection.  For the asymptomatic cases in Italy, samples were collected shortly after exposure to a confirmed case (45M), which is dramatically different for cases in other countries, where samples are collected well after disease onset.

The most dramatic example was samples from health care works (HCWs) in England, who were contacts of the first confirmed case in England (49M from Qatar who remains hospitalized for the 7 months after admission).  Sample collection was more than 10 days after symptoms, when all or most cases had already recovered from mild symptoms.

The data from Italy suggests that detectable MERS-CoV is in the upper respiratory tract, but is quickly cleared, leading to negative test results in the milder cases.  The more severe case develop lower respiratory tract problems, incuding pneumonia.  In cases with a high viral load MERS-CoV can be detected in upper and lower respiratory tract samples, but in many of the cases, only samples from the lower respiratory tract have detectable virus.

The failure to detect MERS-CoV in the earlier mild cases led to a risk assessment by the ECDC which maintained that there were no mild cases because the severe cases were sporadic and due to exposure to an animal reservoir.  ProMED also frequently cited or asked about an animal reservoir, although there is no data supporting such a reservoir for the vast majority cases, which form clusters and have no animal exposure.

WHO also cited a hypothetical animal reservoir and characterized cases as “sporadic”.  More recently WHO has characterized cases as “seemingly” sporadic to reflect the real possibility that the sporadic appearance is linked to testing that targets severe cases and fails to test samples from the lower respiratory tract.

In contrast to the false negatives reported by others, the data from Italy suggests that MERS-CoV transmission is similar to the cases seen over a year ago in the ICU in Jordan, where transmission resemble SARS-Cov transmission in 2003, which was heavily concentrated in HCWs.

Thus, like the claim of an animal reservoir, the data for a genetically changed virus is purely hypothetical, and once again ProMED ignores the testing procedure and cites highly speculative explanations for MERS-CoV transmission, which is similar to transmission seen over a year ago in Jordan, and SARS-CoV transmission seen over 10 years ago worldwide.

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