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The World Health Organization has just posted revised MERS-CoV case definitions for reporting to WHO, based on the latest available information on the virus.
This revision addresses asymptomatic cases, inconclusive test results, as well as expanding the parameters for `probable cases’.
The WHO also released a 21 page protocol for a prospective Case Control study to assess potential risk factors related to human illness caused by Middle East Respiratory Syndrome Coronavirus (MERS- CoV).
Revised interim case definition for reporting to WHO – Middle East respiratory syndrome coronavirus (MERS-CoV)
Interim case definition as of 03 July 2013
These case definitions have been revised based on new information collected since the previous definitions were published. WHO will continue to review and update them as new information becomes available.
Probable case
Three combinations of clinical, epidemiological and laboratory criteria can define a probable case:
- A person with a febrile acute respiratory illness with clinical, radiological, or histopathological evidence of pulmonary parenchymal disease (e.g. pneumonia or Acute Respiratory Distress Syndrome)
AND
Testing for MERS-CoV is unavailable or negative on a single inadequate specimen1
AND
The patient has a direct epidemiologic-link with a confirmed MERS-CoV case2.
- A person with a febrile acute respiratory illness with clinical, radiological, or histopathological evidence of pulmonary parenchymal disease (e.g. pneumonia or Acute Respiratory Distress Syndrome)
AND
An inconclusive MERS-CoV laboratory test (that is, a positive screening test without confirmation)3
AND
A resident of or traveler to Middle Eastern countries where MERS-CoV virus is believed to be circulating in the 14 days before onset of illness.
- A person with an acute febrile respiratory illness of any severity
AND
An inconclusive MERS-CoV laboratory test (that is, a positive screening test without confirmation)3
AND
The patient has a direct epidemiologic-link with a confirmed MERS-CoV case2.Confirmed case
A person with laboratory confirmation of MERS-CoV infection4.
Notes
Inconclusive testing: Patients with an inconclusive initial testing should undergo additional virologic and serologic testing to determine if the patient can be classified as a confirmed MERS-CoV case. It is strongly advised that lower respiratory specimens such as sputum, endotracheal aspirate, or bronchoalveolar lavage fluid be used when possible. If patients do not have signs or symptoms of lower respiratory tract infection and lower track specimens are not available or clinically indicated, both nasopharyngeal and oropharyngeal swab specimens should be collected. If initial testing of a nasopharyngeal swab is negative in a patient who is strongly suspected to have MERS-CoV infection, patients should be retested using a lower respiratory specimen tract or a repeat nasopharyngeal specimen with additional oropharyngeal specimen if lower respiratory tract specimens are not possible, and paired acute and convalescent sera.
Asymptomatic cases: The demonstration of asymptomatic infection is useful for epidemiological investigations and should be pursued as part of case investigations, however, the burden of proof must be higher due to the risk misclassification because of false positive tests due to laboratory contamination. Generally, in most viral infections, an immunological response such as development of specific antibodies would be expected even with mild or asymptomatic infection and as such serological testing may be useful as additional confirmation of the diagnosis. Additional steps to reconfirm asymptomatic cases, or any case in which the diagnosis is suspect, could include re-extraction of RNA from the original clinical specimen and testing for different virus target genes, ideally in an independent laboratory.
1An inadequate specimen would include a nasopharyngeal swab without an accompanying lower respiratory specimen, a specimen that has had improper handling, is judged to be of poor quality by the testing laboratory, or was taken too late in the course of illness.
2A direct epidemiological link may include:
• Close physical contact
• Working together in close proximity or sharing the same classroom environment
• Traveling together in any kind of conveyance
• Living in the same household
• The epidemiological link may have occurred within a 14 day period before or after the onset of illness in the case under consideration.
3Inconclusive tests may include:
• A positive screening test without further confirmation such as testing positive on a single PCR target
• A serological assay considered positive by the testing laboratory.
4Currently confirmatory testing requires molecular diagnostics including either a positive PCR on at least two specific genomic targets or a single positive target with sequencing on a second. However, the interim recommendations for laboratory testing for MERS-CoV should be consulted for the most recent standard for laboratory confirmation (http://www.who.int/csr/disease/coronavirus_infections/en/). See also notes on asymptomatic cases in this document.
