In mid-May of this year, in WHO: Asymptomatic MERS-CoV Case – UAE, we learned of an asymptomatic camel transport truck driver who tested positive for the MERS virus, and was in hospital isolation. Contact tracing was underway and within a couple of days a second asymptomatic case was announced by the Abu Dhabi Health Authority (HAAD) .
While most MERS known MERS infections are serious – and among hospitalized cases the fatality rate in Saudi Arabia runs roughly 44% – we know that some people experience few, if any symptoms.
Since only contacts of known cases are routinely tested (and with a bias towards testing symptomatic cases), it is likely there are more asymptomatic cases out there than we know about. It has even been suggested that they may be responsible for at least part of the `silent’ spread of the disease, although little is known about the ability for asymptomatic cases to infect others (see Study: Possible Transmission From Asymptomatic MERS-CoV Case).
Today the CDC’s EID Journal carries a Dispatch on these two asymptomatic cases. While it doesn’t answer the question of human to human asymptomatic transmission of the virus, it does add to the growing weight of evidence supporting camel-to-human transmission.
It also suggests that people exposed to infected camels can be asymptomatically infected, which highlights the need for more aggressive and proactive testing of high risk groups.
Zulaikha M. Al Hammadi1, Daniel K.W. Chu1, Yassir M. Eltahir, Farida Al Hosani, Mariam Al Mulla, Wasim Tarnini, Aron J. Hall, Ranawaka A.P.M. Perera, Mohamed M. Abdelkhalek, J.S.M. Peiris, Salama S. Al Muhairi , and Leo L.M. Poon
In May 2015 in United Arab Emirates, asymptomatic Middle East respiratory syndrome coronavirus infection was identified through active case finding in 2 men with exposure to infected camels. Epidemiologic and virologic findings suggested zoonotic transmission. Genetic sequences for viruses from the men and camels were similar to those for viruses recently detected in other countries.
Middle East respiratory syndrome (MERS) coronavirus (MERS-CoV) was first detected in humans in 2012 (1). Before 2015, most human infections occurred on the Arabian Peninsula. However, the recent occurrence of MERS in South Korea indicates that this pathogen can cause major outbreaks in other regions (2). Dromedaries are believed to be a source of MERS-CoV (3,4), but only a few case reports provide virologic and epidemiologic evidence that directly supports zoonotic transmission of the virus from dromedaries to humans (5–7). We report the detection of epidemiologically linked MERS-CoV infection in 2 men who had direct contact with infected dromedaries (8,9).
A 29-year-old man (contact 1) transported 8 dromedaries from Oman to United Arab Emirates on May 7, 2015 (Table 1). The same day, as part of a national policy for controlling MERS, samples were collected from the dromedaries at a screening center located at the United Arab Emirates border. The samples were tested by reverse transcription PCR (RT-PCR) on May 10 and found to be positive for the MERS-CoV open reading frame (ORF) 1A and upstream of E genes (10). This finding led local public health authorities to conduct active surveillance on humans who had contact with the infected dromedaries.
A sputum sample collected from contact 1 on May 10, 2015, was tested by RT-PCR on May 12 and found to be positive for MERS-CoV; the man was admitted to a hospital the same day. Follow-up respiratory samples obtained on May 13 and 14 were still RT-PCR–positive, but a sample obtained on May 18 was negative. The patient was asymptomatic at hospital admission and throughout his hospital stay (Technical Appendix[PDF - 95 KB - 3 pages]).
Contact 2 was a 33-year-old man who worked at the screening center mentioned above. He had direct contact with the same group of infected dromedaries during the sampling procedures. A nasal aspirate sample was obtained from the man on May 14, 2015, and found to be RT-PCR positive for MERS-CoV. Contact 2 was hospitalized on May 18. A follow-up sample obtained on May 18 was RT-PCR negative for MERS-CoV. Contact 2 was asymptomatic throughout his hospitalization (Technical Appendix[PDF - 95 KB - 3 pages]).
Samples from 32 other persons were also tested by RT-PCR (Technical Appendix[PDF - 95 KB - 3 pages]). None tested positive.
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After detailing the research findings, and listing some of the limitations of their study, the authors conclude by writing:
MERS-CoV genomic sequences determined in this study are similar to those of viruses detected in 2015 in patients in Saudi Arabia and South Korea with hospital-acquired infections. The infected dromedaries in this study were imported from Oman, which suggests that viruses from this clade are widely circulating on the Arabian Peninsula. Sequence analyses of MERS-CoVs found in South Korea and China do not suggest that viruses from this clade are necessarily more transmissible variants (15). However, given that a single introduction of MERS-CoV from this clade caused >180 human infections in hospital settings (2) and that viruses of this clade are causing other human infections in Saudi Arabia, further phenotypic risk assessment of this particular MERS-CoV clade should be a priority.