Unresolved questions in the zoonotic transmission of MERS

Abstract

The Middle East Respiratory Syndrome-coronavirus (MERS-CoV) is the second of three zoonotic coronaviruses to infect humans since 2002, causing severe pneumonia. Unlike SARS-CoV-1 and SARS-CoV-2, the causes of the severe acute respiratory syndrome and Covid-19, respectively, MERS-CoV is enzootic in dromedary camels, a domestic/companion animal present across Africa, the Middle East and Central or South Asia and is sporadically transmitted to humans. However, it does not transmit readily from human to human except in hospital and household settings. Human MERS disease is reported only from the Arabian Peninsula (and only since 2012 even though the virus was detected in camels from at least the early 1990's) and in travelers from this region. Remarkably, no zoonotic MERS disease has been detected in Africa or Asia, even in areas of high density of MERS-CoV infected dromedaries. Here, we review aspects of MERS biology and epidemiology that might contribute to this lack of correlation between sites of camel infection and human zoonotic disease. Since MERS-CoV or MERS-like CoV have pandemic potential, further investigations into this disparity is critical, to forestall pandemics caused by this virus.

Figure 1

Possible explanations for the lack of locally acquired zoonotic MERS reported from Africa. *Note: Behavioural factors refer to cultural, dietary or other activities that affect the exposure of humans to dromedary camels.

Figure 2

Phylogenetic relationships of MERS-CoV strains circulating in the Arabian Peninsula and Africa (reproduced from Ref. [14^••]). The tree was constructed by the maximum likelihood method usingPhyML. Scale bar indicates the pairwise nucleotide substitutions per site. The virus clade designations are denoted. Taxa labelled with blue and red at the branch tips represents MERS-CoV sequences from human and camels, respectively. ORF3 and ORF4b deletions in the virus genomes are indicated as green and purple boxes respectively. Prototype virus strain EMC is denoted in blue font and other strains from the Arabian Peninsula and Africa that were phenotypically characterized in Ref. [14^••] are denoted with *.