Emerging threats from zoonotic coronaviruses-from SARS and MERS to 2019-nCoV.

ABSTRACT: Coronaviruses are enveloped RNA viruses that are widely detected in mammals and birds, and commonly denoted in etiologies of upper respiratory tract infections in humans.Two potentially dangerous zoonotic coronaviruses have emerged in the past two decades. The severe acute respiratory syndrome coronavirus (SARS-CoV), originating from China, was responsible for the first outbreak that extended from 2002 to 2003. The second outbreak occurred in 2012 in the Middle East and was caused by the Middle East respiratory syndrome coronavirus (MERS-CoV). A new strain of coronavirus, designated as the 2019 novel coronavirus (2019-nCoV), emerged during the third outbreak in Wuhan, China, at the end of 2019. Symptoms of pneumonia with unknown etiology were reported in several patients. The infection was epidemiologically linked to the Huanan seafood market in Wuhan.6 Similar to the SARS-CoV and the MERS-CoV, bats have been denoted as the likely primary reservoirs of the 2019-nCoV based on its similarity to bat coronaviruses.7 The intermediary reservoir is yet to be denoted. The pertinent and critical factor for an emerging virus is its pandemic potential. Efficient human-to-human transmission is a requirement for large-scale spread of a new virus. The proportion of patients with mild symptoms of illness is another important factor that determines our ability to identify infected individuals and to prevent the spread of virus. Identification of transmission chains and subsequent contact tracing are further complicated when several infected individuals remain asymptomatic or mildly symptomatic. A key factor for efficient human-to-human transmission is the ability of the virus to attach to human cells. Coronaviruses use a spike protein for attachment to host cells.8 Apparently, the 2019-nCoV uses the same human angiotensin-converting enzyme 2 receptor as the SARS-CoV,5 whereas the MERS-CoV used dipeptidyl peptidase (also known as CD26). An efficient human-to-human transmission involves multiples routes of transmission, including droplet transfer, direct contact, and indirect contact. A limited human-to-human transmission may require a high infective dose and a significantly close contact with an infected person as prerequisites