Coronaviruses are enveloped RNA viruses, characterized by surface protein spikes, which, under electron microscopy, resemble the sun’s corona. Numerous coronaviruses, first discovered in domestic poultry in the 1930’s, cause respiratory, gastrointestinal, liver, and neurologic diseases in animals.
Human Coronavirus Infection (HCoV)
Only 7 coronaviruses cause disease in humans (HCoV).
Four of the 7 HCoV (HCoV-NL63, -229E, -OC43, and -HKU1) cause mild and self-limiting upper respiratory tract infections, such as the common cold, but can cause severe lower respiratory tract infections, including pneumonia, in infants, the elderly, and the immunocompromised. These HCoV infections show a seasonal pattern with most cases occurring in the winter months in temperate climates.
Three of the 7 HCoV (SARS-CoV, MERS-CoV, and SARS-CoV2) have caused major outbreaks of deadly pneumonia in the 21st century.
The first of these outbreaks, severe acute respiratory syndrome (SARS), first emerged in November 2002 in Guangdong province in southern China and caused an epidemic that spread within months to 29 countries and 6 continents. It sickened over 8,000 people and killed almost 800 worldwide. The majority of cases occurred in China and Hong Kong. In the United States, only 8 people had laboratory-confirmed SARS; all 8 had traveled to areas where SARS-CoV transmission was occurring. The overall case fatality rate was 10%, but varied by age, ranging from < 1% in people aged 24 years and younger to > 50% in those aged 65 years and older.
The source of SARS was palm civets (cat-like mammals) that had been sold as food at local live animal markets in Guangdong. Once introduced into humans, SARS-CoV readily spread person-to-person by large respiratory droplets, aerosols, and by fecal-oral transmission (diarrhea is a common manifestation of the infection).
The next HCoV to cause deadly infection was Middle East Respiratory Syndrome coronavirus (MERS-CoV), which emerged in the Arabian Peninsula in September 2012. MERS-CoV has caused recurrent outbreaks that have sickened over 2,500 people with a case fatality rate of about 35%. Most infected people lived in or recently traveled from the Arabian Peninsula. 85% of cases were reported from Saudi Arabia. The largest MERS outbreak outside the Arabian Peninsula occurred in South Korea in 2015, associated with a traveler returning from the Arabian Peninsula.
MERS-CoV, like SARS-CoV, is a zoonosis, spread by direct or indirect contact with dromedary camels. MERS-CoV also spreads human to human by direct contact, fomites, and respiratory droplets. 42% of all cases in 2018-2019 were linked to clusters of human-to-human transmission in households or healthcare facilities. The source was unknown for 60%. MERS-CoV virus is detected respiratory tract secretions, feces, serum, and urine, and virus has been detected in survivors for a month or more after onset.
The seventh HCoV to be discovered is SARS-CoV2, the cause of an outbreak, named COVID-19, that is currently spreading worldwide. The outbreak began in Wuhan, a city of over 11 million, in Hubei Province, Central China. (Wuhan is home to the Wuhan Institute of Virology, a leading center for coronavirus research, although no connection is suspected between the research and the current outbreak.) SARS-CoV-2 is most closely related, with 96% genetic similarity, to a coronavirus isolated from horseshoe bats that were found in caves in Yunnan, China, over 1000 km (about 621 miles) from Wuhan (1).
1. Zhou P, Yang XL, Wang XG, et al: A pneumonia outbreak associated with a new coronavirus of probable bat origin. Nature 579: 270-273, 2020.