Profese Online, 2020 (vol. 13), issue 1

Coronaviruses are a topical issue due to the ongoing pandemic of COVID-19.

Coronaviruses were discovered in 1937. This coronavirus caused infectious bronchitis in birds and ruined egg producers. The first human coronavirus ever discovered was isolated from a patient with a cold in 1966. Coronaviruses are classified under the order Nidovirales. They are enveloped, large RNA viruses, which infect humans and a wide range of animals. Club-shaped spikes protrude from the lipid envelope, forming an object resembling the solar corona, from which the name of these viruses derives.

The human population is normally infected with four human coronaviruses: 229E and NL63 (Alphacoronaviruses) and OC43 and HKU1 (Betacoronaviruses). The clinical manifestations of these upper respiratory tract infections are referred to as “cold”.

Coronaviruses, originally animal pathogens, can evolve into a strain which can cause serious and fatal disease in humans. An example is the coronaviruses SARS-CoV, MERS-CoV, and the latest to be identified, SARS-CoV2. All three are also beta carotens.

While SARS-CoV infects type II pneumocytes and ciliated epithelial cells with angiotensin-converting enzyme 2 (ACE2) as a receptor, MERS-CoV uses dipeptidyl peptidase 4, a transmembrane glycoprotein, to infect type II pneumocytes and nonciliated bronchial epithelial cells. SARS-CoV2 is also assumed to enter the cell as an ACE2 receptor, similar to SARS-CoV.

SARS

SARS (Severe Acute Respiratory Syndrome) is a disease typically manifesting in humans as a life-threatening pneumonia. SARS-CoV infects nonciliated bronchial epithelial cells and type 2 pneumocytes. In addition to pneumonia, patients often suffer from renal failure. The incubation period ranges from 2 to 10 days; the maximum is 14 days. The virus has been detected in bats and small animals sold in markets, which indicates the possibility of interspecies transmission from bats to other small animals and ultimately to humans. The intermediate hosts are thought to be civets and a raccoon dog. The first cases of SARS occurred in China, Guangdong Province, in November 2002.

These first infections were reported in restaurant workers who prepared wild game as exotic delicacies. Previous findings indicate that SARS-CoV had circulated in bats long before its genetic modification and transmission to the human population. The disease was caused by coronavirus (SARS-CoV) of animal source, which overcame the species barrier due to ecological changes and changes in human behaviour. It adapted to the human population and facilitated human-to-human transmission. The disease spread worldwide, and by July 2003, when SARS disappeared, a total of 8,098 cases of the disease and 774 deaths (WHO) in 26 countries had been recorded. The case-fatality ratio was 9.6%. The epidemiology and ecology of this infection have not been adequately clarified in spite of the end of the SARS pandemic.

(For details, see: Prymula R, Špliňo M. SARS: Syndrom akutního respiračního selhání [Severe Acute Respiratory Syndrome]. Prague: Grada; 2005)

MERS

MERS (Middle East Respiratory Syndrome) is a disease which manifests itself in humans as severe pneumonia, often accompanied by renal failure. The MERS-CoV agent binds to DPP4 receptors of airway epithelial cells. Said receptor contains only about 20% of these cells.

Like SARS-CoV, it infects non-ciliated airway epithelial cells and type 2 pneumocytes. The incubation period is 2 to 5 days.

The cause of this infection has been demonstrated in bats, which suggests a possible origin of the disease. MERS-CoV was transmitted to humans from dromedary camels as an intermediate host. The virus has lived in the animals for at least the last 30 years, based on antibody samples obtained from camels in 1983. Furthermore, the camel MERS-CoV strains have been demonstrated to be almost identical to the human MERS-CoV strains.

The first cases of MERS occurred in Saudi Arabia in September 2012. A total of 2,506 cases across 27 countries were recorded between then and January 2020, of which 862 died. The case-fatality ratio was 34.4%, which is thrice the SARS ratio. Human-to-human transmission of MERS-CoV is, unlike SARS-CoV, less easy and has only been documented in cases of close contact with an infected patient in healthcare settings. In February 2020, another 18 cases of MERS were reported from Saudi Arabia, of whom 5 died. Suspected cases of MERS emerged in the Czech Republic and Slovakia in 2015, but neither was confirmed.

COVID-19

COVID-19 is a disease whose symptoms may have various clinical forms in humans, ranging from asymptomatic infection to severe pneumonia. The incubation period is, like in the case of SARS, 2-10 days, with the maximum of 14 days. Like with SARS and unlike with MERS, person-to-person transmission is common. Genetic analysis of SARS-CoV-2 has found it had identity to 88% bat-derived SARSr-CoV, 79% identity to SARS-CoV, and only 50% identity to MERS-CoV. These findings suggest that this is a new virus, different from SARS-CoV and MERS-CoV, and most likely derived from bats, like SARS-CoV and MERS-CoV. The virus enters the cell by binding to the peptidase angiotensin convertase (ACE2), which is strongly expressed on alveolar epithelial type II cells and the ciliated bronchial epithelium. Chinese researchers, who studied bats in Yunnan Province for five years, sequenced 11 newly discovered bat-derived SARSr-CoV strains. Some of these were able to bind to the human ACE2 receptor due to mutated spike protein. The intermediate host has not been identified yet.

Infection with this new type of SARS-CoV-2 coronavirus was first reported in Wuhan, Hubei, China, in December 2019. The first cases were linked to the Huanan Seafood Wholesale Market in Wuhan, which also sold wildlife such as birds, snakes, monkeys, bats, and rabbits. Raw meat intended for human consumption came into contact with live animals at this market selling live animals and seafood from southern China .

COVID-19 became widespread across all inhabited continents in early 2020 and was declared a pandemic by the World Health Organization on 11 March. The first case in the Czech Republic was confirmed on 1 March 2020, and as of 2 July 2020, the country recorded 12,046 infections and 349 virus-related deaths.
Much remains unknown about SARS-CoV-2, however. It is unclear whether it originated through natural processes or was engineered and leaked from a laboratory. Immunity, infectivity of asymptomatic cases, and many other problems also remain unresolved. A number of vaccines are being prepared, yet the question is whether they can protect humans against COVID-19.

(For more information, see news and the World Health Organization (WHO): http://www.who.int)

Prof. MUDr. Vladimír Janout, CSc.

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