Morphology and ultrastructure
Members of the genera Coronavirus and Torovirus are round-shaped. The genetic material is represented by single-stranded positive-sense RNA, which is closely associated with the main nucleoprotein (N), forming a structure with helical symmetry.
A lipid envelope (supercapsid) with embedded proteins forming club-shaped peplomers is located over the nucleocapsid:
- Glycoprotein S (Spike): a key surface antigen (in the form of trimers) responsible for attachment and inducing the production of protective virus-neutralizing antibodies.
- Protein M (Membrane): the main membrane protein of the lipoprotein envelope.
- Protein E (Envelope): a small envelope protein.
- Hemagglutinin-esterase (HE): present only in certain strains, forming shortened projections.
Life cycle and cultivation
The virus enters the host cell via endocytosis. Reproduction occurs exclusively in the cytoplasm (intracellular inclusions are not formed). Viral particles accumulate in vacuoles near the membranes of the endoplasmic reticulum (ER). Assembly of new virions is carried out by budding into the cavities of the ER and Golgi apparatus, after which they leave the cell via exocytosis.
In laboratory conditions, primary epithelial cells and human embryonic cell cultures are used for cultivation (and chicken embryos for avian strains). Important condition: the optimal cultivation temperature must be lowered to 33–35 °C.
Epidemiology and Clinical Presentation
An infected person is the main source of infection, and the leading transmission mechanism is aerogenic. The incidence has a pronounced winter-spring seasonality.
Clinical forms vary from mild to extremely severe:
- Typical course: the virus multiplies in the upper respiratory tract mucosa. The disease lasts 5–7 days, accompanied by rhinitis and sneezing, while body temperature most often remains normal.
- Severe course: development of atypical pneumonia (e.g., SARS syndrome). Characterized by high fever and damage to the lower respiratory tract.
- Extra-respiratory forms: gastroenteritis (diarrheal syndrome) or damage to the nervous system may develop.
The main danger lies in the addition of a secondary viral or bacterial infection.
Stability in the external environment
Coronaviruses are relatively stable in the external environment. At room temperature, they remain viable for several days, and persist on surfaces for up to 3 hours (in secretions — up to 2 days). Low temperatures and lyophilization (freeze-drying) are well tolerated by the virus.
The presence of a lipid supercapsid makes the virion extremely sensitive to fat solvents and detergents. They are also rapidly inactivated by heat, formaldehyde, and various oxidizers.
Laboratory diagnostics
Nasopharyngeal swabs are usually taken for testing. Isolation of the virus itself (virological method) is labor-intensive and rarely used.
- Rapid diagnostics: immunofluorescence assay (IFA) allows for the rapid detection of antigens directly in the mucous membrane cells.
- Serological method: is the primary method, but is retrospective in nature. From the 10th day of the disease, IFA (indirect immunofluorescence) is used to assess the rise in IgG titer in paired sera. Starting from the third week, ELISA is used to detect IgM and IgG class antibodies.
Specific prophylaxis has not been developed, and treatment is aimed at relieving symptoms.
Briefly: answers to topical questions
What host cell receptors do coronaviruses use for attachment and entry?
To attach to target cells, coronaviruses bind to cell surface glycoprotein receptors. This interaction is determined by the active centers of the virus and dictates its cell tropism.
What drugs are used for etiotropic treatment of coronavirus infection?
Direct-acting antiviral drugs are used for the etiotropic treatment of coronavirus infection.
- Umifenovir — inhibits fusion, preventing the viral lipid envelope from fusing with the cell membrane. Active against coronaviruses, including the causative agent of severe acute respiratory syndrome (SARS).
To memorize the main envelope antigenic proteins, use the acronym S-M-E-N: S (Spike), M (Membrane), E (Envelope), and N (Nucleocapsid — nucleoprotein beneath the envelope).
Class and exam questions
Where exactly does coronavirus replication take place inside the cell?
The process takes place in the cytoplasm of the infected cell. Virions massively accumulate in vacuoles located near the endoplasmic reticulum membranes.
Why are coronaviruses destroyed by ordinary soap?
Their outer envelope (supercapsid) consists of lipids. Detergents (including soap) and other lipid solvents easily destroy this layer, completely inactivating the virus.
What antibodies are formed in response to the infection?
During the humoral immune response, the body produces agglutinating, precipitating, and virus-neutralizing antibodies. The latter play the main protective role.
What else to study on this topic
- Virus entry mechanism: features of receptor-mediated endocytosis
- Specifics of the antigenic structure: differences between the four main human coronavirus groups
- Pathogenesis of SARS syndrome at the tissue level
- Interpretation of serological test results (indirect IFA and ELISA) over the course of the disease
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