Morphology and Ultrastructure
Virions of the genera Coronavirus and Torovirus are roughly spherical. The genetic material is represented by a single-stranded positive-sense RNA closely bound to the main nucleoprotein (N), forming a helical nucleocapsid structure.
Surrounding the nucleocapsid is a lipid envelope (supercapsid) embedded with proteins that form club-shaped peplomers:
- Spike (S) glycoprotein: A key surface antigen (as trimers) responsible for attachment and induction of protective virus-neutralizing antibodies.
- Membrane (M) protein: The main transmembrane protein of the lipoprotein envelope.
- Envelope (E) protein: A small structural envelope protein.
- Hemagglutinin-esterase (HE): Present only in certain strains, forming shorter surface projections.
Life Cycle and Cultivation
The virus enters the host cell via endocytosis. Replication occurs exclusively in the cytoplasm (without forming intracellular inclusion bodies). Viral particles accumulate in vacuoles near the membranes of the endoplasmic reticulum (ER). Assembly of new virions occurs via budding into the lumen of the ER and Golgi apparatus, after which they exit the cell via exocytosis.
- In laboratory settings, primary epithelial cells and human embryonic cell cultures are used (and chicken embryos for avian strains).
- Key condition: The optimal cultivation temperature must be lowered to 33–35 °C.
Epidemiology and Clinical Features
The primary source of infection is an ill human, and the main transmission mechanism is airborne. The incidence shows a marked winter-spring seasonality.
Clinical manifestations range from mild to critically severe:
- Typical course: The virus multiplies in the upper respiratory mucosa. The illness lasts 5–7 days, accompanied by rhinitis and sneezing, with body temperature usually remaining normal.
- Severe course: Development of atypical pneumonia (e.g., SARS). Characterized by high fever and involvement of the lower respiratory tract.
- Extrapulmonary forms: Gastroenteritis (diarrheal syndrome) or nervous system involvement may develop.
The main danger lies in secondary viral or bacterial superinfections.
Environmental Stability
Coronaviruses possess moderate environmental stability. At room temperature, they remain viable for several days, and on surfaces for up to 3 hours (in clinical secretions, up to 2 days). Low temperatures and lyophilization (freeze-drying) are well tolerated.
The presence of a lipid envelope makes the virion extremely sensitive to lipid solvents and detergents. They are also rapidly inactivated by heat, formaldehyde, and various oxidizing agents.
Laboratory Diagnostics
Nasopharyngeal swabs are typically collected for testing. Direct viral isolation (virological method) is labor-intensive and rarely used.
- Rapid diagnostics: Immunofluorescence assay (IFA) allows rapid detection of antigens directly in mucosal epithelial cells.
- Serological testing: This is the mainstay, though retrospective. Starting from the 10th day of illness, indirect immunofluorescence (IFA) is used to assess rising IgG titers in paired sera. From the third week onward, ELISA is used to detect IgM and IgG antibodies.
Specific prophylaxis is not widely available; treatment is primarily supportive.