Taxonomy and main families
A vast number of antigenic variants of viruses capable of causing respiratory diseases circulate in nature. Most of them were isolated in the 1950s and 1960s. All pathogens are divided into five main families:
- Paramyxoviridae: includes human parainfluenza viruses, as well as respiratory syncytial virus (RSV).
- Coronaviridae: various coronaviruses (including SARS-associated virus).
- Picornaviridae: the most numerous group, represented by rhinoviruses (more than 113 serotypes are known).
- Reoviridae: respiratory reoviruses.
- Adenoviridae: adenoviruses. Serotypes 3, 4, and 7 are most frequently registered, whereas infection outbreaks are predominantly associated with types 12 and 21.
Comparative characteristics of virions
The morphology, size, and genetic apparatus of the pathogens vary considerably. Viral particle sizes range from the smallest (20 nm in rhinoviruses) to very large (200 nm in paramyxoviruses).
Genetic apparatus (types of nucleic acids):
- The only ones possessing linear double-stranded DNA are representatives of the Adenoviridae family.
- The remaining groups contain RNA. Furthermore, single-stranded linear (+)RNA is found in Picornaviridae and Coronaviridae; single-stranded linear (-)RNA is present in Paramyxoviridae; and double-stranded segmented RNA characterizes the Reoviridae family.
Capsid structure and presence of an envelope (supercapsid):
- Non-enveloped (simple) viruses. They lack an outer lipoprotein envelope (supercapsid). These include adeno-, rhino-, and reoviruses. Their nucleocapsid has an icosahedral (cubic) type of symmetry, and the virion itself resembles an icosahedron.
- Enveloped (complex) viruses. They are coated with a supercapsid. This group includes paramyxoviruses and coronaviruses. Their virion shape is spherical, and the nucleocapsid symmetry is helical. Due to the presence of lipids in the envelope, such viruses are extremely sensitive to ether.
Antigenic structure, properties, and reproduction
Pathogens are identified using an antigen system. Group antigens are characteristic of an entire genus, whereas type-specific antigens allow precise identification within the genus and determination of the specific serotype.
Most ARI viruses possess hemagglutinating activity—the ability to agglutinate erythrocytes. The diagnostic method of HI assay (hemagglutination inhibition assay) is based on blocking this activity with antibodies. However, there are exceptions: respiratory syncytial virus, rhinoviruses, and the SARS virus do not possess this ability.
For cytological diagnosis of infection (when searching for intracellular inclusions), it is critically important to know the localization of viral particle assembly:
- In adenoviruses, assembly occurs strictly in the nucleus of the cell.
- In all other ARI pathogens, this process is localized in the cytoplasm.
Mechanisms of cell egress and laboratory diagnostics
The completion process of the replicative cycle depends on the virion structure. Non-enveloped viruses employ an "explosive" mechanism that ends in lysis (death and destruction) of the host cell. Enveloped viruses exit via budding: during detachment from the infected cell, they form their outer lipoprotein envelope from host membrane elements.
Basics of Laboratory Diagnostics: The main method for detecting acute respiratory viral infection (ARVI) pathogens (the "gold standard") remains their cultivation on cell cultures. Specific, highly sensitive tissues are selected for each taxonomic group (e.g., HeLa cells or embryonic kidney cells for adenoviruses; tracheal cells for coronaviruses).
The presence of a virus in the culture is judged by the development of the cytopathic effect (CPE). However, relying solely on CPE is not recommended: this sign is not always pathognomonic (specific), which complicates accurate visual identification. To reliably determine the pathogen, virologists use the neutralization test (NT) in cell culture. Its essence lies in adding type-specific antibodies: if they successfully suppress the cytolytic action of the virus, the serotype is considered proven.
Briefly: answers to topical questions
Which surface antigens determine the type-specificity of paramyxoviruses and adenoviruses?
- Type-specific antigens allow for the identification of viruses within a genus, i.e., determining the serotype.
- For paramyxoviruses, the sources describe surface envelope glycoproteins: G — mediates adhesion, F — mediates membrane fusion and syncytium formation; for the mumps virus, the spikes HN and F are indicated, as well as the transmembrane protein SH.
- For adenoviruses, sources confirm the presence of type-specific immunity and serotypes, but the specific surface antigens determining type-specificity are not named in the cited sources.
What laboratory diagnostic methods are used to detect ARVI pathogens?
- Virological method — isolation of pathogens on cell culture with evaluation of the cytopathic effect and subsequent identification via neutralization test, DFA, or ELISA.
- Serological method — examination of paired sera using HI test, CFT, NT, and IHA to detect an increase in antibody titers.
- Molecular genetic method — nucleic acid amplification via PCR and LAMP to detect genetic material of pathogens in nasopharyngeal and oropharyngeal swabs.
Which cellular receptors do rhinoviruses use for adsorption onto the epithelium?
- Intercellular adhesion molecule 1 (intercellular adhesion molecule 1) — a receptor of molecular nature used by rhinoviruses to bind to epithelial cells, fibroblasts, and endothelium.
- Capsid proteins — structural elements of the virus that specifically bind to the ICAM-1 adhesion receptor to penetrate sensitive cells and fibroblasts.
What specific complications does respiratory syncytial virus cause in young children?
- Lower respiratory tract involvement — for RSV, the sources indicate involvement of both the upper and lower respiratory tracts.
- Possible related manifestations: bronchitis, bronchiolitis, bronchial obstruction syndrome.
- Destruction of the respiratory epithelium during acute respiratory infections creates conditions for the onset of a secondary infection.
To easily remember simple (non-enveloped) ARVI viruses, use the mnemonic: Adenoviruses, rhinoviruses, reoviruses. All of them lack a supercapsid, have an icosahedral shape, and exit the cell "abruptly" via explosive lysis.
Class and exam questions
Can the measles virus or mumps virus be considered ARVI pathogens?
No. Although these viruses are transmitted via airborne droplets and also affect the respiratory tract, true ARVI pathogens are considered to be only those whose primary replication occurs exclusively in the respiratory epithelium.
Why are enveloped ARVI viruses destroyed by ether?
Enveloped viruses (e.g., paramyxoviruses and coronaviruses) have an outer supercapsid consisting of host cell membrane lipids. Ether dissolves the lipids, completely inactivating the viral particle.
What is the essence of the neutralization test (NT) in the diagnosis of ARVI?
The NT is used for precise identification of the virus serotype. Specific antibodies are added to the culture of infected cells; if they neutralize the virus and prevent cell death (cytolytic action), it means the antibodies correspond to the given pathogen.
What else to study on this topic
- Cell lines in virology: HeLa cells and embryonic cultures
- Pathogenesis of ARVI: development of intoxication syndrome and risk of bacterial complications
- Mechanisms of hemagglutination and application of the HI test in laboratory practice
- Cytopathic effect (CPE): types of degenerative changes in cells
- Virus budding process: formation of the supercapsid
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