Classification and Taxonomy
The family Orthomyxoviridae derives its name from the Greek words orthos (meaning "straight") and myxa (translating to "mucus"). Structurally, they are complex viruses: an additional membrane called a supercapsid surrounds their nucleocapsid. They contain RNA as their genetic material.
Within the family, several prominent genera are distinguished, differing by their hosts and transmission routes:
- Thogotovirus are arboviruses transmitted by ticks.
- Isavirus is a genus comprising fish pathogens (specifically causing infectious salmon anemia).
- Human pathogens are represented by three genera: Influenzavirus A, Influenzavirus B, and Influenzavirus C.
Features of Influenza A Virus
Of all family members, the genus Influenzavirus A poses the greatest threat to public health. This pathogen possesses three critical characteristics that make it a focus of intense study for epidemiologists worldwide.
- Anthropozoonotic potential. The virus is capable of infecting not only humans but also various animal species, establishing a vast natural reservoir of infection.
- High antigenic diversity. The pathogen constantly alters its structure, hindering the establishment of robust and long-lasting population immunity.
- Epidemiological hazard. Due to its variability and ability to cross species barriers, this virus type regularly triggers large-scale epidemics.
Characteristics of Influenza as a Disease
Influenza is an acute infectious viral disease. The very origins of the terms denoting this illness aptly describe its nature. The French word grippe translates to "to seize" or "to scratch," pointing to a sudden, sharp onset of symptoms. The historical name influenza originates from Italian and translates to "influence."
The clinical presentation of the disease consists of a characteristic triad of manifestations:
- Involvement of respiratory tract tissues (the respiratory system).
- High fever accompanied by pronounced general intoxication of the body.
- Disruption of normal cardiovascular and nervous system functions.
History of Discovery and Isolation
Before the era of scientific virology, symptoms characteristic of influenza were already documented in pre-scientific times. Descriptions of similar epidemics can be found in the works of Hippocrates and the Roman historian Titus Livius.
The chronology of the direct isolation of influenza viruses is as follows:
- 1933 (United Kingdom): Researchers W. Smith, C. Andrewes, and P. Laidlaw isolated the human influenza virus for the first time in history. They used nasopharyngeal washings from patients to infect laboratory ferrets. This isolate was subsequently classified as type A.
- 1936 (USSR): Type A virus was successfully isolated by Soviet scientists — A.A. Smorodintsev in Leningrad and L.A. Zilber in Moscow.
- 1940: T. Francis and T. Magill discovered the influenza B pathogen.
- 1947: R. Taylor first isolated the influenza C virus.
Briefly: answers to topical questions
Which influenza virus structures are responsible for antigenic variability and adhesion?
The surface protein hemagglutinin is responsible for the adhesion of the influenza virus to susceptible cells, whereas the exact structures determining antigenic variability are not specified in the sources. The surface structures of the virion include:
- Hemagglutinin (HA) — a trimer that performs attachment and fusion functions. The large HA1 subunit is responsible for adsorption to specific receptors.
- Neuraminidase (NA) — a tetramer that modifies receptors for viral entry via endocytosis.
Influenza A virus is characterized by significant antigenic diversity, although the structural basis of this phenomenon is not detailed.
What are the main stages of influenza virus replication in the host cell?
The main stages of influenza replication encompass sequential processes from adsorption to the release of new virions, with the complete cycle taking 5–6 hours.
- Adsorption — binding of hemagglutinin to cellular receptors.
- Penetration — occurs via endocytosis after receptor modification by neuraminidase.
- Uncoating — release of the viral genome.
- Component synthesis — nucleic acid replication and protein synthesis (unlike most RNA viruses, the influenza virus replicates in the nucleus rather than the cytoplasm).
- Assembly — formation of progeny virions.
- Release — exit from the cell, in which neuraminidase participates by preventing virion aggregation.
Influenza virus types were discovered strictly in alphabetical order: A in 1933, B in 1940, and C in 1947.
Class and exam questions
Which genera of orthomyxoviruses cause disease in humans?
Three genera are pathogenic to humans: Influenzavirus A, Influenzavirus B, and Influenzavirus C.
Why is the influenza A virus considered the most dangerous?
It exhibits significant antigenic diversity and can infect both humans and animals (possessing an anthropozoonotic potential). This accounts for its maximal epidemiological hazard.
What constitutes the clinical triad in influenza?
The triad includes respiratory tract involvement, fever with general intoxication, and dysfunction of the nervous and cardiovascular systems.
What else to study on this topic
- Mechanisms of Antigenic Diversity Formation in Influenza A Virus
- Structure of the Supercapsid in Complex Viruses
- Life Cycle of the Influenza Virus in the Host Cell
- Pathogenesis of the Intoxication Syndrome in Influenza
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