Etiology: properties of the influenza virus
The infectious agent is the influenza virus (Influenzavirus). There are three main serotypes of the virus: A, B, and C.
A key role in characterizing the pathogen is played by its surface antigens — neuraminidase (N) and hemagglutinin (H). The virus possesses a unique antigenic variability provided by recombination and reassortment processes in the genome.
The evolution of the virus is realized through two main mechanisms that must be considered in the epidemic process:
- Antigenic "drift" — gradual point mutations.
- Antigenic "shift" — radical genome alteration with the emergence of new subtypes.
It is precisely due to these changes that new strains emerge capable of rapidly spreading worldwide, such as the current pandemic strain of the pH1N1 influenza virus.
Features of the epidemiological process
Acute respiratory infections (ARIs) rank first in the global structure of infectious morbidity, and the proportion of influenza among them exceeds 40%. Due to viral mutations (predominantly group A), morbidity regularly takes on the character of epidemics and pandemics. The socio-demographic consequences of such outbreaks are colossal: a significant part of the working-age population is incapacitated.
Traditional risk groups for severe disease and high mortality include:
- Older age groups.
- Patients with comorbid chronic cardiovascular and respiratory diseases.
- Children (high mortality rates are often registered among them).
In clinical practice, collecting an epidemiological history is critically important for making a diagnosis of "Influenza". The development of the disease during the epidemic peak season or contact with sick individuals (e.g., outbreaks in dormitories) are compelling arguments for hospitalizing the patient in an infectious diseases hospital.
H1N1 influenza pandemic (2009)
In April 2009, a new threat emerged — the H1N1 subtype influenza virus. Initially, it was called "swine" or Mexican flu because the first cases were registered at a pig farm in New Mexico.
The infection spread at an extraordinary speed: while on April 29 the virus was present in only 9 countries, by July 1 it had already swept 120 states. This prompted WHO to announce the onset of the first influenza pandemic of the 21st century. Globally, between 17.4 and 18.5 thousand deaths from laboratory-confirmed pH1N1 influenza were recorded.
Specifics of the H1N1 pandemic strain
The 2009 pandemic demonstrated a number of features atypical for classical influenza:
- Seasonality shift. The rise in morbidity began abnormally early — in September-October instead of the traditional December-January.
- Change in age structure. The virus involved all ages, but most frequently affected children and young adults.
- New risk groups for fatal outcome. Pregnant women and patients with metabolic syndrome were added to them. Particularly high mortality was noted in the under-50 age group (among people born after 1957).
- Clinical severity. The virus was distinguished by a more frequent initial involvement of the lower respiratory tract. This led to severe complications in young people and children — the development of progressive pneumonia and acute respiratory distress syndrome (ARDS).
Briefly: answers to topical questions
To which virus family does the influenza pathogen belong?
The influenza pathogen belongs to the family Orthomyxoviridae (orthomyxoviruses). These are complex, RNA-containing pneumotropic viruses possessing a supercapsid.
The family pathogenic to humans includes three main genera:
- Influenza A virus (Influenzavirus A) — possesses zoonotic potential and presents the greatest epidemiological hazard.
- Influenza B virus (Influenzavirus B).
- Influenza C virus (Influenzavirus C).
The family name originates from the Greek words orthos (straight, correct) and myxa (mucus), which reflects the ability of viruses to adsorb onto the membrane of mucus-producing epithelial cells of the respiratory tract.
What are the main mechanisms and routes of transmission of the influenza virus?
The primary transmission mechanism of the influenza virus is aerosol.
Routes of transmission:
- Airborne droplet route.
- Contact-household route — the possibility of this transmission route remains debatable.
The source of infection is a sick person on the 4th–5th day of illness. Under specific conditions, the source can be mammals, in particular pigs, and birds.
The portal of entry for the infection is the mucous membrane throughout the entire respiratory tract.
What is the structure of the influenza virus genome?
The influenza virus genome is represented by a single-stranded segmented minus-sense RNA helix. Such structural segmentation predisposes to genetic recombinations (exchange of genome segments), which is the main cause of the pathogen's antigenic variability.
Structural features of the genome depend on the virus type:
- Types A and B viruses — consist of 8 RNA segments.
- Type C virus — consists of 7 RNA segments.
A distinctive feature of coding is that each RNA segment encodes the synthesis of strictly one protein, therefore the number of proteins produced in the virion equals the number of genomic segments.
The surface antigens of the influenza virus are easy to remember by the letters in the strain names (e.g., H1N1): H — hemagglutinin, N — neuraminidase.
Class and exam questions
Which type of influenza virus most frequently causes pandemics?
The most frequent etiological factor of global epidemics and pandemics is the influenza A virus.
What are antigenic "shift" and "drift"?
These are phenomena of antigenic variability of the influenza virus. They ensure the emergence of new strains to which the population has no established immunity.
What was the main clinical feature of the 2009 H1N1 pandemic?
The virus initially affected the lower respiratory tract, which often led to severe pneumonia and ARDS in children and young adults.
What else to study on this topic
- Pathogenesis of the infection: cytopathic effect of the virus, viremia, and mechanisms of intoxication.
- Leading clinical syndromes: intoxication, catarrhal, hemorrhagic.
- Complications of influenza: infectious-toxic shock, hemorrhagic pulmonary edema, cerebral edema.
- Laboratory diagnostics: rapid methods and mandatory testing of paired sera (CF, HI).
- Differential diagnostics with parainfluenza, adenovirus, rhinovirus, and RSV infection.
Master the complete topic in the «Infectious Diseases» course
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