What causes the flu?

Causes of flu

Influenza pathogens are RNA-containing viruses from the orthomyxovirus family, 80-120 nm in diameter. They have a lipoprotein membrane formed by the glycoproteins neuraminidase (N) and hemagglutinin (H). According to the nucleoprotein (NP) and matrix (M) protein, influenza viruses are classified into 3 antigenically independent types A, B and C. According to the antigenic variants of glycoproteins H and N, subtypes of influenza A virus are distinguished. Currently, 16 subtypes of hemagglutinin (HI-H16) and 12 subtypes of neuraminidase (N1-N12) are known; virus strains are usually designated by a short antigenic formula: H1N1, H2N1, H3N2, etc.

In humans, the disease is caused mainly by viruses with the hemagglutinin subtypes HI, H2, H3 and neuraminidase - N1, N2. In recent years, cases of the disease caused by avian influenza viruses with the antigenic formula H5N13, H7N7 have been registered. Variations in the structure of hemagglutinin and neuraminidase lead to the emergence of new serological variants of the influenza virus. Changes in the antigenic structure can occur by antigenic drift or antigenic shift. Antigenic drift is minor changes in the structure of the antigen (usually hemagglutinin), caused by point mutations. With antigenic shift, a complete replacement of the hemagglutinin and / or neuraminidase subtype (much less often) occurs, probably due to genetic recombination between strains of the human and animal viruses. Influenza virus type B is much less variable, and the type C virus is characterized by constancy of the antigenic structure.

Pathogenesis of influenza

In the pathogenesis of influenza, the epitheliotropic and general toxic effects of the virus are of primary importance. Getting into the mucous membranes of the upper respiratory tract by airborne dust or droplets with small aerosol particles, the virus penetrates into the ciliated epithelial cells, where it reproduces. Reproduction of the virus in epithelial cells leads to their dystrophy, necrosis and desquamation. In addition to the ciliated epithelium, the influenza virus can affect alveolar macrophages, goblet cells and alveolocytes. Through damaged epithelial barriers, the virus and tissue decay products penetrate into the bloodstream, exerting a general toxic effect.

Hemodynamic disorders occur in various organs and systems, which is the leading link in the pathogenesis of severe forms of the disease. Expressed circulatory disorders in the central nervous system lead to encephalopathy, in the lungs - to segmental or widespread hemorrhagic edema, in the mesentery - to abdominal syndrome, etc.

Epidemiology of influenza

Influenza is a widespread infection with epidemic and pandemic morbidity. In the interepidemic period, morbidity is maintained by sporadic cases and local outbreaks. During an epidemic/pandemic, natural immunization of the majority of the population and a decrease in the susceptible population occur, which leads to a rapid decrease in morbidity. The most widespread diseases are associated with type A virus, type B virus usually causes local outbreaks, and type C virus - sporadic cases. Influenza epidemics occur almost annually and are caused by the emergence of new virus strains due to antigenic drift. Influenza pandemics are relatively rare, occurring every 10-20 years or more, and are associated with a change in the virus subtype due to antigenic shift.

The source of infection is a sick person who releases the virus into the environment from the end of the incubation period and throughout the febrile period. After the 5th-7th day of illness, the concentration of the virus in the exhaled air decreases sharply, and the patient poses practically no danger to others. Patients with latent and subclinical forms of the disease who continue to lead an active lifestyle and can infect a large number of people pose a significant epidemiological danger. Influenza viruses are unstable in the environment and are sensitive to high temperatures, drying, and insolation. They also quickly die when exposed to disinfectant solutions.

The infection is transmitted by airborne droplets. In rare cases, the infection can be transmitted through household items (pacifiers, toys, underwear, dishes, etc.). Susceptibility to influenza is universal. Children in the first months of life are relatively resistant to influenza, which is associated with passive immunity received from the mother (even newborns can get sick if the mother does not have protective antibodies). After the flu, a persistent type-specific immunity is formed, repeated diseases are caused by infection with a new serovar of the virus.

Bird flu virus infection occurs in people who care for poultry. In such cases, several people may become infected; family outbreaks have been described with children becoming infected, and the disease was extremely severe. Bird flu virus does not spread from person to person. Chickens, ducks, and turkeys are the direct source of infection for people; viruses are introduced into households mainly by wild waterfowl.