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What causes the flu?
Medical expert of the article
Last reviewed: 20.11.2021
Causes of influenza
Influenza pathogens are RNA-containing viruses from the family of orthomixoviruses, 80-120 nm in diameter. They have a lipoprotein coat formed by glycoproteins with 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. Antigenic variants of the glycoproteins H and N are subtyped by the influenza A virus. Currently, 16 hemagglutinin subtypes (HI-H16 ) and 12 subtypes of neuraminidase (N1-N12), virus strains are usually denoted by a short antigenic formula: H1N1, H2N1, H3N2, etc.
The disease in humans is caused mainly by viruses with subtypes of hemagglutinin HI, H2, NS and neuraminidase - N1, N2. In recent years, cases of disease caused by avian influenza viruses with the antigenic formula H5N13, H7N7 have been reported. Varying the structure of hemagglutinin and neuraminidase results in the appearance of new serological variants of the influenza virus. Changes in the antigenic structure can occur through antigenic drift or antigenic shift. Antigenic drift - minor changes in the structure of the antigen (more often hemagglutinin), caused by point mutations. Under the antigenic shifter, the hemagglutinin and / or neuraminidase subtype is completely replaced (significantly less often), probably due to genetic recombination between human and animal virus strains. The influenza type B virus is much less variable, and for the type C virus, the antigenic structure is constant.
The pathogenesis of influenza
In the pathogenesis of influenza, the epitheliotropic and general toxic effects of the virus are of paramount importance. Getting airborne or dusty with small aerosol particles on the mucous membranes of the upper respiratory tract, the virus enters the ciliated epithelial cells, where it is reproduced. Reproduction of the virus in epithelial cells leads to their dystrophy, necrosis and desquamation. In addition to 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, having 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 the phenomena of encephalopathy, in the lungs to segmental or widespread hemorrhagic edema, in the mesentery to abdominal syndrome, etc.
Epidemiology of influenza
Influenza is a pervasive infection with epidemic and pandemic diseases. In the interepidemic period, the incidence is supported by sporadic cases and local outbreaks. In the course of the epidemic / pandemic, natural immunization of the majority of the population occurs and a decrease in the susceptible population, which leads to a rapid decrease in the incidence. The most common diseases are associated with type A virus, type B virus usually causes local outbreaks, and type C virus sporadic cases. Influenza epidemics occur almost every year and are caused by the appearance of new strains of the virus due to antigenic drift. Influenza pandemics are relatively rare, after 10-20 years or more, and are associated with a change in the virus subtype due to antigenic seizure.
The source of infection is a sick person who secretes the virus into the external environment from the end of the incubation period and the entire febrile period. After the 5th-7th day of illness, the concentration of the virus in the exhaled air drops sharply, and the patient practically does not pose a danger to others. Significant epidemiological danger is represented by patients with erased and subclinical forms of the disease, which continue to lead an active lifestyle and can infect a large number of people. Influenza viruses are unstable in the environment and are sensitive to the effects of high temperatures, drying, insolation. They also quickly die with the action of disinfectant solutions.
Transmission of infection is carried out by airborne droplets. In rare cases, transmission of infection through household items (nipples, toys, linens, dishes, etc.) is possible. Susceptibility to the flu is universal. Children of the first months of life are relatively resistant to influenza, which is associated with passive immunity received from the mother (in the absence of protective antibodies in the mother, even newborns may be sick). After the influenza, persistent type-specific immunity is formed, repeated diseases are caused by infection with a new serovar of the virus.
Infection with the avian influenza virus occurs in persons caring for poultry. In such cases, several people may be infected, family outbreaks with infection of children are described, in which the disease was extremely difficult. The transmission of the bird flu virus from person to person does not occur. An immediate source of infection for humans are chickens, ducks, turkeys; The introduction of viruses into households is carried out mainly by wild waterfowl.