Antigenic structure. Typical Ags of influenza A viruses are hemagglutinin and neuraminidase; The classification of influenza viruses is based on the combination of these proteins. Influenza viruses and influenza Clinical forms of influenza
Anergy clonal– the state of functional unresponsiveness of individual lymphocyte clones to specific antigens.
Antigen-antibody complex (immune complex)– product of the antigen-antibody reaction. It is of great importance in the pathogenesis of many diseases.
Antigen-antibody reaction– specific interaction of an antigen with the corresponding antibody.
Antigenic variability– change in specific surface antigens of an organism within a biological species. It manifests itself most intensely in influenza viruses.
Antigenic modulation– disappearance of surface antigens under the influence of antibodies.
Antigen specificity– structural features that distinguish a specific antigen from the individual, antigenic composition of the immunized organism; antigen specificity does not imply the ability of carriers of this specificity to cause an immune response; haptens have antigen specificity, reacting with pre-existing antibodies, but cannot themselves cause their formation. Antigen specificity gives an antigen the ability to selectively react with specific antibodies or sensitized lymphocytes.
Antigenicity– the ability of a substance to specifically interact with products of the immune response.
Antigenic determinants (epitopes)- specific sections of the antigen molecule to which specific antibodies are produced and with which the products of the immune response react.
Antigenic drift– a gradual change in the antigenic specificity of viral structures (HA and NA), occurring over several years, caused by spontaneous point mutations.
Antigenic shift– changes in the entire antigenic structure of hemagglutinin or neuraminidase. This process leads to the emergence of new subtypes of viruses. Antigenic shift is based on mechanisms of genetic recombination between individual subtypes of viruses.
Antigen presenting cells– highly specialized cells capable of absorbing and processing antigen, as well as presenting peptide antigenic fragments on the cell surface in combination with molecules of MHC classes I or II; main antigen-presenting cells: macrophages, dendritic cells, B-lymphocytes.
Antigen recognition B-cell receptor (surface immunoglobulin - sIg)– surface monomeric form of immunoglobulin belonging to the IgM class; is able to interact with a free antigen that is not associated with any additional molecules.
Antigen recognition T cell receptor (TCR)-heterodimer expressed on the surface of T cells in complex with single-domain C3 proteins; the main function is the recognition of an immunogen (antigenic peptide + molecules of MHC classes I or II) on the surface of an antigen-presenting or virus-infected cell.
Antigens– substances that induce an immune response.
Histocompatibility antigens (HLA) are antigens encoded by the major histocompatibility complex. Histocompatibility antigens contribute to the recognition of foreign antigens, play a decisive role in the cooperation of immunocompetent cells in the development of humoral and cellular immunity, and are the main structures in the implementation of transplantation immunity reactions. They are represented by molecules of two classes.
Synthetic antigens– artificially synthesized antigens.
T-dependent antigens– antigens, the development of an immune response to which requires the participation of helper T-lymphocytes.
Antigens T-independent– antigens, the development of an immune response to which does not require the participation of helper T-lymphocytes.
Antiserum– serum containing specific antibodies.
Antibodies– immune proteins that are formed in the body in response to the arrival of an antigen and have the ability to specifically interact with it.
Immune antibodies– antibodies produced by the body as a result of an infection or produced in response to immunization with any antigen.
Monoclonal antibodies– antibodies produced by one clone of antibody producers. These are antibodies of the same class, subclass and specificity. Antibodies usually produced in the body in response to antigenic stimulation are the products of the activity of several clones of antibody producers.
Antibodies are normal– antibodies contained in the serum of healthy individuals, the production of which is not associated with previous infection or any immunization.
Anticoagulants are drugs for the prevention of blood clots in the insular system during atrial fibrillation.
Antibody-dependent cellular cytotoxicity (ADCC)– destruction of target cells coated with antibodies by effector cells having an Fc receptor.
The influenza virus has two antigenic complexes:
· S antigen(soluble, from lat. solution– dissolve) is represented by nucleocapsid proteins, is type-specific, stable, non-infectious ( NP protein capable of fixing complement, therefore detected in RSC).
· V antigen(from lat. viral– viral) – strain-specific, consists of hemagglutinin and neuraminidase, located on spines, determines virulence (detected in RTGA).
Variability of viruses flu .
The internal structures of the virus are shielded from the action of the external environment and do not change. Variability is inherent in supercapsid antigens, and hemagglutinins and neuraminidase change independently of each other due to 2 genetic mechanisms - drift And shift.
Antigenic drift(from English drift– slow course) causes minor changes caused by a point mutation, mostly in the structure of hemagglutinin. This leads to the development of strain differences that do not go beyond the subtype. As a result of antigenic drift, epidemics(frequency – every 1-3 years).
Shift(from English shift-leap) is a complete replacement of a gene, which leads to the appearance of a new antigenic variant of the virus. It is believed that shift is the result of genetic recombination, i.e. exchange of genetic information between human and animal viruses entering the same cell, which leads to a change in subtype H or N (and sometimes both). This variability may lead to the emergence of new viral variants that can cause pandemic(frequency – every 10-20-40 years).
Influenza viruses B and C lack shift variability, therefore influenza B virus causes epidemics, A influenza C virus– sporadic diseases or small flashes.
Features of virus reproduction.
1. Adsorption on the receptors of sensitive cells containing sialic acid with the help of hemagglutinins.
2. Penetration into the cell by receptor endocytosis, followed by fusion of the virus membranes with the wall of the cell vacuole and the formation of an endosome.
3. Deproteinization: the virus is first freed from the supercapsid, then from the capsid proteins.
4. Eclipse phase (NA replication and viral protein synthesis): viral RNA penetrates the cell cytoplasm, then into the nucleus, where the product necessary for transcription and translation is present. This is where RNA is synthesized. Capsid proteins NP, P1, P2, P3 and M are synthesized in the cytoplasm on ribosomes.
5. Nucleocapsid assembly occurs in the cytoplasm of the cell (RNA and viral proteins recognize each other and self-assemble).
6. Exit from the cell is carried out by budding or explosion (lysis), while a supercapsid is formed from the cytoplasmic membrane of the cell.
Influenza A/H1N1 as a typical emerging infection: General characteristics of influenza viruses, variability, emergence of new pandemic strains
Influenza viruses - RNA viruses - belong to the family. Orthomyxoviridae and are divided into viruses A, B and C (Table 1).
Table 1.
Comparative characteristics of influenza viruses
| Criteria | Type A | Type B | Type C |
| Severity of the disease | ++++ | ++ | + |
| Natural reservoir | Eat | No | No |
| Human pandemics | Calls | Doesn't call | Doesn't call |
| Human epidemics | Calls | Calls | Does not cause (only sporadic diseases) |
| Antigenic changes | Shift, drift | Drifting | Drifting |
| Segmented genome | Yes | Yes | Yes |
| Sensitivity to rimantadine | Sensitive | Not sensitive | Not sensitive |
| Sensitivity to zanamivir | Sensitive | Sensitive | - |
| Surface glycoproteins | 2 (HA, NA) | 2 (HA, NA) | 1(HA) |
The influenza virus has a spherical shape and size of 80-120 nm. The core is a single-stranded negative strand of RNA, consisting of 8 fragments that encode 11 viral proteins.
Influenza A viruses are widespread in nature and infect both humans and a wide range of mammals and birds. Influenza viruses types B and C have been isolated only from humans.
Epidemially significant are 2 subtypes of influenza A virus - H3N2 and H1N1 and influenza virus type B (A.A. Sominova et al., 1997; O.M. Litvinova et al., 2001). The result of such co-circulation was the development of influenza epidemics of various etiologies in different countries during the same epidemic season. The heterogeneity of the population of epidemic viruses also increases due to the divergent nature of the variability of influenza viruses, which leads to the simultaneous circulation of viruses belonging to different evolutionary branches (O.M. Litvinova et al., 2001). Under these conditions, prerequisites are created for the simultaneous infection of humans by various pathogens, which leads to the formation of mixed populations and reassortment both between viruses of co-circulating subtypes and among strains within the same subtype (O.I. Kiselev et al., 2000).
The classification of influenza virus types is based on antigenic differences between two surface glycoproteins - hemagglutinin (HA) and neuraminidase (NA). According to this classification, influenza viruses are divided into 3 types - influenza viruses type A, type B and type C. There are 16 HA subtypes and 9 NA subtypes.
Rice. 1. Classification of influenza A viruses and types of animals and birds - intermediate and final hosts in the chain of transmission of infection to humans.
Subtype 16 (H16) of hemagglutinin was recently discovered
Note: ∗ NA 7 and NA 7-NA8 were also detected in horses
In Fig. 1 shows the subtypes of influenza A viruses and their intermediate hosts and natural reservoirs (migratory birds). The main hosts of influenza A viruses include those species that are associated with influenza.
In the human population, only three subtypes of influenza A viruses have been identified so far: HA1, HA2 and HA3. Moreover, viruses contain only two types of neuraminidase - NA1 and NA2 (Fig. 1). Their stable circulation has been proven over the past century, starting with the 1918 pandemic (R.G. Webster et al., 1978; K.G. Nicholson et al., 2003).
Influenza A viruses (to a lesser extent B) have the ability to change the structure of NA and NA. The influenza A virus is characterized by two types of variability:
- point mutations in the viral genome with a corresponding change in HA and NA (antigenic drift);
- complete replacement of one or both surface glycoproteins (NA and NA) of the virus through reassortment/recombination (antigenic shift), as a result of which a fundamentally new variant of the virus appears that can cause influenza pandemics.
For influenza B virus, antigenic variability is limited only by drift, because it apparently has no natural reservoir among birds and animals. The influenza C virus is characterized by greater stability of the antigenic structure and only local outbreaks and sporadic cases of the disease are associated with it.
Of some interest emergence of new strains of influenza virus in the human population and associated pandemics (Fig. 2). In Fig. Figure 2 presents the main antigenic shifts associated with pan-epidmias of the twentieth century caused by influenza A viruses:
- in 1918, the pandemic was caused by the H1N1 virus;
- in 1957 - H2N2 strain A/Singapore/1/57;
- in 1968 - H3N2 strain A/Hong Kong/1/68;
- in 1977 - H1N1 strain A/USSR/1/77 (many scientists did not consider this as a pandemic, but with the appearance of this strain, a situation arose with the simultaneous co-circulation of 2 strains of influenza A virus - H3N2 and H1N1).
In 1986, in China, the A/Taiwan/1/86 virus caused a widespread epidemic of influenza A/H1N1, which lasted until 1989. Drift variants of this virus survived until 1995, causing local outbreaks and sporadic cases of the disease. According to the results of molecular biological studies, multiple mutations arose in the genome of the A/H1N1 virus during these years. In 1996, two antigenic variants of the A/H1N1 influenza virus appeared: A/Bern and A/Beijing, their feature was not only antigenic, but also geographic disunity. Thus, in Russia, the influenza A/Bern virus took an active part in the influenza epidemic of 1997-98. During the same season, circulation of strains of the A/Beijing virus was registered in the east of the country. Subsequently, in 2000-2001. influenza A/H1N1 virus became the causative agent of the influenza epidemic in Russia. Modern influenza A/H1N1 viruses have low immunogenic activity; fresh isolated virus isolates interact only with the erythrocytes of mammals (human group 0 and guinea pigs).
Rice. 2. The emergence of new strains of influenza virus in the human population and associated pandemics
Influenza A viruses have undergone significant genetic changes over the past century, resulting in global pandemics with high mortality rates in humans. The largest influenza pandemic (H1N1) was in 1918-1919. ("Spaniard"). The virus, which appeared in 1918, has undergone a pronounced drift; its initial (Hsw1N1) and final (H1N1) variants are considered shift. The virus caused a devastating epidemic that claimed 20 million lives (half of the dead were young people aged 20 to 50 years (M.T. Osterholm, 2005).
Research by J.K. Tanbenberger et al., (2005) showed that the virus that caused the 1918 pandemic was not a reassortant between the avian influenza virus and the human influenza virus - all 8 genes of the H1N1 virus were more similar to variants of the avian virus than to the human one (Fig. .3). Therefore, according to R.B. Belshe (2005) avian influenza virus must infect (bypassing the intermediate host) humans, transmitted from person to person.
It is important to note that avian influenza viruses “participate” in the emergence of new “human” influenza viruses, which are characterized by high pathogenicity and the ability to cause pandemics (E.G. Deeva, 2008). These viruses (H1N1, H2N2 and H3N2) had a different set of internal genes, the origin of which indicates their phylogenetic relationship with avian and swine viruses.
What are the mechanisms of origin of pandemic strains and what biological characteristics are necessary for the emergence of a highly pathogenic virus with pandemic potential?
Influenza A viruses are characterized by a high frequency of occurrence of reassortants as a result of mixed infection, which is due to the segmentation of the viral genome. The predominance of a reassortant of a certain gene composition is considered the result of selection, in which from an extensive set of different reassortants the one that is most adapted to reproduction under given conditions is selected (N.L. Varich et al., 2009). Strain-specific properties of genomic segments can have a strong influence on the gene composition of reassortants under non-selective conditions. In other words, a distinctive feature of influenza viruses is that frequent and unpredictable mutations occur in eight of the gene segments, especially the HA gene. Reassortment plays an important role in the emergence of new viral variants, particularly in the origin of pandemic strains. And sometimes the possibility of a virus with higher virulence emerging during a pandemic cannot be ruled out.
Modern research has shown that the gene structure of the new A/H1N1 virus is complex and, as we noted in the introduction, its composition includes the genes of swine flu that affects pigs in North America; genes for swine flu, which affects pigs in Europe and Asia; avian influenza genes; human influenza genes. Essentially, the genes for the new virus come from four different sources. A micrograph of the influenza A/H1N1 virus is shown in Fig. 4.
Rice. 4. Microphotograph of influenza A/H1N1 virus
WHO published “Guidelines for influenza laboratories” and presented new data on the viral gene sequence and their length of the reassortant new influenza A/H1N1 virus (isolate A/California/04/2009): HA, NA, M, PB1, PB2, RA, NP, NS. These data indicate the formation of a new pandemic variant of the virus, creating universal vulnerability to infection due to the lack of immunity. It is becoming clear that pandemic variants of the influenza virus arise through at least two mechanisms:
- reassortment between animal/avian and human influenza viruses;
- direct adaptation of the animal/avian virus to humans.
To understand the origin of pandemic influenza viruses, it is important to study the properties of the natural reservoir of infection and the evolutionary paths of this family of viruses when changing hosts. It is already well known and can be argued that waterfowl are a natural reservoir of influenza A viruses (adapted to these intermediate hosts for many centuries), as evidenced by the carriage of all 16 HA subtypes of this virus. Through bird feces, which can survive in water for more than 400 days (Bird flu..., 2005), viruses can be transmitted to other animal species when drinking water from a reservoir. (K. G. Nicholson et al., 2003). This is confirmed by phylogenetic analysis of nucleic acid sequences of different subtypes of influenza A viruses from different hosts and from different geographical regions.
Analysis of nucleoprotein gene sequences showed that avian influenza viruses evolved with the emergence of 5 specific host lineages: viruses of wild and domestic horses, gulls, pigs and humans. Moreover (!) the human and swine influenza viruses form a so-called sister group, which indicates their close relationship and, naturally, a common origin. The predecessor of the human influenza viruses and the classic swine virus appear to have been entirely of avian origin. In the countries of Central Asia, for known reasons, pork is not popular, and these animals are practically absent from livestock farming. This leads to the fact that (unlike China, for example), this region does not have the main intermediate host in the domestic animal population - pigs, therefore the probability of the “emergence” of pandemic viruses in the Central Asian region is lower than in China, which practically follows from data on the analysis of their origin (Avian influenza, 2005). A permanent source of genes for pandemic influenza viruses exists (in a phenotypically unchanged state) in the natural reservoir of viruses of waterfowl and migratory birds (R.G. Welster, 1998). It should be borne in mind that the predecessors of the viruses that caused the Spanish flu pandemic (1918), as well as the viruses that were the source of the genome of the Asia/57 and Hong Kong/68 pandemic strains, still circulate among the wild bird population with minor mutational changes (Influenza birds..., 2005).
Comments
(visible only to specialists verified by the MEDI RU editorial team)
The first mention of influenza was noted many centuries ago - back in 412 BC
AD a description of the influenza-like illness was made by Hippocrates. Also
influenza-like outbreaks were noted in 1173. First documented
a flu pandemic that has killed many
lives, happened in 1580.
In 1889-1891, a moderate pandemic occurred, caused by a virus of the H3N2 type.
The infamous "Spanish Flu" caused by the H1N1 virus occurred in 1918-1920.
This is the worst known pandemic
Taking more than 20 million lives. From "Spanish flu"
20-40% of the world's population was seriously affected. Death was extremely
fast. A person could still be absolutely healthy in the morning, but by noon he would fall ill and
died by nightfall. Those who did not die in the first days often died from complications,
caused by influenza, such as pneumonia. An unusual feature of the "Spanish flu" was
that it often affected young people (usually influenza primarily
children and the elderly suffer).
The causative agent of the disease, the influenza virus, was discovered by Richard Shope in 1931.
The influenza A virus was first identified by English virologists Smith,
Andrews and Laidlaw (National Institute for Medical Research, London) in 1933
year. Three years later, Francis isolated the influenza B virus.
In 1940, an important discovery was made - the influenza virus could be
cultured on chicken embryos. Thanks to this, new
opportunities for studying the influenza virus.
Influenza C virus was first isolated by Taylor in 1947.
There was a pandemic in 1957-1958
Which was called the "Asian flu", caused by the H2N2 virus. Pandemic
began in February 1957 in the Far East and quickly
spread throughout the world. In the US alone, people died during this pandemic.
more than 70,000 people.
In 1968-1969, a moderately severe "Hong Kong flu" occurred, caused by
H3N2 virus. The pandemic began in Hong Kong in early 1968. Most often
The virus affected older people over 65 years of age. Total number
The death toll from this pandemic was 33,800.
A relatively mild pandemic occurred in 1977-1978
Called the "Russian" flu. Influenza virus (H1N1) that caused this pandemic
already caused an epidemic in the 50s.
Therefore, those born after 1950 were the first to suffer.
Influenza pathogens belong to the orthomyxovirus family, which includes 3 genera of viruses influenza: A, B, C. Influenza viruses contain RNA, an outer shell in which 2 antigens are located - hemagglutinin and neuraminidase, which can change their properties, especially in type A virus. Changes in hemagglutinin and neuraminidase cause the emergence of new subtypes of the virus that usually cause more severe and widespread diseases.
According to the International Nomenclature, the designation of virus strains includes the following information: genus, place of isolation, isolate number, year of isolation, type of hemagglutinin (H) and neuraminidase (N). For example, A/Singapore/l/57/H2N2 denotes a genus A virus isolated in 1957 in Singapore, which has the H2N2 antigen variant.
Influenza pandemics are associated with type A viruses. Influenza B viruses do not cause pandemics, but local “waves” of increased incidence may affect one or more countries. Influenza C viruses cause sporadic cases of illness. Influenza viruses are resistant to low temperatures and freezing, but quickly die when heated.
Orthomyxoviruses - influenza viruses A, B, C
Structural features.
Orthomyxoviruses are enveloped (supercapsid, “dressed”) viruses, the average size of virions is from 80 to 120 nm. Virions are spherical in shape. The genome is represented by single-stranded segmented (fragmented) negative RNA. The virion has a supercapsid containing two glycoproteins protruding above the membrane in the form of protrusions (spikes) - hemagglutinin (HA) and neuraminidase (NA). Influenza A viruses have 17 antigenically different types of hemagglutinin and 10 types of neuraminidases.
Classification of influenza viruses is based on the differences between nucleoprotein antigens (division into viruses A, B and C) and surface proteins HA and NA. The nucleoprotein (also called S-antigen) is constant in its structure and determines the type of virus (A, B or C). Surface antigens (hemagglutinin and neuraminidase - V-antigens), on the contrary, are variable and determine different strains of the same type of virus. Changes in hemagglutinin and neuraminidase cause the emergence of new subtypes of the virus, which usually cause more severe and more widespread diseases
Main functions of hemagglutinin:
Recognizes the cellular receptor - mucopeptide;
Responsible for the penetration of the virion into the cell, ensuring the fusion of the membranes of the virion and the cell; (Hemagglutinin provides the ability of the virus to attach to the cell.)
Its antigens have the greatest protective properties. Changes in antigenic properties (antigenic drift and shift) contribute to the development of epidemics caused by new Ag variants of the virus (against which herd immunity has not been sufficiently developed).
Neuraminidase responds for the dissemination of virions, together with hemagglutinin determines the epidemic properties of the virus.
Neuraminidase is responsible, firstly, for the ability of a viral particle to penetrate the host cell, and, secondly, for the ability of viral particles to exit the cell after reproduction.
The nucleocapsid consists of 8 vRNA segments and capsid proteins that form a helical strand.
Life cycle of the virus.
Replication of orthomyxoviruses is primarily realized in the cytoplasm of the infected cell; viral RNA synthesis occurs in the nucleus. In the nucleus, three types of virus-specific RNA are synthesized on vRNA: positive template mRNAs (a template for the synthesis of viral proteins), full-length complementary cRNA (a template for the synthesis of new negative virion RNAs) and negative virion vRNAs (the genome for newly synthesized virions).
Viral proteins are synthesized on polyribosomes. Next, viral proteins in the nucleus bind to vRNA, forming a nucleocapsid. The final stage of morphogenesis is controlled by the M protein. The nucleocapsid, passing through the cell membrane, is first covered with M protein, then with the cellular lipid layer and supercapsid glycoproteins HA and NA. The reproduction cycle lasts 6-8 hours and ends with the budding of newly synthesized virions.
Antigenic variability.
(Antigenic variability of influenza viruses. The variability of the influenza virus is well known. This variability of antigenic and biological properties is a fundamental feature of influenza viruses types A and B. Changes occur in the surface antigens of the virus - hemagglutinin and neuraminidase. Most likely this is an evolutionary mechanism of virus adaptation to ensure survival. New virus strains, unlike their predecessors, are not bound by specific antibodies that accumulate in the population. There are two mechanisms of antigenic variability: relatively small changes (antigenic drift) and strong changes (antigenic shift).
The modern division of orthomyxoviruses into genera (or types A, B and C) is associated with the antigenic properties of the main nucleocapsid proteins (nucleocapsid protein - phosphoprotein NP) and the viral envelope matrix (M protein). In addition to differences in NP and M proteins, orthomyxoviruses are distinguished by the highest antigenic variability due to the variability of the surface proteins HA and NA. There are two main types of changes - antigenic drift and antigenic shift.
Antigenic drift is caused by point mutations that change the structure of these proteins. The main regulator of the epidemic process during influenza is population (collective) immunity. As a result of its formation, strains with altered antigenic structure (primarily hemagglutinin) are selected, against which antibodies are less effective. Antigenic drift maintains the continuity of the epidemic process.
(Antigenic drift - occurs between pandemics in all types of viruses (A, B and C). These are minor changes in the structure of surface antigens (hemagglutinin and neuraminidase) caused by point mutations in the genes that encode them. Typically, such changes occur every year. As a result, epidemics occur, since protection from previous contacts with the virus remains, although it is insufficient.)
However, another form of antigenic variability has been discovered in influenza A viruses - antigenic shift(shift) associated with a change from one type of hemagglutinin (or neuraminidase) to another, i.e. on the emergence of a new antigenic variant of the virus. This is rarely observed and is associated with the development of pandemics. Over the entire known history of influenza, only a few antigenic phenotypes have been identified that cause influenza epidemics in humans: HoN1, H1N1, H2N2, H3N2, i.e. only three types of hemagglutinin (HA1-3) and two neuraminidase (NA 1 and 2). Influenza viruses type B and C cause disease only in humans, influenza A viruses cause disease in humans, mammals and birds. The most variable influenza A viruses have the greatest epidemic role. Influenza C viruses lack neuraminidase; these viruses usually cause a milder clinical picture.
There is an opinion that antigenic shift is the result of genetic exchange (recombination) between human and animal influenza viruses. It has not yet been definitively established where, during the inter-epidemic period - outside the human population (in birds or mammals) or in the human population (due to long-term persistence, local circulation) viruses that have temporarily exhausted their epidemic capabilities are preserved.
Birds are considered the primary and main hosts of influenza A viruses, in which, unlike humans, viruses with all 17 types of HA and 10 types of NA are common. Wild ducks are the natural hosts of influenza A viruses, in which the pathogen is located in the gastrointestinal tract and does not cause noticeable damage to the hosts. Viruses exhibit their pathogenic properties when they move to other birds and mammals. Among mammals, the greatest importance is attached to pigs, which are considered an intermediate host and are compared to a “mixing vessel”.
(Modern human influenza viruses are weakly transmitted to animals. All influenza A pandemics since 1930 began in China, the main gateway of spread is Siberia (mass migrations of birds).
Н1N1- 1930 Identified in humans, pigs, whales (1972), domestic and wild birds. The famous “Spanish flu” pandemic is associated with it. This type has become widespread again since 1977.
H2N2 has been detected since 1957. in humans and birds. Epidemics associated with these viruses came periodically. Now both types are identified in parallel.
H3N2 was identified in 1963. (Hong Kong).
Virus A/Singapore/1/57 (H2N2) has three genes from Eurasian avian influenza viruses, virus A/Hong Kong/1/68 (H3N2) contains 6 genes from the “Singapore” virus and two from birds. These data confirm that humanity receives new epidemic types of influenza A viruses from birds, the primary host. The immediate forecast is the possibility of the emergence of new epidemic variants of the influenza A virus that have hemagglutinin HA5 or 7 (replacement of one or two amino acids in their structure is sufficient).)
The first strain of influenza virus isolated from humans had the antigenic formula H0N1 (1933), and already in 1947 serovar H1N1 was isolated; Over the past 30 years, serovars H2N2 and H3N2 have been isolated.
Theories of the origin of pandemic influenza virus strains. All influenza pandemics were caused by influenza A viruses that underwent shift. The 1918 influenza pandemic was caused by a virus with the H1N1 phenotype (about 20 million people died); pandemic of 1957 – H2N2 virus (more than half of the world’s population was ill); 1968 – by the H3N2 virus. To explain the reasons for such a sharp change in the types of influenza A viruses, 2 hypotheses have been proposed.
According to the zooanthroponotic hypothesis (authors Webster and Tembol), the virus that caused the pandemic, after the emergence of immunity to it, passes to populations of mammals or birds. Then, as a result of genetic recombinations (facilitated by a fragmented genome) between human and animal influenza A viruses, a recombinant strain arises with a new type of hemagglutinin, to which people do not yet have immunity, and it causes a new influenza pandemic.
According to the anthroponotic hypothesis put forward by Francis (USA) and A.A. Smorodintsev (USSR), a virus that has exhausted its epidemic capabilities does not disappear, but continues to circulate in the community of people without noticeable outbreaks or persists for a long time in the human body. In 10-20 years, when a new generation of people who do not have immunity to it appears, this virus returns and becomes the cause of a new pandemic. This hypothesis is confirmed by the fact that the influenza A virus with the H1N1 phenotype, which disappeared in 1957 when it was replaced by the H2N2 virus, reappeared after a twenty-year absence in 1977. In addition, according to serological archeology, the 1889 pandemic was obviously caused by a virus with the H2N2 phenotype, since in 1957 antibodies to it were found only in people over 70 years of age who were not sick during the 1957 pandemic. Finally, it was established that influenza in humans was and is caused by type A viruses only 3 or 4 phenotypes (H1N1 (H0N1), H2N2, H3N2). At the same time, a new epidemiological feature of influenza has emerged: if previously each new pandemic variant completely replaced its predecessor, then since 1977, viruses with the H1N1 and H3N2 phenotypes seem to coexist in a community, probably possessing, until a certain time, equal epidemic potential. Antigenic drifts and shifts of influenza A viruses are the main obstacle to the development of effective vaccines.
Cultivation of influenza A viruses. Influenza A viruses are cultivated in chicken embryos and cell cultures. In chicken embryos, influenza A viruses reproduce within 36-48 hours in the amniotic and allantoic cavities at a temperature of 37˚C. The most sensitive to the influenza A virus are primary cultures of human embryonic kidney cells and some animals. Reproduction of the virus in these cultures is accompanied by a mild cytopathic effect, reminiscent of spontaneous cell degeneration.
Influenza A viruses can be cultured in mice, monkeys and primates. Mice are not naturally infected by influenza viruses, but influenza A viruses can be adapted to mice. The virus replicates in the upper and lower respiratory tract and, once it has adapted, can cause pneumonia and death in mice. The A2G mouse species is not susceptible to influenza and carries a dominant allele (Mx gene), which encodes an interferon-inducible protein that inhibits the influenza virus.
Epidemiology.Host range and virus spread. Influenza A viruses typically cause disease in humans, pigs, horses, and rarely in birds. Since 1933, 3 hemagglutinin serotypes have been identified in humans, 2 in pigs, and 2 in horses. All 16 serotypes were found in waterfowl, mainly wild ducks; they do not usually cause disease in these hosts, but serve as reservoirs of influenza A genetic information. Influenza viruses (of avian origin) have been isolated accidentally from mink and whales.
Evolution. The results of the analysis of the sequences of each of the genes of influenza A viruses confirmed the assumption that there are 5 specific lineages from the hosts, and that the origin from birds is in evolutionary stagnation. Influenza A viruses currently circulating in humans may have evolved approximately 150 years ago from avian influenza viruses. The catastrophic Spanish Flu of 1918 is thought to have originated in pigs (with confirmation that the pig is an intermediate host). Highly virulent strains disappeared but amplified the current emergence of human influenza. Influenza A viruses circulating in pigs and horses also originate from avian sources.
Epidemiology. The source of infection for influenza is waterfowl of the goose and gull families - the original reservoir of influenza viruses for bird and mammal species. Avian influenza viruses are usually spread by fecal contamination of water (mechanism of transmission - fecal-oral, transmission route - predominantly water, in rare cases - aerogenous). Transmission of influenza from pigs to humans and vice versa has been proven. Transmission of the pathogen from birds to pigs and horses and indirect transmission to humans from birds through pigs is possible, as well as transmission of the pathogen from chickens to humans. Among people, the source of infection is a person with influenza and a virus carrier. The transmission mechanism is aerogenic, carried out primarily by airborne droplets. A sick person becomes infectious 24 hours before the onset of the main symptoms of the disease and poses an epidemiological danger for 48 hours after their disappearance.
Pathogenesis influenza includes all 7 stages characteristic of the pathogenesis of cyclic viral diseases (chapter “Viral infectious process”).
Influenza A viruses attach to the squamous and ciliated epithelium of the upper respiratory tract through hemagglutinin. Primary reproduction of viruses occurs in epithelial cells. Reproduction occurs at an exceptionally high speed, which is achieved due to the fragmented genome of influenza A viruses and explains the short incubation period of 1-2 days. The speed of viral reproduction is facilitated by the spread of many hundreds of virions prepared by just one infected cell. The viruses then enter the bloodstream and spread throughout the body. Under the influence of viruses, the proteolysis system is activated and the capillary endothelium is damaged, which leads to increased vascular permeability, hemorrhages and additional damage to the tissues of various organs (trachea, bronchi, myocardium, lungs, brain, kidneys). Influenza A viruses, entering the blood, cause suppression of hematopoiesis and the immune system, leukopenia and a hypersuppressive variant of immunodeficiency develop. Damage to the ciliated epithelium of the respiratory tract is accompanied by its destruction, which is the entrance gate for the penetration of bacteria into the lungs. There is a danger of developing bacterial superinfection – bronchitis, pneumonia. The NS1 protein of influenza A virus is capable of inducing apoptosis in sensitive cells.
Immunity provided by the interferon system, natural killer cells, T-killers and specific antibodies. Interferons (mainly α-interferons) inhibit virus reproduction in epithelial cells and also stimulate the functional activity of natural killer cells. The latter destroy virus-infected cells, which helps eliminate the pathogen from the body. Influenza viruses themselves are inducers of interferon production, but interferon production is significantly inhibited at high infectious doses of the virus.
Antigen-specific killer T cells destroy virus-infected epithelial cells, thereby providing immunoglobulins with access to influenza viruses. The latter, interacting with the antigenic determinants of viruses, form a CEC. With low affinity of immunoglobulins, inactivation of the virus in the CEC does not occur, which can cause infection of healthy epithelial cells. Elimination of CEC from the body of a patient with influenza is carried out by the macrophage system. The completeness of phagocytosis of CECs depends on their molecularity: large-molecular CECs are eliminated most intensively, while medium and small CECs can circulate for a long time in the internal environment of the body, settling in parenchymal organs (kidneys, lungs, brain), as well as microcirculatory vessels, which causes additional defeat.
Secretory immunoglobulins A, displayed on the surface of the mucous membranes of the upper respiratory tract, cause inactivation of influenza viruses and promote their phagocytosis. The main protective antibodies for influenza are secretory Ig A and serum Ig M and G to the hemagglutinin molecule, which neutralize the infectivity of viruses and are responsible for the formation of resistance to infection. The humoral response to hemagglutinin is specific for a given type of virus, but antigenic drift allows viruses to avoid inactivation by antibodies. Neuraminidase antibodies do not prevent infection, but they do reduce the spread of viruses in the body. Immunity to influenza is genus- and species-specific, lasting for many decades.
Clinic. The flu begins acutely with chills, fever (39-40˚C), headache, weakness, aching bones and joints, nasal congestion with scanty discharge, and unproductive cough. More severe influenza may develop if primary influenza pneumonia or secondary bacterial pneumonia occurs. The duration of the disease in adults is on average 7 days. Children who get the flu for the first time in their lives can harbor the virus for 13 days.
Laboratory diagnostics influenza includes virusoscopic, virological and serological diagnostic methods. The material for the study is smears, secretions and washings from the nasopharynx, blood, cerebrospinal fluid and sectional material.
Express diagnostics. The antibody immunofluorescence method is used (direct Coombs method). Allows you to conduct research within 2-3 hours from the moment of taking the material. Columnar epithelial cells of the mucous membrane of the inferior turbinate and posterior pharyngeal wall are selected with dry cotton swabs and placed in a medium for transporting viruses. In the laboratory, the swabs are wrung out and the suspension is centrifuged. Smears are prepared from the cell sediment on glass slides. During post-mortem detection of influenza virus antigens, prints of pieces of lung tissue are made, and preparations are also prepared from the mucous membrane of the trachea and bronchi, scraping off epithelial cells. The drugs are treated with anti-influenza immunoglobulins loaded with fluorochromes, incubated for an hour, and then washed with saline. Virus-infected cells, upon specific interaction with immunoglobulins and their subsequent examination in a fluorescent microscope, exhibit luminescence. The localization and nature of the glow depend on the stage of development of the influenza virus in cells, as well as on the period of occurrence of influenza infection. In the first days of the disease, the antigen is more often localized in the nuclei of columnar epithelial cells with simultaneous diffuse or granular luminescence of the cytoplasm in the same or other cells. Often a uniform homogeneous glow of the entire cell is detected. If the infection subsides, luminescence is most often observed in the cytoplasm or in its part in the form of individual granules. By comparing the number of affected cells in the field of view with the duration of the disease, the greatest number of them (4-10) is noted in the first days of the disease than in subsequent days. Diagnostic is the specific glow of 5 or more columnar epithelial cells with a brightness of at least “++”.
Indirect hemadsorption reaction is based on the ability of columnar epithelial cells of the upper respiratory tract affected by the influenza virus to adsorb red blood cells sensitized with anti-influenza antibodies on their surface. A 0.25% suspension of sensitized sheep erythrocytes is used. After 30 minutes of exposure at room temperature, a Goryaev counting chamber is filled with a cell suspension and microscoped in a light microscope using phase-contrast optics. If 4-5 columnar epithelial cells with two or more red blood cells adsorbed on them are detected in the test preparation, the reaction is considered positive.
Passive hemagglutination reaction. The influenza virus antigen in the test material is detected using an erythrocyte antibody diagnostic, which is combined with a washout freed from mucus and heterohemagglutinins in a ratio of 1:20. When hemagglutination occurs, the reaction is considered positive. Can be used as an express diagnostic method ELISA.
Virological method. The material to be studied is swabs from the nasopharynx, sectional material, and cerebrospinal fluid. Viruses are isolated from chicken embryos and cell cultures.
Isolation of influenza viruses in chicken embryos is the most accessible. 10-11-day-old chicken embryos are infected with infectious material in a volume of 0.1 ml in the amniotic cavity or 0.2 ml in the allantoic cavity, after which the embryos are kept at 33-34˚C for 72 hours (optimal conditions for the reproduction of viruses A and B ). The virus-containing material obtained from chicken embryos is examined for the presence of the hemagglutination phenomenon in the hemadsorption reaction (HRA) with erythrocytes of chickens or guinea pigs. If the results of X-ray analysis are negative, after the passages the study of the material is completed. If there is agglutination of erythrocytes, titration of the hemagglutinating virus is carried out in the RGA.
Each viral antigen is titrated in 2 parallel rows of plate wells in dilutions from 1:10 to 1:2560. The titer difference in 2 rows should not exceed one twofold dilution. If it is higher, the titration should be repeated. By calculating the titer (in the case of a twofold difference), its arithmetic mean value is calculated. After determining the hemagglutinating titer of the freshly isolated influenza virus, its working dilution is prepared, containing 4 HAU (hemagglutinating units) in a given volume. Isolated influenza virus is identified in the HRTHA (hemagglutination inhibition reaction) using commercial diagnostic sera against influenza pathogens A1 (H1N1), A2 (H2N2), A3 (H3N2), B and C. In addition, the type of influenza virus can be determined in the complement fixation reaction (RSK).
Isolation of influenza viruses in cell cultures. Single-layer trypsinized cell cultures of human embryos and one-day-old chicks are used. CPD in case of infection with influenza viruses is characterized by degeneration of the cell layer. CPE appears from 3 to 10 days from the moment of infection. Influenza A viruses develop more slowly in cell culture; their CPD is expressed in the appearance of scalloped cells or cells with vacuolization of the cytoplasm, which are exfoliated during the process of degeneration. Hemadsorption is detected much earlier than CPD. If the titer of hemagglutinins in the culture liquid is 1:8 or more, influenza viruses isolated on cell culture are identified in the RTGA; if the titer is less than 1:8, the collected material is cultured to increase the titer, or identification is carried out in the hemadsorption inhibition reaction (RTGAds) on infected culture. To inhibit hemadsorption, the titer of the immune serum must be no less than 1:160 and correspond to the antigenic structure of the isolated virus. The type of influenza virus hemagglutinin in the virological research method is determined in the RTGA, the subtype of neuraminidase is determined in the reaction of inhibition of neuraminidase activity.
Serological method is based on identifying an increase in the titer of anti-influenza antibodies in the dynamics of the disease. For serological diagnosis of influenza, RNGA, RSK, RTGA are used, with the last 2 reactions being the most common. In the process of serological testing of blood sera in the RTGA, successive two-fold dilutions of them in isotonic sodium chloride solution, influenza antigen (4 HAE) and a suspension of erythrocytes of chickens or humans with blood group O (I) are used. For serological diagnosis of influenza, RSK is carried out under the same conditions as for the identification of isolated viruses. In the RNGA, antibodies are detected using standard lyophilized antigen erythrocyte diagnostic kits.
Treatment. Used to treat influenza α-interferons, which have the greatest antiviral effect (reaferon - human genetically engineered recombinant α 2 -interferon, which is prescribed from 500,000 to 1,000,000 units 3 times a day intramuscularly for 5-7 days). Inducers of endogenous interferon production (mefenamic acid, amizone) also have an antiviral effect. The chemotherapy drugs rimantadine and amantadine have a pronounced antiviral effect. Anti-influenza immunoglobulins are also used to treat influenza. In case of complicated influenza infection, broad-spectrum antibiotics are indicated.
Prevention.Nonspecific prevention influenza includes early identification, isolation and sanitation of the source of infection (sick person), as well as disruption of the mechanism and route of transmission of influenza viruses. For this purpose, an anti-epidemic regime is established in organized groups (separation of persons, wearing masks, compliance with the anti-epidemic regime in hospital departments). In areas of infection, ultraviolet irradiation and wet cleaning of premises using disinfectants are carried out. Mass nonspecific prevention includes the use of inducers of endogenous interferon production (amizon, mefenamic acid).
Specific prevention includes immunoglobulin prophylaxis And vaccine prophylaxis. Donor anti-influenza immunoglobulins are used in accordance with age-specific doses. The following types of influenza vaccines are used for vaccine prophylaxis: 1) live attenuated influenza vaccine (allantoic and cultural); 2) killed whole virion influenza vaccine; 3) subvirion influenza vaccine; 4) subunit influenza vaccine containing only hemagglutinin and neuraminidase.
The most effective are subunit influenza vaccines, among which there is a monovaccine (H1N1), a divaccine (H1N1+H3N2) and a trivaccine (A/H1N1+A/H3N2+B). Currently, foreign influenza vaccines “Fluarix” (Belgium), “Vaxigripp” (France) and “Influvac” (Holland) are widely used in Ukraine. These vaccines are subvirion influenza trivaccines. One dose of the vaccine (0.5 ml) is administered intramuscularly or subcutaneously, immunity develops within 3-4 weeks. Vaccination is carried out during periods of greatest risk of epidemics. Vaccination is most indicated for young and elderly people, as well as employees of medical institutions. Vaccination should be carried out 3-4 weeks before the outbreak of the epidemic.
