Abstract: Swine flu is one of the most common respiratory infectious diseases in the world. It causes more serious damage when mixed with other pathogens. Its immune mechanisms are mainly mucosal immunity and humoral immunity. Both intranasal and intramuscular injections can produce highly effective immune responses. The current swine influenza virus subtypes are mainly H1N1, H1N2 and H3N2. The commonly used vaccines are killed vaccines. Because of the large number of subtypes of swine flu viruses, the susceptibility of antigens to antigens, and the spread of viruses among species, many challenges have been raised for the prevention of influenza diseases and even for human health. Research on nucleic acid vaccines, synthetic peptide vaccines, and genetically engineered vaccines has provided broad prospects for the prevention of swine flu.
Key words: Swine influenza virus; Antigen drift; Immunization Swine influenza is one of the most common swine infectious diseases in the world, spreading rapidly and often causing an outbreak of acute respiratory disease. The typical clinical symptoms are runny nose, cough and dyspnea, accompanied by fever, drowsiness, and concomitant anorexia and weight loss. Because it rarely causes death in the case of infection alone, it has not received sufficient attention in the country. The number of swine farms immunized with the Swine influenza virus (SIV) vaccine is very small. However, when SIV is mixed with Porcine reproductive respiratory syndrome virus (PRRSV), circovirus, and Mycoplasma pneumoniae, it will significantly increase the degree of damage and mortality of these diseases. In addition, SIV is also the main pathogen of Porcine respiratory disease complex (PRDC). The damage caused by co-infection or secondary infection of SIV with other pathogens has become one of the main reasons constraining the economic efficiency of farms. Because swine flu has a special role in the spread, spread, and molecular variability of bird flu and human flu, it also has very important significance for public health.
1 The main subtype of swine influenza virus
1.1 Main Basis for Classification of Influenza Virus Subtypes
SIV is a member of Influenza A virus in the genus Influenza virus of the family Orthomyxoviridae, and its genome is a single-stranded partial strand RNA of 8 fragments. Fragment 5 encodes the major structural protein, nucleocapsid protein (NP), whose main function is to form RNP complexes with viral RNA to stabilize viral RNA from RNase degradation. Since NPs are highly antigenic and difficult to mutate, influenza viruses can be divided into A, B, and C 3 types depending on the NP antigenicity. Although the nucleotide sequence of the NP gene of type A SIV was significantly different from that of other types (homology rate was 60%), the SIV strain showed a high degree of conservation among the different strains (homology rate up to 90%). Therefore, RT-PCR using the NP gene as a template can be specifically used for the detection of type A SIV. RNA fragments 4 and 6 of SIV encode hemagglutinin (HA) and neuraminidase (NA), respectively, which are the basis for classification of influenza virus subtypes. Currently, there are 15 HA subtypes of influenza virus and 9 subtypes of NA subtype [1]. Although a wide variety of influenza virus serotypes can be formed between different subtypes. However, there are currently only H1N1, H1N2, and H3N2 serotypes of the worldwide epidemic SIV. There are also sporadic reports of other serotypes, such as British H1N7 [2], American H4N6 [3], Chinese H9N2 [4] and H5N1 [5], but none have caused a large-scale epidemic.
1.2 History and current status of swine flu epidemic at home and abroad
Since the first occurrence of swine flu in the United States in 1918, it has caused numerous pandemics worldwide. The SIV was first isolated and identified by Shope in 1931. This was the classical H1N1 strain that caused the swine flu epidemic in the early 20th century. Unlike the United States, swine flu in the European continent occurred relatively late until the outbreak of the end of the 1970s, although the epidemic strain is also H1N1, but with the classical SIV in the United States on the antigenic and hereditary All have significant differences, and have a high homology with H1N1 from ducks. In order to distinguish between the two different H1N1 strains, the latter is often referred to as avian type H1N1. These two kinds of H1N1 were then introduced into Asia and other countries around the world and they have long been popular. In 1969, the H3N2 subtype SIV was first isolated in Taiwan. In 1970, the H3N2 subtype strain of SIV was also isolated in Hong Kong. This popular H3N2 strain and its variants spread to the rest of the world over the next decade or so. In 1984, the outbreak of swine influenza caused by human-like H3N2 subtype strains was reported for the first time in continental Europe, and pigs across the European continent showed high levels of antibodies. In 1988, many swine influenza outbreaks occurred in many pig farms in the United States that had been vaccinated with the H1N1 flu vaccine. The pathogen was identified as an H3N2 subtype strain. Since then, H1N1 and H3N2 have become the predominant two SIV serotypes in the world [6]. Studies have shown that H1N2 subtype SIV occurred in Japan, Korea, and the United States in recent years as a recombinant product of H1N1 and H3N2, because the double-immunized pigs of the H1N1 and H3N2 vaccines have a cross-protective effect against H1N2 subtype SIV infection [7]. The serological and etiological investigation results of swine influenza in China have shown that there are severe swine influenza epidemics in many provinces and cities across the north and south of China. The main subtypes are H1N1 and H3N2 [8-10]. It is worth noting that these epidemiological investigations are based on the occurrence of typical influenza in pig farms. In recent years, swine flu has become more and more in the direction of multi-pathogen mixed infection. When SIV is co-infected with PRRSV, Actinobacillus pleuropneumoniae, and Mycoplasma pneumoniae etc., its harmfulness will be significantly increased and the mortality rate will be significantly higher [11-13].
2 Swine influenza virus host specificity and its variation??
2.1 Molecular Biology of Influenza Virus Host Specificity
Interspecies transmission is one of the salient features of SIV. In the course of influenza virus infection, HA plays a role in recognizing and adsorbing cellular receptors, which is a major determinant of host specificity. The cellular receptors on the tracheal mucosa of different animals bind specifically to the receptor binding sites on the HA molecule. HAs of different influenza viruses have different sialidase molecular structures and bind to different sialic acid receptors, but this combination Cross-cutting. The receptor for the avian influenza virus and the horse influenza virus is sialic acid α2,3-galactoside, and the receptor for the human influenza virus is sialic acid α2,6-galactoside, and the swine influenza virus has both receptors. Affinity, the respiratory tract of pigs also have these two receptors [14]. In other words, pigs can also infect human, poultry, and horse flu viruses. Swine flu viruses can also infect these animals. Of course, the adsorption and infection of influenza virus also requires the participation of host cell membrane surface glycoproteins and other factors [15]. The Spanish flu in 1918, the Asian flu in 1957, and the Hong Kong flu in 1968 all had a close relationship with the occurrence of swine flu at the time. Viet Nam and Thailand have recently reported that many people have died of bird flu virus infection. There is no conclusive evidence on whether pigs are involved in the spread from poultry to humans. Fortunately, no animal influenza viruses have been found to be transmitted among humans. evidence. Therefore, swine flu not only restricts the economic benefits of pig farms, but also poses a serious threat to the health of the entire aquaculture industry and even humans.
2.2 Genetic variation of swine influenza virus.
Antigenic variation is another significant feature of the influenza virus. HA and NA are located on the surface of the viral envelope and are the most highly variable antigen proteins. Antigenic variations of influenza virus include antigen drift and antigen shifts. The magnitude of antigenic mutations is large and often leads to the emergence of new subtypes. Type A influenza virus has 8 genome segments. When 2 or more different virus particles simultaneously infect a host cell, the 8 genome segments of different virus particles can be exchanged at random during the process of virus proliferation. Generate a new subtype of influenza virus. Therefore, there may be a large difference in antigenicity between different strains of the same subtype. The H3N2 SIV, which is prevalent in North America, is a recombinant containing human, porcine, and avian influenza virus gene fragments [16]. The H1N2 swine influenza virus that is prevalent in Japan is the NA recombination of avian HA and human influenza viruses. Into [17]. HA and NA can also cause small mutations due to spontaneous point mutations, which we call antigen drift. The reason for the occurrence of antigenic shift is that the RNA polymerase needed for synthesizing the influenza virus RNA lacks a corrective function. Nucleotide point mutation in the process of virus replication may cause the amino acid variation it encodes. When the amino acid variation reaches a certain degree, it causes the antigen. The change in the determinants, ie, the change in antigenicity. The antigenic variation of the SIV can help it escape the established immunity of the host and bring greater difficulties to the prevention of swine flu. Sequencing nucleotide sequences of different strains isolated from various parts of the world and using biological software to analyze the lineage and genetic variation among these strains is one of the hot spots for the study of swine influenza viruses.
3 The immune mechanism and prevention of swine flu
3.1 The immune mechanism of swine flu
Swine flu mainly consists of mucosal immunity and humoral immunity, and T cells also participate in certain ways. The respiratory tract is the main route of SIV infection, so mucosal immunity is crucial for the prevention of swine flu. The mucosal immune system is a relatively independent immune system in the body. In the animal's bronchus, there is a collection of lymphoid nodules, diffuse immune cells and dendritic cells, which can complete a series of immune reactions such as antigen presentation and immune response. Mucosal epithelial lymphocytes include M cells, B cells, T cells, and other immune cells. M cells are squamous epithelial cells. SIV binds to the cell membrane of M cells, enters the cytoplasm to form phagocytic vesicles and is digested, and then the antigenic epitopes are transmitted to the underlying lymphoid tissues to generate an immune response. [18] . SIV can also be absorbed by another mucosal epithelial cell, the antigen-presenting epithelial cell, and completes antigen presentation by MHCII, and induces specific antibodies to B cells with the participation of CD4+ T cells [19]. The immunoglobulin produced by B cells is mainly IgA. Intramuscular injection can also induce the body to produce a high level of humoral immune response, protect the body from lung tissue damage, but generally can not increase the titers of IgA in the respiratory mucosa [20].
3.2 Swine Flu Vaccine
For a long time, the immunization of swine flu has not received enough attention in all countries. However, by the mid-1990s, the occurrence and prevalence of porcine respiratory disease syndrome changed this fact. The occurrence of PRRS and PRDC has caused serious losses in many pig farms, and the research and application of SIV vaccines have been extensively initiated in various countries. The SIV vaccines currently in use on the market are all inactivated vaccines, with a unit seedling and double seedlings of H1N1 and H3N2. In order to obtain long-term sustained immune effects, many research institutes at home and abroad are conducting studies on swine influenza genetic engineering seedlings. It was found that amino acids 92-105, 127-133, and 183-195 in HA of H3N2 are effective epitopes, and the synthetic peptide vaccine prepared by using the repeat sequences of these three antigenic epitopes immunized mice. Both rabbits and rabbits obtained high titer antiserum and inhibited SIV formation of plaques on MDCK cell lines [21]. Genetically engineered vaccines expressing HA and NP using adenovirus as a vector have also been successfully constructed. They can rapidly promote the production of immunity and have a good protective effect on piglets up to 3 weeks of age. This vaccine is particularly suitable for the lack of maternal antibody protection. Piglets [22]. Liposomal encapsulated DNA vaccines encoding HA can induce IgA, IgG humoral responses, and increase intratracheal IgA titers when inoculated via the respiratory route [20]. In order to enhance the immune effect of the vaccine, certain immune adjuvants should be added to the vaccine. IL-1 enhances the IgM response and proliferation of CD4+ T cells, relieves acute inflammation of the SIV-infected lung, and increases survival of diseased animals [23]. When using interferon as a mucosal adjuvant, SIV vaccine can not only provide long-term sustained specific immunity, but also increase the proportion of macrophages in the nasal mucosa. This may be a new mechanism of action of type I interferon [ twenty four]. The prevailing SIV in the world today is dominated by H1N1 and H3N2. In many places, H1N2 is also a new subtype. At the same time, pigs immunized with these two subtypes have cross protection against H1N2 subtype SIV infection [25]. Porcine respiratory disease syndrome usually occurs between 14 and 17 weeks of age, and we are customarily referred to as the 18-week-old wall. In order to maintain a uniform and orderly antibody level in the herd, protection by maternal antibodies is an effective way.
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