Polio virus

The poliovirus genome is represented by single-stranded non-fragmented RNA consisting of 7.5-8 thousand nucleotides, its molecular weight is 2.5 MD. The organization of virion RNA has the following features that determine the nature of its behavior in the cell:

• coding sequences account for about 90% of the total length;
• between the 5' end and the beginning of the reading frame is the so-called 5' untranslated region, which accounts for about 10% of the length of the RNA; this region contains from 6 to 12 AUG initiation codons;
• the poliovirus genomic RNA does not contain a cap at the 5' end; instead, a small virus-specific glycoprotein is covalently linked to the 5' end of the RNA, which is cleaved by a cellular enzyme before translation;
• under the influence of virion RNA, the synthesis of protein factors necessary for the initiation of cap-dependent translation is suppressed in the cell, as a result of which cap-independent translation of viral proteins occurs very actively;
• The 5-untranslated region of poliovirus RNA contains a special regulatory element that ensures its cap-independent translation. A relationship has been established between the neurovirulence of the virus and the degree of activity of this regulatory element, which determines the intensity of synthesis of viral proteins, especially in nerve cells.

The virion mass is 8-9 MD. The virus has a spherical shape. The symmetry type is cubic. The virion capsid is formed by four proteins, 60 copies of each. Three of them - VP1, VP2, VP3 - form the outer surface of the capsid, and VP4 - the inner one, so it is not visible from the outside.

The virion envelope is formed from 12 compact structures called pentamers, since they contain 5 molecules of each protein. The pentamers are arranged like a mountain, the top of which is occupied by VP1, and its base is formed by VP4; the VP2 and VP3 proteins alternately surround the foot. The virion genome is very tightly enclosed in its central cavity. The envelope proteins play a role in recognizing the host cell receptor, in attaching the virion to it, and in releasing virion RNA inside the cell. The virion does not have hemagglutinating properties. The ability of the polio virus to cause paralysis is also apparently associated with one of the envelope proteins. They, the proteins, also determine the immunogenic properties of the virus. According to antigenic features, polio viruses are divided into three types: I, II, III.

Poliovirus type I has the greatest pathogenicity for humans: all significant poliomyelitis epidemics were caused by this type. Poliovirus type III causes epidemics less frequently. Poliovirus type II more often causes a latent form of infection.

Intracellular reproduction of the virus. The interaction of the virus with the cell consists of the following stages:

• virus adsorption;
• penetration into the cell, accompanied by the destruction of the capsid and the release of genomic RNA.

Being positive, vRNA is directly translated into virus-specific proteins. One of these proteins - non-structural - is RNA replicase, with the participation of which the replication of vRNA occurs according to the scheme:

VRNA -> cRNA -> vRNA.

The structural proteins, all four, are synthesized as an initial single polypeptide chain, which then undergoes a cascade of proteolysis and is finally split into four proteins VP1-VP4. This cleavage is apparently catalyzed by the viral protein itself and is necessary for the formation of newly formed virions. The newly synthesized vRNA is incorporated into the capsid, and virion formation is complete. The newly synthesized virions exit the cell. Up to 150,000 virions are synthesized from one virion in the cell.

The word poliomyelitis translated into Russian means inflammation of the gray matter of the brain (Greek polios - gray, myelitis - inflammation of the spinal cord). The fact is that the most important biological property of polioviruses is their tropism to nervous tissue, they affect the motor cells of the gray matter of the spinal cord.

Pathogenesis and symptoms of poliomyelitis

The entry point for poliomyelitis is the mucous membrane of the pharynx, stomach and intestines. Primary virus reproduction occurs in them, and therefore, several days after infection, it can be detected in the pharyngeal mucus and feces. After reproduction in the epithelial cells, the virus penetrates the regional lymph nodes and then into the blood. In other words, following the alimentary stage of the disease, viremia with hematogenous dissemination of the pathogen occurs. Symptoms of poliomyelitis in these two stages are usually absent. Only sometimes viremia is accompanied by a short-term increase in temperature and mild malaise, this characterizes the so-called "minor" disease, it ends with recovery and the formation of post-infection immunity. However, polioviruses can overcome the blood-brain barrier and penetrate the central nervous system, resulting in the development of a "major" disease. The virus-induced death of motor neurons in the anterior horns of the spinal cord leads to the development of paralysis of skeletal muscles, as a result of which the patient either dies or remains disabled for life.

There are four main clinical forms of poliomyelitis:

• abortive (minor illness);
• non-paralytic (meningeal), manifested by serous meningitis;
• paralytic;
• inapparent (hidden).

Depending on the location of the lesion, the paralytic form is divided into spinal, bulbar, pontine (pons) and other, rarer forms.

The course of poliomyelitis is determined by the size of the infective dose, the degree of neurovirulence of the virus and the immune status of the organism. The lesions are found in the anterior horns of the spinal cord, most often in the area of the lumbar expansion, in the motor cells of the reticular formation of the medulla oblongata and the pons, the cerebellum, in the motor and premotor areas of the cerebral cortex.

Immunity to Polio

After the disease (including in a latent form), strong lifelong immunity remains, caused by virus-neutralizing antibodies and immune memory cells.

Epidemiology of poliomyelitis

The source of infection is only humans. Although the virus multiplies in the epithelial and lymphoid tissues of the upper respiratory tract, the airborne route of infection does not play a significant role due to the absence of catarrhal phenomena. The main route of infection is fecal-oral. The virus is excreted with feces in large quantities from the end of the incubation period (the last 3-7 days) to the 40th day of the disease, and in some cases - several months.

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Polio Treatment

Treatment of acute poliomyelitis should be comprehensive and carried out taking into account the stage and form of the disease. In paralytic forms, it is especially important to follow an early orthopedic regimen. The main significance in the treatment of poliomyelitis belongs to correct and long-term gymnastics. Patients with respiratory disorders should be under special supervision of specially trained personnel. There is no specific treatment for poliomyelitis.

Specific prophylaxis of poliomyelitis

By the middle of the 20th century, poliomyelitis had become a formidable epidemic disease, periodically affecting thousands and tens of thousands of people, of whom about 10% died, and 40% were left with lifelong paralysis. The only reliable weapon against this disease could only be the polio vaccine and the creation of herd immunity with its help. To do this, it was necessary to develop methods that would allow the virus to accumulate in the required quantity. And the persistent efforts of scientists finally bore fruit. In the late 1940s - early 1950s, methods were developed for obtaining single-layer cell cultures (first primary trypsinized, then transplanted), which were widely used to grow viruses, and therefore real conditions arose for creating a vaccine against poliomyelitis. It should be noted that the development of methods for obtaining cell cultures was of great importance for the development of virology. In the 50s of the 20th century, two vaccines against poliomyelitis were created:

• Formaldehyde-inactivated vaccine of J. Salk.
• A. Sebin's live vaccine from attenuated strains of poliovirus types I, II and III.

Large-scale production of a live vaccine was first mastered in our country in the 1950s. Immediately (since 1959), mass vaccination of children against poliomyelitis with this vaccine was started. Both vaccines - killed and live - are quite effective, however, in our country, preference is given to the live vaccine, since the vaccine strains, multiplying in the epithelial cells of the intestinal tract, are released into the external environment and, circulating in groups, displace wild strains of polioviruses. According to the WHO recommendation, vaccinations against poliomyelitis are mandatory and are carried out starting from the age of 3 months and up to 16 years. Since the live vaccine, although extremely rare, causes complications, vaccinations are now recommended to be carried out with the inactivated Salk vaccine. With the help of existing vaccines, the incidence of poliomyelitis in all countries of the world can and should be reduced to isolated cases, i.e., it has become possible to sharply reduce it.