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Poliomyelitis - Causes and Pathogenesis
Medical expert of the article
Last reviewed: 04.07.2025
Causes of Polio
Poliomyelitis is caused by an RNA-containing poliovirus of the Picornaviridae family, genus Enterovirus, 15-30 nm in size. There are three known serotypes of the virus: I - Brunhilda (isolated from a sick monkey with this nickname), II - Lansing (isolated in the town of Lansing) and III - Leon (isolated from a sick boy named McLeon). All types are similar in structure and differ in the nucleotide sequence. Two type-specific antigens of polioviruses have been identified: N (native), which is found in intact virions containing RNA, and H (heated), which is released from capsids that do not contain RNA. The H antigen initiates the primary antibody reaction in humans, which is subsequently replaced by a reaction to the N antigen. Reproduction of the virus occurs in the cytoplasm of affected cells.
The virus is stable in the environment. It survives for a long time at low temperatures (up to several years in frozen form): several months in feces, waste water, milk and vegetables. It is resistant to pH fluctuations, is slightly sensitive to alcohol, and is well preserved in 50% glycerin. Polio virus is quickly inactivated by chlorine-containing substances (3-5% chloramine), 15% sulfuric and 4% hydrochloric acids, iodine solutions, potassium permanganate, copper sulfate, corrosive sublimate and under the influence of ultraviolet rays. It dies instantly when boiled.
[ Pathogenesis of poliomyelitis
Polioviruses enter the human body through the mucous membrane of the gastrointestinal tract and nasopharynx, where primary replication of the virus occurs. In the absence of dissemination of the pathogen, the infectious process occurs as a carrier. If hematogenous and lymphogenous spread of the pathogen occurs, but the virus does not penetrate the central nervous system, abortive forms of the disease develop. When the virus overcomes the BBB, a meningeal or paralytic form of the disease develops. Polioviruses have a high tropism for the gray matter of the brain and spinal cord. Most often, large motor neurons of the anterior horns of the spinal cord are affected, less often - motor nuclei of the cranial nerves, brainstem, etc. Lesions are accompanied by an inflammatory reaction and dystrophic changes leading to the death of neurons and the development of paresis and paralysis of the peripheral type (atony, areflexia, atrophy or hypotension, hypotrophy, hyporeflexia). Preservation of some neurons and restoration of the function of damaged neurons determines the possibility of subsequent partial or complete restoration of muscle functions. Death of patients occurs as a result of paralysis of the respiratory muscles or respiratory center, bulbar disorders, and the addition of secondary aspiration pneumonia.
Epidemiology of poliomyelitis
The source and reservoir of the pathogen is a person (patient or virus carrier). The virus is excreted with nasopharyngeal mucus during the incubation period and up to the 5th day after the onset of the disease, with feces - from several weeks to 3-4 months. The patient is most infectious in the acute period of poliomyelitis.
The main mechanism of transmission of the virus is the feco-oral route, which is realized by water, food and contact-household routes. Airborne transmission is possible in the first days of the disease and the initial period of virus carriage. In tropical countries, cases of the disease are registered throughout the year, in countries with a temperate climate, summer-autumn seasonality is noted. Children under 3 years of age are most susceptible to infection, but adults can also get sick. When infected, asymptomatic infection or abortive form of poliomyelitis most often develops, and only in one out of 200 cases - typical paralytic forms of poliomyelitis. After the infection, persistent type-specific immunity is developed. Passive immunity received from the mother lasts for the first six months of life.
Before vaccination, in the early 1950s, poliomyelitis was registered in more than one hundred countries of the world. Thanks to the global campaign to eradicate poliomyelitis through mass vaccination with the inactivated Salk vaccine and the live Sabin vaccine, which has been carried out by WHO since 1988, it became possible to completely get rid of this disease. According to WHO statistics, since 1988 the number of poliomyelitis cases has fallen from 350 thousand to several hundred per year. During this period, the list of countries where cases of this disease were registered decreased from 125 to six. Currently, cases of poliomyelitis are registered in India, Nigeria and Pakistan, which account for 99% of those infected. as well as in Egypt, Afghanistan, Niger. Due to the use of a live oral vaccine, there is a wide circulation of vaccine strains of poliovirus, which in a non-immune group can restore their virulence and cause paralytic poliomyelitis.
Specific prevention of poliomyelitis is carried out with a polyvalent (prepared from three types of attenuated virus) oral live vaccine (live Sabin vaccine) according to the vaccination calendar from the age of 3 months three times with an interval of 45 days. Revaccination - at 18, 20 months and 14 years. Oral live vaccine is one of the least reactogenic vaccines. It is easy to use. forms local specific immunity of the gastrointestinal mucosa. Live Sabin vaccine is contraindicated in febrile conditions and primary immunodeficiency. For individuals with immunodeficiency, it is advisable to use the inactivated polio vaccine, which is registered in Russia in the form of the drug "Imovax Polno" and as part of the vaccine "Tetrakok 05".
Early isolation of patients with poliomyelitis is mandatory for 40 days from the onset of the disease. Final disinfection and extended epidemiological examination are carried out in the outbreak area. Contact persons are observed for 21 days. Quarantine is introduced in children's institutions for the same period. Immediate vaccination of children under 7 years of age who were vaccinated out of schedule and all identified unvaccinated persons, regardless of age, is mandatory.