Poliomyelitis: causes and pathogenesis
Last reviewed: 23.04.2024
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Causes of poliomyelitis
The causes of poliomyelitis are the RNA-containing poliovirus of the family Picornaviridae of the genus Enterovirus, 15-30 nm in size. There are 3 serotypes of the virus: I - Brunhilda (isolated from a sick monkey with such a nickname), II - Lansing (isolated in Lansing) and III - Leon (isolated from the sick boy MacLeon). All types are close in their structure and differ in the sequence of nucleotides. Two type-specific antigens of polioviruses are identified: N (native), which is found in intact virions containing RNA, and H (warmed), which is released from capsules that do not contain RNA. H-antigen initiates in humans a primary antibody reaction, followed by a subsequent reaction to the N-antigen. Reproduction of the virus occurs in the cytoplasm of the affected cells.
The virus is stable in the environment. It is stored for a long time at low temperature (in the frozen state - up to several years): several months - in faeces, sewage, milk and vegetables. Resistant to fluctuations in pH, insensitive to alcohol, well retained in 50% glycerol. The polio virus is rapidly inactivated by chlorine-containing substances (3-5% chloramine), 15% sulfuric and 4% hydrochloric acid, solutions of iodine, potassium permanganate, copper sulfate, mercury, and ultraviolet rays. At boiling dies instantly.
[Pathogenesis of poliomyelitis
Polioviruses enter the human body through the mucous membrane of the gastrointestinal tract and the nasopharynx, in which the primary replication of the virus takes place. If there is no dissemination of the pathogen, the infection process proceeds according to the type of carrier. If hematogenous and lymphogenous spread of the pathogen occurs. But the virus does not penetrate into the central nervous system, abortive forms of the disease develop. When the GEB virus is overcome, the meningeal or paralytic form of the disease develops. Polioviruses have high tropicity to the gray matter of the brain and spinal cord. The most common motor neurons of the anterior horns of the spinal cord, less often the motor nuclei of the cranial nerves, the brainstem, etc. The lesions are accompanied by an inflammatory reaction and dystrophic changes leading to the death of neurons and the development of paresis and paralysis by peripheral type (atony, areflexia, atrophy or hypotension, hypotrophy, hyporeflexia). Preservation of a part of neurons and restoration of function of the damaged neurons determines possibility of the subsequent partial or full restoration of functions of muscles. The death of patients occurs as a result of paralysis of the respiratory muscles or respiratory center, bulbar disorders, and the attachment of secondary aspiration pneumonia.
Epidemiology of poliomyelitis
The source and reservoir of the pathogen is a person (a patient or a 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 contagious in the acute period of poliomyelitis.
The main mechanism of transmission of the virus is fecal-oral, which is realized by water, food and contact-household ways. Airborne droplet infection is possible in the early days of the disease and the initial period of virus carrying. In tropical countries, cases are recorded throughout the year, in countries with a temperate climate, summer-autumn seasonality is observed. Children are most susceptible to infection before the age of 3, but adults can also get sick. When infected, asymptomatic infection or the abortive form of poliomyelitis most often develops, and only in one of 200 cases - typical paralytic forms of poliomyelitis. After the transferred infection, resistant type-specific immunity is produced. Passive immunity, received from the mother, is preserved during the first half of life.
Prior to vaccination, in the early 1950s, poliomyelitis was registered in more than 100 countries worldwide. Thanks to a global campaign to eradicate polio by mass vaccination with inactivated Salk vaccine and Sabin's live vaccine, which has been carried out by WHO since 1988, it became possible to completely eliminate this disease. According to WHO statistics, since 1988 the number of cases of poliomyelitis has fallen from 350 thousand to several hundred per year. During this period, the list of countries in which cases of this disease were recorded. Decreased from 125 to six. Currently, cases of poliomyelitis are registered in India, Nigeria and Pakistan, accounting for 99% of the cases. As well as in Egypt, Afghanistan, Niger. In connection with the use of live oral vaccine, a wide circulation of vaccine strains of poliovirus is observed, which in a non-immune group can restore their virulence and cause paralytic poliomyelitis.
Specific prevention of poliomyelitis is carried out with polyvalent (prepared from three types of attenuated virus) oral live vaccine (live Sabin vaccine) according to the vaccination schedule from 3 months of age three times with an interval of 45 days. Revaccination - at 18, 20 months and 14 years. Oral live vaccine is one of the most low-reagent vaccines. It is easy to use. Forms local specific immunity of the gastrointestinal mucosa. Sabin's live vaccine is contraindicated in febrile conditions and primary immunodeficiency. It is advisable for people with immunodeficiency to use inactivated polio vaccine, which is registered in Russia in the form of Imovax Full and as part of the Tetrakok 05 vaccine.
Obligatory early isolation of patients with polio for 40 days from the onset of the disease. The focus is on final disinfection and an expanded epidemiological survey. The contact persons are observed for 21 days. In children's institutions for the same period, quarantine is introduced. Immediate vaccination of children under 7 years of age, vaccinated in violation of the schedule, and all identified unvaccinated, regardless of age, is mandatory.