Properties of the Virus
The causative agent of poliomyelitis belongs to the species Enterovirus C, genus Enterovirus, family Picornaviridae. There are three known serotypes of the virus (1, 2, and 3). All of them are capable of causing human disease and are pathogenic to primates.
An important feature is the absence of cross-immunity: if a person has recovered from an infection caused by the first serotype, they are not protected against the second and third.
Pathogenesis of Infection
Human susceptibility to the virus is very high. Infection occurs through the mucous membranes of the respiratory and gastrointestinal tracts.
- Primary replication. The virus multiplies in the lymph nodes of the pharyngeal ring and the small intestine. At this stage, the pathogen is already actively excreted in feces and nasopharyngeal mucus, although symptoms are still absent.
- Dissemination. From the lymphatic system, the virus enters the bloodstream (viremia occurs) and then penetrates the central nervous system.
- CNS involvement. The virus destroys the cells of the anterior horns of the spinal cord, leading to the loss of motor functions—causing flaccid paralysis of the limbs and torso.
Clinical Course
On average, the incubation period lasts from 7 to 14 days. The disease can present in several forms:
- Abortive: a mild course without involvement of the nervous system.
- Meningeal: meninges are affected, but there is no paralysis.
- Paralytic: the most severe and rare form (1% of cases). Most commonly caused by serotype 1 virus.
The disease often begins with general infectious symptoms (fever, vomiting, sore throat). A biphasic course is characteristic: after the initial mild malaise, there is a temporary improvement, followed by a sharp deterioration with the development of paralysis.
Diagnosis
During the patient's life, stool and nasopharyngeal swabs are tested; post-mortem, brain tissue and lymph nodes are examined.
- Virological method (gold standard): the virus is isolated in cell cultures, cytopathic effect (CPE) is observed, and it is identified in a neutralization assay (NT) with type-specific sera.
- Strain differentiation: ELISA, PCR, or NT are used to determine whether the disease was caused by a "wild" (pathogenic) strain or a vaccine strain.
- Serological method: rising antibody titers are assessed in paired sera.
Specific Prophylaxis (Vaccines)
Two types of vaccines are used to prevent poliomyelitis:
- Inactivated Polio Vaccine (IPV): administered parenterally. It generates humoral immunity but does not create local gut immunity (the virus can still replicate there).
- Oral Polio Vaccine (OPV): contains attenuated (weakened) strains. In addition to systemic immunity, it establishes robust local mucosal immunity (secretory IgA), which blocks viral replication.
Thanks to OPV, the WHO launched a global polio eradication initiative in 1988, leading to the elimination of the infection in many countries.