Poliovirus

Virus properties

The causative agent of poliomyelitis belongs to the species Enterovirus C of the genus Enterovirus of the family Picornaviridae. Three serotypes of the virus are known (1, 2, and 3). All of them are capable of causing disease in humans and are pathogenic for monkeys.

An important feature is the absence of cross-immunity: if a person has had an infection caused by the first serotype, they are not protected against the second and third.

Pathogenesis of infection

Natural human susceptibility to the virus is very high. Infection occurs through the mucous membranes of the respiratory tract and gastrointestinal tract.

1. Primary replication. The virus multiplies in the lymph nodes of the pharyngeal ring and small intestine. At this stage, the pathogen is already actively excreted with feces and nasopharyngeal mucus, although symptoms are still absent.
2. Dissemination. From the lymphatic system, the virus enters the bloodstream (viremia occurs) and then penetrates the central nervous system.
3. CNS involvement. The virus destroys cells in the anterior horns of the spinal cord, causing motor functions to fail — flaccid paralysis of the limbs and trunk develops.

Clinical course

On average, the incubation period lasts from 7 to 14 days. The disease can occur in several forms:

• Abortive: mild course without involvement of the nervous system.
• Meningeal: the meninges are affected, but there is no paralysis.
• Paralytic: the most severe and rare form (1% of cases). Most commonly caused by type 1 virus.

The disease often begins with general infectious symptoms (fever, vomiting, sore throat). A biphasic course is characteristic: after the initial mild malaise, a temporary improvement occurs, followed by a sharp deterioration with the development of paralysis.

Diagnostics

During the patient's life, stool and nasopharyngeal swabs are examined; postmortem, brain tissue and lymph nodes are analyzed.

• Virological method (gold standard): the virus is isolated in cell cultures, the cytopathic effect (CPE) is recorded, and it is identified using the neutralization test (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: an increase in antibody titer is assessed in paired sera.

Specific prevention (Vaccines)

Two types of vaccines are used to prevent poliomyelitis:

1. Inactivated poliovirus vaccine (IPV): administered parenterally. It generates humoral immunity but does not create local protection in the gut (the virus can still multiply there).
2. Oral poliovirus vaccine (OPV): contains attenuated (weakened) strains. In addition to systemic immunity, it generates powerful local immunity (secretory IgA) on the mucous membranes, which blocks virus replication.

It is thanks to the OPV that in 1988 the WHO launched the global poliomyelitis eradication program, which led to the disappearance of the infection in many countries (no cases have been registered in the Russian Federation since 2002).

Class and exam questions

What else to study on this topic

• Mechanisms of secretory immunity (IgA) formation in the gut
• Features of the cytopathic effect (CPE) of enteroviruses in cell culture
• Neutralization test: methodology and interpretation of results
• WHO global strategy for polio eradication: current status
• Differential diagnostics of wild and vaccine strains using PCR

Other topics in the section "Picornaviruses (enteroviruses, poliomyelitis, Coxsackie, ECHO, rhinoviruses, hepatitis A)"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.