Poliovirus are from the family Picornavirus and the Enterovirus genus. Poliomyelitis is caused by one of the three types of polioviruses. It is an acute infectious diseases with or without signs of CNS involvement. It causes infantile paralysis and acute anterior poliomyelitis.
The disease (Poliomyelitis) has probably been around for centuries and was described as a separate clinical entity during the nineteenth century. Epidemics of poliomyelitis has been observed in Europe, Africa and North America with high prevalence among young children. It was also documented among older children and adolescents. In tropical and subtropical climates, poliomyelitis is prevalent throughout the year while in temperate countries, cases are seen mainly during summer and autumn.
Poliovirus is stable at pH values between 3 and 9, resistant to lipid solvents and rather slowly inactivated at room temperature reason why the virus may remain infectious for several days in milk, food, faeces, water and sewage.
Structure of Poliovirus
- As seen with other enteroviruses, it a naked virus measuring 28 nm
- Icosahedral in structure
- Single stranded RNA of positive (mRNA) within a capsid composed of 60 capsomeres
- Capsid made up of four protein (VP1-4)
Replication of Poliovirus
- RNA transcription initiates viral replication into negative strands which acts as templates for new viral RNAs
- A large polyproteins is made from the viral RNA which later is cleaved to generate the capsid protein VP1-4 with a range of other proteins.
- Assembly of new virons finally takes place in the cytoplasm.
- Releases and lysis of the cytoplasm
- Faecal-oral route
- Usually 1-2 weeks but may vary from 4 to more than a months
- The infected patient can infect susceptible hosts from some days before illness and for one to several weeks after the illness.
Pathogenesis of Poliomyelitis
- Poliomyelitis typically starts with a prodromal phase of a few days duration. Infected individual usually start complaining of fever and myalgia. Another feature is constipation. This phase is term “Minor disease”.
- The Minor disease phase usually follow by an interval of few days when the temperature becomes normal and the patient seems to recover.
- Following minor disease phase is the biphasic phase where the infected individual temperature increases again with the development of paralysis and frequently aseptic meningitis. This phase is especially common in children.
- The biphasic phase is usually characterized by hyperirritability followed by increases tendon reflexes which may last from several hours to a few days and potentially leading to paralytic stage with loss of tendon reflexes.
- Development of paralysis may take some hours or a few days. The paralysis is flaccid and most frequently affects the extremities but can also affect any voluntary muscles.
- Patient may also exhibit sensory disturbances especially during the initial phase of paralytic stage.
- External factors such as vaccination, strenuous exercise, operations and even pregnancy may increase the incidence, site and severity of paralytic disease.
- Bulbar poliomyelitis which is a form of paralytic poliomyelitis affecting neurons of the medulla oblongata of the brain stem may also occur alone or as a mixed bulbospinal form. This may lead to involvement of cranial nerves with paralysis of the pharyngeal muscles and dysphagia and affecting respiratory muscle followed by dyspnea.
- Bulbar poliomyelitis is often accompanied by lesions of the respiratory and circulatory centers which can lead to respiratory failure, circulatory shock and hypotension.
Signs and symptoms of Poliomyelitis
Minor systemic diseases usually follow by an interval of few days with temperature becoming normal and patient seems to be recovering. Symptoms includes:
Local diseases manifest when fever increases followed by
- Paralysis (often results in persistent lameness)
- Aseptic meningitis (recover in 1–2 weeks)
- Fever and above symptoms last for 1–2 weeks.
- The paralysis reaches a maximum within 2–3 days.
- More than 50% of cases recover during the subsequent weeks or months but the remaining patients will suffer from residual deficits in one or more muscles.
- The overall lethality of poliomyelitis has been 5 to 10%, but is substantially reduced by maintaining patients in respirators.
Complications of Poliomyelitis
- Encephalitis and myocarditis may occur during the acute stage.
- Respiratory failure
- Obstruction of airways
- Involvement of the autonomic nervous system.
Laboratory diagnosis of Poliomyelitis
- Isolation of viral RNA in faecal sample and throat swab by inoculation into cell culture. The presence of poliovirus is shown by degeneration of cultured cells within a few days
- Detection of Poliovirus Ig M and or Ig G antibody by Eliza
- Detection of poliovirus RNA in faecal samples by PCR. This method can distinguish between wild strains and vaccine strains
Treatment and prophylaxis
- No specific treatment,
- Immunoglobulin is of no practical value.
- Vaccine, either attenuated or inactivated, gives more than 90% protection.
Source: A Practical Guide to Clinical Virology. Edited by L. R. Haaheim, J. R. Pattison and R. J. Whitley