SRI HARI 08/05/2025
POLIOMYELITIS
INTRODUCTION
- Poliomyelitis is a viral infectious disease caused by poliovirus
- It mainly affects young children
- They can lead to acute accid paralysis due to involvement of nervous system
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MORPHOLOGY
- Polioviruses are type of enteroviruses, belong to the family Picornaviridae.
- Simple in structure, appear very small (28–30 nm size) and nonenveloped.
- They are spherical shaped and have icosahedral symmetry
- Capsid is composed of 60 subunits, each consisting of four viral proteins (VP1-
VP4), except parechoviruses(have three proteins)
- Possess single-stranded positive sense linear RNA
ANTIGENIC TYPES
- Polioviruses can be classi ed into wild polioviruses
- They cause natural disease and vaccine derived poliovirus (VDPV), which are the vaccine strains
that have regained neurovirulence and are capable of producing disease in man.
WILD POLIOVIRUS
There are three wild poliovirus strains: Wild poliovirus type 1 (WPV1), wild poliovirus type 2 (WPV2)
and wild poliovirus type 3 (WPV3).
- All three are identical, produce similar manifestations and severity of illness However, they are
genetically and immunologically-distinct; differ from each other in VP1 region.
- The antibody response is type-speci c and not cross-protective.
- Therefore, each strain need to be eradicated individually ,Currently all the natural cases are
caused by WPV1. It has also been the common serotype to cause poliomyelitis till now
- Both WPV2 and WPV3 are globally eradicated, in the years 1999 and 2019 respectively.
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PATHOGENESIS
- Polioviruses are transmitted by feco-oral route (most common), followed by
respiratory droplets via inhalation or rarely by conjunctival contact
- They multiply in intestinal epithelial cells, submucosal lymphoid tissues, tonsils
and Peyer’s patches
- Viral entry into the host cells is mediated by binding to CD155 receptors present
on the host cell surface
Spread to CNS/spinal cord:
- Hematogenous spread (most common): Virus spreads to the regional lymph
nodes and spills over to the bloodstream (primary viremia). After further
multiplying in the reticuloendothelial system, the virus enters the bloodstream
again, causing secondary viremia. Then it is carried to the spinal cord and brain
- Neural spread: Virus may also spread directly through nerves.
CLINICAL MANIFESTATIONS
Clinical Manifestations:
- Incubation period is usually 7–14 days. The manifestations may range from
asymptomatic stage to the most severe paralytic stage.
- Inapparent infection: Following infection, the majority (91–96%) of cases are
asymptomatic
- Abortive infection: About 5% of patients develop minor symptoms such as fever,
malaise, sore throat, anorexia,myalgia, and headache
- Nonparalytic poliomyelitis: It is seen in 1% of patients,presented as aseptic
meningitis
RISK FACTORS
- Paralytic disease is more common among:
- Older children and adults
- Pregnant women
- Following heavy muscular exercise
- Persons undergoing trauma at the time of CNS symptoms
- Tonsillectomy: It predisposes to bulbar poliomyelitis
- IM injections: They increase the risk of paralysis inthe involved limb.
LAB DIAGNOSIS
Virus isolation
- Specimen: Poliovirus may be recovered from the throat swabs
- Transport: Specimens should be kept frozen during transport to the laboratory
- nCell line: Primary monkey kidney cells are the most recommended cell lines
Antibody Detection
- Neutralization test is the recommended method, which detects neutralizing antibodies
- Only rst infection with poliovirus produces strictly type-speci c responses
Molecular Method
- Real-time multiplex reverse-transcriptase PCR has been developed using primers from VP1 region
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VACCINE
- Both inactivated and live attenuated polio vaccines are available
INJECTABLE POLIO VACCINE (IPV, Salk Vaccine)
- Preparation: Virus is grown in monkey kidney cell line and inactivated by formalin.
- Dose: IPV can be given either as (i) full dose (0.5 mL/dose), or (ii) as fractional dose (f-IPV): 1/5th of full
dose (0.1 mL).
Advantages:
- IPV is much safer than OPV, safer even in immunocompromised people
- It does not cause vaccine-associated paralytic polio(VAPP)
- It is more stable, does not require stringent storage conditions.
Disadvantages:
- It does not provide herd immunity: Being inactivated vaccine, it cannot spread by feco-oral route.
- It is relatively expensive than OPV.
ORAL POLIO VACCINE (OPV, Sabin Vaccine)
- OPV is available as (i) trivalent OPV, (ii) bivalent OPV, and (iii) monovalent OPV.
Advantages:
- OPV has the several advantages over IPV
- Herd immunity: OPV strains being live, can shed in the feces and spread in the
community by feco-oralroute, hence, it can induce herd immunity. It can provide both
individual and community protection, OPV is the vaccine of choice during epidemics
- Local immunity: OPV induces mucosal IgA production, hence provides local or
mucosal immunity.
- Cheaper than IPV, Easy to administer.
Disadvantages:
- Safety: OPV is otherwise safe, but it is risky to give in immunocompromised people,
during pregnancy, and in old age
- Stability: OPV is unstable vaccine, requires stringent conditions such as: (i) by storage
at –20°C, (ii) by maintaining pH <7 .
- OPV can cause vaccine-associated paralytic polio-myelitis (VAPP) and vaccine-derived
polioviruses.
EPIDEMIOLOGY
- Reservoir: Man is the only known reservoir. Most cases are subclinical
- Clinical-subclinical ratio: For every clinical case, there may be 1,000 children
and 75 adults of subclinical cases. There are no chronic carriers. However,
immunode cient individuals may excrete the virus for longer periods
- Source: Infective materials such as stool and oropharyngeal secretions are the
sources of infection
- Age: Younger children and infants are more susceptible to infection than adults.
However in developed countries, there is shift of age; affecting older children
- Period of communicability: Patients are infectious, shedding the virus in the
feces from 7–10 days before the onset of symptoms up to 2–3 weeks thereafter,
sometimes as long as 3–4 months.
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