Polio Virus

Name the viruses included in the Enterovirus group of the family Picornaviridae. Describe in detail the morphology, antigenic properties, pathogenesis, and laboratory diagnosis of poliovirus. (20 marks)(2013)

Poliovirus — 20 Marks Essay

Introduction

Poliovirus is an important member of the genus Enterovirus, family Picornaviridae, and is the causative agent of poliomyelitis. It is an important model virus for studying RNA-virus replication and was one of the first human viruses to be cultivated in tissue culture. Fenner and White emphasize the historical importance of poliovirus in the development of modern virology, including its cultivation in non-neural cell cultures and subsequent development of the Salk and Sabin vaccines.

1. Enteroviruses of the family

Picornaviridae

Traditional classification — important for examination

The human enterovirus group traditionally includes:

1. Polioviruses — types 1, 2 and 3

2. Coxsackie A viruses — types 1–24, with some numbers absent

3. Coxsackie B viruses — types 1–6

4. Echoviruses — traditionally numbered enteroviruses

5. Other enteroviruses — including enterovirus 68–71 and subsequently numbered enteroviruses

Jawetz describes the older classification as polioviruses 1–3, Coxsackie A and B viruses, echoviruses, and numbered enteroviruses.

Fenner and White similarly list Coxsackieviruses A and B, echoviruses, enteroviruses 68–71 and polioviruses 1–3 within the genus Enterovirus.

Modern classification

Molecular classification now places human enteroviruses into Enterovirus A, B, C and D, while rhinoviruses are also within the genus Enterovirus. Thus, the older names such as Coxsackievirus and echovirus are retained mainly as clinically familiar names.

Important taxonomic points:

• Hepatitis A virus was formerly called enterovirus type 72 but is now in genus Hepatovirus.

• Echoviruses 22 and 23 are now Parechoviruses.

2. Classification of Poliovirus

Characteristic	Poliovirus
Family	Picornaviridae
Genus	Enterovirus
Genome	Positive-sense ssRNA
Envelope	Absent
Symmetry	Icosahedral
Size	Approximately 28–30 nm
Serotypes	3 — types 1, 2 and 3
Main disease	Poliomyelitis

3. Morphology and structure

A. Virion

Poliovirus is a small, non-enveloped, icosahedral virus, approximately 30 nm in diameter. On electron microscopy it appears as a smooth, rounded particle with little surface detail.

The capsid consists of 60 protomers, each containing four structural proteins:

• VP1

• VP2

• VP3

• VP4

VP1, VP2 and VP3 form the external surface of the capsid, whereas VP4 is located internally.

The capsid has characteristic canyon-like depressions, which participate in receptor binding and are also important antigenically. Fenner and White illustrate the poliovirus capsid by electron microscopy and structural models showing VP1, VP2, VP3 and internal VP4.

B. Genome

The genome is:

• Single-stranded RNA

• Positive-sense

• Approximately 7–8 kb

• Infectious by itself

• VPg is covalently attached to the 5′ end

• Polyadenylated at the 3′ end

• Contains a long 5′ untranslated region with an internal ribosome entry site (IRES).

The RNA acts directly as messenger RNA after entering the cytoplasm.

C. Replication

Replication occurs entirely in the cytoplasm.

The viral RNA is translated into a single large polyprotein, which is cleaved by viral proteases into structural and nonstructural proteins. The P1 region gives rise to the capsid proteins, while P2 and P3 encode proteins required for replication.

4. Antigenic properties

Three antigenic types

There are three antigenic/serologic types of poliovirus:

1. Poliovirus type 1

2. Poliovirus type 2

3. Poliovirus type 3

Jawetz specifically states that there are three antigenic types.

Important antigenic features

• The three types are antigenically distinct.

• Neutralizing antibodies are predominantly type-specific.

• Infection with one type produces strong and usually lifelong immunity to that type.

• There is only limited heterotypic protection between types.

• Important neutralizing epitopes are located on the surface capsid proteins, particularly VP1.

• VP1 contains several virus-neutralizing epitopes; antibodies directed against these epitopes interfere with viral attachment/entry.

Thus, immunity against one poliovirus serotype does not reliably protect against the other two serotypes.

5. Pathogenesis

Route of infection

The major route is the fecal–oral route.

The mouth is the portal of entry. Virus may also be transmitted through contaminated food and water and by close person-to-person contact.

Humans are the major reservoir. Virus is shed in large quantities in the feces, making sanitation and hygiene important determinants of transmission.

Stepwise pathogenesis

1. Primary multiplication

After ingestion, poliovirus first multiplies in:

• Oropharynx

• Tonsils

• Regional lymphoid tissue

• Peyer's patches

• Small intestine

Fenner and White specifically emphasize multiplication in the pharynx and small intestine, particularly lymphoid tissue such as tonsils and Peyer's patches.

2. Primary viremia

Virus enters the bloodstream, producing a primary viremia.

At this stage, infection may be completely asymptomatic or produce a nonspecific febrile illness.

Neutralizing antibodies develop early and may prevent further dissemination.

3. Secondary viremia and dissemination

In a small proportion of patients, virus escapes local control and produces a more significant viremia.

It may reach the:

• Central nervous system

• Skeletal muscles indirectly through neural damage

• Other tissues

The nervous system is the critical target responsible for paralytic disease.

4. CNS invasion

Poliovirus can reach the CNS by:

• Hematogenous spread

• Spread along peripheral nerves/axons

The virus has a marked tropism for particular neurons, especially motor neurons.

Fenner and White describe poliovirus entry through its receptor CD155/PVR and subsequent intracellular delivery of viral RNA.

5. Destruction of motor neurons

The characteristic lesion is destruction of motor neurons, especially:

• Anterior horn cells of the spinal cord

• Motor nuclei of the brainstem in bulbar disease

Viral multiplication causes neuronal injury and death, followed by inflammatory changes.

Poliovirus does not primarily multiply in skeletal muscle. Muscle atrophy and weakness are secondary to destruction of the motor neurons supplying the muscles.

6. Clinical spectrum

Most poliovirus infections are subclinical.

The clinical spectrum is:

A. Inapparent infection

• No symptoms

• Most common form

B. Abortive/minor poliomyelitis

• Fever

• Malaise

• Headache

• Sore throat

• Nausea/vomiting

• Occasionally gastrointestinal symptoms

C. Non-paralytic poliomyelitis

Essentially an aseptic meningitis syndrome:

• Fever

• Headache

• Neck and back stiffness

• Muscle pain

• Meningeal irritation

D. Paralytic poliomyelitis

The characteristic manifestation is:

Acute flaccid paralysis due to lower motor neuron destruction.

Types include:

• Spinal poliomyelitis — predominantly limb paralysis

• Bulbar poliomyelitis — cranial nerve and brainstem involvement

• Bulbospinal poliomyelitis — combined disease

Respiratory muscle involvement can cause respiratory failure and death.

Fenner and White emphasize that paralysis is an uncommon complication of an otherwise generally mild infection; when paralysis develops, it is typically flaccid and may predominantly affect the legs.

7. Laboratory diagnosis

A. Specimens

The most important specimens are:

1. Stool — most important

• Poliovirus is readily isolated from feces.

• Virus may remain detectable for several weeks after onset.

• Stool specimens are therefore particularly important for poliovirus surveillance.

2. Throat swab/throat specimen

• Useful particularly during the first week of illness.

• Virus disappears from the throat relatively early.

3. CSF

• Poliovirus is rarely recovered from CSF.

• Therefore, a negative CSF culture does not exclude poliomyelitis.

Fenner and White specifically state that virus is readily isolated from feces for up to about six weeks and may be recovered from respiratory secretions during the first week, but is rarely isolated from CSF.

B. Virus isolation

Historically, isolation in cell culture has been an important diagnostic method.

Cell culture

Specimens are inoculated into susceptible:

• Human cell cultures

• Monkey/simian cell cultures

Poliovirus grows rapidly and produces characteristic cytopathic effects (CPE).

Jawetz describes CPE appearing within approximately 3–6 days after inoculation.

Typical CPE includes:

• Cell retraction

• Increased refractility

• Cytoplasmic granularity

• Nuclear pyknosis

• Cell destruction and detachment

Fenner and White also emphasize the rapid destruction of cultured cells following poliovirus infection.

8. Identification and typing

Once an enterovirus is isolated, it must be identified and typed.

A. Neutralization test

The isolate is identified and typed by neutralization with type-specific antisera.

This was traditionally an important method for distinguishing poliovirus types 1, 2 and 3.

B. Molecular methods

Modern diagnosis relies heavily on:

• RT-PCR

• Real-time RT-PCR

• Sequencing of PCR products

• VP1-region sequencing for molecular typing

RT-PCR is faster than conventional culture and allows detection and molecular characterization of enteroviruses.

For poliovirus surveillance, it is particularly important to distinguish:

• Wild poliovirus

• Vaccine-derived poliovirus

• Other enteroviruses

All poliovirus isolates have traditionally been referred to specialized/reference laboratories for detailed characterization.

9. Serology

Serology has a limited role in routine diagnosis.

Paired sera may demonstrate a significant rise in antibody titre during infection.

Important points:

• Neutralizing antibodies appear early.

• They are predominantly type-specific.

• A single antibody titre is generally less useful than demonstration of a rise in titre.

• Serology is much less important than direct detection/isolation of virus for acute poliomyelitis surveillance.

Jawetz describes the use of paired serum specimens to demonstrate a rise in antibody titre and notes that subsequent infection can induce antibodies against antigens shared among the three types.

10. Summary of laboratory diagnosis

Test	Specimen	Importance
RT-PCR/real-time RT-PCR	Stool, respiratory specimens ± other appropriate samples	Rapid and sensitive
Virus isolation	Mainly stool; throat early in illness	Important for confirmation/characterization
Cell culture	Stool/throat	CPE in susceptible cells
Neutralization	Culture isolate	Serotyping
VP1 sequencing	Viral RNA/isolate	Molecular typing
Serology	Paired sera	Demonstration of rising antibody titre
CSF culture	CSF	Usually negative/low yield

11. Important exam points

Remember the sequence:

Fecal–oral entry → oropharynx & intestine → tonsils/Peyer's patches → viremia → CNS invasion → anterior horn cell destruction → acute flaccid paralysis.

Characteristic virus:

Poliovirus = small +ssRNA, non-enveloped, icosahedral, ~30 nm, VP1–VP4, 3 serotypes.

Most important diagnostic specimen:

STOOL

Most important molecular test:

RT-PCR followed by molecular typing/sequencing

Characteristic neurological lesion:

Destruction of anterior horn motor neurons

Characteristic paralysis:

Acute flaccid paralysis

Sources synthesized for this answer

1. Jawetz, Melnick & Adelberg's Medical Microbiology, 26th ed. — Chapter 36, Picornaviruses (Enterovirus and Rhinovirus Groups), especially pp. 527–533: classification, virion structure, poliovirus properties, antigenic types, pathogenesis and laboratory diagnosis.

2. Fenner and White's Medical Virology — Chapter 32, Picornaviruses, especially pp. 447–459: virion structure, poliovirus replication, pathogenesis, clinical disease, laboratory diagnosis and molecular typing.

3. Fields Virology, 7th ed., Volume 2: DNA Viruses (2021) — this particular uploaded volume is a DNA-virus volume and does not contain a dedicated Picornaviridae/poliovirus chapter.

For a 20-mark university answer, the highest-yield headings to reproduce are: Classification → Morphology → Antigenic properties → Pathogenesis/pathology → Clinical spectrum → Laboratory diagnosis → Summary.