6. The Pathogenicity of Viruses - Poliovirus

Question 1

The Polio virus

Answer 1

The Polio virus

Question 2

What disease is caused by the polio virus?

Answer 2

Poliomyelitis

Question 3

What type of virus is the polio virus?

Answer 3

Human enterovirus

Question 4

What viral family does the poliovirus belong to?

Answer 4

Picornaviridae

Question 5

Describe the genome of the polio virus.

Answer 5

Single stranded positive sense RNA molecule, about 7500 nucleotides long.

Question 6

What receptor on the host cell surface does the poliovirus bind to in order to fascilitate attachment?

Answer 6

CD155

Question 7

How does poliovirus enter the host cell?

Answer 7

The mechanism is not fully under stood but it is suspected to enter either via the formation of a pore by which the viral RNA is injected into the host cell or by receptor mediated endocytosis.

Question 8

Where does the poliovirus replication cycle take place?

Answer 8

The exact replication cycle of the poliovirus is not fully understood however poliovirus has been identified in lyphoid tissue such as tonsils and lymphoid aggregates known as Peyer’s patches in the ileum and small intestine of the gastrointestinal tract, therefore some studies suggest these locations to be the site of replication.

Question 9

Why does transcription not occur for the poliovirus?

Answer 9

Poliovirus genome is a positive sense RNA strand and therefore this strand can be directed translated using host cell machinery as if it were mRNA to produce its protein products. Transcription therefore does not occur.

Question 10

Describe the process of translation in the poliovirus.

Answer 10

the poliovirus has an extremely long (>700 nucleotides in length) 5’ end termed the internal ribosome entry site which is associated with the facilitation of translation.Translation results in the production of one extremely long polypeptide.

Question 11

What occurs to the single large poliovirus polypeptide post translation?

Answer 11

This single polypeptide is autocleaved by viral proteases 2Apro and 3Cpro into ~10 individual viral proteins

Question 12

What are the viral protein products of the poliovirus and what are their function?

Answer 12

1. 3Dpol: RNA polymerase copies vRNA to multiply the genome.2. 2Apro: cleave viral polypeptide 3. 3Cpro/3CDpro: cleave viral polypeptide4. 3B: part of protein complex for viral replication5. 2BC: part of protein complex for viral replication6. 2B: part of protein complex for viral replication7. 2C: part of protein complex for viral replication8. 3AB: part of protein complex for viral replication9. 3A: part of protein complex for viral replication10. VPO: which is further cleaved into VP2 & VP4 and VP1 & VP3 forms viral capsid.

Question 13

How many serotypes of poliovirus are identified?

Answer 13

1. PV12. PV23. PV3

Question 14

What species can be infected by the poliovirus?

Answer 14

Humans due to the presence of CD155 receptor.

Question 15

How is polioviris transmitted?

Answer 15

Poliovirus infection occurs via the fecal-oral route meaning the virus is ingested , repliction occurs in the alimentary canal and the virus is shed in the feces of infected individuals.

Question 16

What other sites are known to be capable of infection by the poliovirus?

Answer 16

Brown fatreticuloendothelial tissuemuscleMotor neurons of CNS

Question 17

What is a consequence of CNS motor neuron infection by poliovirus?

Answer 17

paralysis

Question 18

What three theories are proposed to suggest how poliovirus may enter the CNS?

Answer 18

1. The virion passes directly from the blood into the CNS by crossing the blood-brain barrier independent of CD155.2. Virions enter the blood to peripheral tissues where they are transported to the CNS through retrograde axonal transport.3. Virions are imported into the CNS via infected monocytes and macrophages.

Question 19

How does the poliovirus evade the immune system?

Answer 19

1. Capable of surviving in acidic conditions of the gastrointestinal tract allowing the virus to infect the host and spead throughout the body via the lymphatic system. 2. It replicates very quickly overwhelming the host organs before an immune response is mounted.

Question 20

Mention two complications of poliovirus infection.

Answer 20

1. Possible spread to motor neurons causing poliomyelitis or to spinal cord causing encephalitis. 2. Muscle weakness 30-40 years after resolution due to lingering damage to motor neurons.

Question 21

What is known about polio virus release from the cell?

Answer 21

Very little is known regarding the mechanism however it is known that enteroviruses are generally considereing to be highly lytic and therefore it is suspected that viroids are released from host cells upon lysis. It estimated that each dying host cell releases up to 10, 000 polio virions

Question 22

What is the typical incubation period for a poliovirus infection?

Answer 22

3-35 days