Lec 6 Virology Positive single strand rna viruses

Lec 6 Virology

Positive single strand RNA viruses

*Introduction

Viruses with a positive-strand RNA genome (that is, one that can serve

as a messenger RNA in the infected cell) include the viral families

Picornaviridae, Togaviridae, Flaviviridae, Caliciviridae, and

Coronaviridae .

*Molecular structure

The viruses in these families cause a broad spectrum of

diseases but share the following features :

1-they replicate in the cytoplasm .

2-genomic RNAs serve as messenger RNAs and are infectious.

3-genomic RNAs are nonsegmented.

4-virions do not contain any enzymes.

5-virus-specified proteins are synthesized as polyproteins

that are processed by viral and cellular proteases .

* Replication of positive- ssRNA virus genome:

1- These viruses are positive-strand RNA genome (that is, one that can serve as a messenger RNA in the infected cell.

2- Translation: translation of mRNA viral lead to synthesis of a single, long polyprotein, which is processed by viral proteases into nonstructural proteins( enzymes) including the viral RNA dependent RNA polymerase needed to synthesize template (negative RNA )for produce multiple copies of progeny positive-strand RNA .

3- New positive-strand RNA genome (can serve as a messenger RNA)

and translation for non structural proteins ( e.g : capsid protein)

4- Assembly new progeny positive-strand RNA genome and capsid .

5- Releasing of viruses particles by budding or fusion with cell membrane of infected cells .



Figure (1) Replication of positive ssRNA viruses

Note : this figure presence in page : 239 from Microbiology book 2013

*Families of positive ssRNA viruses

A-Picornaviridae :(pico= small)

Picornaviruses are small, naked (nonenveloped), icosahedral viruses which contain a single-stranded, nonsegmented RNA

genome and four structural proteins. Picornaviridae are divided into five genera: common in human called enteroviruses, rhinoviruses, and hepatoviruses

1- enteroviruses:(example 1-poliovirus)

*Epidemiology: Individuals are infected with enteroviruses by

ingestion of contaminated food or water. Enteroviruses are stable

at the low pH of the stomach, replicate in the gastrointestinal

(GI)tract, and are excreted in the stool. (diagnosis).Thus, these viruses can be transmitted by the fecal–oral route. The virus can replicate in a variety of tissues. For example, after replicating in the oropharynx

and intestinal tract lymphoid tissue, enteroviruses can enter the

bloodstream and, thereby, spread to various target organs (for

example, poliovirus spreads to the central nervous system (CNS).

*Pathogenesis

All enteroviruses can cause CNS disease. For example,

enteroviruses are currently the major recognizable cause of acute

aseptic meningitis syndrome.

1- poliovirus infection or Poliomyelitis is an acute illness in which the poliovirus selectively destroys the lower motor neurons of the spinal cord and brainstem, resulting in flaccid,asymmetric weakness or paralysis.

2- Respiratory paralysis may also occur .

3- Poliomyelitis should be considered in any unimmunized person

with the combination of fever, headache, neck and back pain.

Figure (2) Central nervous system invasion by poliovirus.

Note : this figure presence in page : 285 from Microbiology book 2013

B- Rhinoviruses :

*Epidemiology :Rhinoviruses cause the common-cold syndrome . They differ in two important respects from enteroviruses. First rhinoviruses are acid labile. Second rhinoviruses replicate in the nasal passages, have an optimal temperature for replication that is lower than that of enteroviruses.This permits rhinoviruses to replicate efficiently at temperatures several degrees below body temperature. Rhinovirus replication is similarto that of the poliovirus .Because there are more than 100 serotypes of rhinoviruses, development of a vaccine is impractical.

*Transmission

Studies have shown that in addition to being spread by respiratory droplets, rhinoviruses can also be spread by hand-to hand contact. Therefore, hand washing at appropriate preventive measure.

Figure (3) Pathogenesis of the common cold showing stages from infection to recovery. Note : this figure presence in page : 286 from Microbiology book 2013

2- Togaviridae : (example Rubivirus)

The togaviruses are enveloped, icosahedral viruses that contain a positive-sense, single-stranded RNA genome and generally three structural proteins. The capsid (C) protein encloses the viral RNA, forming the nucleocapsid, and the two other proteins (E1 and E2) are glycoprotein that form the hemagglutinin-containing viral spikes that project from the lipid bilayer.

*Rubella virus :

The member of the Rubivirus genus is rubella virus.

*Transmission :

Respiratory secretions of an infected person are the primary vehicles for rubella virus transmission.

*Clinical signs and pathogenesis :

1-Rubella causes a mild clinical syndrome that is characterized by a generalized maculopapular rash and lymphadenopathy

2- severe damage possible to the developing fetus (especially in the first trimester) woman is infected during pregnancy (congenital rubella).

3- This damage can include congenital heart disease, hepatitis, and abnormalities related to the CNS, such as mental retardation, motor

dysfunction, and deafness.

Note : Fetal damage resulting from rubella infection is preventable by use of the live attenuated rubella vaccine.

Figure (4) Pathology of rubella virus infection.

Note : this figure presence in page : 288 from Microbiology book 2013

Questions expected about lectures 5and 6 :

1-A- mention the classical and modern method used for diagnosis rotavirus

B- mention the molecular stucture for picornaviridae .

C- How the poliovirus infect the central nervous system by diagram .

2- Choose the best correct answer :

A- one from below viruses caused congenital is :

a- Rhinovirus b- CMV c- Rubella virus d- b+c

B- Which of the following groups RNA viruses are common

causes of viral meningitis?

a. Rhinoviruse b. Caliciviruses c. Hepatitis C virus

d. Enteroviruses