Wednesday, June 1, 2011

Introduction to Clinical virology

Definition of virus

Virus is a small infectious agent (10-30 nm) in diameter. It is metabolically inert, since it cannot replicate outside of a living cell I,e needs a living media (tissue culture) for isolation. Because viruses are non-motile, they are entirely dependent on external physical factors for chance movement and spread to infect other susceptible cellsIt is composed of nucleic acid either DNA or RNA and a capsid. Some viruses acquire envelope from the host.


Viruses are broadly classified primarily upon the type of genomic nucleic acid, eg. DNA or RNA ,

and then further by the number of strands of nucleic acid (eg. double-stranded DNA, double-stranded RNA or single-stranded RNA, with a positive or negative "sense" of that single strand).

Retroviruses are a special category of RNA viruses that require reverse transcription of their RNA to DNA and then integration of that DNA into the host cell genome before replication can take place. They carry a reverse transcriptase enzyme as part of the virion.

Classification of viruses into families based on chemical & physical properties

A) DNA viruses

 -Adenovirus, Upper and lower respiratory infections, hemorrhagic cystitis, keratoconjunctivitis.

 -Herpes Viruses

• Human herpesvirus 1 (herpes simplex virus type 1)

• Human herpesvirus 2 (herpes simplex virus type 2)

These cause:

gingiva stomatitis - vesicles and ulcers in the mouth

herpes genitalis - vesicles and ulcers on genitalia

herpes labialis (cold sores, fever blisters) - vesicles and ulcers of lips

herpes gladiatorum - clusters of vesicles and ulcers on skin



whitlow (felon) a purulent infection involving the pulp of the distal phalanx of a finger

• Human herpesvirus 3 (varicella-zoster virus) - this virus causes:

 chickenpox (varicella) - epithelial cell infection resulting in an exanthem of macules, papules, pustules, vesicles and shallow ulcers

 shingles (zoster) - peripheral nerve cell infection with an eruption in the overlying epidermis

• Human herpesvirus 4 (Epstein-Barr virus) - This virus causes:

 infectious mononucleosis (b-lymphocyte infection)

 Burkitt's lymphoma

 oropharyngeal carcinoma

• Human herpesvirus 5 (cytomegalovirus) - this is the agent of:

 cytomegalovirus mononucleosis - similar to, but milder than, infectious mononucleosis

 cytomegalic inclusion disease (salivary gland disease) - a generalized and often fatal disease of the newborn

• Human herpesvirus 6 (human b-lymphotrophic virus) - causes exanthem subitum (fourth disease, Duke disease or roseola infantum and possibly multiple sclerosis)

• Human herpesvirus 8 - causes Kaposi's sarcoma in AIDS patients

• Human herpesvirus 7 - causes a cryptic infection of the T-helper cell

 -EBV---Mononucleosis, associated with Burrkitt s lymphoma and nasopharyngeal carcinoma.

 -HBV—Hepatitis B

 -B19 virus - causes fifth disease (erythema infectiosum), bone marrow aplasia and polyarthralgia. Target tissue is the erythroid cell

B) RNA viruses

 Influenza virus---Influenza

 Measles virus---measles

 Mumps virus---mumps and orcitis

 Rubella virus---Rubella and congenital malformation as cardio vascular and neurological.

 Parainfluenza-----Bronchiolitis in infants and croups inyoung children

 RSV------Bronchiolitis and pneumonia in infants


 HCV----Hepatitis C and complications

 HDV----hepatitis

 HAV---Hepatitis A in children

 Coxsachie viruses------aseptic meningitis, myocarditis and pericarditis

 Rota virus----Diarrhea in young children

 Rhinovirus------common cold

Laborator Diagnosis OF VIRUSES

Collection of specimens for viral disease diagnosis:

-Swabs of the lesion sites and transport in viral transport media

-Aspiration of secretions or exudates

-Excreta such as urine or stool.

-Biopsy samples obtained by needle aspirations, open exploration or endoscopy.

-Blood samples for antigen detection of some viruses, for serological tests, and for PCR.

Diagnosis of viral infections

Viruses can be studied in a number of direct and indirect ways and all these methods can be applied in a diagnostic situation, ie. is this patient infected with a particular virus? There are two approaches:

Detection and demonstration of the virus itself; and

The study of the host's response to that virus

Detection of virus :


II-Detection of viral antigen

III-Detection of viral DNA or RNA.


The types of cell culture are

-Monolayer cultures Primary


Continuous line

- Tissue culture

-Organ culture

Detection of viral growth by

1-Characteristic cytopathic effect.

2-Haemadsorption (attachment of RBCS to the surface of virus infected cells) Some viruses (eg. myxo- and paramyxoviruses, including influenza) have the property of haemagglutination (causing red blood cells to stick together ) which can be used to detect and quantitate the virus (by haemagglutination).

3-Interference with the formation of cytopathic effect e.g neutralisation of viral infectivity by antibodies can be used to detect and quantify either virus or specific antibody to that virus.

4-Adecrease in acid production by infected and dying cells.

5- Recent method depend on use of fluorescent antibodies to the infective virus antigen, or by use of enzyme linked immunosorbant assay (ELISA).

II-Viral antigens can be detected by a wide range of serological techniques utilizing polyclonal or monoclonal antibodies. Techniques include precipitation, agglutination, immunofluorescence, ELISA, complement fixation and radio immuno assays.

III-Molecular techniques of both protein chemistry and nucleic acid biochemistry have greatly improved the specificity of virus diagnostic procedures. Methods include:-

• polyacrylamide gel electrophoresis (PAGE) of protein fragments

• western blotting, and identification of specific proteins with labelled probes

• polymerase chain reaction (PCR), to amplify specific segments of viral nucleic acid

• Southern blotting, and DNA hybridisation with labelled probes

• sequencing of portions of the viral genome

• restriction fragment length polymorphisms of viral nucleic acid


-The application of sophisticated molecular technology has enabled the generation of diagnostic assays for viruses that have not yet been visualized or cultured. Hepatitis C virus is the prime example. This RNA virus has never been cultured, but portions of its genome were extracted from blood known to be infectious for hepatitis C.

-Molecular biology methods have been used to compare degrees of relatedness of similar organisms and to build phylogenetic trees ("family trees" based on genomic similarities). The ability to detect and sequence portions of a viral genome permits genetic markers of specific sub-strains to be identified. This has led to the new science of genetic epidemiology (ie. disease tracing).

The study of the host's response to that virus. Techniques include precipitation, agglutination, immunofluorescence, ELISA, complement fixation and radio immuno assays can be used to detect specific antibodies to those viruses in the patient's serum. Identification of different classes of antibodies (IgG and IgM) can aid in differentiating between a current infection and immunity.
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