General Properties Of Viruses And Laboratory Diagnosis Of Viral Diseases Question And Answers

General Properties Of Viruses

 

Question 1. Write notes on the general characteristics of the virus.
Or
Discuss the general characteristics of viruses.
Answer:

Viruses are unicellular, ultramicroscopic particles containing either RNA or DNA.

General Properties of Viruses:

• Do not possess cellular organization
• Contains only one type of nucleic acid, either RNA or DNA but never both
• Lack of enzymes necessary for protein and nucleic acid synthesis and so depend upon synthetic machinery of host cells
• They multiply by a complex process and by binary fusion.
• They are sensitive to interferons
• They are obligatory parasites.

Read And Learn More: Microbiology Question And Answers

Laboratory Diagnosis Of Viral Diseases

Question 1. Write a short note on laboratory diagnosis of viral diseases.
Answer:

The following are the methods used in diagnosing viral diseases:

1. Direct demonstration of virus and its components
2. Isolation of virus: Viruses can be isolated and identified during the course of the disease.
3. Detection of specific antibodies.

1. Direct Demonstration of Virus and its Components

• Various microscopic methods used for demonstrating viruses are:
• Demonstration of cytopathic changes inside the infected cells, for example, Tzanck cells in the herpes simplex virus
• Demonstration of the inclusion bodies inside stained smear under a light microscope
• Demonstration of the virus by electron microscope or by immunoelectron microscopy
• Demonstration of virus inside the lesions by direct immunofluorescence test.

Electron Microscopy: The detection of viruses by electron microscopy is being used increasingly, especially for viruses that are difficult to culture. Clinical applications of electron microscopy include the detection of rotavirus and hepatitis A virus in fecal specimens.

Immunoelectron Microscopy The sensitivity of electron microscopy can be increased by adding specific antibodies to the specimen to aggregate the virus particles. These aggregates can be observed under electron microscopy.

Fluorescent Microscopy: Direct or indirect fluorescent antibody techniques can be used to detect viruses or viral antigens in vesicle fluid, cell cultures, or frozen tissue sections. The method is also useful for the rapid diagnosis of respiratory infections caused by paramyxoviruses, orthomyxoviruses, adenoviruses, and herpesviruses.

Light Microscopy: Inclusion bodies in tissue sections may be detected by light microscopy. Demonstration of inclusion bodies helps in the diagnosis of some viral infections.

Viral Antigens: These may be detected by enzyme-linked immunosorbent assay (ELISA), radioimmunoassay, and latex agglutination.

Nucleic Acid Probes: Enzyme-labeled or radiolabelled nucleic acid sequence complementary to unique regions in the nucleic acid sequence of a virus is known as a nucleic acid probe. Two strands of the target DNA molecule in the clinical specimens are first separated and then allowed to hybridize with a labeled single-stranded DNA or RNA probe.

After hybridization, this hybridized labeled probe can be detected by different methods depending on the type of label attached to the probe. For example, hybridized enzyme-labeled probes can be detected by color detection by using the appropriate substrate. Cytomegalovirus, papillomavirus, and Epstein-Barr virus have been identified by the use of nucleic acid probes.

Polymerase Chain Reaction: With a PCR technique, a target DNA sequence can be amplified to the point where it can readily be identified using labeled probes in a hybridization assay. Thus, viral DNA extracted from a very small number of virions or infected cells can be detected.

The technique can be used for the diagnosis of infections caused by HIV-1, HIV-2, human papillomaviruses, herpes simplex virus, hepatitis B virus, rubella virus, and Epstein-Barr virus.

2. Isolation of Virus It is the common method used for diagnosing viral infections. Specimens should be collected and sent to the laboratory as soon as possible. In general, viruses are grown by inoculation into animals, eggs, or cell cultures after the specimen should be processed to remove contaminants.

If the viral antigen is present in abundance in the lesions, it is demonstrated by serological methods, i.e.

• Immunofluorescence test
• Precipitation in gel (immunodiffusion)
• Radioimmunoassay
• Enzyme-linked immunosorbent assay
• Countercurrent immunoelectrophoresis
• Immunoperoxidase staining.

Methods of Detection of Nucleic Acid of Viruses:

• Nucleic acid probes (DNA probe analysis)
• Polymerase chain reaction (PCR)
• Reverse transcription polymerase chain reaction (RT-PCR).

3. Detection of Specific Antibodies Demonstrating the rise in titer of antiviral antibodies during the course of the disease is strong evidence of the presence of the aetiological agent. In such cases paired sera are collected from the patient, acute samples should be collected early while convalescent samples should be collected 10 to 14 days later.

Serological techniques used for the detection of specific antibodies are neutralization, ELISA, hemagglutination inhibition test, complement fixation test, immunofluorescence, and latex agglutination test.