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DIAGNOSIS OF VIRUS INFECTIONS

DIAGNOSIS OF VIRUS INFECTIONS

The laboratory procedure used in diagnosis of viral diseases include:

I- Direct detection of viruses:

their antigens, or their nucleic acids in clinical specimens. Can be achieved by different technique:

1- Light microscope: This can be used to visualize some large viruses e.g. pox viruses in which elementary bodies can be seen in skin lesions. Inclusion bodies can also be seen under the light microscope in several viral infections. In rabies. Intracytoplasmic inclusions called “Negri bodies” can be detected in nerve cells.

2- Electron microscope: is used to demonstrate virus particles in vesicular fluid or tissue extracts treated numbers of particles (10^9/ml) are present.

3- Immunoelectron microscopy (IEM): Addition of specific antisera to the clinical material leads to aggregation of virus, so it can be detected more readily than separate virus particles e.g. diagnosis of HAV and rotavirus in stools.

4- Immunoflurescence microscopy: Detection of virus in smears from lesion using fluorescien labeled specific antisera e.g. diagnosis of rabies in brain smears.

5- Solid phase immunoassays: Both radio immunoassay (RTA) and enzyme linked immunosorbent assay (ELISA) can be used for detection of H.A and rotavirus in stools and hepatitis B antigens in blood.

6- Nucleic acid hybridization: using DNA probes, it is possible to detect virus nucleic acid in pathologic specimens or in tissue samples. The probe which is a single strand of the nucleic acid of the virus in question will hypridize with labeled and can be easily detected.

7- Polymerase chain reaction (PCR): This technique involves amplification of a short sequence of a target DNA or RNA (which may be in low concentration e.g. one copy) leading to accumulation of large amounts of that sequence, so it can be easily detected.

PCR can be used to determine the quantity of viruses in patient’s blood i.e. virus load e.g. in HIV patients.

This is used to monitor the course of the disease and to evaluate treatment and prognosis.

II- Isolation of viruses:

Isolation of virus from clinical specimens by inoculation on tissue culture, chich emryo or laboratory animals according to the virus in question.

III- Serologic detection of antiviral antibodies:

Serologic diagnosis of virus infections can be established by detecting a rising antibody titer to the virus. The first sample should be collected early after onset, the second 102 weeks later. If paired sera are not available or rapid diagnosis is needed, as in diagnosis of rubella in early pregnancy, detection of Igm antibodies to the virus is restored to. The detection of Igm is a single serum sample, indicates recent infection.

The presence of IgM antibodies to any virus e.g. CMV or rubella in the new born serum indicates infection in intero (IgM does not cross the placenta).

The tests used for serologic diagnosis of viral infections include:

• Virus neutralization
• Complement fixation
• Haemagglutination inhibition
• Radio immunoassay (RIA)
• ELISA

DIAGNOSIS OF VIRUS INFECTIONS

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