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LABORATORY DIAGNOSIS OF VIRAL INFECTIONS.
In developing countries, virological specimens will need to be
transferred from district laboratories to regional or central virology
laboratories.
Diagnostic Methods in Virology
1. Direct Examination
2. Indirect Examination (Virus Isolation)
3. Serology
1-Direct Examination
This is now becoming a widely used-and fast- method of virus diagnosis. Virus
or viruses' antigen is detected in lesions, fluid, tissues, or excretions from the
patient and a result can be obtained within an hour or two of receipt of
specimen.
1. Serology (Antigen Detection)
Immunofluorescence, ELISA etc.
2. Electron Microscopy:
a- Morphology of virus particles
b- Immune electron microscopy
3. Light Microscopy
a- Histological appearance
b- Inclusion bodies
4. Viral Genome Detection
a. Hybridization.
c. Polymerase Chain Reaction (PCR)
Electron Microscopy
•106 virus particles per ml required for visualization,
50,000 - 60,000 magnification normally used.
•Viruses may be detected in the following specimens.
1. Faeces
Rotavirus, Adenovirus
2. Vesicle Fluid
HSV, VZV
3. Skin scrapings
papillomavirus.
Immune Electron Microscopy
The sensitivity and specificity of EM may be enhanced by immune
electron microscopy.
There are two variants:-
a. Classical Immune electron microscopy (IEM)
1. The sample is treated with specific anti-sera before being put
up for EM.
2. Viral particles present will be agglutinated and thus congregate
together by the antibody.
b. Solid phase immune electron microscopy (SPIEM)
1. The grid is coated with specific anti-sera.
2.Virus particles present in the sample will be absorbed onto the
grid by the antibody.
Problems with Electron Microscopy:
•Expensive equipment.
•Expensive maintenance.
•Require experienced observer.
•Sensitivity often low.
2-Indirect Examination
1.Cell Culture
2. Eggs
3. Animals
cytopathic effect (CPE)
haemabsorption
immunofluorescence
Haemagglutination
Inclusion bodies
disease or death
Virus Isolation
Cell Cultures are most widely used for virus isolation.
there are 3 types of cell cultures:
1-Primary cell cultures:
are widely acknowledged as the best cell culture systems
available since they support the widest range of viruses.
However, they are very expensive and it is often difficult to
obtain a reliable supply.
2-Semi-continuous cells.
3-Continuous cells :
are the most easy to handle but the range of viruses supported
is often limited.
Growing virus may produce:
1. Cytopathic Effect (CPE) - such as the ballooning of
cells or syncytia formation, may be specific or nonspecific.
2. Haemadsorption - cells acquire the ability to stick to
mammalian red blood cells.
Problems with cell culture
•Long period (up to 4 weeks) required for result.
•Often very poor sensitivity, sensitivity depends on a large
extent on the condition of the specimen.
•Susceptible to bacterial contamination.
•Susceptible to toxic substances which may be present in the
specimen.
•Many viruses will not grow in cell culture e.g. Hepatitis B,
Diarrhoeal viruses, parvovirus, papillomavirus.
Rapid Culture Techniques:
Rapid culture techniques are available whereby viral antigens
are detected 2 to 4 days after inoculation. The CMV DEAFF
test is the best example, whereby
•The cell sheet is grown on individual cover
slips in a plastic bottle.
•Following inoculation, the bottle then is
spun at a low speed for one hour (to speed
up the adsorption of the virus) and then
incubated for 2 to 4 days.
•The cover slip is then taken out and
examined for the presence of CMV early
antigens by immunofluorescence.
3- Serology
Detection of rising titres of antibody between acute and
convalescent stages of infection, or the detection of IgM in
primary infection. It is far the most widely used way to diagnose
virus infection.
Criteria for diagnosing
Primary Infection:
-4 fold or more increase
in titre of IgG or total
antibody between acute
and convalescent sera.
Criteria for diagnosing
Reinfection:
-fold or more increase in titre
of IgG or total antibody
between acute and
convalescent sera.
- Presence of IgM
- Absence or slight increase in
IgM
Serological events following primary infection and reinfection. Note that in
reinfection, IgM may be absent or only present transiently at a low level.
Tests used in serology:
The old but well-tried and reliable technique of complement
fixation test is now being replaced by more sensitive assays,
especially those which detect virus-specific IgM. But, note, the
complement fixation test is still an indispensable technique in
virus laboratory.
Below are some of the most widely used tests:
1. Enzyme linked immunosorbent assay (ELSA):
Now widely used and often available as
commercially produced kits, because of its:
•Sensitivity.
•Low cost compared with other serological
techniques.
•Because it is possible to test large numbers of specimens at the same time.
At present ELISA is used mainly to diagnose infections causes by:
•Rotaviruses.
•Hepatitis B virus and hepatitis C surface antigen.
Virus + patient's serum
.Add enzyme-labelled anti-human IgM antiserum
Incubate
Stop reaction
Add substrate
Measure reaction by colour intensity in optical density
reader
Calculate as positive or negative reaction by comparison with controls
Anti-human IgM (or anti IgG) antibodies is used to detect specific IgM (or IgG) in
the serum under test. Labeled anti-human antibody is used to detect the virus
antibody: the label is an enzyme which reacts with a suitable substrate (the most
used is alkaline phosphatase: paranitrophenyl phosphate) to produce a visible
colour change.